Enhanced High-Temperature Oxidation Resistance of Mg-Gd-Zn-Zr Alloy via YSZ Thermal Barrier Coating with a Plasma Electrolytic Oxidation Bond Layer

Enhanced High-Temperature Oxidation Resistance of Mg-Gd-Zn-Zr Alloy via YSZ Thermal Barrier Coating with a Plasma Electrolytic Oxidation Bond Layer | 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 Enhanced High-Temperature Oxidation Resistance of Mg-Gd-Zn-Zr Alloy via YSZ Thermal Barrier Coating with a Plasma Electrolytic Oxidation Bond Layer Xuanyi He, Yinhua Shao, Yichen Li, Wei Zhang, Jinlong Wang, Minghui Chen, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7136434/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 02 Dec, 2025 Read the published version in npj Materials Degradation → Version 1 posted 10 You are reading this latest preprint version Abstract Magnesium alloys suffer from significant limitations in high-temperature applications due to poor oxidation resistance, primarily attributed to the non-protective nature of their native MgO scale (Pilling-Bedworth ratio, PBR = 0.81). This study investigates a novel thermal barrier coating (TBC) system, YSZ/PEO/Mg, designed to enhance the high-temperature performance of a Mg-Gd-Zn-Zr alloy. The system consists of an atmospheric plasma sprayed (APS) 8YSZ top coat deposited onto a plasma electrolytic oxidation (PEO) bond layer applied to the Mg substrate. For comparison, a YSZ coating deposited directly on the Mg substrate (YSZ/Mg) was also prepared. Both TBC systems exhibited stability during 100-hour cyclic oxidation at 200°C. However, under cyclic oxidation at 400°C for 100 minutes, the YSZ/Mg coating experienced catastrophic spallation due to interfacial oxidation and thermal stress, exposing the substrate. In contrast, the YSZ/PEO/Mg system maintained excellent integrity. Crucially, a continuous and protective gadolinium oxide (Gd₂O₃) thermally grown oxide (TGO) layer formed at the Mg/PEO interface during high-temperature exposure. Furthermore, the porous structure of the PEO layer facilitated mechanical interlocking of the YSZ top coat, significantly enhancing interfacial bonding strength. These results demonstrate that the YSZ/PEO/Mg TBC architecture, leveraging the synergistic effects of the PEO bond coat and the protective Gd₂O₃ TGO, provides an effective solution for significantly improving the high-temperature oxidation resistance of magnesium alloys. This approach is particularly promising for demanding applications such as aerospace thermal protection systems. Physical sciences/Chemistry Physical sciences/Energy science and technology Physical sciences/Engineering Physical sciences/Materials science Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction The exceptional low density of magnesium makes it highly attractive for aerospace lightweighting, offering substantial potential for reducing fuel consumption and increasing payload capacity 1 . However, the broader application of magnesium alloys is significantly constrained by inherent material limitations, including a relatively low melting point, high chemical reactivity (particularly with oxygen and moisture), and fundamentally inadequate resistance to elevated temperatures and corrosive environments 2 , 3 . These characteristics drive the rapid formation of a native magnesium oxide (MgO) scale upon air exposure, a process that accelerates dramatically at elevated temperatures. Crucially, MgO possesses a Pilling-Bedworth ratio (PBR) of only 0.81, signifying that the volume of oxide formed is less than the volume of metal consumed. This volumetric deficit impedes the formation of a fully coherent and adherent scale. Furthermore, the MgO scale is intrinsically porous and exhibits a non-protective, typically cubic crystalline structure that fails to act as an effective diffusion barrier 3 . Overcoming this inherent weakness necessitates the development of advanced strategies specifically designed to enhance the high-temperature oxidation resistance of magnesium alloys for demanding aerospace applications. Surface modification techniques represent a crucial pathway for mitigating these intrinsic limitations and unlocking the potential of magnesium alloys. A range of methods, including laser surface treatment 4 – 6 , chemical conversion coatings 7 – 9 , and plasma electrolytic oxidation (PEO) 10 – 13 , have been extensively investigated. However, the primary focus of these studies has largely centered on improving corrosion resistance at ambient or moderately elevated temperatures. Research specifically dedicated to enhancing high-temperature oxidation resistance (typically above 300°C) using these surface engineering approaches remains comparatively scarce. In this context, Thermal Barrier Coatings (TBCs) emerge as a particularly promising strategy. TBCs are specifically engineered to provide dual functionality: significant thermal insulation to lower the substrate operating temperature and a barrier against oxidizing atmospheres. For instance, Fan et al. 14 fabricated a multi-layered TBC system consisting of an 8 wt.% yttria-stabilized zirconia (8YSZ) top coat and a NiCrAlY bond coat on a magnesium alloy. Their results demonstrated the system's thermal insulation capability: when the coating surface was held at 200°C, the underlying substrate temperature was significantly reduced to 146°C. Under sustained thermal loading reaching equilibrium, the substrate stabilized at approximately 450°C while the coating surface reached about 530°C. This substantial temperature gradient directly translates to a marked improvement in the substrate's resistance to high-temperature oxidation. Despite their demonstrated potential for thermal management and oxidation protection, conventional TBC systems applied to magnesium alloys often suffer from inadequate thermal shock resistance, limiting their long-term durability. A primary driver of this failure is the significant mismatch in the coefficients of thermal expansion (CTE) between the ceramic top coat, metallic bond coats (if used), and the underlying magnesium alloy substrate. This CTE disparity induces substantial cyclic thermal stresses concentrated at critical bond interfaces during repeated heating and cooling cycles. Over time, this stress accumulation can lead to crack initiation and propagation, ultimately resulting in coating delamination and spallation 15 , 16 . The problem is particularly acute for magnesium due to its inherently high CTE, generating more severe interfacial stresses than in systems based on lower CTE substrates like nickel superalloys. Research efforts, such as those by Fan 15 , 16 , have explored strategies to mitigate this issue, including introducing Ni–P electroplated layers or thermally formed Mg-Al intermetallic diffusion layers as interlayers. While these approaches successfully doubled the thermal shock life compared to baseline systems, the fundamental challenge of achieving optimal interfacial bonding strength persists. Residual stresses and potential weak points at interfaces remain significant concerns for long-term reliability under extended service conditions involving repeated thermal transients. Surface morphology, particularly roughness, is a well-established factor critically influencing the bonding strength and durability of TBCs, especially those deposited via thermal spray processes like Atmospheric Plasma Spraying (APS), which rely heavily on mechanical interlocking. Eriksson 17 demonstrated that increasing the surface roughness of the underlying layer (bond coat or substrate) significantly extends the thermal fatigue life of APS-applied TBCs. The rougher surface provides a larger effective bonding area and facilitates superior mechanical keying of the deposited splats. Consequently, an intermediate layer characterized by both high surface roughness and intrinsically strong adhesion to the magnesium substrate holds considerable promise as an effective bond coat for TBC systems. Plasma Electrolytic Oxidation (PEO) treatment inherently produces precisely such a structure on magnesium alloys. The PEO process generates a thick, hard, in-situ grown oxide ceramic coating that is directly integrated with the substrate metal. This coating exhibits a characteristic bi-layer structure: a relatively dense and adherent inner layer adjacent to the substrate, and a more porous outer layer 18 . Crucially, the coating formation involves complex plasma-chemical reactions at the metal/electrolyte interface, resulting in a coating that is chemically bonded, rather than merely mechanically attached, to the underlying alloy. Furthermore, PEO coatings are renowned for significantly enhancing the substrate's corrosion resistance 13 , 19 . These attributes – strong chemical bonding, inherent corrosion protection, and a rough, porous outer morphology – collectively contribute to PEO coatings exhibiting outstanding adhesion strength to magnesium alloy substrates 20 , 21 . These advantages make a PEO layer serving as a bond coat for an APS-deposited 8YSZ top coat a highly viable and effective approach to substantially improve the thermal resistance and durability of magnesium alloys. Building directly upon this compelling rationale and the demonstrated potential of the PEO/YSZ combination, this study introduces and investigates a novel thermal barrier coating architecture specifically designed for a Mg-Gd-Zn-Zr alloy system. The core innovation lies in utilizing a plasma electrolytic oxidation (PEO)-generated oxide ceramic layer as the primary bond coat. Upon this robust and rough PEO foundation, a conventional 8YSZ top coat is deposited using Atmospheric Plasma Spraying (APS), forming the complete YSZ/PEO/Mg coating system. To evaluate the performance benefits of the novel PEO bond coat, this research directly compares the YSZ/PEO/Mg system against a conventional 8YSZ coating deposited directly onto the bare Mg-Gd-Zn-Zr alloy substrate (YSZ/Mg). The comprehensive investigation encompasses detailed microstructural characterization of both systems, systematic evaluation of thermal shock lifetime under cyclic heating/quenching, thorough analysis of failure mechanisms causing degradation and spallation, and detailed elucidation of the oxidation resistance mechanism within the YSZ/PEO/Mg system. Results Microstructure of as-prepared TBC Figure 1 presents the fracture cross-sectional morphologies of the two coating systems. In Fig. 1 a, both the YSZ/PEO and PEO/Mg interfaces exhibit an undulating profile. In contrast, the YSZ/Mg interface in Fig. 1 b is predominantly straight. The characteristic morphology of the APS-deposited YSZ layer, featuring microcracks and porosity, is evident in both figures. During the APS process, precursor powder particles are melted and propelled towards the substrate. Upon impact with the cooled substrate surface, these molten droplets undergo rapid solidification, resulting in the formation of a porous and rough coating structure 22 . Figure 2 displays the cross-sectional morphologies of the as-deposited samples. For the YSZ/PEO/Mg system (Fig. 2 a and 2 b), the YSZ top coat is embedded within the porous outer region of the PEO layer. This interlocking structure significantly enhances interfacial bonding strength with the top coat, achieving the intended design objective. Conversely, the YSZ layer deposited directly onto the substrate in the YSZ/Mg sample (Fig. 2 c and 2 d) shows a characteristic morphology, including a distinct oxide layer at the interface. Oxidation behavior of the TBCs Figure 3 presents the cross-sectional morphologies of both coating systems after cyclic oxidation at 200°C for 100 h. The images demonstrate that the coatings remain well-bonded in both systems. In Fig. 3 b, partial penetration of the YSZ into the PEO layer is observed, increasing the interfacial contact area. An oxidized region is visible at the YSZ/substrate interface in Fig. 3 d; however, it cannot be determined whether this oxidation occurred during the APS deposition process or the subsequent oxidation experiment. Figure 4 displays the surface morphologies of both samples after cyclic oxidation at 400°C for 100 min. The coating on the YSZ/PEO/Mg sample (Fig. 4 a) maintains its integrity. In contrast, the YSZ layer on the YSZ/Mg sample (Fig. 4 b) exhibits partial detachment. EDS analysis (Table 2 ) shows that Region 1 corresponds to the intact YSZ top coat. In Area 2, however, the atomic percentage of Zr decreased significantly from 20.1–4.9%, while the atomic percentage of Mg increased markedly from 1.9–34.6%. This compositional shift indicates that the YSZ coating in Area 2 has spalled, exposing the underlying magnesium alloy substrate. These results demonstrate that samples lacking a bond coat cannot provide stable protection at this temperature. Table 2 EDS results of “1” and “2” in Fig. 4 (at.%) Components O Mg Zr Y “1” 76.5 1.9 20.1 1.5 “2” 57.8 34.6 4.9 0.7 Figure 5 shows the cross-sectional morphologies of both systems after cyclic oxidation at 400°C for 100 min. For the YSZ/PEO/Mg sample (Figs. 5 a and 5 b), the interfaces remain well-bonded. A thin, continuous, bright-contrast band is evident at the Mg/PEO interface. The composition and phase identity of this band were analyzed. In the YSZ/Mg sample (Fig. 5 c), catastrophic spallation of the YSZ layer has occurred. Within the separation area (Fig. 5 d), an oxidized region containing discontinuous thermally grown oxide (TGO) is visible on the Mg substrate. This spallation is inferred to result primarily from thermal stress induced by the mismatch in coefficients of thermal expansion 22 . The oxidation occurring at the interface further weakened the bonding strength. Figure 6 presents TEM micrographs and diffraction patterns of the PEO/Mg interface. Elemental mapping indicates that Gd is predominantly concentrated within the region demarcated by the yellow dashed line. While the presence and distribution of O, Si, Mg, and other elements in this region are less distinct, analysis confirms Gd enrichment within this bright-contrast band. Analysis of the diffraction patterns identifies the phase of the bright-contrast band as Gd₂O₃. Therefore, it is concluded that this thin band corresponds to a Gd₂O₃ TGO layer formed at the interface between the substrate and the PEO layer. Figure 7 shows the YSZ/PEO interface morphology of the YSZ/PEO/Mg sample after cyclic oxidation at 400°C for 100 min. The micrograph indicates that the interface remains well-bonded with no observable interdiffusion zone or significant degradation. Discussion The preparing process of two TBCs After the APS process, the substrate of Mg/YSZ sample is slightly oxidized in Fig. 2 d. There is no heat affected layer(HAL)on the substrate. Nevertheless, the oxidized area has negative effect on the bonding strength of the interface. The PEO layer consists of MgO and magnesium silicate 23 , 24 , which comfirmed by EDS in Fig. 6 . The magnesium alloy substrate generates discharge sparks under high voltage, which react in situ in the substrate, undergo diffusion bonding and chemical bonding, and exhibit good bonding strength 25 . Multilayer structure of PEO layer reported by Guo 23 can not be observed clearly in this study. The porous structure provides a channel for YSZ penetrating into PEO layer which can be observed in Fig. 3 b. This structure of YSZ embedded in the PEO layer has a bigger bonding strength than YSZ layer on the bare Mg substrate due to the increased contact area. The oxidation mechanism of two TBCs The Pilling-Bedworth Ratio (PBR) is a critical criterion for assessing the protective integrity of an oxide scale. It is defined as the ratio of the volume of oxide formed to the volume of metal consumed during oxidation: where ρ denotes density, oxi refers to the oxide, and m refers to the metal. A PBR less than 1 indicates insufficient oxide volume to fully cover the metal surface, while a PBR exceeding 2 typically leads to excessive compressive stresses within the oxide film, promoting spallation. Consequently, a protective and adherent oxide film generally exhibits a PBR between 1 and 2. For pure magnesium, the densities of Mg and MgO are 1.74 g/cm³ and 3.58 g/cm³, respectively 3 . The resulting PBR of MgO is calculated as 0.81, confirming its inability to form a protective oxide scale. Gadolinium (Gd), a rare earth element with a large atomic diameter similar to Yttrium (Y), is expected to follow an oxidation mechanism analogous to that reported for Mg-Y alloys by Fan 26 . During the initial high-temperature oxidation stage of Mg-Gd alloys, both Gd and Mg oxidize simultaneously. Subsequently, due to the significantly faster outward diffusion rate of Mg²⁺ cations compared to Gd³⁺ cations, the rare earth oxide becomes overgrown by MgO, which also covers the underlying metal substrate. Concurrently, inward-diffusing oxygen anions oxidize Gd at the oxide/substrate interface, explaining the formation of Gd₂O₃ beneath the outer MgO layer. Given the densities of Gd (7.901 g/cm³) and Gd₂O₃ (7.407 g/cm³), the PBR for Gd₂O₃ is calculated as 1.29. This value suggests Gd₂O₃ has the potential to form a protective oxide layer. A continuous and dense Gd₂O₃ layer on the Mg substrate would therefore enhance oxidation resistance. While no significant morphological changes were observed in either TBC system after oxidation at 200°C for 100 hours, catastrophic spallation of the YSZ layer occurred on the YSZ/Mg sample after exposure at 400°C for 100 minutes. In contrast, the PEO/YSZ sample demonstrated superior thermal resistance. Figure 6 b reveals the formation of a continuous Gd₂O₃ thermally grown oxide (TGO) layer at the interface between the Mg substrate and the PEO layer. The oxidation of the Mg substrate involves the following equilibria: $$M{\text{g}}+1/2{O_2}=MgO$$ 2 $$2Gd+3/2{O_2}=G{d_2}{O_3}$$ 3 Substituting thermodynamic parameters 27 at 400°C yields: $$\Delta {G_{MgO}}= - 583.7KJ/mol$$ 4 $$\Delta {G_{G{d_2}{O_3}}}= - 1718.2KJ/mol$$ 5 Thermodynamic analysis confirms that the initial oxidation stage involves both Gd and Mg. Outward diffusion of Mg and Gd atoms in the alloy proceeds faster than inward oxygen diffusion, leading to simultaneous external oxidation forming MgO and Gd₂O₃ on the surface. However, the diffusion rate of Gd³⁺ cations within the oxide film is lower than that of Mg²⁺ due to Gd's larger atomic mass. As oxidation progresses, the more mobile Mg²⁺ readily reacts with O²⁻ anions, forming MgO that overgrows the initially formed Gd₂O₃ and covers the substrate. The porous structure and low PBR (0.81) of MgO prevent it from forming a fully protective barrier, allowing O²⁻ anions to diffuse inwards through the scale. Under the low oxygen partial pressure conditions at the oxide/metal interface, these O²⁻ anions can react with Gd³⁺ cations, driven by the highly negative Gibbs free energy of Gd₂O₃ formation (Eq. 5 ), leading to internal Gd₂O₃ formation. Gd₂O₃, possessing the Bixbyite crystal structure, exhibits significant oxygen anion conductivity but presents a substantial barrier to cation diffusion. Consequently, in the later stages of oxidation, the diffusion rate of O²⁻ anions through the oxide scale becomes the rate-controlling step. As oxidation continues, the Gd₂O₃ layer, with its favorable PBR of 1.29, develops into a continuous, dense, and protective oxide barrier on the substrate. Above all, the YSZ/PEO/Mg thermal barrier coating system, constructed by introducing a plasma electrolytic oxidation (PEO) interlayer on the magnesium alloy substrate, significantly enhances high-temperature oxidation resistance. The core mechanism lies in: the unique porous structure of the PEO layer facilitates mechanical interlocking of the YSZ coating, substantially enhancing interfacial bonding strength; more importantly, during high-temperature oxidation, the gadolinium (Gd) element in the substrate undergoes selective oxidation at the PEO layer/magnesium matrix interface, forming a continuous and dense Gd₂O₃ thermally grown oxide (TGO) layer with an ideal Pilling-Bedworth ratio (PBR = 1.29). Future work requires a focused and in-depth investigation into the long-term evolution behavior of the Gd₂O₃ barrier layer, systematic optimization of PEO process parameters to precisely control the continuity of the TGO layer and the interfacial bonding state, and assessment of the evolutionary shift in oxidation kinetics following gadolinium element depletion. The main conclusions are as follows: The results demonstrate that both the YSZ/Mg and YSZ/PEO/Mg coating systems maintain stability during 100 hours of exposure at 200°C. Following oxidation at 400°C for 100 minutes, the YSZ/Mg sample exhibited extensive coating spallation. In contrast, the YSZ/PEO/Mg samples retained significant adhesion to the substrate after the same exposure period, with a Gd₂O₃ thermally grown oxide (TGO) layer identified at the PEO/substrate interface. For the novel TBC system developed in this study, the coating maintains robust bonding with the substrate following both long-term low-temperature (200°C) and high-temperature (400°C) oxidation. This performance confirms that the YSZ/PEO/Mg structural design is functionally viable and demonstrates excellent oxidation resistance. Methods Materials The substrate material used in this study was an Mg-Gd-Zn-Zr alloy (provided by Shanghai Jiao Tong University, Shanghai, China), with a chemical composition as shown in Table 1 . The samples had dimensions of 25 mm × 15 mm × 10 mm and were initially ground using SiC abrasive papers to a grit size of 400. Subsequently, they were washed with distilled water and alcohol before being dried in cold air. Table 1 Chemical composition of magnesium alloy (wt.%) Elements Gd Zn Zr Mg wt% 14 2.3 1 Bal. The PEO bond coating was manufactured using a plasma electrolytic oxidation system developed by the Institute of Metal Research (IMR). The electrolyte composition consisted of 20 g/L Na₂SiO₃, 2 g/L NaOH, and 5 g/L KF. The alloy samples were subjected to a constant current mode (20 mA/cm²) for 15 minutes to deposit the coating. A cooling system maintained the electrolyte temperature below 30°C. Subsequently, the YSZ top coating was prepared using a Metco 7700 plasma spray system (Praxair, USA). Before air plasma spraying, the substrates were grit-blasted to enhance surface roughness and ultrasonically cleaned for 5 minutes. The Metco 204NS powder (Oerlikon Metco, Westbury, NY) was used to deposit an 8YSZ coating onto both the bare substrate surfaces and the PEO-coated surfaces. Cyclic oxidation YSZ/PEO/Mg and YSZ/Mg specimens underwent thermal cycling between room temperature and peak temperatures of 200°C and 400°C in laboratory air using an automatically controlled circulating heating furnace. For the 200°C cycle, specimens were heated in the furnace for one hour followed by 10 minutes of air cooling. For the 400°C cycle, heating lasted five minutes followed by 10 minutes of air cooling. Specimens were then removed to examine coating microstructure changes and TGO growth. Characterization The phase constituent was characterized by XRD (X’Pert PRO, PANa-lytic Co., Almelo, Holland, Cu Kα radiation at 40 kV). The obtained X-ray diffraction patterns were recorded in the 2θ range of 10–90°, and a step-scanning mode was employed with a step size of 0.02°. Field-emission scanning electron microscopy (FE-SEM, Inspect F50, FEI Co., Hillsboro, OR, US) coupled with an energy dispersive spectrometer (EDS, X-Max, Oxford instruments Co., Oxford, UK) was used to examine the morphologies and microstructures of the surface and cross-section of the oxidized samples. Transmission electron microscopy (TEM; JEM-2100F, JEOL) coupled with energy-dispersive X-ray spectroscopy (EDS) was employed for detailed analysis. Declarations COMPETING INTERESTS The authors declare no competing interests. Author Contribution X.H.: Conceptualization, methodology, investigation, writing. Y.S. & Y.L.: Ivestigation. W.Z.: Manufacturing, resources. J.W.:Writing – review & editing, formal analysis, investigation. M.C.: Funding acquisition, resources. S.Z.: resources, supervision. F.W.: supervision. Acknowledgement This project is financially supported by the National Natural Science Foundation of China under Grant (51801021 and 51671053), Fundamental Research Funds for the Central Universities (N N25DCG001) and Ministry of Industry and Information Technology Project (MJ-2017-J-99). Data availability All data included in this study are available upon request by contact with the corresponding author. References Ishihara, S., Notoya, H., Okada, A., Nan, ZY. & Goshima, T. Effect of electroless-Ni-plating on corrosion fatigue behavior of magnesium alloy. Surf. Coat. Technol. 202 , 2085-2092 (2008). Cui, X. M., Yu, Z. L., Liu, F., Du, Z. X. & Bai, P. C. 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Supplementary Files Graphicabstract.docx Cite Share Download PDF Status: Published Journal Publication published 02 Dec, 2025 Read the published version in npj Materials Degradation → Version 1 posted Editorial decision: Revision requested 15 Sep, 2025 Reviews received at journal 13 Sep, 2025 Reviewers agreed at journal 02 Sep, 2025 Reviews received at journal 01 Aug, 2025 Reviewers agreed at journal 24 Jul, 2025 Reviewers agreed at journal 23 Jul, 2025 Reviewers invited by journal 22 Jul, 2025 Editor assigned by journal 17 Jul, 2025 Submission checks completed at journal 17 Jul, 2025 First submitted to journal 16 Jul, 2025 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. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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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-7136434","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":489986863,"identity":"daa3088f-79ff-4a61-a5e1-4339673e74a6","order_by":0,"name":"Xuanyi He","email":"","orcid":"","institution":"Northeastern University","correspondingAuthor":false,"prefix":"","firstName":"Xuanyi","middleName":"","lastName":"He","suffix":""},{"id":489986864,"identity":"9454e233-fd35-40c9-9f2e-cc7d229f4439","order_by":1,"name":"Yinhua Shao","email":"","orcid":"","institution":"Northeastern University","correspondingAuthor":false,"prefix":"","firstName":"Yinhua","middleName":"","lastName":"Shao","suffix":""},{"id":489986865,"identity":"a1845a9b-ecba-42c0-910b-0bdca886a1fb","order_by":2,"name":"Yichen Li","email":"","orcid":"","institution":"Northeastern University","correspondingAuthor":false,"prefix":"","firstName":"Yichen","middleName":"","lastName":"Li","suffix":""},{"id":489986866,"identity":"dabad67f-e80b-42f0-b512-62643f5e718f","order_by":3,"name":"Wei Zhang","email":"","orcid":"","institution":"Chinese Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Wei","middleName":"","lastName":"Zhang","suffix":""},{"id":489986867,"identity":"c3b60125-022e-4bac-891c-4112fced0df8","order_by":4,"name":"Jinlong Wang","email":"data:image/png;base64,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","orcid":"","institution":"Northeastern University","correspondingAuthor":true,"prefix":"","firstName":"Jinlong","middleName":"","lastName":"Wang","suffix":""},{"id":489986870,"identity":"4077a52a-b4a8-4aed-925f-f7cda7d375f3","order_by":5,"name":"Minghui Chen","email":"","orcid":"","institution":"Northeastern University","correspondingAuthor":false,"prefix":"","firstName":"Minghui","middleName":"","lastName":"Chen","suffix":""},{"id":489986874,"identity":"9537d502-5bfb-4f59-8b3f-f8fb69732c49","order_by":6,"name":"Shenglong Zhu","email":"","orcid":"","institution":"Chinese Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Shenglong","middleName":"","lastName":"Zhu","suffix":""},{"id":489986877,"identity":"30297c7c-eea4-44ad-b8f8-d59dab025a2f","order_by":7,"name":"Fuhui Wang","email":"","orcid":"","institution":"Northeastern University","correspondingAuthor":false,"prefix":"","firstName":"Fuhui","middleName":"","lastName":"Wang","suffix":""}],"badges":[],"createdAt":"2025-07-16 06:23:40","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7136434/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7136434/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41529-025-00711-6","type":"published","date":"2025-12-02T15:58:14+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":87480744,"identity":"4781489d-2c5d-437c-9f29-d80d0337c59a","added_by":"auto","created_at":"2025-07-24 09:54:05","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":514287,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSEM micrograph of as-deposited samples under different modes\u003c/strong\u003e. \u003cstrong\u003ea\u003c/strong\u003e YSZ/ PEO/ Mg sample under SE mode. \u003cstrong\u003eb\u003c/strong\u003e YSZ/ PEO/ Mg sample under A+B mode.\u003cstrong\u003e c\u003c/strong\u003eYSZ/ Mg sample under SE mode. \u003cstrong\u003ed\u003c/strong\u003e YSZ/ Mg sample under A+B mode.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7136434/v1/87fc6b0c19695a0da12405fa.png"},{"id":87480765,"identity":"96ac255c-1031-4596-93e4-0ad7b19426f7","added_by":"auto","created_at":"2025-07-24 09:54:07","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":425383,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCross-sectional morphologies of the as-deposited samples.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003ecross-sectional morphology of YSZ/ PEO/ Mg sample. \u003cstrong\u003eb\u003c/strong\u003e high magnification morphology of a. \u003cstrong\u003ec\u003c/strong\u003e cross-sectional morphology of YSZ/ Mg sample. \u003cstrong\u003ed\u003c/strong\u003ehigh magnification morphology of c.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7136434/v1/122c892c6070f8b291815376.png"},{"id":87480772,"identity":"f766d8e0-f01e-4675-bdfd-aec8d5e7780a","added_by":"auto","created_at":"2025-07-24 09:54:07","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":436672,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCross-sectional morphologies of YSZ/ PEO/ Mg and YSZ/ Mg samples after cyclical oxidation at 200°C for 100 h. a\u003c/strong\u003e cross-sectional morphology of YSZ/ PEO/ Mg sample. \u003cstrong\u003eb\u003c/strong\u003e the high magnification morphology of a. \u003cstrong\u003ec\u003c/strong\u003ecross-sectional morphology of YSZ/ Mg sample. \u003cstrong\u003ed\u003c/strong\u003e the high magnification morphology of c.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7136434/v1/33686148b71227afd0a518e7.png"},{"id":87480742,"identity":"2eb836d1-eb50-4f0d-8660-cb65f45063a7","added_by":"auto","created_at":"2025-07-24 09:54:04","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":186636,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSurface morphologies of YSZ/ PEO/ Mg and YSZ/ Mg samples after cyclical oxidation at 400°C for 100 min. a\u003c/strong\u003esurface morphology of YSZ/ PEO/ Mg sample. \u003cstrong\u003eb\u003c/strong\u003e cross-sectional morphology of YSZ/ Mg sample.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7136434/v1/024d598d674da4a9157125d9.png"},{"id":87480761,"identity":"5ebf4696-199d-4404-be63-6d3f045c666c","added_by":"auto","created_at":"2025-07-24 09:54:06","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":346398,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCross-sectional morphologies of YSZ/ PEO/ Mg and YSZ/ Mg samples after cyclical oxidation at 400°C for 100 min.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e cross-sectional morphology of YSZ/ PEO/ Mg sample. \u003cstrong\u003eb\u003c/strong\u003e the high magnification morphology of a. \u003cstrong\u003ec\u003c/strong\u003ecross-sectional morphology of YSZ/ Mg sample. \u003cstrong\u003ed\u003c/strong\u003e the high magnification morphology of c.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-7136434/v1/4785fb659e147c876a2d0548.png"},{"id":87480770,"identity":"831c1b91-31ca-4c78-bc9e-3cc87cc5793e","added_by":"auto","created_at":"2025-07-24 09:54:07","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":392882,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eElemental mappings and diffraction spots of TGO of YSZ/ PEO/ Mg sample.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-7136434/v1/3bd4019a99aa826e8809f166.png"},{"id":87481074,"identity":"6adbef65-e3e7-4eb7-ba87-8d0f3902428e","added_by":"auto","created_at":"2025-07-24 10:02:06","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":25814,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eInterface morphology of YSZ and PEO of YSZ/ PEO/ Mg sample after cyclical oxidation at 400°C for 100 min by TEM.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-7136434/v1/de5a5aefb28850d61c56038d.png"},{"id":97723973,"identity":"bbd19036-6c07-4862-ae6e-1d75cc35e38c","added_by":"auto","created_at":"2025-12-08 16:10:23","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3192047,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7136434/v1/ec1af82e-cd05-4c70-99b7-919a6c71ed9a.pdf"},{"id":87480743,"identity":"4103c553-d108-47a1-ae7e-3fbec5f8aa4e","added_by":"auto","created_at":"2025-07-24 09:54:05","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":5498721,"visible":true,"origin":"","legend":"","description":"","filename":"Graphicabstract.docx","url":"https://assets-eu.researchsquare.com/files/rs-7136434/v1/8261d6c870ad2663d1f19a43.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Enhanced High-Temperature Oxidation Resistance of Mg-Gd-Zn-Zr Alloy via YSZ Thermal Barrier Coating with a Plasma Electrolytic Oxidation Bond Layer","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe exceptional low density of magnesium makes it highly attractive for aerospace lightweighting, offering substantial potential for reducing fuel consumption and increasing payload capacity\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. However, the broader application of magnesium alloys is significantly constrained by inherent material limitations, including a relatively low melting point, high chemical reactivity (particularly with oxygen and moisture), and fundamentally inadequate resistance to elevated temperatures and corrosive environments\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. These characteristics drive the rapid formation of a native magnesium oxide (MgO) scale upon air exposure, a process that accelerates dramatically at elevated temperatures. Crucially, MgO possesses a Pilling-Bedworth ratio (PBR) of only 0.81, signifying that the volume of oxide formed is less than the volume of metal consumed. This volumetric deficit impedes the formation of a fully coherent and adherent scale. Furthermore, the MgO scale is intrinsically porous and exhibits a non-protective, typically cubic crystalline structure that fails to act as an effective diffusion barrier\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. Overcoming this inherent weakness necessitates the development of advanced strategies specifically designed to enhance the high-temperature oxidation resistance of magnesium alloys for demanding aerospace applications.\u003c/p\u003e\u003cp\u003eSurface modification techniques represent a crucial pathway for mitigating these intrinsic limitations and unlocking the potential of magnesium alloys. A range of methods, including laser surface treatment\u003csup\u003e\u003cspan additionalcitationids=\"CR5\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e, chemical conversion coatings\u003csup\u003e\u003cspan additionalcitationids=\"CR8\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e, and plasma electrolytic oxidation (PEO)\u003csup\u003e\u003cspan additionalcitationids=\"CR11 CR12\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e, have been extensively investigated. However, the primary focus of these studies has largely centered on improving corrosion resistance at ambient or moderately elevated temperatures. Research specifically dedicated to enhancing high-temperature oxidation resistance (typically above 300\u0026deg;C) using these surface engineering approaches remains comparatively scarce. In this context, Thermal Barrier Coatings (TBCs) emerge as a particularly promising strategy. TBCs are specifically engineered to provide dual functionality: significant thermal insulation to lower the substrate operating temperature and a barrier against oxidizing atmospheres. For instance, Fan et al.\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e fabricated a multi-layered TBC system consisting of an 8 wt.% yttria-stabilized zirconia (8YSZ) top coat and a NiCrAlY bond coat on a magnesium alloy. Their results demonstrated the system's thermal insulation capability: when the coating surface was held at 200\u0026deg;C, the underlying substrate temperature was significantly reduced to 146\u0026deg;C. Under sustained thermal loading reaching equilibrium, the substrate stabilized at approximately 450\u0026deg;C while the coating surface reached about 530\u0026deg;C. This substantial temperature gradient directly translates to a marked improvement in the substrate's resistance to high-temperature oxidation.\u003c/p\u003e\u003cp\u003eDespite their demonstrated potential for thermal management and oxidation protection, conventional TBC systems applied to magnesium alloys often suffer from inadequate thermal shock resistance, limiting their long-term durability. A primary driver of this failure is the significant mismatch in the coefficients of thermal expansion (CTE) between the ceramic top coat, metallic bond coats (if used), and the underlying magnesium alloy substrate. This CTE disparity induces substantial cyclic thermal stresses concentrated at critical bond interfaces during repeated heating and cooling cycles. Over time, this stress accumulation can lead to crack initiation and propagation, ultimately resulting in coating delamination and spallation\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. The problem is particularly acute for magnesium due to its inherently high CTE, generating more severe interfacial stresses than in systems based on lower CTE substrates like nickel superalloys. Research efforts, such as those by Fan\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e, have explored strategies to mitigate this issue, including introducing Ni\u0026ndash;P electroplated layers or thermally formed Mg-Al intermetallic diffusion layers as interlayers. While these approaches successfully doubled the thermal shock life compared to baseline systems, the fundamental challenge of achieving optimal interfacial bonding strength persists. Residual stresses and potential weak points at interfaces remain significant concerns for long-term reliability under extended service conditions involving repeated thermal transients.\u003c/p\u003e\u003cp\u003eSurface morphology, particularly roughness, is a well-established factor critically influencing the bonding strength and durability of TBCs, especially those deposited via thermal spray processes like Atmospheric Plasma Spraying (APS), which rely heavily on mechanical interlocking. Eriksson\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e demonstrated that increasing the surface roughness of the underlying layer (bond coat or substrate) significantly extends the thermal fatigue life of APS-applied TBCs. The rougher surface provides a larger effective bonding area and facilitates superior mechanical keying of the deposited splats. Consequently, an intermediate layer characterized by both high surface roughness and intrinsically strong adhesion to the magnesium substrate holds considerable promise as an effective bond coat for TBC systems. Plasma Electrolytic Oxidation (PEO) treatment inherently produces precisely such a structure on magnesium alloys. The PEO process generates a thick, hard, in-situ grown oxide ceramic coating that is directly integrated with the substrate metal. This coating exhibits a characteristic bi-layer structure: a relatively dense and adherent inner layer adjacent to the substrate, and a more porous outer layer\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. Crucially, the coating formation involves complex plasma-chemical reactions at the metal/electrolyte interface, resulting in a coating that is chemically bonded, rather than merely mechanically attached, to the underlying alloy. Furthermore, PEO coatings are renowned for significantly enhancing the substrate's corrosion resistance\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. These attributes \u0026ndash; strong chemical bonding, inherent corrosion protection, and a rough, porous outer morphology \u0026ndash; collectively contribute to PEO coatings exhibiting outstanding adhesion strength to magnesium alloy substrates\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e. These advantages make a PEO layer serving as a bond coat for an APS-deposited 8YSZ top coat a highly viable and effective approach to substantially improve the thermal resistance and durability of magnesium alloys.\u003c/p\u003e\u003cp\u003eBuilding directly upon this compelling rationale and the demonstrated potential of the PEO/YSZ combination, this study introduces and investigates a novel thermal barrier coating architecture specifically designed for a Mg-Gd-Zn-Zr alloy system. The core innovation lies in utilizing a plasma electrolytic oxidation (PEO)-generated oxide ceramic layer as the primary bond coat. Upon this robust and rough PEO foundation, a conventional 8YSZ top coat is deposited using Atmospheric Plasma Spraying (APS), forming the complete YSZ/PEO/Mg coating system. To evaluate the performance benefits of the novel PEO bond coat, this research directly compares the YSZ/PEO/Mg system against a conventional 8YSZ coating deposited directly onto the bare Mg-Gd-Zn-Zr alloy substrate (YSZ/Mg). The comprehensive investigation encompasses detailed microstructural characterization of both systems, systematic evaluation of thermal shock lifetime under cyclic heating/quenching, thorough analysis of failure mechanisms causing degradation and spallation, and detailed elucidation of the oxidation resistance mechanism within the YSZ/PEO/Mg system.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003e\u003cb\u003eMicrostructure of as-prepared TBC\u003c/b\u003e\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e presents the fracture cross-sectional morphologies of the two coating systems. In Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea, both the YSZ/PEO and PEO/Mg interfaces exhibit an undulating profile. In contrast, the YSZ/Mg interface in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb is predominantly straight. The characteristic morphology of the APS-deposited YSZ layer, featuring microcracks and porosity, is evident in both figures. During the APS process, precursor powder particles are melted and propelled towards the substrate. Upon impact with the cooled substrate surface, these molten droplets undergo rapid solidification, resulting in the formation of a porous and rough coating structure\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e displays the cross-sectional morphologies of the as-deposited samples. For the YSZ/PEO/Mg system (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb), the YSZ top coat is embedded within the porous outer region of the PEO layer. This interlocking structure significantly enhances interfacial bonding strength with the top coat, achieving the intended design objective. Conversely, the YSZ layer deposited directly onto the substrate in the YSZ/Mg sample (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed) shows a characteristic morphology, including a distinct oxide layer at the interface.\u003c/p\u003e\u003cp\u003e\u003cb\u003eOxidation behavior of the TBCs\u003c/b\u003e\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e presents the cross-sectional morphologies of both coating systems after cyclic oxidation at 200\u0026deg;C for 100 h. The images demonstrate that the coatings remain well-bonded in both systems. In Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb, partial penetration of the YSZ into the PEO layer is observed, increasing the interfacial contact area. An oxidized region is visible at the YSZ/substrate interface in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed; however, it cannot be determined whether this oxidation occurred during the APS deposition process or the subsequent oxidation experiment.\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e displays the surface morphologies of both samples after cyclic oxidation at 400\u0026deg;C for 100 min. The coating on the YSZ/PEO/Mg sample (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea) maintains its integrity. In contrast, the YSZ layer on the YSZ/Mg sample (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb) exhibits partial detachment. EDS analysis (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e2\u003c/span\u003e) shows that Region 1 corresponds to the intact YSZ top coat. In Area 2, however, the atomic percentage of Zr decreased significantly from 20.1\u0026ndash;4.9%, while the atomic percentage of Mg increased markedly from 1.9\u0026ndash;34.6%. This compositional shift indicates that the YSZ coating in Area 2 has spalled, exposing the underlying magnesium alloy substrate. These results demonstrate that samples lacking a bond coat cannot provide stable protection at this temperature.\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 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eEDS results of \u0026ldquo;1\u0026rdquo; and \u0026ldquo;2\u0026rdquo; in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e (at.%)\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eComponents\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eO\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eMg\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eZr\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eY\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u0026ldquo;1\u0026rdquo;\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e76.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e1.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e20.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e1.5\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u0026ldquo;2\u0026rdquo;\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e57.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e34.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e4.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e0.7\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=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows the cross-sectional morphologies of both systems after cyclic oxidation at 400\u0026deg;C for 100 min. For the YSZ/PEO/Mg sample (Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb), the interfaces remain well-bonded. A thin, continuous, bright-contrast band is evident at the Mg/PEO interface. The composition and phase identity of this band were analyzed. In the YSZ/Mg sample (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec), catastrophic spallation of the YSZ layer has occurred. Within the separation area (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed), an oxidized region containing discontinuous thermally grown oxide (TGO) is visible on the Mg substrate. This spallation is inferred to result primarily from thermal stress induced by the mismatch in coefficients of thermal expansion\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e. The oxidation occurring at the interface further weakened the bonding strength.\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e presents TEM micrographs and diffraction patterns of the PEO/Mg interface. Elemental mapping indicates that Gd is predominantly concentrated within the region demarcated by the yellow dashed line. While the presence and distribution of O, Si, Mg, and other elements in this region are less distinct, analysis confirms Gd enrichment within this bright-contrast band. Analysis of the diffraction patterns identifies the phase of the bright-contrast band as Gd₂O₃. Therefore, it is concluded that this thin band corresponds to a Gd₂O₃ TGO layer formed at the interface between the substrate and the PEO layer.\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows the YSZ/PEO interface morphology of the YSZ/PEO/Mg sample after cyclic oxidation at 400\u0026deg;C for 100 min. The micrograph indicates that the interface remains well-bonded with no observable interdiffusion zone or significant degradation.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003e\u003cb\u003eThe preparing process of two TBCs\u003c/b\u003e\u003c/p\u003e\u003cp\u003eAfter the APS process, the substrate of Mg/YSZ sample is slightly oxidized in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed. There is no heat affected layer(HAL)on the substrate. Nevertheless, the oxidized area has negative effect on the bonding strength of the interface.\u003c/p\u003e\u003cp\u003eThe PEO layer consists of MgO and magnesium silicate\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e, which comfirmed by EDS in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. The magnesium alloy substrate generates discharge sparks under high voltage, which react in situ in the substrate, undergo diffusion bonding and chemical bonding, and exhibit good bonding strength\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e. Multilayer structure of PEO layer reported by Guo\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e can not be observed clearly in this study. The porous structure provides a channel for YSZ penetrating into PEO layer which can be observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb. This structure of YSZ embedded in the PEO layer has a bigger bonding strength than YSZ layer on the bare Mg substrate due to the increased contact area.\u003c/p\u003e\u003cp\u003e\u003cb\u003eThe oxidation mechanism of two TBCs\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThe Pilling-Bedworth Ratio (PBR) is a critical criterion for assessing the protective integrity of an oxide scale. It is defined as the ratio of the volume of oxide formed to the volume of metal consumed during oxidation:\u003c/p\u003e\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\" width=\"257\" height=\"66\"\u003e\u003c/p\u003e\u003cp\u003ewhere ρ denotes density, oxi refers to the oxide, and m refers to the metal. A PBR less than 1 indicates insufficient oxide volume to fully cover the metal surface, while a PBR exceeding 2 typically leads to excessive compressive stresses within the oxide film, promoting spallation. Consequently, a protective and adherent oxide film generally exhibits a PBR between 1 and 2. For pure magnesium, the densities of Mg and MgO are 1.74 g/cm³ and 3.58 g/cm³, respectively\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. The resulting PBR of MgO is calculated as 0.81, confirming its inability to form a protective oxide scale.\u003c/p\u003e\u003cp\u003eGadolinium (Gd), a rare earth element with a large atomic diameter similar to Yttrium (Y), is expected to follow an oxidation mechanism analogous to that reported for Mg-Y alloys by Fan\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. During the initial high-temperature oxidation stage of Mg-Gd alloys, both Gd and Mg oxidize simultaneously. Subsequently, due to the significantly faster outward diffusion rate of Mg²⁺ cations compared to Gd³⁺ cations, the rare earth oxide becomes overgrown by MgO, which also covers the underlying metal substrate. Concurrently, inward-diffusing oxygen anions oxidize Gd at the oxide/substrate interface, explaining the formation of Gd₂O₃ beneath the outer MgO layer. Given the densities of Gd (7.901 g/cm³) and Gd₂O₃ (7.407 g/cm³), the PBR for Gd₂O₃ is calculated as 1.29. This value suggests Gd₂O₃ has the potential to form a protective oxide layer. A continuous and dense Gd₂O₃ layer on the Mg substrate would therefore enhance oxidation resistance.\u003c/p\u003e\u003cp\u003eWhile no significant morphological changes were observed in either TBC system after oxidation at 200°C for 100 hours, catastrophic spallation of the YSZ layer occurred on the YSZ/Mg sample after exposure at 400°C for 100 minutes. In contrast, the PEO/YSZ sample demonstrated superior thermal resistance. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb reveals the formation of a continuous Gd₂O₃ thermally grown oxide (TGO) layer at the interface between the Mg substrate and the PEO layer.\u003c/p\u003e\u003cp\u003eThe oxidation of the Mg substrate involves the following equilibria:\u003c/p\u003e\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$M{\\text{g}}+1/2{O_2}=MgO$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$2Gd+3/2{O_2}=G{d_2}{O_3}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eSubstituting thermodynamic parameters\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e at 400°C yields:\u003c/p\u003e\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$\\Delta {G_{MgO}}= - 583.7KJ/mol$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ5\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ5\" name=\"EquationSource\"\u003e\n$$\\Delta {G_{G{d_2}{O_3}}}= - 1718.2KJ/mol$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\u003c/div\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThermodynamic analysis confirms that the initial oxidation stage involves both Gd and Mg. Outward diffusion of Mg and Gd atoms in the alloy proceeds faster than inward oxygen diffusion, leading to simultaneous external oxidation forming MgO and Gd₂O₃ on the surface. However, the diffusion rate of Gd³⁺ cations within the oxide film is lower than that of Mg²⁺ due to Gd's larger atomic mass. As oxidation progresses, the more mobile Mg²⁺ readily reacts with O²⁻ anions, forming MgO that overgrows the initially formed Gd₂O₃ and covers the substrate.\u003c/p\u003e\u003cp\u003eThe porous structure and low PBR (0.81) of MgO prevent it from forming a fully protective barrier, allowing O²⁻ anions to diffuse inwards through the scale. Under the low oxygen partial pressure conditions at the oxide/metal interface, these O²⁻ anions can react with Gd³⁺ cations, driven by the highly negative Gibbs free energy of Gd₂O₃ formation (Eq.\u0026nbsp;\u003cspan refid=\"Equ5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), leading to internal Gd₂O₃ formation. Gd₂O₃, possessing the Bixbyite crystal structure, exhibits significant oxygen anion conductivity but presents a substantial barrier to cation diffusion. Consequently, in the later stages of oxidation, the diffusion rate of O²⁻ anions through the oxide scale becomes the rate-controlling step. As oxidation continues, the Gd₂O₃ layer, with its favorable PBR of 1.29, develops into a continuous, dense, and protective oxide barrier on the substrate.\u003c/p\u003e\u003cp\u003eAbove all, the YSZ/PEO/Mg thermal barrier coating system, constructed by introducing a plasma electrolytic oxidation (PEO) interlayer on the magnesium alloy substrate, significantly enhances high-temperature oxidation resistance. The core mechanism lies in: the unique porous structure of the PEO layer facilitates mechanical interlocking of the YSZ coating, substantially enhancing interfacial bonding strength; more importantly, during high-temperature oxidation, the gadolinium (Gd) element in the substrate undergoes selective oxidation at the PEO layer/magnesium matrix interface, forming a continuous and dense Gd₂O₃ thermally grown oxide (TGO) layer with an ideal Pilling-Bedworth ratio (PBR = 1.29). Future work requires a focused and in-depth investigation into the long-term evolution behavior of the Gd₂O₃ barrier layer, systematic optimization of PEO process parameters to precisely control the continuity of the TGO layer and the interfacial bonding state, and assessment of the evolutionary shift in oxidation kinetics following gadolinium element depletion. The main conclusions are as follows:\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003col\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eThe results demonstrate that both the YSZ/Mg and YSZ/PEO/Mg coating systems maintain stability during 100 hours of exposure at 200°C.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eFollowing oxidation at 400°C for 100 minutes, the YSZ/Mg sample exhibited extensive coating spallation. In contrast, the YSZ/PEO/Mg samples retained significant adhesion to the substrate after the same exposure period, with a Gd₂O₃ thermally grown oxide (TGO) layer identified at the PEO/substrate interface.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eFor the novel TBC system developed in this study, the coating maintains robust bonding with the substrate following both long-term low-temperature (200°C) and high-temperature (400°C) oxidation. This performance confirms that the YSZ/PEO/Mg structural design is functionally viable and demonstrates excellent oxidation resistance.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003c/ol\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cb\u003eMaterials\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThe substrate material used in this study was an Mg-Gd-Zn-Zr alloy (provided by Shanghai Jiao Tong University, Shanghai, China), with a chemical composition as shown in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The samples had dimensions of 25 mm × 15 mm × 10 mm and were initially ground using SiC abrasive papers to a grit size of 400. Subsequently, they were washed with distilled water and alcohol before being dried in cold air.\u003c/p\u003e\u003cdiv class=\"gridtable\"\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\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eChemical composition of magnesium alloy (wt.%)\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003c/colgroup\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eElements\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eGd\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eZn\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eZr\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eMg\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ewt%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e14\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eBal.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/table\u003e\u003c/div\u003e\u003cp\u003eThe PEO bond coating was manufactured using a plasma electrolytic oxidation system developed by the Institute of Metal Research (IMR). The electrolyte composition consisted of 20 g/L Na₂SiO₃, 2 g/L NaOH, and 5 g/L KF. The alloy samples were subjected to a constant current mode (20 mA/cm²) for 15 minutes to deposit the coating. A cooling system maintained the electrolyte temperature below 30°C.\u003c/p\u003e\u003cp\u003eSubsequently, the YSZ top coating was prepared using a Metco 7700 plasma spray system (Praxair, USA). Before air plasma spraying, the substrates were grit-blasted to enhance surface roughness and ultrasonically cleaned for 5 minutes. The Metco 204NS powder (Oerlikon Metco, Westbury, NY) was used to deposit an 8YSZ coating onto both the bare substrate surfaces and the PEO-coated surfaces.\u003c/p\u003e\u003cp\u003e\u003cb\u003eCyclic oxidation\u003c/b\u003e\u003c/p\u003e\u003cp\u003eYSZ/PEO/Mg and YSZ/Mg specimens underwent thermal cycling between room temperature and peak temperatures of 200°C and 400°C in laboratory air using an automatically controlled circulating heating furnace. For the 200°C cycle, specimens were heated in the furnace for one hour followed by 10 minutes of air cooling. For the 400°C cycle, heating lasted five minutes followed by 10 minutes of air cooling. Specimens were then removed to examine coating microstructure changes and TGO growth.\u003c/p\u003e\u003cp\u003e\u003cb\u003eCharacterization\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThe phase constituent was characterized by XRD (X’Pert PRO, PANa-lytic Co., Almelo, Holland, Cu Kα radiation at 40 kV). The obtained X-ray diffraction patterns were recorded in the 2θ range of 10–90°, and a step-scanning mode was employed with a step size of 0.02°. Field-emission scanning electron microscopy (FE-SEM, Inspect F50, FEI Co., Hillsboro, OR, US) coupled with an energy dispersive spectrometer (EDS, X-Max, Oxford instruments Co., Oxford, UK) was used to examine the morphologies and microstructures of the surface and cross-section of the oxidized samples. Transmission electron microscopy (TEM; JEM-2100F, JEOL) coupled with energy-dispersive X-ray spectroscopy (EDS) was employed for detailed analysis.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eCOMPETING INTERESTS\u003c/h2\u003e\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eX.H.: Conceptualization, methodology, investigation, writing. Y.S. \u0026amp; Y.L.: Ivestigation. W.Z.: Manufacturing, resources. J.W.:Writing \u0026ndash; review \u0026amp; editing, formal analysis, investigation. M.C.: Funding acquisition, resources. S.Z.: resources, supervision. F.W.: supervision.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThis project is financially supported by the National Natural Science Foundation of China under Grant (51801021 and 51671053), Fundamental Research Funds for the Central Universities (N N25DCG001) and Ministry of Industry and Information Technology Project (MJ-2017-J-99).\u003c/p\u003e\u003ch2\u003eData availability\u003c/h2\u003e\u003cp\u003eAll data included in this study are available upon request by contact with the corresponding author.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eIshihara, S., Notoya, H., Okada, A., Nan, ZY. \u0026amp; Goshima, T. Effect of electroless-Ni-plating on corrosion fatigue behavior of magnesium alloy. Surf. Coat. Technol. \u003cstrong\u003e202\u003c/strong\u003e, 2085-2092 (2008).\u003c/li\u003e\n\u003cli\u003eCui, X. M., Yu, Z. L., Liu, F., Du, Z. X. \u0026amp; Bai, P. C. 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Formation of oxygen bubbles and its influence on current efficiency in micro-arc oxidation process of AZ91D magnesium alloy. Thin Solid Films \u003cstrong\u003e485\u003c/strong\u003e, 53-58 (2005).\u003c/li\u003e\n\u003cli\u003eFan, J. F., Yang, G. C., Zhou, Y. H., Wei, Y. H. \u0026amp; Xu, B. S. Selective oxidation and the third-element effect on the oxidation of Mg-Y alloys at high temperatures. Metall. Mater. Trans. \u003cstrong\u003e40\u003c/strong\u003e, 2184\u0026ndash;2189 (2009).\u003c/li\u003e\n\u003cli\u003eDean J. A. 6 \u0026ndash; Thermodynamic Properties. Lange\u0026rsquo;s Handbook of Chemistry (15th Edition), McGraw Hill (2003) 90-115\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"npj-materials-degradation","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npjmatdeg","sideBox":"Learn more about [npj Materials Degradation](http://www.nature.com/npjmatdeg/)","snPcode":"41529","submissionUrl":"https://submission.springernature.com/new-submission/41529/3","title":"npj Materials Degradation","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-7136434/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7136434/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eMagnesium alloys suffer from significant limitations in high-temperature applications due to poor oxidation resistance, primarily attributed to the non-protective nature of their native MgO scale (Pilling-Bedworth ratio, PBR\u0026thinsp;=\u0026thinsp;0.81). This study investigates a novel thermal barrier coating (TBC) system, YSZ/PEO/Mg, designed to enhance the high-temperature performance of a Mg-Gd-Zn-Zr alloy. The system consists of an atmospheric plasma sprayed (APS) 8YSZ top coat deposited onto a plasma electrolytic oxidation (PEO) bond layer applied to the Mg substrate. For comparison, a YSZ coating deposited directly on the Mg substrate (YSZ/Mg) was also prepared. Both TBC systems exhibited stability during 100-hour cyclic oxidation at 200\u0026deg;C. However, under cyclic oxidation at 400\u0026deg;C for 100 minutes, the YSZ/Mg coating experienced catastrophic spallation due to interfacial oxidation and thermal stress, exposing the substrate. In contrast, the YSZ/PEO/Mg system maintained excellent integrity. Crucially, a continuous and protective gadolinium oxide (Gd₂O₃) thermally grown oxide (TGO) layer formed at the Mg/PEO interface during high-temperature exposure. Furthermore, the porous structure of the PEO layer facilitated mechanical interlocking of the YSZ top coat, significantly enhancing interfacial bonding strength. These results demonstrate that the YSZ/PEO/Mg TBC architecture, leveraging the synergistic effects of the PEO bond coat and the protective Gd₂O₃ TGO, provides an effective solution for significantly improving the high-temperature oxidation resistance of magnesium alloys. This approach is particularly promising for demanding applications such as aerospace thermal protection systems.\u003c/p\u003e","manuscriptTitle":"Enhanced High-Temperature Oxidation Resistance of Mg-Gd-Zn-Zr Alloy via YSZ Thermal Barrier Coating with a Plasma Electrolytic Oxidation Bond Layer","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-24 09:53:35","doi":"10.21203/rs.3.rs-7136434/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-09-16T03:07:44+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-13T13:03:49+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"195918182298970142839208817320197946624","date":"2025-09-02T17:29:31+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-08-01T10:32:15+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"174540539365298320746201628875790550535","date":"2025-07-24T09:46:30+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"327187381543710529032379373057447908550","date":"2025-07-23T22:31:14+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-07-22T09:12:05+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-07-18T00:52:27+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-07-17T11:07:59+00:00","index":"","fulltext":""},{"type":"submitted","content":"npj Materials Degradation","date":"2025-07-16T06:18:01+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"npj-materials-degradation","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npjmatdeg","sideBox":"Learn more about [npj Materials Degradation](http://www.nature.com/npjmatdeg/)","snPcode":"41529","submissionUrl":"https://submission.springernature.com/new-submission/41529/3","title":"npj Materials Degradation","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"7a8342e3-fc90-46ef-8747-a141eb1b8197","owner":[],"postedDate":"July 24th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":52022433,"name":"Physical sciences/Chemistry"},{"id":52022434,"name":"Physical sciences/Energy science and technology"},{"id":52022435,"name":"Physical sciences/Engineering"},{"id":52022436,"name":"Physical sciences/Materials science"}],"tags":[],"updatedAt":"2025-12-08T16:03:46+00:00","versionOfRecord":{"articleIdentity":"rs-7136434","link":"https://doi.org/10.1038/s41529-025-00711-6","journal":{"identity":"npj-materials-degradation","isVorOnly":false,"title":"npj Materials Degradation"},"publishedOn":"2025-12-02 15:58:14","publishedOnDateReadable":"December 2nd, 2025"},"versionCreatedAt":"2025-07-24 09:53:35","video":"","vorDoi":"10.1038/s41529-025-00711-6","vorDoiUrl":"https://doi.org/10.1038/s41529-025-00711-6","workflowStages":[]},"version":"v1","identity":"rs-7136434","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7136434","identity":"rs-7136434","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}