Unveiling the role of Ta in Cr-Ta-Ti modified silicide coatings for Nb alloy with superior oxidation resistance up to 1500 ℃

Unveiling the role of Ta in Cr-Ta-Ti modified silicide coatings for Nb alloy with superior oxidation resistance up to 1500 ℃ | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Unveiling the role of Ta in Cr-Ta-Ti modified silicide coatings for Nb alloy with superior oxidation resistance up to 1500 ℃ Hao Jiang, Yanqiang Qiao, Weiping Zhang, Xiping Guo, Qifan You, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9197546/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 8 You are reading this latest preprint version Abstract To address the insufficient high temperature oxidation resistance of Nb521 alloy in engineering applications, Cr-Ta-Ti modified silicide coatings with different Ta contents were fabricated on its surface via slurry sintering. The oxidation behavior and elemental diffusion characteristics of the coatings were systematically investigated at 1400–1500 ℃, and the mechanism by which Ta affects the microstructural evolution and oxidation resistance of the coatings was revealed. The results show that the Cr-Ta-Ti modified silicide coating exhibits a distinct multilayer structure, which from top to bottom consists of a (Nb,Cr,Ti,Ta)Si 2 /Cr 4 Nb 2 Si 5 composite layer, a dense NbSi 2 layer, a NbSi 2 /Nb 5 Si 3 layer, and a Nb 5 Si 3 transition layer. The introduction of Ta influences the diffusion of Si, Cr, and other elements, resulting in a reduced coating thickness. An appropriate Ta content (5Ta) can markedly decrease the growth rate of the oxide scale, providing effective protection for the substrate for at least 150 h at 1400 ℃ and 100 h at 1500 ℃, respectively. The dissolution of Ta into SiO 2 and TiO 2 enhances the high temperature viscosity and thermal stability of the oxide scale, restrains the inward diffusion of oxygen and the outward diffusion of coating elements, thereby significantly improving the high temperature oxidation resistance of the coating. Nb521 Silicide coating Slurry sintering Ta modification High temperature oxidation resistance Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 1. Introduction To meet the urgent demands of next-generation aero-engines with high thrust-to-weight ratios and reusable spacecraft for extreme hot-section components (service temperature > 1500 ℃), developing material systems with both ultra-high temperature mechanical properties and long-term oxidation resistance has become an imperative [ 1 , 2 ]. Due to their high melting points, low densities, excellent high temperature strengths, and good processability, Nb based alloys are considered ideal structural materials in the ultra-high temperature field [ 3 – 7 ]. Currently, Nb based alloys widely studied and applied in the aerospace sector (such as C103, Nb521, Nb-Si based alloys, and Nb-Ti-Al based alloys) inherently form expansive oxides, such as Nb 2 O 5 , during the oxidation process, which are detrimental to their oxidation performance [ 8 ]. Consequently, Nb based alloys must rely on compatible high temperature anti-oxidation coatings for protection when utilized in aerobic environments [ 9 – 13 ]. Among various coating systems, silicide coatings have emerged as one of the most extensively researched and successfully engineered protective strategies, as they can form a continuous, dense SiO 2 protective scale with an extremely low oxygen diffusion rate during high temperature oxidation [ 14 , 15 ]. Early studies predominantly focused on single silicide coatings such as MoSi 2 and NbSi 2 [ 16 , 17 ]. However, these generally suffer from intermediate-temperature "pesting" oxidation failure, cracking caused by coefficient of thermal expansion (CTE) mismatch with the substrate, and the decomposition and degradation of silicide phases under long-term high temperature [ 18 ]. To overcome these bottlenecks, elemental modification and structural composite design have been widely adopted to mitigate these issues [ 19 – 24 ]. For instance, preparing multi-element modified silicide coatings by introducing elements such as Cr, Ti, and Al can alleviate the aforementioned problems to a certain extent [ 25 – 27 ]. Under ultra-high temperature service conditions, coatings face immense challenges. For example, the SiO 2 scale exhibits insufficient thermal stability at ultra-high temperature, presenting a significant risk of high temperature volatilization [ 28 ]. Furthermore, exposure to ultra-high temperature aggravates elemental interdiffusion within the coating, which can easily induce the premature decomposition of key protective phases including NbSi 2 and exhausts the constrained silicon reservoir within the coating [ 29 ]. In addition, the interdiffusion and degradation at the coating/substrate interface are prominent. The continuous inward diffusion of Si into the substrate not only leads to Si depletion in the coating but also forms a brittle silicide layer at the interface, acting as a mechanical weak link [ 30 ]. Introducing refractory elements or high temperature stabilizing elements is an effective approach to enhance the oxidation resistance of coatings at higher temperature [ 31 , 32 ]. For example, Wang et al. [ 33 ] prepared a Y 2 O 3 -modified MoSi 2 silicide coating using supersonic atmospheric plasma spraying. Their research demonstrated that Y 2 O 3 can regulate the microstructure of the generated glassy oxide scale during oxidation, thereby improving the high temperature oxidation resistance of the coating. Wang et al. [ 20 ] fabricated a (Nb,Ti,Mo)Si 2 -(Ti,Nb,Mo) 5 Si 3 composite coating on the surface of C103 alloy using slurry sintering method. The coating maintained its structural integrity after oxidation for 100 h at 1400 ℃ and 1500 ℃, respectively. Ta and Nb are refractory metals of the same group with similar atomic radii and chemical properties, allowing Ta to dissolve into the niobium silicide phase to form a stable solid solution [ 34 , 35 ]. This solid solution effect not only strengthens the coating via lattice distortion but also draws significant attention in the field of ultra-high temperature protective coatings due to extremely high melting point (~ 3017 ℃) of Ta and superior high temperature stability. Existing studies have confirmed that Ta can dissolve into the SiO 2 glass network formed at high temperature, which not only significantly reduces the oxygen permeability of liquid SiO 2 [ 36 ] but also enhances the bond strength of Si-O bonds [ 37 , 38 ]. These effects are of paramount importance for enhancing the compactness of the oxide scale and extending its long-term thermally stable service life. However, studies also indicate that the protective performance of a single Ta-Si-O oxide scale is less than ideal. For instance, Bahr et al. [ 39 ] found that a single TaSi 2 coating forms a discontinuous and porous Ta 2 O 5 +SiO 2 mixed oxide scale at high temperature (> 850 ℃), which may conversely act as a rapid diffusion channel for oxygen ions, accelerating the coating's oxidation rate. It is evident that simple Ta substitution or addition does not consistently yield excellent protective effects and may even induce adverse impacts under certain circumstances. To overcome the shortcomings of single element modification, it is feasible to design the co-addition of Ta with metallic elements (e.g., Ti, Cr, Mo, W, Hf) or active elements (e.g., B, Y). Through the multi-element synergistic effect, it is promising to break through the performance bottlenecks of singly modified systems. Nevertheless, current research on multi-element modification mainly focuses on Cr-Ti or Mo-W systems [ 27 , 40 ], and the application of Cr-Ta-Ti ternary synergistic modification in silicide coatings has not yet been reported. There is a lack of systematic experimental data and in-depth mechanistic analyses addressing key scientific issues, such as the microstructural evolution laws of multi-element modified silicide coatings and the formation mechanisms of mixed oxide scales under high temperature environments. In view of this, the present study aims to systematically investigate the microstructure, high temperature oxidation behavior, and intrinsic mechanisms of Cr-Ta-Ti modified silicide coatings on the surface of Nb521 alloy. The Cr-Ta-Ti modified silicide coatings were successfully fabricated on the Nb521 alloy using the slurry sintering method, which is widely applied in industry. Through systematic long-term isothermal oxidation experiments at 1400 ℃ and 1500 ℃, the oxidation kinetics, microstructural evolution of the oxide scale, and coating degradation behavior before and after modification were comparatively analyzed. Emphasis was placed on the effects of Ta content on the phase formation, microstructure, and diffusion behavior of key elements (Si, Cr, Ti, Nb) in the coatings. Finally, the functional mechanisms of Ta modification on the composition, structure, adhesion, and oxygen barrier properties of the mixed oxide scale were discussed. The results of this study not only provide a new perspective for understanding the oxidation protection mechanisms of multi-element silicide coatings modified by refractory elements but also offer crucial theoretical bases and feasible technical pathways for developing long-life, highly reliable protective coatings for Nb alloys applied in extreme ultra-high temperature environments. 2. Materials and methods 2.1 Substrate material In this work, Nb521 alloy with a nominal composition of Nb-5W-2Mo-1Zr (wt.%) was used. The alloy ingot was cut into specimens with dimensions of 15 mm × 6 mm × 3 mm using a wire electrical discharge machine. All specimen surfaces, edges and corners were gradually ground and polished with SiC sandpapers from 80# to 800# to reduce stress concentration. Subsequently, the specimens were ultrasonically cleaned in ethanol for 15 min and dried for later use. 2.2 Coating process Pure Si powder (purity ≥ 99%, particle size 74 µm), Cr powder (purity ≥ 99.9%, particle size 2–5 µm), Ti powder (purity ≥ 99.9%, particle size 1–3 µm) and Ta powder (purity ≥ 99.95%, mesh < 325) were selected as raw materials. The corresponding powders were weighed according to the mass ratio Si:Cr:Ti:Ta = 15:4:1:x (x = 0, 5, 10, 15), mixed with ethanol and polyvinyl butyral (PVB), and ball milled for 4 h to prepare a uniform slurry. For convenience, the Cr-Ta-Ti modified silicide coatings with different Ta contents are denoted as 0Ta, 5Ta, 10Ta and 15Ta, respectively. The pretreated substrate specimens were immersed in the slurry for 3–5 s, taken out and dried in air for 10–15 min, and this process was repeated three times. The coated specimens were dried in a vacuum oven at 100 ℃ for 4 h. Finally, all samples were sintered at 1450 ℃ for 0.5 h in an ultra-high vacuum furnace and then cooled to room temperature with the furnace. 2.3 Oxidation test Isothermal oxidation tests were carried out in a muffle furnace at 1400 ℃ and 1500 ℃ for 1-150 h. After oxidation, the samples were directly taken out of the furnace and cooled naturally in air. The mass changes before and after oxidation were measured using an electronic balance with an accuracy of 0.1 mg 2.4 Analyzing methods Scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (SEM/EDS, TESCAN MIRA 3 and Inca X-sight) and X-ray diffraction (XRD, Panalytical X’Pert PRO, Cu Kα) were employed to analyze the microstructure, phase composition and elemental distribution of the samples before and after oxidation. Elemental distribution of the samples was characterized by electron probe microanalysis (EPMA). 3. Results 3.1 Microstructure of the Cr-Ta-Ti modified silicide coating Figure 1 and 2 show the surface XRD patterns and surface BSE images of the Cr-Ta-Ti modified silicide coatings with different Ta contents, respectively. According to the XRD analysis results, all coatings are mainly composed of the NbSi 2 phase and a small amount of the Cr 4 Nb 2 Si 5 phase. As seen from the surface images of the coatings (Fig. 2 ), with the increase of Ta content, the surface roughness of the coatings gradually increases, and the number of pores also increases accordingly. After the addition of Ta, a white Ta-rich phase (Sites 3 and 4 in Fig. 2 ) appears on the coating surface, which remains the NbSi 2 phase. Furthermore, a small amount of incompletely reacted residual slurry can be observed in the samples, as indicated by Site 6 in Fig. 2 d and EDS composition in Table 1 . (a) 0Ta, (b) 5Ta, (c)10Ta and (d)15Ta. Table 1 Chemical compositions of the marked sites in Fig. 2 , determined by EDS. Sites Composition (at.%) Si Nb Cr Ti Ta 1 69.9 21.2 5.8 3.1 - 2 71.7 18.3 5.9 3.6 0.5 3 66.0 19.5 8.5 3.3 2.7 4 69.7 12.6 9.7 3.7 4.3 5 69.6 17.4 8.7 3.8 0.5 6 44.4 4.2 23.1 13.0 15.3 7 67.4 23.1 4.3 3.8 1.4 Figure 3 presents the cross-sectional BSE morphologies of the Cr-Ta-Ti modified silicide coatings with different Ta contents. There is no significant difference in the overall structure of all coatings, which are composed of two layers: a main layer and a transition layer. The main layer can be further divided into two sub-layers: the upper layer consists of a gray phase and a dispersed dark-gray granular phase, while the lower layer is a single gray phase. Combining the EDS compositional analysis (Table 2 ) and XRD results (Fig. 1 ), it can be determined that the upper gray phase is the (Nb,Cr,Ti,Ta)Si 2 phase (Sites 1, 2, 6, and 7 in Table 2 ). Moreover, as the Ta content increases, the Ta concentration in the (Nb,Cr,Ti,Ta)Si 2 phase increases synchronously, albeit with a limited increment. The dispersed dark-gray phase is identified as the Cr 4 Nb 2 Si 5 phase (Site 3 in Table 2 ). The lower single gray phase is the NbSi 2 phase (Site 4 in Table 2 ), and the transition layer is composed of Nb 5 Si 3 phase. Between the single NbSi 2 layer and the Nb 5 Si 3 layer, a mixed layer composed of NbSi 2 +Nb 5 Si 3 is found. Evidently, the coating consists of a (Nb,Cr,Ti,Ta)Si 2 +Cr 4 Nb 2 Si 5 layer, a single NbSi 2 layer, a NbSi 2 +Nb 5 Si 3 mixed layer, and a Nb 5 Si 3 transition layer from top to bottom. It is noteworthy that the coating thickness gradually decreases with the increase of Ta content. The thicknesses of the main layers for the 0Ta, 5Ta, 10Ta, and 15Ta coatings are approximately 122, 113, 90, and 77 µm, respectively. Table 2 Chemical compositions of the marked sites in Fig. 3 , determined by EDS. Sites Composition (at.%) Si Nb Cr Ti Ta 1 68.7 20.6 6.9 3.8 - 2 69.9 22.7 5.0 1.2 1.2 3 48.0 13.2 33.7 5.1 - 4 67.7 32.3 - - - 5 38.4 61.6 - - - 6 66.7 26.1 4.2 1.2 1.8 7 68.3 22.7 4.4 2.6 2.0 Figure 4 displays the cross-sectional BSE image of 5Ta coating and the elemental mapping of the corresponding region. It can be seen that the Nb element exhibits a three-layer distribution in the coating above the substrate. The bottom most part of the coating, which has a lower Si content, is the Nb 5 Si 3 transition layer, above which lies the NbSi 2 and Nb 5 Si 3 mixed layer. The Si and Nb elements are distributed uniformly in the single NbSi 2 layer. There is a distinct Cr-enriched region in the upper layer of the coating, primarily corresponding to the Cr 4 Nb 2 Si 5 phase. The Ti element is also enriched here and is mainly distributed in the upper layer of the coating. The Ta element is distributed relatively uniformly throughout the coating, but its content is relatively higher within the range from the coating surface down to 30 µm. Mo, W, and Zr are substrate elements and are mainly concentrated in the substrate. 3.2 Oxidation behavior of the coating 3.2.1 Oxidation kinetics The mass changes per unit area of the Cr-Ta-Ti modified silicide coatings with different Ta contents after oxidation at 1400 ℃ and 1500 ℃ are shown in Fig. 5 . The 0Ta coating exhibits a characteristic of continuous weight loss during oxidation at 1400 ℃, whereas 5Ta coating only shows a slight weight gain in the early stage of oxidation and also experiences weight loss in the later stage. After 150 h of oxidation, there is no significant difference in the mass change per unit area between the two coatings. The oxidation weight gain of 10Ta and 15Ta coatings increases sharply, indicating that the coatings have failed. Under the 1500 ℃ oxidation condition, the specific oxidation weight gain of 0Ta coating first drops significantly and then turns to a slow increase after 10 h of oxidation, with a mass change per unit area of 1.2 mg/cm² at 100 h. The 5Ta coating initially shows a weight gain, begins to continuously lose weight after 5 h, and slowly recovers after 50 h, resulting in a mass change per unit area of -0.3 mg/cm² after 100 h of oxidation. Figure 6 illustrates the macroscopic morphologies and the corresponding BSE images of the failure regions for the Cr-Ta-Ti modified silicide coatings with different Ta contents after oxidation at 1400 ℃ and 1500 ℃. Obviously, after 50 h of oxidation at 1400 ℃, a large amount of oxides of the substrate element Nb appears on the surface of 15Ta coating (Fig. 6 d), and its specific oxidation weight gain increases sharply (Fig. 5 a), indicating coating failure. This can probably be attributed to the relatively thin thickness of 15Ta coating, making it difficult to provide long-term protection for the substrate under high temperature conditions. After 50 h of oxidation, the coating has essentially degraded completely, generating a large amount of non-protective oxides (Nb 2 O 5 ), leading to the loss of protective capability. After oxidation at 1400 ℃ for 100 h (Fig. 5 c), the surface of 10Ta coating also generates a substantial amount of white Nb 2 O 5 products. Combined with the cross-sectional observation of the coating, these oxides grow outward from the interior of the substrate, and the substrate has undergone obvious oxidation, resulting in the destruction of the coating structure. In contrast, the overall structures of the 0Ta and 5Ta coatings remain relatively intact after long-term isothermal oxidation. For 0Ta coating, after 150 h of oxidation at 1400 ℃, its surface exhibits a yellowish-brown appearance with dark-brown spots in localized regions. Microstructural characterization reveals the presence of a large number of mixed oxide particles of Nb 2 O 5 and TiO 2 in the dark-brown regions. At this point, the coating has essentially failed, and obvious cracks are observed at the coating/substrate interface. A mixed oxide scale of SiO 2 -Nb 2 O 5 -TiO 2 with a thickness of approximately 27.3 µm has formed on the coating surface. After further oxidation at 1500 ℃ for 100 h, the entire surface of 0Ta coating turns yellowish-brown. After oxidation at 1400 ℃, the surface of 5Ta coating is entirely brown, while after oxidation at 1500 ℃, its surface is completely covered by a dense glassy dark-brown oxide scale. 3.2.2 Oxidation products Figure 7 shows the XRD patterns of 5Ta coating after oxidation at different temperature for different time. After oxidation, the phase composition of the coating surface is mainly NbSi 2 , SiO 2 , Cr 2 O 3 and TiO 2 . At 1400 ℃ and 1500 ℃, the oxidation products of the coating are primarily SiO 2 , Cr 2 O 3 , and TiO 2 . With prolonged oxidation time, the diffraction peaks of SiO 2 and TiO 2 gradually intensify, whereas those of Cr 2 O 3 show a weakening trend. No diffraction peaks of Nb 2 O 5 and Ta 2 O 5 are detected throughout the entire oxidation process. 3.2.3 Microstructure of the coating oxidized at 1400 ℃ Table 3 Chemical compositions of the marked sites in Fig. 8 , determined by EDS. Sites Composition (at.%) Si Nb Cr Ti Ta O 1 2.7 - 32.6 0.5 - 64.2 2 27.7 9.0 2.1 1.7 - 59.5 3 16.5 5.1 4.7 10.5 - 63.2 4 25.8 3.4 0.5 2.0 - 68.3 5 1.0 1.0 0.6 18.1 - 79.3 6 26.6 - - 1.6 - 71.8 7 2.5 1.4 1.5 15.1 1.8 77.7 Figure 8 presents the surface BSE images of 5Ta coating after oxidation at 1400 ℃ for different time. As can be seen from Fig. 8 a, after 1 h of oxidation, the oxide scale formed on the coating surface is not dense. Combining the XRD analysis results (Fig. 7 ) and the EDS data at Site 1 (Fig. 8 and Table 3 ), the oxides formed on the coating surface are mainly Cr 2 O 3 . This indicates that during the oxidation process, the Cr element continuously diffuses outward and reacts with oxygen. When the oxidation time was extended to 5 h, white granular TiO 2 begins to appear on the coating surface (Site 3 in Fig. 8 and Table 3 ). Simultaneously, the content of SiO 2 (Site 4 in Fig. 8 and Table 3 ) also increases significantly. Obviously, the content of the Cr 2 O 3 phase formed on the coating surface in the early stage of oxidation gradually decreases, which is due to the transformation of Cr 2 O 3 into volatile CrO 3 at high temperature (Figs. 7 and 8 ) [ 41 , 42 ]. As oxidation time extends, the oxide scale formed on the coating surface gradually becomes denser and more compact. When the oxidation time reaches 20 h, the coating surface is completely covered by a dense oxide scale. This oxide scale mainly consists of SiO 2 and fine TiO 2 particles dispersed within it. At this stage, the intrusion rate of oxygen into the substrate is drastically reduced, leading to a significant decrease in the oxidation rate. In the later stage of oxidation, the surface products of the coating remain basically unchanged, mainly composed of SiO 2 and TiO 2 . Furthermore, with the further extension of oxidation time, both the content and size of the TiO 2 particles on the coating surface exhibit a continuously increasing trend. Additionally, the EDS compositional analysis at Site 7 in Fig. 8 (Table 3 ) reveals that the TiO 2 particles contain small amounts of Cr and Nb, indicating that the Cr and Nb elements exist in the TiO 2 particles in the form of a solid solution. Table 4 Chemical compositions of the marked sites in Fig. 9 , determined by EDS. Sites Composition (at.%) Si Nb Cr Ti Ta O 1 - - 40.2 - - 59.8 2 69.9 22.7 1.2 5.0 1.2 - 3 24.9 4.7 3.2 0.9 0.3 66.0 4 24.6 1.9 - 0.7 - 72.8 5 63.2 28.2 5.8 2.8 - - 6 37.3 33.1 25.8 3.8 - - 7 67.2 32.8 - - - - 8 4.3 - 23.8 9.4 - 62.5 9 46.3 30.6 16.4 6.7 - - 10 5.0 12.4 15.3 3.1 - 64.2 11 68.6 31.4 - - - - 12 46.5 46.0 7.5 - - - 13 63.5 36.5 - - - - 14 38.2 59.5 2.3 - - - Figure 9 shows the cross-sectional BSE images of 5Ta coating after oxidation at 1400 ℃ for different time. Similar to the variation pattern of the coating surface, after 1 h of oxidation, the pore regions on the coating surface are filled with a large amount of Cr 2 O 3 (Site 1 in Fig. 9 ). At this time, the overall structure of the coating has not undergone obvious alterations, and the Cr 4 Nb 2 Si 5 phase can be clearly identified from the locally magnified morphology (Site 3 in Fig. 9 ). As the oxidation time extends to 5 h, the oxide content on the coating surface further increases, while the Cr 4 Nb 2 Si 5 phase gradually decreases. Meanwhile, a white degradation region is observed between the (Nb,Cr,Ti,Ta)Si 2 layer and the single NbSi 2 layer (Fig. 9 c). With further extension of the oxidation time, the content of SiO 2 formed on the coating surface continuously increases (Site 4 in Fig. 9 ), and the white degradation region inside the coating further expands, which is mainly composed of (Nb,Cr,Ti) 5 Si 3 phase (Table 4 ). Upon oxidation to 20 h, the coating surface is essentially covered by a continuous oxide scale, within which Cr 2 O 3 embedded in SiO 2 can still be observed (Site 8 in Fig. 9 ). At this stage, the coating has undergone significant degradation, and a two-phase region of (Nb,Cr,Ti,Ta)Si 2 /(Nb,Cr,Ti) 5 Si 3 can be observed beneath the (Nb,Cr,Ti,Ta)Si 2 layer. As the oxidation time continues to prolong, the oxide scale on the coating surface continuously thickens, and the (Nb,Cr,Ti) 5 Si 3 degradation region also expands continuously. Conversely, the (Nb,Cr,Ti,Ta)Si 2 layer and the single NbSi 2 layer continuously degrade and decrease. After 150 h of oxidation, a dense oxide scale with a thickness of approximately 22 µm has formed on the coating surface, which is mainly composed of SiO 2 , TiO 2 , and Cr 2 O 3 (Site 10 in Fig. 9 ). Notably, the dissolved Cr, Ti, and Ta atoms in the original (Nb,Cr,Ti,Ta)Si 2 layer can no longer be detected (Site 11 in Fig. 9 ). This indicates that during the long-term oxidation process, the Cr, Ti, and Ta elements continuously diffuse outward and undergo oxidation reactions, thereby driving the gradual transformation of the (Nb,Cr,Ti,Ta)Si 2 phase into the pure NbSi 2 phase. 3.2.4 Microstructure of the coating oxidized at 1500 ℃ Figure 10 shows the surface BSE images of 5Ta coating after oxidation at 1500 ℃ for different time. After 1 h of oxidation, the coating surface was immediately covered by a large amount of Cr 2 O 3 , while abundant pore defects still remained. With prolonged oxidation time, the content of Cr 2 O 3 on the coating surface gradually decreased, whereas that of SiO 2 exhibited an increasing trend. When the oxidation time reached 20 h, Cr 2 O 3 was barely detectable on the surface, which was fully covered by a continuous glassy SiO 2 layer. In addition, the content of TiO 2 embedded in the SiO 2 matrix also increased gradually. With further extension of oxidation time, the surface oxide scale became progressively denser, and both the content and size of TiO 2 particles increased. The morphological evolution of the scale was generally consistent with that observed at 1400 ℃. Figure 11 shows the cross-sectional BSE images of 5Ta coating after oxidation at 1500 ℃ for different time. After 1 h of oxidation, the coating surface was covered by a Cr 2 O 3 layer with a thickness of approximately 3 µm. When oxidized for 5 h, a considerable amount of SiO 2 phase was formed on the coating surface besides Cr 2 O 3 . At this stage, a (Nb,Cr,Ti)₅Si₃ degraded layer with a thickness of about 15 µm was observed in the coating. After 10 h of oxidation, the Cr 2 O 3 content on the coating surface decreased remarkably. The coating from top to bottom consisted of a mixed SiO 2 -TiO 2 -Cr 2 O 3 oxide scale, a (Nb,Cr,Ti,Ta)Si 2 layer, a (Nb,Cr,Ti) 5 Si 3 layer, a single NbSi 2 layer, and a Nb 5 Si 3 layer. With further oxidation up to 20 h, the scale thickened continuously and was mainly composed of SiO 2 and TiO 2 . Similar to the oxidation behavior at 1400 ℃, both the (Nb,Cr,Ti,Ta)Si 2 layer and the single NbSi 2 layer degraded continuously and reduced in content. After 100 h of oxidation, the single NbSi 2 layer was completely depleted, and (Nb,Cr,Ti,Ta)Si 2 was fully transformed into pure NbSi 2 (Fig. 11 f). To investigate the elemental diffusion behavior during high temperature oxidation, EPMA elemental mapping was performed on the cross-section of 5Ta coating after oxidation at 1500 ℃ for 1 h and 100 h, as shown in Figs. 12 and 13 , respectively. As can be seen from Fig. 12 , after oxidation at 1500 ℃ for 1 h, Nb and Si in the coating exhibited a three-layered distribution, corresponding to the (Nb,Cr,Ti,Ta)Si 2 /Cr 4 Nb 2 Si 5 layer, the single NbSi 2 layer, and the Nb 5 Si 3 layer. Cr was mainly distributed in Cr 4 Nb 2 Si 5 phase and was significantly enriched in the surface oxide. Notably, the overall Cr concentration in the coating was considerably lower than that before oxidation (Fig. 4 ). Ti was primarily concentrated in the (Nb,Cr,Ti,Ta)Si 2 /Cr 4 Nb 2 Si 5 layer. O was mainly enriched in the surface oxide, while Ta was predominantly distributed in the upper (Nb,Cr,Ti,Ta)Si 2 phase, with a small amount also detected in the surface oxide scale. W, Mo, and Zr were mostly retained in the substrate. As shown in Fig. 13 , after oxidation at 1500 ℃ for 100 h, the thickness of the scale increased substantially. Compared with the 1 h oxidized sample, the Si content in the residual coating decreased sharply, indicating significant outward diffusion of Si. Meanwhile, the Cr and Ti contents in the coating were obviously reduced. Ti was enriched in the scale as TiO 2 , in which the enrichment of Nb and Cr could also be detected, suggesting that a certain amount of Nb and Cr was dissolved in TiO 2 . In addition, Ta was also enriched in the TiO 2 phase and was prominently distributed in the white degraded Nb 5 Si 3 phase in the upper residual coating. It is worth noting that W and Mo were significantly enriched at the coating transition layer/substrate interface. 4. Discussion 4.1 Formation mechanism of Cr-Ta-Ti modified silicide coating A schematic diagram illustrating the formation mechanism of the Cr-Ta-Ti modified silicide coating is displayed in Fig. 14 . Due to the low melting point of Si (< 1420 ℃) [ 43 ] and its high mass fraction of 75 wt% in the slurry, Si melted rapidly to form a liquid phase when the sintering temperature exceeded its melting point. Therefore, the coating formation was mainly dominated by the inward diffusion of liquid Si into the substrate. At the initial stage of reaction, liquid Si came into contact with the Nb substrate and reacted rapidly to produce a Nb 5 Si 3 transition layer. Subsequently, Nb 5 Si 3 further reacted with the inward-diffusing Si and transformed into NbSi 2 . Driven by the Si concentration gradient, this reaction front continuously propagated toward the substrate interior. Meanwhile, Cr, Ti, and part of Ta in the slurry also participated in the reaction. Cr and Ti diffused inward together with Si and dissolved into the as-formed NbSi 2 lattice driven by the concentration gradient. With the reaction proceeding, the coating thickness increased rapidly. Since the solid solubility of Cr in NbSi 2 was limited [ 44 ], Cr 4 Nb 2 Si 5 precipitated as a secondary phase when the local Cr content exceeded its solubility limit. In contrast, Ti remained dissolved in NbSi 2 owing to its relatively low content. Eventually, the coating exhibited a typical four-layered structure from top to bottom: a mixed (Nb,Cr,Ti,Ta)Si 2 /Cr 4 Nb 2 Si 5 layer, a single NbSi 2 layer, a NbSi 2 /Nb 5 Si 3 layer, and a Nb 5 Si 3 transition layer. Notably, increasing the Ta content in the slurry did not significantly improve the dissolved Ta concentration inside the coating (sites 6 and 7 in Fig. 3 ). This phenomenon is mainly attributed to the kinetic constraints of diffusion. Firstly, due to its relatively large atomic radius [ 45 ], Ta exhibits substantially higher diffusion resistance in silicide lattices and grain boundaries in comparison with Cr and Ti, which in turn leads to a slower migration rate toward the substrate. Secondly, as a high melting point refractory metal, Ta exhibits a significantly lower thermal diffusivity at the sintering temperature of 1450 ℃ than Cr and Ti, which are close to a molten state [ 46 , 47 ]. This indicates that the diffusion of Ta acts as the rate-limiting step in the present sintering system. Simply increasing the Ta content in the slurry cannot overcome its diffusion kinetic barrier. Instead, excessive unreacted Ta tends to accumulate in the upper coating, hindering the effective diffusion of Si and other active elements and eventually reducing the coating thickness (Fig. 3 ). 4.2 Oxidation mechanism of Cr-Ta-Ti modified silicide coating Based on the above analysis of oxidation kinetics and oxide scale evolution, the oxidation process of the Cr-Ta-Ti modified silicide coatings can be divided into three stages. At the initial oxidation stage (1–5 h), oxygen reacts with (Nb,Cr,Ti,Ta)Si 2 on the coating surface, rapidly forming amorphous SiO 2 . Meanwhile, driven by the concentration gradient, a large amount of Cr diffuses outward and reacts with oxygen to form Cr 2 O 3 . EPMA elemental mapping (Fig. 12 ) confirms that most Cr 2 O 3 is distributed at the coating pores at this stage. In addition, a small amount of Ta is uniformly distributed in the oxide scale without forming discrete oxide particles of Ta, and no diffraction peaks of Ta 2 O 5 are observed in the XRD patterns (Fig. 7 ). This indicates that Ta is incorporated into the SiO 2 glass network in the form of solid solution during oxidation. Previous studies have demonstrated that Ta ion doping can significantly improve the high temperature viscosity of the SiO 2 glass phase and effectively reduce the diffusion coefficient of oxygen ions in the SiO 2 glass network [ 48 , 49 ]. Such an in-situ modification effect endows the as-formed oxide scale with favorable compactness and oxygen barrier properties, suppressing the rapid inward penetration of oxygen. Although Cr tends to diffuse outward and form volatile Cr 2 O 3 , the Ta-doped high viscosity oxide scale can inhibit the excessive volatilization of Cr to a certain extent, maintaining a more intact oxide scale structure. At the middle oxidation stage (5–20 h), the further oxidative volatilization of Cr 2 O 3 at high temperatures (especially 1500 ℃) and the increased formation of SiO 2 become the dominant factors controlling the oxidation behavior. As seen from the oxidation kinetic curves (Fig. 5 b), the coating without Ta shows a remarkable mass gain during the middle oxidation stage, whereas the mass change of the 5Ta coating continues to decrease slowly. In this system, the mass gain is attributed to the reaction between the silicide coating and oxygen to form oxides, while the mass loss results from the continuous further oxidation and volatilization of Cr 2 O 3 . The difference in oxidation kinetics reveals that the oxidation rate of the 5Ta coating is lower than that of the Ta-free coating, which is mainly ascribed to two reasons. First, the SiO 2 oxide scale doped with Ta exhibits superior high temperature thermal stability compared with pure SiO 2 . Its viscosity is less affected by temperature fluctuations, enabling it to effectively cover the microporous defects left by Cr 2 O 3 volatilization and showing excellent self-healing ability. Second, Ta dissolved in the SiO 2 glassy oxide scale acts as a skeleton-strengthening component, which improves the overall corrosion resistance of the oxide scale [ 50 ] and reduces the oxidation rate. Accordingly, the SiO 2 -Cr 2 O 3 mixed oxide scale formed at this stage is denser than that of the unmodified coating, which can effectively block the oxygen transport pathways. This is consistent with the experimental results of the significantly reduced oxidation mass gain rate (Fig. 5 ) and the decreased oxide scale thickness (Fig. 15 ) [ 51 ]. However, the advantage of the 5Ta coating in the oxidation kinetic curve is not obvious at 1400 ℃, indicating that the effect of refractory Ta cannot be fully exerted at relatively low temperatures. The higher the temperature, the more significantly Ta functions. At the late oxidation stage (20–100 h), the coating undergoes continuous degradation. The high-silicide layer gradually degrades due to the sustained diffusion and consumption of Si. Once this layer is completely depleted, the coating will lose its protective capability and fail because sufficient Si can no longer be supplied to the surface layer. Therefore, a sufficient coating thickness is required to achieve long-term protection of the substrate in high-temperature oxidation environments. The 10Ta and 15Ta coatings are prone to failure during this stage (Fig. 5 ), with insufficient thickness being a major contributing factor. In contrast, for the 5Ta coating, even after oxidation at 1500 ℃ for 100 h, the NbSi 2 layer still retains a thickness of 23.7 µm (Fig. 11 ), and the coating surface is completely covered by a mixed SiO 2 -TiO 2 -Cr 2 O 3 oxide scale with a thickness of approximately 27.4 µm. The mixed oxide scale on the coating surface can significantly suppress the inward diffusion of oxygen, and the silicide beneath the oxide scale can continuously supply Si to the outer layer. From the distribution of Ta at the late oxidation stage, obvious enrichment of Ta in TiO 2 is observed. It has been reported [ 34 ] that the introduction of Ta leads to the substitution of tetravalent Ti by pentavalent Ta, which reduces the concentration of Ti vacancies and O vacancies in TiO 2 , thereby inhibiting the inward diffusion of oxygen and the outward diffusion of Ti. This indicates that the defect reaction triggered by Ta 5+ entering the TiO 2 lattice effectively reduces the overall concentration of Ti vacancies and O vacancies in TiO 2 . Since oxygen vacancies serve as the main diffusion pathways for oxygen ions in oxides, their markedly decreased concentration directly hinders the diffusion rate of oxygen ions through TiO 2 grains or grain boundaries. Meanwhile, the reduced point defects render the TiO 2 solid solution grains more structurally intact and denser, further enhancing the thermal stability and oxygen diffusion barrier property of this oxide. In addition, the stable Ta-O structure exerts a pinning effect in the oxide scale, suppressing the excessive flow and structural relaxation of the SiO 2 glass phase at ultra-high temperatures, and ensuring the structural integrity of the oxide scale during long-term service. Accordingly, at the late stage, Ta improves the overall protective performance of the coating through the above mechanisms, leading to a significantly reduced oxide scale thickness (Fig. 15 ). By comparative analysis with the Cr-Ti modified silicide coating without Ta addition [ 51 ], the introduction of Ta optimizes the formation process, microstructure, and high temperature stability of the oxide scale at 1400 ℃ and 1500 ℃. Although the two coatings share the same oxidation stage division (dominated by Cr in the early stage and Si in the later stage), the addition of Ta significantly enhances the protective efficiency of the coating via its unique thermodynamic and kinetic effects, as directly evidenced by the obviously reduced oxide scale thickness. In summary, the core high temperature oxidation mechanism of the Cr-Ta-Ti modified silicide coating lies in achieving longer-lasting protection with a thinner and denser oxide scale by increasing the oxide scale viscosity, restraining elemental diffusion, and forming an oxide scale with low oxygen permeability. This mechanism significantly improves the oxidation resistance of the coating under extreme conditions at 1400 ℃ and 1500 ℃. 5. Conclusions (1) The Cr-Ta-Ti modified silicide coatings exhibit a distinct layered structure, which from top to bottom consists of a (Nb,Cr,Ti,Ta)Si 2 /Cr 4 Nb 2 Si 5 composite layer, a dense NbSi 2 layer, a NbSi 2 /Nb 5 Si 3 layer and a Nb 5 Si 3 transition layer. The introduction of Ta suppresses the diffusion of Si and other active elements during sintering, resulting in a reduced total coating thickness. (2) The addition of an appropriate amount of Ta can significantly slow down the growth rate of the oxide scale. Specifically, the 5Ta coating achieves the optimal comprehensive performance, providing effective protection to the substrate for at least 150 h at 1400 ℃ and 100 h at 1500 ℃. Coatings with an excessively high Ta content are prone to spallation failure during oxidation, and the insufficient coating thickness is one of the primary factors inducing this failure behavior. (3) The oxidation process of the Cr-Ta-Ti modified silicide coating is controlled by the diffusion of Cr and Si. Ta “pinned” in SiO 2 and TiO 2 not only significantly improves the high temperature viscosity of the oxide scale, but also effectively inhibits the inward diffusion of oxygen ions and the outward diffusion of other elements. This dual mechanism significantly reduces the oxide scale thickness and thereby enhances the oxidation resistance of the coating. Declarations Author Contribution Hao Jiang: Writing - original draft, Validation, Investigation, Data curation. Yanqiang Qiao: Writing – review & editing, Validation, Su pervision, Resources, Conceptualization. Weiping Zhang: Validation, Investigation. Xiping Guo: Writing – review & editing, Supervision. Qifan You: Investigation, Supervision. Xuan Li: Writing – review & editing, Supervision, Methodology. Acknowledgement This work was financially supported by the National Natural Science Foundation of China (No. 52571099), the Key R&D Program of Jiangxi Province (No. 20261BCE310021), the Key R&D Program of Xianyang (L2025-ZDYF-KTH-001) and the General Project of Sichuan Provincial Natural Science Foundation (No. 2026NSFSC0331). References Guo L, He WT, Chen WB, Xue ZL, He J, Guo YQ, Wu Y, Gao LH, Li DQ, Zhang Z, Wei LL, Gao YY, Zhang TH, Qiao J, Li QH, Guo HB (2023) Progress on high-temperature protective coatings for aero-engines. Surf Sci Technol 4:1 Liu YK, Liu Z, Fei YJ, Wang CH, Yang HR (2025) Study on the effect of multi-rare-earth-doping on thermal barrier coatings' thermophysical and fluorescence properties. Surf Sci Technol 3:40 Zhang YY, Fu T, Yu LH, Cui KK, Wang J, Shen FQ, Zhang X, Zhou KC (2023) Anti-corrosion coatings for protecting Nb-based alloys exposed to oxidation environments: A review. Met Mater Int 29:1-17 Chen DZ, Zhu JQ, Gong WT, Zhang XF, Xu C, Wang S, Wang QB, Xu Q, Wang Q, Chen RR, Fu HZ (2026) Combined effect of NdNbTiO 6 precipitation and fine microstructure on oxidation resistance of Nb-Si alloys at 1250 ℃. Corros Commun, doi: https://doi.org/10.1016/j.corcom.2026.03.011 Zhang X, Fu T, Cui KK, Zhang YY, Shen FQ, Wang J, Yu LH, Mao HB (2021) The protection, challenge, and prospect of anti-oxidation coating on the surface of niobium alloy. Coatings 11:742 Joshi K, Kumar P (2024) Strength behavior of niobium-based refractory systems. JOM 76:6277-6301 Tang XH, Zhao G, Liu J, Wang Q, Bai XD, Wang LF (2024) High-temperature antioxidant silicate coating of low-density Nb-Ti-Al alloy: A review. High Temp Mater Processes 43: 20240029 Majumdar S, Kishor J, Paul B, Hubli RC, Chakravartty JK (2015) Isothermal oxidation behavior and growth kinetics of silicide coatings formed on Nb-1Zr-0.1C alloy. Corros Sci 95:100-109 Narita T, Kato Y, Narita T, Ara M (2023) Advances in diffusion barrier coatings for high-temperature applications. High Temp Corros Mater 100:399-411 Chen DZ, Gong WT, Xu C, Zhang YJ, Hao R, Yang ZB, Chen RR, Fu HZ (2025) Oxidation behavior of Ge-modified Nb-Si alloys at 1250 ℃: Self-generated NbGe 2 layer with enhanced oxidation resistance. Appl Sur Sci Advances 30:100874 Fu T, Zhan SR, Zhang YY, Shen FQ, Wang H (2025) Preparation of Si-rich NbSi 2 coating on Nb surface by hot dip silicon-plating technology to improve the high-temperature oxidation resistance of pure Nb substrate. Ceram Int 51:23341-23353 Wang XY, Zhu L, Yang YJ, Zhang BJ, Kiryukhantsev-Korneev PV, Levashov EA, Ren XR, Ji X, Feng PZ, Wang XH (2025) Upcycling waste MoSi 2 into high-performance composite coatings for protecting refractory alloys across a wide temperature range. Int J Refract Met Hard Mater 132:107295 Xu BB, Guo XP, Qiao YQ (2024) Effects of TiB 2 contents on the microstructure and oxidation behavior of Mo-Si-B composite coatings. J Alloys Compd 1005:175967 Majumdar S, Sharma IG (2011) Oxidation behavior of MoSi 2 and Mo(Si,Al) 2 coated Mo-0.5Ti-0.1Zr-0.02C alloy. Intermetallics 19:541-545 Choi YJ, Yoon JK, Kim GH, Yoon WY, Doh JM, Hong KT (2017) High temperature isothermal oxidation behavior of NbSi 2 coating at 1000-1450 ℃. Corros Sci 129:102-114 Li LF, Guo XP, Qiao YQ (2023) Formation and oxidation resistance of MoSi 2 coating for Nb-Si based alloy prepared by slurry sintering method. J Alloys Compd 938:168456 Alam MZ, Rao AS, Das DK (2010) Microstructure and high temperature oxidation performance of silicide coating on Nb-based alloy C-103. Oxid Met 73:513-530 Li Z, Zhang P, Li X, Chen C, He Y, Feng P (2025) A comparison of multi-component silicide (Ta,Mo,W)Si 2 coatings on Nb alloy prepared by low-temperature reactive sintering using different powder feedstocks. Mater Today Commun 42:111338 Shao W, Cui YW, Zhou CG (2019) Diffusion paths of silicide coatings on Nb-Si-based alloys during pack cementation process. Metall Materi Trans A 50A:2945-2955 Wang X, Zhang W, Wang TY, Yang T, Han CX, Yang F (2025) In-situ formation mechanism and long-term isothermal oxidation behaviors at 1400 ℃ and 1500 ℃ of (Nb,Ti,Mo)Si 2 -(Ti,Nb,Mo) 5 Si 3 biphasic compositionally-complex ceramic composite coating on C103 alloy. Corros Sci 251:112914 Zhang YF, Xiao LR, Zeng DL, Li X, Zhou XJ, Zhao G, He WK, Li S, Xiao YX, Zhao XJ, Liu SA, Cai ZY (2022) A novel niobium based oxidation protective coating with three lines of defense at ultra-high temperature. Corros Sci 206: 110515 Ge YL, Wang YM, Chen JC, Zou YC, Guo LX, Ouyang J, Jia DC, Zhou Y (2018) An Nb 2 O 5 -SiO 2 -Al 2 O 3 /NbSi 2 /Nb 5 Si 3 multilayer coating on Nb-Hf alloy to improve oxidation resistance. J Alloys Compd 745:271-281 Wang Z, Wang YM, Wang SQ, Zou YC, Chen GL, Wen L, Ouyang JH, Jia DC, Zhou Y (2022) ZrSi 2 /SiO 2 -Nb 2 O 5 /NbSi 2 multi-layer coating formed on niobium alloy by HAPC combined with LPDS: Microstructure evolution and high temperature oxidation behavior. Corros Sci 206:110460 Wang Z, Wang YM, Wang SQ, Zou YC, Chen GL, Wen L, Zhang GX, Zhao LN, Ouyang JH, Jia DC, Zhou Y (2022) Design and preparation of MoSi 2 /SiO 2 -Nb 2 O 5 /NbSi 2 multilayer coating on Nb alloy: Microstructure and hot corrosion behavior. Corros Sci 209:110733 Jiang H, Qiao YQ, Zhang WP, Guo XP (2024) Characterization of microstructure and oxidation behavior of Al modified MoSi 2 coating on Nb-Si based alloy. Surf Interfaces 51:104739 Li LF, Guo XP, Qiao YQ (2023) A novel composite Si-Mo-Ti coating with Ti concentration gradient for Nb-Si based ultrahigh temperature alloy prepared by slurry sintering. Corros Sci 225:111632 An DY, Liu CF, Zhao S, Zhang HP, Zou YC, Tang ZW, Xiao P, Dai JM, Wang YM (2023) High-temperature oxidation resistance performance of Si-Ti-Cr silicide coating on Nb-Hf alloy surface in constant oxidation and thermal shock. Rare Met Mater Eng 52:1219-1226 Cheng CY, Xie W, Li HJ, Fu QG (2021) Evaporation behavior of SiO 2 glass doped with various transition metal oxides. J Am Ceram Soc 104:3130-3138 Li JL, Wang W, Zhou CG (2017) Oxidation and interdiffusion behavior of a germanium-modified silicide coating on an Nb-Si-based alloy. Int J Miner Metall Mater 24:289-296 Sankar M, Prasad VVS, Baligidad RG, Alam MZ, Das DK, Gokhale AA (2015) Microstructure, oxidation resistance and tensile properties of silicide coated Nb-alloy C-103. Mat Sci Eng. A-struct 645:339-346 Wu JH, Wu WX, Sun AY, Zhu JY, Tang HQ, Shen F, Lei SY (2025) Study on high temperature oxidation resistance of HfSi 2 modified MoSi 2 coating. J Mater Res Technol 37:242-251 Ou HK, Liu YF, Fan KF, Zhang YY, Guo LX, Liu B, Sun J, Fu QG (2025) Long term protection of silicide coating at 1700 ℃ in air with HfO 2 modification. Corros Sci 255:113122 Wang CC, Li KZ, He DY, Shi XH (2020) Evolution behavior of rare-earth yttria modified silicide oxidation-resistant coating at 1700 ℃. J Eur Ceram Soc 40:4419-4427 Yu JY, Chen DZ, Xu FD, Wang S, Cui XY, Gong WT, Chen RR (2024) Simultaneous improving high temperature oxidation resistance and room temperature toughness of Nb-Si based alloy via appropriate Ta content addition strategy. Int J Refract Met Hard Mater 124:106800 Liu XJ, Luo H, Lu Y, Han JJ, Li J, Guo YH, Huang YX, Wang CP (2017) Thermodynamic Assessment of the Nb-Si-Ta System. J Phase Equilib Diffus 38:897-905 He S, Liu Y, Ma Z, Zhu S, Liu L, Mu G, Li Y (2024) Study on anti-ablation properties of HfB 2 -TaSi 2 coatings prepared by atmospheric plasma spraying. J Alloys Compd 999:175069 Xu Y, Zheng W, Dai M, Xu X, Hong D, Zhong X, Niu Y, Zheng X (2023) Effect of TaSi 2 addition on long-term ablation behavior of HfB 2 -SiC coating. J Eur Ceram Soc 43:5802-5813 Xu Y, Dai M, Yu L, Huang S, Zhong X, Niu Y, Zeng Y, Zheng X (2025) Ablation behavior and mechanism of modified HfB 2 -SiC coatings. Surf Coat Technol 496:131725 Bahr A, Richter S, Hahn R, Wojcik T, Podsednik M, Limbeck A, Ramm J, Hunold O, Kolozsvari S, Riedl H (2023) Oxidation behaviour and mechanical properties of sputter-deposited TMSi 2 coatings (TM = Mo, Ta, Nb). J Alloys Compd 931:167532 Zhang YF, Liu SN, Zhou XJ, Zhao G, Liu J, Shen HT, Cai ZY, Zhao XJ, Xiao LR (2022) Ultra-high temperature oxidation behavior of ZrB 2 /YSZ modified Si-Mo-W coating with a diffusion barrier on niobium alloy. Corros Sci 195:109977 Wang Z, Zhang YP, Zhou SH, Wang ZY, Yao YR, Wang AY, Li M, Ke PL (2024) Oxidation resistance of Mo/Cr bilayer coating on Zr alloy in a 1200 ℃ steam environment. Corros Commun 14:49-57 Yin L, Ma JY, Yang F, Nie YJ, Meng LX, Wang YZ, Zheng LW, Shi QX, Liang W (2025) Influence of precipitates on the initial oxidation behavior of GH4169 superalloy at 1000 ℃. Corros Commun 19:138-148 Dorner F, Sukurma Z, Dellago C, Kresse G (2018) Melting Si: Beyond density functional theory. Phys Rev Lett 121:195701 Sekido N, Aizawa R, Ueno S (2019) Effect of Cr Addition on the phase equilibria and oxidation behavior of NbSi 2 . Mater Trans 60:666-673 Tsakiropoulos P (2017) On the Nb silicide based alloys: Part I - The bcc Nb solid solution. J Alloys Compd 708:961-971 Ansel D, Thibon I, Boliveau M, Debuigne J (1998) Interdiffusion in the body cubic centered β-phase of Ta-Ti alloys. Acta Mater 46:423-430 Roy S, Prasad S, Divinski SV, Paul A (2014) Diffusion pattern in MSi 2 and M 5 Si 3 silicides in group VB (M = V, Nb, Ta) and VIB (M = Mo, W) refractory metal-silicon systems. Philos Mag 94:1508-1528 Fattah-alhosseini A, Chaharmahali R, Dikici B, Kaseem M (2025) A comprehensive review of plasma electrolytic oxidation (PEO) of tantalum (Ta): Mechanisms, properties, and applications. Int J Refract Met Hard Mater 128:107059 Kane KA, Pint BA, Mitchell D, Haynes JA (2021) Oxidation of ultrahigh temperature ceramics: kinetics, mechanisms, and applications. J Eur Ceram Soc 41:6130-6150 Luo X, Zhang XX, Yang X, Zuo YM, Huang QZ (2025) Effect of titanium and silicon oxides migration on oxide structure evolution and oxidation mechanism of (Zr 0.25 Hf 0.25 Ti 0.25 Ta 0.25 )C-30SiC at 1600 ℃. Ceram Int 51:21936-21950 Jiang H, Qiao YQ, Zhang WP, Guo XP, Gao JC, Li LF, Nan X (2026) Temperature-dependent oxidation behavior and interfacial diffusion dynamics of multilayered silicide coating on Nb521 alloy (1200-1500 ℃). Surf Interfaces 86:108689 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 27 Apr, 2026 Reviews received at journal 20 Apr, 2026 Reviewers agreed at journal 09 Apr, 2026 Reviewers agreed at journal 05 Apr, 2026 Reviewers invited by journal 05 Apr, 2026 Editor assigned by journal 31 Mar, 2026 Submission checks completed at journal 31 Mar, 2026 First submitted to journal 23 Mar, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9197546","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":617949387,"identity":"9000edfb-6abd-490a-a6ff-a68e7367f276","order_by":0,"name":"Hao Jiang","email":"","orcid":"","institution":"Northwestern Polytechnical University","correspondingAuthor":false,"prefix":"","firstName":"Hao","middleName":"","lastName":"Jiang","suffix":""},{"id":617949390,"identity":"8f6dde09-cd41-4811-a801-348f54f36373","order_by":1,"name":"Yanqiang 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08:24:14","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9197546/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9197546/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":106551162,"identity":"3c8f27cf-d1f6-484e-ae22-8d0377b807c0","added_by":"auto","created_at":"2026-04-09 18:16:34","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1188058,"visible":true,"origin":"","legend":"\u003cp\u003eSurface XRD patterns of Cr-Ta-Ti modified silicide coatings with different Ta contents.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-9197546/v1/d057f7bcd608a89091832dfc.png"},{"id":106725193,"identity":"1c60d4bf-b77f-407e-a058-f87bdca0e02e","added_by":"auto","created_at":"2026-04-12 18:31:46","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":737451,"visible":true,"origin":"","legend":"\u003cp\u003eSurface BSE images of Cr-Ta-Ti modified silicide coatings with different Ta contents:\u003c/p\u003e\n\u003cp\u003e(a) 0Ta, (b) 5Ta, (c)10Ta and (d)15Ta.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-9197546/v1/8faf850820a0d5ec286a6faa.png"},{"id":106551164,"identity":"f778a552-3659-444b-9213-622b71cc05d7","added_by":"auto","created_at":"2026-04-09 18:16:34","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1861677,"visible":true,"origin":"","legend":"\u003cp\u003eCross-sectional BSE images of Cr-Ta-Ti modified silicide coatings with different Ta contents: (a) 0Ta, (b) 5Ta, (c)10Ta and (d)15Ta.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-9197546/v1/a73f49a91d9be5767783cc07.png"},{"id":106725270,"identity":"09ae169c-4448-49a5-b6af-dbd2ab21a610","added_by":"auto","created_at":"2026-04-12 18:32:10","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":841408,"visible":true,"origin":"","legend":"\u003cp\u003eCross-sectional BSE image of 5Ta coating and the corresponding EPMA elemental mappings.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-9197546/v1/5745a80b55b60ac82ee7d0c3.png"},{"id":106551166,"identity":"491885da-bb05-47c4-b0e7-2a1fba8799bf","added_by":"auto","created_at":"2026-04-09 18:16:34","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":427480,"visible":true,"origin":"","legend":"\u003cp\u003eOxidation mass gain per unit area of the Cr-Ta-Ti modified silicide coatings with different Ta contents after oxidation at 1400 ℃ and 1500 ℃.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-9197546/v1/b69cb0de3970c75c9de56480.png"},{"id":106727307,"identity":"b2bf071d-cd8f-4476-9856-60adb5cf9bb5","added_by":"auto","created_at":"2026-04-12 18:38:38","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":2329810,"visible":true,"origin":"","legend":"\u003cp\u003eMacroscopic morphologies (a) and corresponding cross-sectional BSE morphologies of the failure regions (b, c, d) of Cr-Ta-Ti modified silicide coatings with different Ta contents after oxidation.\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-9197546/v1/6282597f10b097efd6c7c52d.png"},{"id":106551168,"identity":"63a6350c-3adb-4cbf-b9ad-a4fcf68e6a86","added_by":"auto","created_at":"2026-04-09 18:16:34","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":639367,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns of 5Ta coating after oxidation at 1400 ℃ (a) and 1500 ℃ (b) for different time.\u003c/p\u003e","description":"","filename":"image7.png","url":"https://assets-eu.researchsquare.com/files/rs-9197546/v1/4c7e089edcbb9bc102c542fd.png"},{"id":106725178,"identity":"461ff719-17d4-4f50-a964-8403a8227e14","added_by":"auto","created_at":"2026-04-12 18:31:41","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":2954852,"visible":true,"origin":"","legend":"\u003cp\u003eSurface BSE images of 5Ta coating after oxidation at 1400 ℃ for (a) 1 h, (b,c) 5 h, (d) 10 h, (e) 20 h, (f) 50 h, (g) 100 h and (h,i) 150 h.\u003c/p\u003e","description":"","filename":"image8.png","url":"https://assets-eu.researchsquare.com/files/rs-9197546/v1/1c5d806888687289ad1c1c80.png"},{"id":106726080,"identity":"61950a3d-31fc-48a8-9233-8bbf9a792d88","added_by":"auto","created_at":"2026-04-12 18:35:13","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":2121944,"visible":true,"origin":"","legend":"\u003cp\u003eCross-sectional BSE images of oxide scales for 5Ta coating after oxidation at 1400 ℃ for (a,b) 1 h, (c) 5 h, (d) 10 h, (e,f) 20 h, (g) 50 h, (h) 100 h and (i) 150 h.\u003c/p\u003e","description":"","filename":"image9.png","url":"https://assets-eu.researchsquare.com/files/rs-9197546/v1/2771f0baeca31b619dca997c.png"},{"id":106551170,"identity":"21342750-0163-47b8-8abb-957d1b7eaa34","added_by":"auto","created_at":"2026-04-09 18:16:34","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":1974639,"visible":true,"origin":"","legend":"\u003cp\u003eSurface BSE images of 5Ta coating after oxidation at 1500 ℃ for (a) 1 h, (b) 5 h, (c) 10 h, (d) 20 h, (e) 50 h and (f) 100 h.\u003c/p\u003e","description":"","filename":"image10.png","url":"https://assets-eu.researchsquare.com/files/rs-9197546/v1/1584219916d0c116eb6448b4.png"},{"id":106725037,"identity":"2fe0a941-e8fc-4c52-b3ce-85b5c8b624eb","added_by":"auto","created_at":"2026-04-12 18:31:08","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":1693499,"visible":true,"origin":"","legend":"\u003cp\u003eCross-sectional BSE images of the oxide scales for 5Ta coating after oxidation at 1500 ℃ for (a) 1 h, (b) 5 h, (c) 10 h, (d) 20 h, (e) 50 h and (f) 100 h.\u003c/p\u003e","description":"","filename":"image11.png","url":"https://assets-eu.researchsquare.com/files/rs-9197546/v1/6d30306434152f61cfd2ea98.png"},{"id":106551171,"identity":"88794aa3-2f9f-40dd-95c1-9412921b850e","added_by":"auto","created_at":"2026-04-09 18:16:34","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":2229854,"visible":true,"origin":"","legend":"\u003cp\u003eEPMA elemental mappings of 5Ta coating after oxidation at 1500 ℃ for 1 h.\u003c/p\u003e","description":"","filename":"image12.png","url":"https://assets-eu.researchsquare.com/files/rs-9197546/v1/83b954ac850eda0f2ade4c6e.png"},{"id":106725177,"identity":"10ea13b2-6dc9-4e84-96d5-56e801ab5d19","added_by":"auto","created_at":"2026-04-12 18:31:41","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":1945334,"visible":true,"origin":"","legend":"\u003cp\u003eEPMA elemental mappings of 5Ta coating after oxidation at 1500 ℃ for 100 h.\u003c/p\u003e","description":"","filename":"image13.png","url":"https://assets-eu.researchsquare.com/files/rs-9197546/v1/7572e0b55d15f94057c29809.png"},{"id":106551173,"identity":"336c5061-3dca-4884-8721-6571ca77705d","added_by":"auto","created_at":"2026-04-09 18:16:34","extension":"png","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":186942,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of the formation mechanism of Cr-Ta-Ti modified silicide coating.\u003c/p\u003e","description":"","filename":"image14.png","url":"https://assets-eu.researchsquare.com/files/rs-9197546/v1/bfb36230902fc0a6fa7d718a.png"},{"id":106551175,"identity":"3944c296-68f6-48ad-88e7-08c08eb5a564","added_by":"auto","created_at":"2026-04-09 18:16:34","extension":"png","order_by":15,"title":"Figure 15","display":"","copyAsset":false,"role":"figure","size":192499,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of oxide scale thickness between 0Ta coating [51] and 5Ta coating.\u003c/p\u003e","description":"","filename":"image15.png","url":"https://assets-eu.researchsquare.com/files/rs-9197546/v1/a9dedc439d209dac161300ad.png"},{"id":106728449,"identity":"a1b333d8-6bb1-4845-836c-57e114291c48","added_by":"auto","created_at":"2026-04-12 18:42:48","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":22464259,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9197546/v1/f935b1ee-edcd-4937-9a15-87c92eb8264a.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Unveiling the role of Ta in Cr-Ta-Ti modified silicide coatings for Nb alloy with superior oxidation resistance up to 1500 ℃","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eTo meet the urgent demands of next-generation aero-engines with high thrust-to-weight ratios and reusable spacecraft for extreme hot-section components (service temperature\u0026thinsp;\u0026gt;\u0026thinsp;1500 ℃), developing material systems with both ultra-high temperature mechanical properties and long-term oxidation resistance has become an imperative [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Due to their high melting points, low densities, excellent high temperature strengths, and good processability, Nb based alloys are considered ideal structural materials in the ultra-high temperature field [\u003cspan additionalcitationids=\"CR4 CR5 CR6\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Currently, Nb based alloys widely studied and applied in the aerospace sector (such as C103, Nb521, Nb-Si based alloys, and Nb-Ti-Al based alloys) inherently form expansive oxides, such as Nb\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e, during the oxidation process, which are detrimental to their oxidation performance [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Consequently, Nb based alloys must rely on compatible high temperature anti-oxidation coatings for protection when utilized in aerobic environments [\u003cspan additionalcitationids=\"CR10 CR11 CR12\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAmong various coating systems, silicide coatings have emerged as one of the most extensively researched and successfully engineered protective strategies, as they can form a continuous, dense SiO\u003csub\u003e2\u003c/sub\u003e protective scale with an extremely low oxygen diffusion rate during high temperature oxidation [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Early studies predominantly focused on single silicide coatings such as MoSi\u003csub\u003e2\u003c/sub\u003e and NbSi\u003csub\u003e2\u003c/sub\u003e [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. However, these generally suffer from intermediate-temperature \"pesting\" oxidation failure, cracking caused by coefficient of thermal expansion (CTE) mismatch with the substrate, and the decomposition and degradation of silicide phases under long-term high temperature [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. To overcome these bottlenecks, elemental modification and structural composite design have been widely adopted to mitigate these issues [\u003cspan additionalcitationids=\"CR20 CR21 CR22 CR23\" citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. For instance, preparing multi-element modified silicide coatings by introducing elements such as Cr, Ti, and Al can alleviate the aforementioned problems to a certain extent [\u003cspan additionalcitationids=\"CR26\" citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. Under ultra-high temperature service conditions, coatings face immense challenges. For example, the SiO\u003csub\u003e2\u003c/sub\u003e scale exhibits insufficient thermal stability at ultra-high temperature, presenting a significant risk of high temperature volatilization [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. Furthermore, exposure to ultra-high temperature aggravates elemental interdiffusion within the coating, which can easily induce the premature decomposition of key protective phases including NbSi\u003csub\u003e2\u003c/sub\u003e and exhausts the constrained silicon reservoir within the coating [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. In addition, the interdiffusion and degradation at the coating/substrate interface are prominent. The continuous inward diffusion of Si into the substrate not only leads to Si depletion in the coating but also forms a brittle silicide layer at the interface, acting as a mechanical weak link [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIntroducing refractory elements or high temperature stabilizing elements is an effective approach to enhance the oxidation resistance of coatings at higher temperature [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. For example, Wang et al. [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e] prepared a Y\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-modified MoSi\u003csub\u003e2\u003c/sub\u003e silicide coating using supersonic atmospheric plasma spraying. Their research demonstrated that Y\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e can regulate the microstructure of the generated glassy oxide scale during oxidation, thereby improving the high temperature oxidation resistance of the coating. Wang et al. [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e] fabricated a (Nb,Ti,Mo)Si\u003csub\u003e2\u003c/sub\u003e-(Ti,Nb,Mo)\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e composite coating on the surface of C103 alloy using slurry sintering method. The coating maintained its structural integrity after oxidation for 100 h at 1400 ℃ and 1500 ℃, respectively. Ta and Nb are refractory metals of the same group with similar atomic radii and chemical properties, allowing Ta to dissolve into the niobium silicide phase to form a stable solid solution [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. This solid solution effect not only strengthens the coating via lattice distortion but also draws significant attention in the field of ultra-high temperature protective coatings due to extremely high melting point (~\u0026thinsp;3017 ℃) of Ta and superior high temperature stability. Existing studies have confirmed that Ta can dissolve into the SiO\u003csub\u003e2\u003c/sub\u003e glass network formed at high temperature, which not only significantly reduces the oxygen permeability of liquid SiO\u003csub\u003e2\u003c/sub\u003e [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e] but also enhances the bond strength of Si-O bonds [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. These effects are of paramount importance for enhancing the compactness of the oxide scale and extending its long-term thermally stable service life.\u003c/p\u003e \u003cp\u003eHowever, studies also indicate that the protective performance of a single Ta-Si-O oxide scale is less than ideal. For instance, Bahr et al. [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e] found that a single TaSi\u003csub\u003e2\u003c/sub\u003e coating forms a discontinuous and porous Ta\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e+SiO\u003csub\u003e2\u003c/sub\u003e mixed oxide scale at high temperature (\u0026gt;\u0026thinsp;850 ℃), which may conversely act as a rapid diffusion channel for oxygen ions, accelerating the coating's oxidation rate. It is evident that simple Ta substitution or addition does not consistently yield excellent protective effects and may even induce adverse impacts under certain circumstances. To overcome the shortcomings of single element modification, it is feasible to design the co-addition of Ta with metallic elements (e.g., Ti, Cr, Mo, W, Hf) or active elements (e.g., B, Y). Through the multi-element synergistic effect, it is promising to break through the performance bottlenecks of singly modified systems. Nevertheless, current research on multi-element modification mainly focuses on Cr-Ti or Mo-W systems [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e], and the application of Cr-Ta-Ti ternary synergistic modification in silicide coatings has not yet been reported. There is a lack of systematic experimental data and in-depth mechanistic analyses addressing key scientific issues, such as the microstructural evolution laws of multi-element modified silicide coatings and the formation mechanisms of mixed oxide scales under high temperature environments.\u003c/p\u003e \u003cp\u003eIn view of this, the present study aims to systematically investigate the microstructure, high temperature oxidation behavior, and intrinsic mechanisms of Cr-Ta-Ti modified silicide coatings on the surface of Nb521 alloy. The Cr-Ta-Ti modified silicide coatings were successfully fabricated on the Nb521 alloy using the slurry sintering method, which is widely applied in industry. Through systematic long-term isothermal oxidation experiments at 1400 ℃ and 1500 ℃, the oxidation kinetics, microstructural evolution of the oxide scale, and coating degradation behavior before and after modification were comparatively analyzed. Emphasis was placed on the effects of Ta content on the phase formation, microstructure, and diffusion behavior of key elements (Si, Cr, Ti, Nb) in the coatings. Finally, the functional mechanisms of Ta modification on the composition, structure, adhesion, and oxygen barrier properties of the mixed oxide scale were discussed. The results of this study not only provide a new perspective for understanding the oxidation protection mechanisms of multi-element silicide coatings modified by refractory elements but also offer crucial theoretical bases and feasible technical pathways for developing long-life, highly reliable protective coatings for Nb alloys applied in extreme ultra-high temperature environments.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Substrate material\u003c/h2\u003e \u003cp\u003eIn this work, Nb521 alloy with a nominal composition of Nb-5W-2Mo-1Zr (wt.%) was used. The alloy ingot was cut into specimens with dimensions of 15 mm \u0026times; 6 mm \u0026times; 3 mm using a wire electrical discharge machine. All specimen surfaces, edges and corners were gradually ground and polished with SiC sandpapers from 80# to 800# to reduce stress concentration. Subsequently, the specimens were ultrasonically cleaned in ethanol for 15 min and dried for later use.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Coating process\u003c/h2\u003e \u003cp\u003ePure Si powder (purity\u0026thinsp;\u0026ge;\u0026thinsp;99%, particle size 74 \u0026micro;m), Cr powder (purity\u0026thinsp;\u0026ge;\u0026thinsp;99.9%, particle size 2\u0026ndash;5 \u0026micro;m), Ti powder (purity\u0026thinsp;\u0026ge;\u0026thinsp;99.9%, particle size 1\u0026ndash;3 \u0026micro;m) and Ta powder (purity\u0026thinsp;\u0026ge;\u0026thinsp;99.95%, mesh\u0026thinsp;\u0026lt;\u0026thinsp;325) were selected as raw materials. The corresponding powders were weighed according to the mass ratio Si:Cr:Ti:Ta\u0026thinsp;=\u0026thinsp;15:4:1:x (x\u0026thinsp;=\u0026thinsp;0, 5, 10, 15), mixed with ethanol and polyvinyl butyral (PVB), and ball milled for 4 h to prepare a uniform slurry. For convenience, the Cr-Ta-Ti modified silicide coatings with different Ta contents are denoted as 0Ta, 5Ta, 10Ta and 15Ta, respectively.\u003c/p\u003e \u003cp\u003eThe pretreated substrate specimens were immersed in the slurry for 3\u0026ndash;5 s, taken out and dried in air for 10\u0026ndash;15 min, and this process was repeated three times. The coated specimens were dried in a vacuum oven at 100 ℃ for 4 h. Finally, all samples were sintered at 1450 ℃ for 0.5 h in an ultra-high vacuum furnace and then cooled to room temperature with the furnace.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Oxidation test\u003c/h2\u003e \u003cp\u003eIsothermal oxidation tests were carried out in a muffle furnace at 1400 ℃ and 1500 ℃ for 1-150 h. After oxidation, the samples were directly taken out of the furnace and cooled naturally in air. The mass changes before and after oxidation were measured using an electronic balance with an accuracy of 0.1 mg\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Analyzing methods\u003c/h2\u003e \u003cp\u003eScanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (SEM/EDS, TESCAN MIRA 3 and Inca X-sight) and X-ray diffraction (XRD, Panalytical X\u0026rsquo;Pert PRO, Cu Kα) were employed to analyze the microstructure, phase composition and elemental distribution of the samples before and after oxidation. Elemental distribution of the samples was characterized by electron probe microanalysis (EPMA).\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Microstructure of the Cr-Ta-Ti modified silicide coating\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e show the surface XRD patterns and surface BSE images of the Cr-Ta-Ti modified silicide coatings with different Ta contents, respectively. According to the XRD analysis results, all coatings are mainly composed of the NbSi\u003csub\u003e2\u003c/sub\u003e phase and a small amount of the Cr\u003csub\u003e4\u003c/sub\u003eNb\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003e5\u003c/sub\u003e phase. As seen from the surface images of the coatings (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), with the increase of Ta content, the surface roughness of the coatings gradually increases, and the number of pores also increases accordingly. After the addition of Ta, a white Ta-rich phase (Sites 3 and 4 in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) appears on the coating surface, which remains the NbSi\u003csub\u003e2\u003c/sub\u003e phase. Furthermore, a small amount of incompletely reacted residual slurry can be observed in the samples, as indicated by Site 6 in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed and EDS composition in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e(a) 0Ta, (b) 5Ta, (c)10Ta and (d)15Ta.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eChemical compositions of the marked sites in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, determined by EDS.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSites\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colspan=\"4\" nameend=\"c6\" namest=\"c3\"\u003e \u003cp\u003eComposition (at.%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"1\" nameend=\"c7\" namest=\"c7\"\u003e\u0026nbsp;\u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSi\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNb\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCr\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eTi\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c7\" namest=\"c6\"\u003e \u003cp\u003eTa\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e69.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e21.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e5.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e3.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c7\" namest=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e71.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e18.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e5.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e3.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c7\" namest=\"c6\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e66.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e19.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e8.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e3.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c7\" namest=\"c6\"\u003e \u003cp\u003e2.7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e69.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e12.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e9.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e3.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c7\" namest=\"c6\"\u003e \u003cp\u003e4.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e69.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e17.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e8.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e3.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c7\" namest=\"c6\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e44.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e23.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e13.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c7\" namest=\"c6\"\u003e \u003cp\u003e15.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e67.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e23.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e3.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c7\" namest=\"c6\"\u003e \u003cp\u003e1.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e presents the cross-sectional BSE morphologies of the Cr-Ta-Ti modified silicide coatings with different Ta contents. There is no significant difference in the overall structure of all coatings, which are composed of two layers: a main layer and a transition layer. The main layer can be further divided into two sub-layers: the upper layer consists of a gray phase and a dispersed dark-gray granular phase, while the lower layer is a single gray phase. Combining the EDS compositional analysis (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) and XRD results (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), it can be determined that the upper gray phase is the (Nb,Cr,Ti,Ta)Si\u003csub\u003e2\u003c/sub\u003e phase (Sites 1, 2, 6, and 7 in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Moreover, as the Ta content increases, the Ta concentration in the (Nb,Cr,Ti,Ta)Si\u003csub\u003e2\u003c/sub\u003e phase increases synchronously, albeit with a limited increment. The dispersed dark-gray phase is identified as the Cr\u003csub\u003e4\u003c/sub\u003eNb\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003e5\u003c/sub\u003e phase (Site 3 in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The lower single gray phase is the NbSi\u003csub\u003e2\u003c/sub\u003e phase (Site 4 in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), and the transition layer is composed of Nb\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e phase. Between the single NbSi\u003csub\u003e2\u003c/sub\u003e layer and the Nb\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e layer, a mixed layer composed of NbSi\u003csub\u003e2\u003c/sub\u003e+Nb\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e is found. Evidently, the coating consists of a (Nb,Cr,Ti,Ta)Si\u003csub\u003e2\u003c/sub\u003e+Cr\u003csub\u003e4\u003c/sub\u003eNb\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003e5\u003c/sub\u003e layer, a single NbSi\u003csub\u003e2\u003c/sub\u003e layer, a NbSi\u003csub\u003e2\u003c/sub\u003e+Nb\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e mixed layer, and a Nb\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e transition layer from top to bottom. It is noteworthy that the coating thickness gradually decreases with the increase of Ta content. The thicknesses of the main layers for the 0Ta, 5Ta, 10Ta, and 15Ta coatings are approximately 122, 113, 90, and 77 \u0026micro;m, respectively.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eChemical compositions of the marked sites in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, determined by EDS.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSites\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"6\" nameend=\"c7\" namest=\"c2\"\u003e \u003cp\u003eComposition (at.%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSi\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNb\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCr\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eTi\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eTa\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"1\" nameend=\"c7\" namest=\"c7\"\u003e\u0026nbsp;\u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e68.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e20.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e3.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c7\" namest=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e69.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e22.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e5.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c7\" namest=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e48.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e13.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e33.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c7\" namest=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e67.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e32.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c7\" namest=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e38.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e61.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c7\" namest=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e66.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e26.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c7\" namest=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e68.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e22.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c7\" namest=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e displays the cross-sectional BSE image of 5Ta coating and the elemental mapping of the corresponding region. It can be seen that the Nb element exhibits a three-layer distribution in the coating above the substrate. The bottom most part of the coating, which has a lower Si content, is the Nb\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e transition layer, above which lies the NbSi\u003csub\u003e2\u003c/sub\u003e and Nb\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e mixed layer. The Si and Nb elements are distributed uniformly in the single NbSi\u003csub\u003e2\u003c/sub\u003e layer. There is a distinct Cr-enriched region in the upper layer of the coating, primarily corresponding to the Cr\u003csub\u003e4\u003c/sub\u003eNb\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003e5\u003c/sub\u003e phase. The Ti element is also enriched here and is mainly distributed in the upper layer of the coating. The Ta element is distributed relatively uniformly throughout the coating, but its content is relatively higher within the range from the coating surface down to 30 \u0026micro;m. Mo, W, and Zr are substrate elements and are mainly concentrated in the substrate.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Oxidation behavior of the coating\u003c/h2\u003e \u003cdiv id=\"Sec10\" class=\"Section3\"\u003e \u003ch2\u003e3.2.1 Oxidation kinetics\u003c/h2\u003e \u003cp\u003eThe mass changes per unit area of the Cr-Ta-Ti modified silicide coatings with different Ta contents after oxidation at 1400 ℃ and 1500 ℃ are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. The 0Ta coating exhibits a characteristic of continuous weight loss during oxidation at 1400 ℃, whereas 5Ta coating only shows a slight weight gain in the early stage of oxidation and also experiences weight loss in the later stage. After 150 h of oxidation, there is no significant difference in the mass change per unit area between the two coatings. The oxidation weight gain of 10Ta and 15Ta coatings increases sharply, indicating that the coatings have failed. Under the 1500 ℃ oxidation condition, the specific oxidation weight gain of 0Ta coating first drops significantly and then turns to a slow increase after 10 h of oxidation, with a mass change per unit area of 1.2 mg/cm\u0026sup2; at 100 h. The 5Ta coating initially shows a weight gain, begins to continuously lose weight after 5 h, and slowly recovers after 50 h, resulting in a mass change per unit area of -0.3 mg/cm\u0026sup2; after 100 h of oxidation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e illustrates the macroscopic morphologies and the corresponding BSE images of the failure regions for the Cr-Ta-Ti modified silicide coatings with different Ta contents after oxidation at 1400 ℃ and 1500 ℃. Obviously, after 50 h of oxidation at 1400 ℃, a large amount of oxides of the substrate element Nb appears on the surface of 15Ta coating (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed), and its specific oxidation weight gain increases sharply (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea), indicating coating failure. This can probably be attributed to the relatively thin thickness of 15Ta coating, making it difficult to provide long-term protection for the substrate under high temperature conditions. After 50 h of oxidation, the coating has essentially degraded completely, generating a large amount of non-protective oxides (Nb\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e), leading to the loss of protective capability. After oxidation at 1400 ℃ for 100 h (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec), the surface of 10Ta coating also generates a substantial amount of white Nb\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e products. Combined with the cross-sectional observation of the coating, these oxides grow outward from the interior of the substrate, and the substrate has undergone obvious oxidation, resulting in the destruction of the coating structure. In contrast, the overall structures of the 0Ta and 5Ta coatings remain relatively intact after long-term isothermal oxidation. For 0Ta coating, after 150 h of oxidation at 1400 ℃, its surface exhibits a yellowish-brown appearance with dark-brown spots in localized regions. Microstructural characterization reveals the presence of a large number of mixed oxide particles of Nb\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e and TiO\u003csub\u003e2\u003c/sub\u003e in the dark-brown regions. At this point, the coating has essentially failed, and obvious cracks are observed at the coating/substrate interface. A mixed oxide scale of SiO\u003csub\u003e2\u003c/sub\u003e-Nb\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e-TiO\u003csub\u003e2\u003c/sub\u003e with a thickness of approximately 27.3 \u0026micro;m has formed on the coating surface. After further oxidation at 1500 ℃ for 100 h, the entire surface of 0Ta coating turns yellowish-brown. After oxidation at 1400 ℃, the surface of 5Ta coating is entirely brown, while after oxidation at 1500 ℃, its surface is completely covered by a dense glassy dark-brown oxide scale.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section3\"\u003e \u003ch2\u003e3.2.2 Oxidation products\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows the XRD patterns of 5Ta coating after oxidation at different temperature for different time. After oxidation, the phase composition of the coating surface is mainly NbSi\u003csub\u003e2\u003c/sub\u003e, SiO\u003csub\u003e2\u003c/sub\u003e, Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and TiO\u003csub\u003e2\u003c/sub\u003e. At 1400 ℃ and 1500 ℃, the oxidation products of the coating are primarily SiO\u003csub\u003e2\u003c/sub\u003e, Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, and TiO\u003csub\u003e2\u003c/sub\u003e. With prolonged oxidation time, the diffraction peaks of SiO\u003csub\u003e2\u003c/sub\u003e and TiO\u003csub\u003e2\u003c/sub\u003e gradually intensify, whereas those of Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e show a weakening trend. No diffraction peaks of Nb\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e and Ta\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e are detected throughout the entire oxidation process.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003e3.2.3 Microstructure of the coating oxidized at 1400 ℃\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eChemical compositions of the marked sites in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e, determined by EDS.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSites\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"7\" nameend=\"c8\" namest=\"c2\"\u003e \u003cp\u003eComposition (at.%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSi\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNb\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCr\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eTi\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eTa\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eO\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e32.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e64.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e27.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e9.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e59.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e16.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e10.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e63.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e25.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e68.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e18.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e79.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e26.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e71.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e15.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e77.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e presents the surface BSE images of 5Ta coating after oxidation at 1400 ℃ for different time. As can be seen from Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea, after 1 h of oxidation, the oxide scale formed on the coating surface is not dense. Combining the XRD analysis results (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e) and the EDS data at Site 1 (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e and Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), the oxides formed on the coating surface are mainly Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. This indicates that during the oxidation process, the Cr element continuously diffuses outward and reacts with oxygen. When the oxidation time was extended to 5 h, white granular TiO\u003csub\u003e2\u003c/sub\u003e begins to appear on the coating surface (Site 3 in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e and Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Simultaneously, the content of SiO\u003csub\u003e2\u003c/sub\u003e (Site 4 in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e and Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) also increases significantly. Obviously, the content of the Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e phase formed on the coating surface in the early stage of oxidation gradually decreases, which is due to the transformation of Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e into volatile CrO\u003csub\u003e3\u003c/sub\u003e at high temperature (Figs.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e and \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e) [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. As oxidation time extends, the oxide scale formed on the coating surface gradually becomes denser and more compact. When the oxidation time reaches 20 h, the coating surface is completely covered by a dense oxide scale. This oxide scale mainly consists of SiO\u003csub\u003e2\u003c/sub\u003e and fine TiO\u003csub\u003e2\u003c/sub\u003e particles dispersed within it. At this stage, the intrusion rate of oxygen into the substrate is drastically reduced, leading to a significant decrease in the oxidation rate. In the later stage of oxidation, the surface products of the coating remain basically unchanged, mainly composed of SiO\u003csub\u003e2\u003c/sub\u003e and TiO\u003csub\u003e2\u003c/sub\u003e. Furthermore, with the further extension of oxidation time, both the content and size of the TiO\u003csub\u003e2\u003c/sub\u003e particles on the coating surface exhibit a continuously increasing trend. Additionally, the EDS compositional analysis at Site 7 in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) reveals that the TiO\u003csub\u003e2\u003c/sub\u003e particles contain small amounts of Cr and Nb, indicating that the Cr and Nb elements exist in the TiO\u003csub\u003e2\u003c/sub\u003e particles in the form of a solid solution.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eChemical compositions of the marked sites in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e, determined by EDS.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSites\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"7\" nameend=\"c8\" namest=\"c2\"\u003e \u003cp\u003eComposition (at.%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSi\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNb\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCr\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eTi\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eTa\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eO\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e40.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e59.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e69.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e22.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e24.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e66.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e24.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e72.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e63.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e28.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e5.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e37.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e33.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e25.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e3.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e67.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e32.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e4.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e23.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e9.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e62.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e46.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e30.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e16.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e6.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e12.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e15.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e3.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e64.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e68.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e31.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e46.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e46.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e7.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e63.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e36.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e38.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e59.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e shows the cross-sectional BSE images of 5Ta coating after oxidation at 1400 ℃ for different time. Similar to the variation pattern of the coating surface, after 1 h of oxidation, the pore regions on the coating surface are filled with a large amount of Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (Site 1 in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e). At this time, the overall structure of the coating has not undergone obvious alterations, and the Cr\u003csub\u003e4\u003c/sub\u003eNb\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003e5\u003c/sub\u003e phase can be clearly identified from the locally magnified morphology (Site 3 in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e). As the oxidation time extends to 5 h, the oxide content on the coating surface further increases, while the Cr\u003csub\u003e4\u003c/sub\u003eNb\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003e5\u003c/sub\u003e phase gradually decreases. Meanwhile, a white degradation region is observed between the (Nb,Cr,Ti,Ta)Si\u003csub\u003e2\u003c/sub\u003e layer and the single NbSi\u003csub\u003e2\u003c/sub\u003e layer (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ec). With further extension of the oxidation time, the content of SiO\u003csub\u003e2\u003c/sub\u003e formed on the coating surface continuously increases (Site 4 in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e), and the white degradation region inside the coating further expands, which is mainly composed of (Nb,Cr,Ti)\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e phase (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Upon oxidation to 20 h, the coating surface is essentially covered by a continuous oxide scale, within which Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e embedded in SiO\u003csub\u003e2\u003c/sub\u003e can still be observed (Site 8 in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e). At this stage, the coating has undergone significant degradation, and a two-phase region of (Nb,Cr,Ti,Ta)Si\u003csub\u003e2\u003c/sub\u003e/(Nb,Cr,Ti)\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e can be observed beneath the (Nb,Cr,Ti,Ta)Si\u003csub\u003e2\u003c/sub\u003e layer. As the oxidation time continues to prolong, the oxide scale on the coating surface continuously thickens, and the (Nb,Cr,Ti)\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e degradation region also expands continuously. Conversely, the (Nb,Cr,Ti,Ta)Si\u003csub\u003e2\u003c/sub\u003e layer and the single NbSi\u003csub\u003e2\u003c/sub\u003e layer continuously degrade and decrease. After 150 h of oxidation, a dense oxide scale with a thickness of approximately 22 \u0026micro;m has formed on the coating surface, which is mainly composed of SiO\u003csub\u003e2\u003c/sub\u003e, TiO\u003csub\u003e2\u003c/sub\u003e, and Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (Site 10 in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e). Notably, the dissolved Cr, Ti, and Ta atoms in the original (Nb,Cr,Ti,Ta)Si\u003csub\u003e2\u003c/sub\u003e layer can no longer be detected (Site 11 in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e). This indicates that during the long-term oxidation process, the Cr, Ti, and Ta elements continuously diffuse outward and undergo oxidation reactions, thereby driving the gradual transformation of the (Nb,Cr,Ti,Ta)Si\u003csub\u003e2\u003c/sub\u003e phase into the pure NbSi\u003csub\u003e2\u003c/sub\u003e phase.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section3\"\u003e \u003ch2\u003e3.2.4 Microstructure of the coating oxidized at 1500 ℃\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e shows the surface BSE images of 5Ta coating after oxidation at 1500 ℃ for different time. After 1 h of oxidation, the coating surface was immediately covered by a large amount of Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, while abundant pore defects still remained. With prolonged oxidation time, the content of Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e on the coating surface gradually decreased, whereas that of SiO\u003csub\u003e2\u003c/sub\u003e exhibited an increasing trend. When the oxidation time reached 20 h, Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e was barely detectable on the surface, which was fully covered by a continuous glassy SiO\u003csub\u003e2\u003c/sub\u003e layer. In addition, the content of TiO\u003csub\u003e2\u003c/sub\u003e embedded in the SiO\u003csub\u003e2\u003c/sub\u003e matrix also increased gradually. With further extension of oxidation time, the surface oxide scale became progressively denser, and both the content and size of TiO\u003csub\u003e2\u003c/sub\u003e particles increased. The morphological evolution of the scale was generally consistent with that observed at 1400 ℃.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e shows the cross-sectional BSE images of 5Ta coating after oxidation at 1500 ℃ for different time. After 1 h of oxidation, the coating surface was covered by a Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e layer with a thickness of approximately 3 \u0026micro;m. When oxidized for 5 h, a considerable amount of SiO\u003csub\u003e2\u003c/sub\u003e phase was formed on the coating surface besides Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. At this stage, a (Nb,Cr,Ti)₅Si₃ degraded layer with a thickness of about 15 \u0026micro;m was observed in the coating. After 10 h of oxidation, the Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e content on the coating surface decreased remarkably. The coating from top to bottom consisted of a mixed SiO\u003csub\u003e2\u003c/sub\u003e-TiO\u003csub\u003e2\u003c/sub\u003e-Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e oxide scale, a (Nb,Cr,Ti,Ta)Si\u003csub\u003e2\u003c/sub\u003e layer, a (Nb,Cr,Ti)\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e layer, a single NbSi\u003csub\u003e2\u003c/sub\u003e layer, and a Nb\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e layer. With further oxidation up to 20 h, the scale thickened continuously and was mainly composed of SiO\u003csub\u003e2\u003c/sub\u003e and TiO\u003csub\u003e2\u003c/sub\u003e. Similar to the oxidation behavior at 1400 ℃, both the (Nb,Cr,Ti,Ta)Si\u003csub\u003e2\u003c/sub\u003e layer and the single NbSi\u003csub\u003e2\u003c/sub\u003e layer degraded continuously and reduced in content. After 100 h of oxidation, the single NbSi\u003csub\u003e2\u003c/sub\u003e layer was completely depleted, and (Nb,Cr,Ti,Ta)Si\u003csub\u003e2\u003c/sub\u003e was fully transformed into pure NbSi\u003csub\u003e2\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ef).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo investigate the elemental diffusion behavior during high temperature oxidation, EPMA elemental mapping was performed on the cross-section of 5Ta coating after oxidation at 1500 ℃ for 1 h and 100 h, as shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e and \u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e, respectively. As can be seen from Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e, after oxidation at 1500 ℃ for 1 h, Nb and Si in the coating exhibited a three-layered distribution, corresponding to the (Nb,Cr,Ti,Ta)Si\u003csub\u003e2\u003c/sub\u003e/Cr\u003csub\u003e4\u003c/sub\u003eNb\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003e5\u003c/sub\u003e layer, the single NbSi\u003csub\u003e2\u003c/sub\u003e layer, and the Nb\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e layer. Cr was mainly distributed in Cr\u003csub\u003e4\u003c/sub\u003eNb\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003e5\u003c/sub\u003e phase and was significantly enriched in the surface oxide. Notably, the overall Cr concentration in the coating was considerably lower than that before oxidation (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Ti was primarily concentrated in the (Nb,Cr,Ti,Ta)Si\u003csub\u003e2\u003c/sub\u003e/Cr\u003csub\u003e4\u003c/sub\u003eNb\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003e5\u003c/sub\u003e layer. O was mainly enriched in the surface oxide, while Ta was predominantly distributed in the upper (Nb,Cr,Ti,Ta)Si\u003csub\u003e2\u003c/sub\u003e phase, with a small amount also detected in the surface oxide scale. W, Mo, and Zr were mostly retained in the substrate.\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e, after oxidation at 1500 ℃ for 100 h, the thickness of the scale increased substantially. Compared with the 1 h oxidized sample, the Si content in the residual coating decreased sharply, indicating significant outward diffusion of Si. Meanwhile, the Cr and Ti contents in the coating were obviously reduced. Ti was enriched in the scale as TiO\u003csub\u003e2\u003c/sub\u003e, in which the enrichment of Nb and Cr could also be detected, suggesting that a certain amount of Nb and Cr was dissolved in TiO\u003csub\u003e2\u003c/sub\u003e. In addition, Ta was also enriched in the TiO\u003csub\u003e2\u003c/sub\u003e phase and was prominently distributed in the white degraded Nb\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e phase in the upper residual coating. It is worth noting that W and Mo were significantly enriched at the coating transition layer/substrate interface.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e4.1 Formation mechanism of Cr-Ta-Ti modified silicide coating\u003c/h2\u003e \u003cp\u003eA schematic diagram illustrating the formation mechanism of the Cr-Ta-Ti modified silicide coating is displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e14\u003c/span\u003e. Due to the low melting point of Si (\u0026lt;\u0026thinsp;1420 ℃) [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e] and its high mass fraction of 75 wt% in the slurry, Si melted rapidly to form a liquid phase when the sintering temperature exceeded its melting point. Therefore, the coating formation was mainly dominated by the inward diffusion of liquid Si into the substrate.\u003c/p\u003e \u003cp\u003eAt the initial stage of reaction, liquid Si came into contact with the Nb substrate and reacted rapidly to produce a Nb\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e transition layer. Subsequently, Nb\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e further reacted with the inward-diffusing Si and transformed into NbSi\u003csub\u003e2\u003c/sub\u003e. Driven by the Si concentration gradient, this reaction front continuously propagated toward the substrate interior. Meanwhile, Cr, Ti, and part of Ta in the slurry also participated in the reaction. Cr and Ti diffused inward together with Si and dissolved into the as-formed NbSi\u003csub\u003e2\u003c/sub\u003e lattice driven by the concentration gradient. With the reaction proceeding, the coating thickness increased rapidly. Since the solid solubility of Cr in NbSi\u003csub\u003e2\u003c/sub\u003e was limited [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e], Cr\u003csub\u003e4\u003c/sub\u003eNb\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003e5\u003c/sub\u003e precipitated as a secondary phase when the local Cr content exceeded its solubility limit. In contrast, Ti remained dissolved in NbSi\u003csub\u003e2\u003c/sub\u003e owing to its relatively low content. Eventually, the coating exhibited a typical four-layered structure from top to bottom: a mixed (Nb,Cr,Ti,Ta)Si\u003csub\u003e2\u003c/sub\u003e/Cr\u003csub\u003e4\u003c/sub\u003eNb\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003e5\u003c/sub\u003e layer, a single NbSi\u003csub\u003e2\u003c/sub\u003e layer, a NbSi\u003csub\u003e2\u003c/sub\u003e/Nb\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e layer, and a Nb\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e transition layer.\u003c/p\u003e \u003cp\u003eNotably, increasing the Ta content in the slurry did not significantly improve the dissolved Ta concentration inside the coating (sites 6 and 7 in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). This phenomenon is mainly attributed to the kinetic constraints of diffusion. Firstly, due to its relatively large atomic radius [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e], Ta exhibits substantially higher diffusion resistance in silicide lattices and grain boundaries in comparison with Cr and Ti, which in turn leads to a slower migration rate toward the substrate. Secondly, as a high melting point refractory metal, Ta exhibits a significantly lower thermal diffusivity at the sintering temperature of 1450 ℃ than Cr and Ti, which are close to a molten state [\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]. This indicates that the diffusion of Ta acts as the rate-limiting step in the present sintering system. Simply increasing the Ta content in the slurry cannot overcome its diffusion kinetic barrier. Instead, excessive unreacted Ta tends to accumulate in the upper coating, hindering the effective diffusion of Si and other active elements and eventually reducing the coating thickness (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003e4.2 Oxidation mechanism of Cr-Ta-Ti modified silicide coating\u003c/h2\u003e \u003cp\u003eBased on the above analysis of oxidation kinetics and oxide scale evolution, the oxidation process of the Cr-Ta-Ti modified silicide coatings can be divided into three stages. At the initial oxidation stage (1\u0026ndash;5 h), oxygen reacts with (Nb,Cr,Ti,Ta)Si\u003csub\u003e2\u003c/sub\u003e on the coating surface, rapidly forming amorphous SiO\u003csub\u003e2\u003c/sub\u003e. Meanwhile, driven by the concentration gradient, a large amount of Cr diffuses outward and reacts with oxygen to form Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. EPMA elemental mapping (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e) confirms that most Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e is distributed at the coating pores at this stage. In addition, a small amount of Ta is uniformly distributed in the oxide scale without forming discrete oxide particles of Ta, and no diffraction peaks of Ta\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e are observed in the XRD patterns (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). This indicates that Ta is incorporated into the SiO\u003csub\u003e2\u003c/sub\u003e glass network in the form of solid solution during oxidation. Previous studies have demonstrated that Ta ion doping can significantly improve the high temperature viscosity of the SiO\u003csub\u003e2\u003c/sub\u003e glass phase and effectively reduce the diffusion coefficient of oxygen ions in the SiO\u003csub\u003e2\u003c/sub\u003e glass network [\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e, \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e]. Such an in-situ modification effect endows the as-formed oxide scale with favorable compactness and oxygen barrier properties, suppressing the rapid inward penetration of oxygen. Although Cr tends to diffuse outward and form volatile Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, the Ta-doped high viscosity oxide scale can inhibit the excessive volatilization of Cr to a certain extent, maintaining a more intact oxide scale structure.\u003c/p\u003e \u003cp\u003eAt the middle oxidation stage (5\u0026ndash;20 h), the further oxidative volatilization of Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e at high temperatures (especially 1500 ℃) and the increased formation of SiO\u003csub\u003e2\u003c/sub\u003e become the dominant factors controlling the oxidation behavior. As seen from the oxidation kinetic curves (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb), the coating without Ta shows a remarkable mass gain during the middle oxidation stage, whereas the mass change of the 5Ta coating continues to decrease slowly. In this system, the mass gain is attributed to the reaction between the silicide coating and oxygen to form oxides, while the mass loss results from the continuous further oxidation and volatilization of Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. The difference in oxidation kinetics reveals that the oxidation rate of the 5Ta coating is lower than that of the Ta-free coating, which is mainly ascribed to two reasons. First, the SiO\u003csub\u003e2\u003c/sub\u003e oxide scale doped with Ta exhibits superior high temperature thermal stability compared with pure SiO\u003csub\u003e2\u003c/sub\u003e. Its viscosity is less affected by temperature fluctuations, enabling it to effectively cover the microporous defects left by Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e volatilization and showing excellent self-healing ability. Second, Ta dissolved in the SiO\u003csub\u003e2\u003c/sub\u003e glassy oxide scale acts as a skeleton-strengthening component, which improves the overall corrosion resistance of the oxide scale [\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e] and reduces the oxidation rate. Accordingly, the SiO\u003csub\u003e2\u003c/sub\u003e-Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e mixed oxide scale formed at this stage is denser than that of the unmodified coating, which can effectively block the oxygen transport pathways. This is consistent with the experimental results of the significantly reduced oxidation mass gain rate (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) and the decreased oxide scale thickness (Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e15\u003c/span\u003e) [\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e]. However, the advantage of the 5Ta coating in the oxidation kinetic curve is not obvious at 1400 ℃, indicating that the effect of refractory Ta cannot be fully exerted at relatively low temperatures. The higher the temperature, the more significantly Ta functions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAt the late oxidation stage (20\u0026ndash;100 h), the coating undergoes continuous degradation. The high-silicide layer gradually degrades due to the sustained diffusion and consumption of Si. Once this layer is completely depleted, the coating will lose its protective capability and fail because sufficient Si can no longer be supplied to the surface layer. Therefore, a sufficient coating thickness is required to achieve long-term protection of the substrate in high-temperature oxidation environments. The 10Ta and 15Ta coatings are prone to failure during this stage (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), with insufficient thickness being a major contributing factor. In contrast, for the 5Ta coating, even after oxidation at 1500 ℃ for 100 h, the NbSi\u003csub\u003e2\u003c/sub\u003e layer still retains a thickness of 23.7 \u0026micro;m (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e), and the coating surface is completely covered by a mixed SiO\u003csub\u003e2\u003c/sub\u003e-TiO\u003csub\u003e2\u003c/sub\u003e-Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e oxide scale with a thickness of approximately 27.4 \u0026micro;m. The mixed oxide scale on the coating surface can significantly suppress the inward diffusion of oxygen, and the silicide beneath the oxide scale can continuously supply Si to the outer layer. From the distribution of Ta at the late oxidation stage, obvious enrichment of Ta in TiO\u003csub\u003e2\u003c/sub\u003e is observed. It has been reported [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e] that the introduction of Ta leads to the substitution of tetravalent Ti by pentavalent Ta, which reduces the concentration of Ti vacancies and O vacancies in TiO\u003csub\u003e2\u003c/sub\u003e, thereby inhibiting the inward diffusion of oxygen and the outward diffusion of Ti. This indicates that the defect reaction triggered by Ta\u003csup\u003e5+\u003c/sup\u003e entering the TiO\u003csub\u003e2\u003c/sub\u003e lattice effectively reduces the overall concentration of Ti vacancies and O vacancies in TiO\u003csub\u003e2\u003c/sub\u003e. Since oxygen vacancies serve as the main diffusion pathways for oxygen ions in oxides, their markedly decreased concentration directly hinders the diffusion rate of oxygen ions through TiO\u003csub\u003e2\u003c/sub\u003e grains or grain boundaries. Meanwhile, the reduced point defects render the TiO\u003csub\u003e2\u003c/sub\u003e solid solution grains more structurally intact and denser, further enhancing the thermal stability and oxygen diffusion barrier property of this oxide. In addition, the stable Ta-O structure exerts a pinning effect in the oxide scale, suppressing the excessive flow and structural relaxation of the SiO\u003csub\u003e2\u003c/sub\u003e glass phase at ultra-high temperatures, and ensuring the structural integrity of the oxide scale during long-term service. Accordingly, at the late stage, Ta improves the overall protective performance of the coating through the above mechanisms, leading to a significantly reduced oxide scale thickness (Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e15\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eBy comparative analysis with the Cr-Ti modified silicide coating without Ta addition [\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e], the introduction of Ta optimizes the formation process, microstructure, and high temperature stability of the oxide scale at 1400 ℃ and 1500 ℃. Although the two coatings share the same oxidation stage division (dominated by Cr in the early stage and Si in the later stage), the addition of Ta significantly enhances the protective efficiency of the coating via its unique thermodynamic and kinetic effects, as directly evidenced by the obviously reduced oxide scale thickness. In summary, the core high temperature oxidation mechanism of the Cr-Ta-Ti modified silicide coating lies in achieving longer-lasting protection with a thinner and denser oxide scale by increasing the oxide scale viscosity, restraining elemental diffusion, and forming an oxide scale with low oxygen permeability. This mechanism significantly improves the oxidation resistance of the coating under extreme conditions at 1400 ℃ and 1500 ℃.\u003c/p\u003e \u003c/div\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003e(1) The Cr-Ta-Ti modified silicide coatings exhibit a distinct layered structure, which from top to bottom consists of a (Nb,Cr,Ti,Ta)Si\u003csub\u003e2\u003c/sub\u003e/Cr\u003csub\u003e4\u003c/sub\u003eNb\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003e5\u003c/sub\u003e composite layer, a dense NbSi\u003csub\u003e2\u003c/sub\u003e layer, a NbSi\u003csub\u003e2\u003c/sub\u003e/Nb\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e layer and a Nb\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e transition layer. The introduction of Ta suppresses the diffusion of Si and other active elements during sintering, resulting in a reduced total coating thickness.\u003c/p\u003e \u003cp\u003e(2) The addition of an appropriate amount of Ta can significantly slow down the growth rate of the oxide scale. Specifically, the 5Ta coating achieves the optimal comprehensive performance, providing effective protection to the substrate for at least 150 h at 1400 ℃ and 100 h at 1500 ℃. Coatings with an excessively high Ta content are prone to spallation failure during oxidation, and the insufficient coating thickness is one of the primary factors inducing this failure behavior.\u003c/p\u003e \u003cp\u003e(3) The oxidation process of the Cr-Ta-Ti modified silicide coating is controlled by the diffusion of Cr and Si. Ta \u0026ldquo;pinned\u0026rdquo; in SiO\u003csub\u003e2\u003c/sub\u003e and TiO\u003csub\u003e2\u003c/sub\u003e not only significantly improves the high temperature viscosity of the oxide scale, but also effectively inhibits the inward diffusion of oxygen ions and the outward diffusion of other elements. This dual mechanism significantly reduces the oxide scale thickness and thereby enhances the oxidation resistance of the coating.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eHao Jiang: Writing - original draft, Validation, Investigation, Data curation. Yanqiang Qiao: Writing \u0026ndash; review \u0026amp; editing, Validation, Su pervision, Resources, Conceptualization. Weiping Zhang: Validation, Investigation. Xiping Guo: Writing \u0026ndash; review \u0026amp; editing, Supervision. Qifan You: Investigation, Supervision. Xuan Li: Writing \u0026ndash; review \u0026amp; editing, Supervision, Methodology.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThis work was financially supported by the National Natural Science Foundation of China (No. 52571099), the Key R\u0026amp;D Program of Jiangxi Province (No. 20261BCE310021), the Key R\u0026amp;D Program of Xianyang (L2025-ZDYF-KTH-001) and the General Project of Sichuan Provincial Natural Science Foundation (No. 2026NSFSC0331).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eGuo L, He WT, Chen WB, Xue ZL, He J, Guo YQ, Wu Y, Gao LH, Li DQ, Zhang Z, Wei LL, Gao YY, Zhang TH, Qiao J, Li QH, Guo HB (2023) Progress on high-temperature protective coatings for aero-engines. Surf Sci Technol 4:1\u003c/li\u003e\n\u003cli\u003eLiu YK, Liu Z, Fei YJ, Wang CH, Yang HR (2025) Study on the effect of multi-rare-earth-doping on thermal barrier coatings\u0026apos; thermophysical and fluorescence properties. Surf Sci Technol 3:40\u003c/li\u003e\n\u003cli\u003eZhang YY, Fu T, Yu LH, Cui KK, Wang J, Shen FQ, Zhang X, Zhou KC (2023) Anti-corrosion coatings for protecting Nb-based alloys exposed to oxidation environments: A review. Met Mater Int 29:1-17\u003c/li\u003e\n\u003cli\u003eChen DZ, Zhu JQ, Gong WT, Zhang XF, Xu C, Wang S, Wang QB, Xu Q, Wang Q, Chen RR, Fu HZ (2026) Combined effect of NdNbTiO\u003csub\u003e6\u003c/sub\u003e precipitation and fine microstructure on oxidation resistance of Nb-Si alloys at 1250 ℃. Corros Commun, doi: https://doi.org/10.1016/j.corcom.2026.03.011\u003c/li\u003e\n\u003cli\u003eZhang X, Fu T, Cui KK, Zhang YY, Shen FQ, Wang J, Yu LH, Mao HB (2021) The protection, challenge, and prospect of anti-oxidation coating on the surface of niobium alloy. Coatings 11:742\u003c/li\u003e\n\u003cli\u003eJoshi K, Kumar P (2024) Strength behavior of niobium-based refractory systems. JOM 76:6277-6301\u003c/li\u003e\n\u003cli\u003eTang XH, Zhao G, Liu J, Wang Q, Bai XD, Wang LF (2024) High-temperature antioxidant silicate coating of low-density Nb-Ti-Al alloy: A review. High Temp Mater Processes 43: 20240029\u003c/li\u003e\n\u003cli\u003eMajumdar S, Kishor J, Paul B, Hubli RC, Chakravartty JK (2015) Isothermal oxidation behavior and growth kinetics of silicide coatings formed on Nb-1Zr-0.1C alloy. Corros Sci 95:100-109\u003c/li\u003e\n\u003cli\u003eNarita T, Kato Y, Narita T, Ara M (2023) Advances in diffusion barrier coatings for high-temperature applications. High Temp Corros Mater 100:399-411\u003c/li\u003e\n\u003cli\u003eChen DZ, Gong WT, Xu C, Zhang YJ, Hao R, Yang ZB, Chen RR, Fu HZ (2025) Oxidation behavior of Ge-modified Nb-Si alloys at 1250 ℃: Self-generated NbGe\u003csub\u003e2\u003c/sub\u003e layer with enhanced oxidation resistance. Appl Sur Sci Advances 30:100874\u003c/li\u003e\n\u003cli\u003eFu T, Zhan SR, Zhang YY, Shen FQ, Wang H (2025) Preparation of Si-rich NbSi\u003csub\u003e2\u003c/sub\u003e coating on Nb surface by hot dip silicon-plating technology to improve the high-temperature oxidation resistance of pure Nb substrate. Ceram Int 51:23341-23353\u003c/li\u003e\n\u003cli\u003eWang XY, Zhu L, Yang YJ, Zhang BJ, Kiryukhantsev-Korneev PV, Levashov EA, Ren XR, Ji X, Feng PZ, Wang XH (2025) Upcycling waste MoSi\u003csub\u003e2\u003c/sub\u003e into high-performance composite coatings for protecting refractory alloys across a wide temperature range. Int J Refract Met Hard Mater 132:107295\u003c/li\u003e\n\u003cli\u003eXu BB, Guo XP, Qiao YQ (2024) Effects of TiB\u003csub\u003e2\u003c/sub\u003e contents on the microstructure and oxidation behavior of Mo-Si-B composite coatings. J Alloys Compd 1005:175967\u003c/li\u003e\n\u003cli\u003eMajumdar S, Sharma IG (2011) Oxidation behavior of MoSi\u003csub\u003e2\u003c/sub\u003e and Mo(Si,Al)\u003csub\u003e2\u003c/sub\u003e coated Mo-0.5Ti-0.1Zr-0.02C alloy. Intermetallics 19:541-545\u003c/li\u003e\n\u003cli\u003eChoi YJ, Yoon JK, Kim GH, Yoon WY, Doh JM, Hong KT (2017) High temperature isothermal oxidation behavior of NbSi\u003csub\u003e2\u003c/sub\u003e coating at 1000-1450 ℃. Corros Sci 129:102-114\u003c/li\u003e\n\u003cli\u003eLi LF, Guo XP, Qiao YQ (2023) Formation and oxidation resistance of MoSi\u003csub\u003e2\u003c/sub\u003e coating for Nb-Si based alloy prepared by slurry sintering method. J Alloys Compd 938:168456\u003c/li\u003e\n\u003cli\u003eAlam MZ, Rao AS, Das DK (2010) Microstructure and high temperature oxidation performance of silicide coating on Nb-based alloy C-103. Oxid Met 73:513-530\u003c/li\u003e\n\u003cli\u003eLi Z, Zhang P, Li X, Chen C, He Y, Feng P (2025) A comparison of multi-component silicide (Ta,Mo,W)Si\u003csub\u003e2\u003c/sub\u003e coatings on Nb alloy prepared by low-temperature reactive sintering using different powder feedstocks. Mater Today Commun 42:111338\u003c/li\u003e\n\u003cli\u003eShao W, Cui YW, Zhou CG (2019) Diffusion paths of silicide coatings on Nb-Si-based alloys during pack cementation process. Metall Materi Trans A 50A:2945-2955\u003c/li\u003e\n\u003cli\u003eWang X, Zhang W, Wang TY, Yang T, Han CX, Yang F (2025) In-situ formation mechanism and long-term isothermal oxidation behaviors at 1400 ℃ and 1500 ℃ of (Nb,Ti,Mo)Si\u003csub\u003e2\u003c/sub\u003e-(Ti,Nb,Mo)\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e biphasic compositionally-complex ceramic composite coating on C103 alloy. Corros Sci 251:112914\u003c/li\u003e\n\u003cli\u003eZhang YF, Xiao LR, Zeng DL, Li X, Zhou XJ, Zhao G, He WK, Li S, Xiao YX, Zhao XJ, Liu SA, Cai ZY (2022) A novel niobium based oxidation protective coating with three lines of defense at ultra-high temperature. Corros Sci 206: 110515\u003c/li\u003e\n\u003cli\u003eGe YL, Wang YM, Chen JC, Zou YC, Guo LX, Ouyang J, Jia DC, Zhou Y (2018) An Nb\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e-SiO\u003csub\u003e2\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/NbSi\u003csub\u003e2\u003c/sub\u003e/Nb\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e multilayer coating on Nb-Hf alloy to improve oxidation resistance. J Alloys Compd 745:271-281\u003c/li\u003e\n\u003cli\u003eWang Z, Wang YM, Wang SQ, Zou YC, Chen GL, Wen L, Ouyang JH, Jia DC, Zhou Y (2022) ZrSi\u003csub\u003e2\u003c/sub\u003e/SiO\u003csub\u003e2\u003c/sub\u003e-Nb\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/NbSi\u003csub\u003e2\u003c/sub\u003e multi-layer coating formed on niobium alloy by HAPC combined with LPDS: Microstructure evolution and high temperature oxidation behavior. Corros Sci 206:110460\u003c/li\u003e\n\u003cli\u003eWang Z, Wang YM, Wang SQ, Zou YC, Chen GL, Wen L, Zhang GX, Zhao LN, Ouyang JH, Jia DC, Zhou Y (2022) Design and preparation of MoSi\u003csub\u003e2\u003c/sub\u003e/SiO\u003csub\u003e2\u003c/sub\u003e-Nb\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e/NbSi\u003csub\u003e2\u003c/sub\u003e multilayer coating on Nb alloy: Microstructure and hot corrosion behavior. Corros Sci 209:110733\u003c/li\u003e\n\u003cli\u003eJiang H, Qiao YQ, Zhang WP, Guo XP (2024) Characterization of microstructure and oxidation behavior of Al modified MoSi\u003csub\u003e2\u003c/sub\u003e coating on Nb-Si based alloy. Surf Interfaces 51:104739\u003c/li\u003e\n\u003cli\u003eLi LF, Guo XP, Qiao YQ (2023) A novel composite Si-Mo-Ti coating with Ti concentration gradient for Nb-Si based ultrahigh temperature alloy prepared by slurry sintering. Corros Sci 225:111632\u003c/li\u003e\n\u003cli\u003eAn DY, Liu CF, Zhao S, Zhang HP, Zou YC, Tang ZW, Xiao P, Dai JM, Wang YM (2023) High-temperature oxidation resistance performance of Si-Ti-Cr silicide coating on Nb-Hf alloy surface in constant oxidation and thermal shock. Rare Met Mater Eng 52:1219-1226\u003c/li\u003e\n\u003cli\u003eCheng CY, Xie W, Li HJ, Fu QG (2021) Evaporation behavior of SiO\u003csub\u003e2\u003c/sub\u003e glass doped with various transition metal oxides. J Am Ceram Soc 104:3130-3138\u003c/li\u003e\n\u003cli\u003eLi JL, Wang W, Zhou CG (2017) Oxidation and interdiffusion behavior of a germanium-modified silicide coating on an Nb-Si-based alloy. Int J Miner Metall Mater 24:289-296\u003c/li\u003e\n\u003cli\u003eSankar M, Prasad VVS, Baligidad RG, Alam MZ, Das DK, Gokhale AA (2015) Microstructure, oxidation resistance and tensile properties of silicide coated Nb-alloy C-103. Mat Sci Eng. A-struct 645:339-346\u003c/li\u003e\n\u003cli\u003eWu JH, Wu WX, Sun AY, Zhu JY, Tang HQ, Shen F, Lei SY (2025) Study on high temperature oxidation resistance of HfSi\u003csub\u003e2\u003c/sub\u003e modified MoSi\u003csub\u003e2\u003c/sub\u003e coating. J Mater Res Technol 37:242-251\u003c/li\u003e\n\u003cli\u003eOu HK, Liu YF, Fan KF, Zhang YY, Guo LX, Liu B, Sun J, Fu QG (2025) Long term protection of silicide coating at 1700 ℃ in air with HfO\u003csub\u003e2\u003c/sub\u003e modification. Corros Sci 255:113122\u003c/li\u003e\n\u003cli\u003eWang CC, Li KZ, He DY, Shi XH (2020) Evolution behavior of rare-earth yttria modified silicide oxidation-resistant coating at 1700 ℃. J Eur Ceram Soc 40:4419-4427\u003c/li\u003e\n\u003cli\u003eYu JY, Chen DZ, Xu FD, Wang S, Cui XY, Gong WT, Chen RR (2024) Simultaneous improving high temperature oxidation resistance and room temperature toughness of Nb-Si based alloy via appropriate Ta content addition strategy. Int J Refract Met Hard Mater 124:106800\u003c/li\u003e\n\u003cli\u003eLiu XJ, Luo H, Lu Y, Han JJ, Li J, Guo YH, Huang YX, Wang CP (2017) Thermodynamic Assessment of the Nb-Si-Ta System. J Phase Equilib Diffus 38:897-905\u003c/li\u003e\n\u003cli\u003eHe S, Liu Y, Ma Z, Zhu S, Liu L, Mu G, Li Y (2024) Study on anti-ablation properties of HfB\u003csub\u003e2\u003c/sub\u003e-TaSi\u003csub\u003e2\u003c/sub\u003e coatings prepared by atmospheric plasma spraying. J Alloys Compd 999:175069\u003c/li\u003e\n\u003cli\u003eXu Y, Zheng W, Dai M, Xu X, Hong D, Zhong X, Niu Y, Zheng X (2023) Effect of TaSi\u003csub\u003e2\u003c/sub\u003e addition on long-term ablation behavior of HfB\u003csub\u003e2\u003c/sub\u003e-SiC coating. J Eur Ceram Soc 43:5802-5813\u003c/li\u003e\n\u003cli\u003eXu Y, Dai M, Yu L, Huang S, Zhong X, Niu Y, Zeng Y, Zheng X (2025) Ablation behavior and mechanism of modified HfB\u003csub\u003e2\u003c/sub\u003e-SiC coatings. Surf Coat Technol 496:131725\u003c/li\u003e\n\u003cli\u003eBahr A, Richter S, Hahn R, Wojcik T, Podsednik M, Limbeck A, Ramm J, Hunold O, Kolozsvari S, Riedl H (2023) Oxidation behaviour and mechanical properties of sputter-deposited TMSi\u003csub\u003e2\u003c/sub\u003e coatings (TM = Mo, Ta, Nb). J Alloys Compd 931:167532\u003c/li\u003e\n\u003cli\u003eZhang YF, Liu SN, Zhou XJ, Zhao G, Liu J, Shen HT, Cai ZY, Zhao XJ, Xiao LR (2022) Ultra-high temperature oxidation behavior of ZrB\u003csub\u003e2\u003c/sub\u003e/YSZ modified Si-Mo-W coating with a diffusion barrier on niobium alloy. Corros Sci 195:109977\u003c/li\u003e\n\u003cli\u003eWang Z, Zhang YP, Zhou SH, Wang ZY, Yao YR, Wang AY, Li M, Ke PL (2024) Oxidation resistance of Mo/Cr bilayer coating on Zr alloy in a 1200 ℃ steam environment. Corros Commun 14:49-57\u003c/li\u003e\n\u003cli\u003eYin L, Ma JY, Yang F, Nie YJ, Meng LX, Wang YZ, Zheng LW, Shi QX, Liang W (2025) Influence of precipitates on the initial oxidation behavior of GH4169 superalloy at 1000 ℃. Corros Commun 19:138-148\u003c/li\u003e\n\u003cli\u003eDorner F, Sukurma Z, Dellago C, Kresse G (2018) Melting Si: Beyond density functional theory. Phys Rev Lett 121:195701\u003c/li\u003e\n\u003cli\u003eSekido N, Aizawa R, Ueno S (2019) Effect of Cr Addition on the phase equilibria and oxidation behavior of NbSi\u003csub\u003e2\u003c/sub\u003e. Mater Trans 60:666-673\u003c/li\u003e\n\u003cli\u003eTsakiropoulos P (2017) On the Nb silicide based alloys: Part I - The bcc Nb solid solution. J Alloys Compd 708:961-971\u003c/li\u003e\n\u003cli\u003eAnsel D, Thibon I, Boliveau M, Debuigne J (1998) Interdiffusion in the body cubic centered \u0026beta;-phase of Ta-Ti alloys. Acta Mater 46:423-430\u003c/li\u003e\n\u003cli\u003eRoy S, Prasad S, Divinski SV, Paul A (2014) Diffusion pattern in MSi\u003csub\u003e2\u003c/sub\u003e and M\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e silicides in group VB (M = V, Nb, Ta) and VIB (M = Mo, W) refractory metal-silicon systems. Philos Mag 94:1508-1528\u003c/li\u003e\n\u003cli\u003eFattah-alhosseini A, Chaharmahali R, Dikici B, Kaseem M (2025) A comprehensive review of plasma electrolytic oxidation (PEO) of tantalum (Ta): Mechanisms, properties, and applications. Int J Refract Met Hard Mater 128:107059\u003c/li\u003e\n\u003cli\u003eKane KA, Pint BA, Mitchell D, Haynes JA (2021) Oxidation of ultrahigh temperature ceramics: kinetics, mechanisms, and applications. J Eur Ceram Soc 41:6130-6150\u003c/li\u003e\n\u003cli\u003eLuo X, Zhang XX, Yang X, Zuo YM, Huang QZ (2025) Effect of titanium and silicon oxides migration on oxide structure evolution and oxidation mechanism of (Zr\u003csub\u003e0.25\u003c/sub\u003eHf\u003csub\u003e0.25\u003c/sub\u003eTi\u003csub\u003e0.25\u003c/sub\u003eTa\u003csub\u003e0.25\u003c/sub\u003e)C-30SiC at 1600 ℃. Ceram Int 51:21936-21950\u003c/li\u003e\n\u003cli\u003eJiang H, Qiao YQ, Zhang WP, Guo XP, Gao JC, Li LF, Nan X (2026) Temperature-dependent oxidation behavior and interfacial diffusion dynamics of multilayered silicide coating on Nb521 alloy (1200-1500 ℃). Surf Interfaces 86:108689\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"surface-science-and-technology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [Surface Science and Technology](https://link.springer.com/journal/44251)","snPcode":"44251","submissionUrl":"https://submission.springernature.com/new-submission/44251/3","title":"Surface Science and Technology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Open","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Nb521, Silicide coating, Slurry sintering, Ta modification, High temperature oxidation resistance","lastPublishedDoi":"10.21203/rs.3.rs-9197546/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9197546/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eTo address the insufficient high temperature oxidation resistance of Nb521 alloy in engineering applications, Cr-Ta-Ti modified silicide coatings with different Ta contents were fabricated on its surface via slurry sintering. The oxidation behavior and elemental diffusion characteristics of the coatings were systematically investigated at 1400\u0026ndash;1500 ℃, and the mechanism by which Ta affects the microstructural evolution and oxidation resistance of the coatings was revealed. The results show that the Cr-Ta-Ti modified silicide coating exhibits a distinct multilayer structure, which from top to bottom consists of a (Nb,Cr,Ti,Ta)Si\u003csub\u003e2\u003c/sub\u003e/Cr\u003csub\u003e4\u003c/sub\u003eNb\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003e5\u003c/sub\u003e composite layer, a dense NbSi\u003csub\u003e2\u003c/sub\u003e layer, a NbSi\u003csub\u003e2\u003c/sub\u003e/Nb\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e layer, and a Nb\u003csub\u003e5\u003c/sub\u003eSi\u003csub\u003e3\u003c/sub\u003e transition layer. The introduction of Ta influences the diffusion of Si, Cr, and other elements, resulting in a reduced coating thickness. An appropriate Ta content (5Ta) can markedly decrease the growth rate of the oxide scale, providing effective protection for the substrate for at least 150 h at 1400 ℃ and 100 h at 1500 ℃, respectively. The dissolution of Ta into SiO\u003csub\u003e2\u003c/sub\u003e and TiO\u003csub\u003e2\u003c/sub\u003e enhances the high temperature viscosity and thermal stability of the oxide scale, restrains the inward diffusion of oxygen and the outward diffusion of coating elements, thereby significantly improving the high temperature oxidation resistance of the coating.\u003c/p\u003e","manuscriptTitle":"Unveiling the role of Ta in Cr-Ta-Ti modified silicide coatings for Nb alloy with superior oxidation resistance up to 1500 ℃","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-09 18:16:28","doi":"10.21203/rs.3.rs-9197546/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-04-27T13:11:44+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-20T15:03:39+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"138422262217996772224022369131795001442","date":"2026-04-09T09:19:18+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"193638566553689182615278157681615919913","date":"2026-04-06T01:56:25+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-04-05T14:29:17+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-03-31T05:54:16+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-03-31T05:53:39+00:00","index":"","fulltext":""},{"type":"submitted","content":"Surface Science and Technology","date":"2026-03-23T08:06:38+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"surface-science-and-technology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [Surface Science and Technology](https://link.springer.com/journal/44251)","snPcode":"44251","submissionUrl":"https://submission.springernature.com/new-submission/44251/3","title":"Surface Science and Technology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Open","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"bca6e70e-e265-4a4c-8427-daf89907cb09","owner":[],"postedDate":"April 9th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-12T08:38:47+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-09 18:16:28","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9197546","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9197546","identity":"rs-9197546","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}