Microstructural and Mechanical Investigation of CYSZ/MCrAlY Thermal Barrier Coatings: A Comparison of APS and SPS Processes

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TBCs play a vital role in high-temperature applications within the aerospace and automotive industries. APS coatings demonstrated greater porosity, increased thermally grown oxide (TGO) formation, and more significant thermal degradation at elevated temperatures. Conversely, SPS coatings were denser with superior adhesion, lower porosity, and enhanced mechanical properties, including higher microhardness. Phase analysis showed APS reduced CeO₂ content and transformed zirconia phases, while SPS preserved the original phase composition, ensuring better structural integrity and thermal stability. Overall, SPS outperformed APS in mechanical strength, phase stability, and oxidation resistance, making it a superior method for producing high-performance TBCs. CYSZ MCrAlY Thermal barrier coating Atmospheric plasma spray Spark plasma sintering Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Highlights 1. SPS TBCs showed higher microhardness than APS coatings. 2. SPS preserved the CeO₂ phase, ensuring thermal stability. 3. SPS coatings had lower porosity and better layer adhesion. 4. SPS TBCs formed a uniform TGO layer for better oxidation resistance. 5. SPS offered superior performance for high-temperature applications. 1. Introduction Thermal barrier coatings (TBCs) are crucial for improving the performance and longevity of components operating in high-temperature environments, especially in aerospace and automotive industries. The production of TBCs can be achieved through various methods. The most used two production methods are atmospheric plasma spraying (APS) and electron beam physical vapour deposition (EB-PVD). While APS coatings offer better thermal insulation due to higher porosity, EB-PVD coatings provide higher thermal diffusivity and structural integrity. Spark plasma sintering (SPS) is an advanced technique used to fabricate TBCs. The method offers several advantages over traditional techniques, including rapid processing times and the ability to create dense, multi-layered structures. SPS TBCs demonstrate better sintering resistance compared to APS and EBPVD coatings. This is crucial for maintaining mechanical properties and thermal conductivity at high temperatures. [ 1 – 3 ]. This study will investigate the comparison of APS and SPS methods for fabricating TBC and its microstructure and mechanical properties. TBCs are primarily composed of ceramic materials, such as Yttria-Stabilized Zirconia (YSZ), which provide excellent thermal insulation and protect underlying substrates from thermal fatigue and oxidation [ 4 , 5 ]. These coatings are commonly applied to turbine blades in gas turbines, where they help maintain efficiency by allowing for higher operating temperatures [ 6 , 7 ]. The effectiveness of TBCs is influenced by their microstructure, which can be tailored through various deposition techniques, including plasma spraying and electron beam physical vapour deposition (EB-PVD) [ 4 , 8 ]. However, these traditional methods often face challenges related to coating porosity and adhesion, which can compromise performance. Spark plasma sintering has gained attention as a promising alternative for producing TBCs due to its unique advantages. SPS allows for the simultaneous densification of the coating and the substrate, leading to improved adhesion and reduced porosity compared to conventional methods [ 8 ]. The process involves applying a pulsed electric current to the powder material, which facilitates rapid heating and densification at lower temperatures [ 8 , 9 ]. This method has been shown to produce TBCs with enhanced mechanical properties and thermal stability, making them suitable for high-temperature applications [ 8 , 10 ]. For instance, Nozahic et al. demonstrated that self-healing TBC systems fabricated by SPS exhibited superior performance under thermal cycling conditions, highlighting the potential of this technique for advanced applications [ 9 ]. Despite its advantages, the application of SPS in TBC production is not without challenges. The control of microstructural features during the sintering process is critical, as variations can lead to differences in thermal conductivity and mechanical properties [ 8 ]. Additionally, the cost of SPS equipment and the need for specialized knowledge can limit its widespread adoption in industrial settings [ 10 ]. Nevertheless, ongoing research continues to explore the optimization of SPS parameters to enhance the quality and performance of TBCs, making it a viable option for future advancements in thermal protection technologies. In this study, commonly used thermal barrier coating material Ceria-Yttria-Stabilized Zirconia (CYSZ) and NiCoCrAlY bond coat material was produced functionally graded by APS and SPS methods to investigate for microstructure and mechanical properties. 2. Experimental Procedures Ceria-yttria stabilized zirconia (CYSZ, Metco 205NS: ZrO₂, 24CeO₂, 2.5Y₂O₃, particle size range − 90 ± 16 µm) and NiCoCrAlY (Sulzer Metco, Amdry 9951: Ni 23Co 20Cr 8.5Al 4Ta 0.6Y, particle size range ± 37 µm) were utilized as ceramic top coat powders (Fig. 1 c-d). The ceramic top coats were produced in a functionally graded, 5-layered design using both APS and SPS methods (Fig. 1 (a)). NiCoCrAlY was also applied as the bond coat layer. The CYSZ and MCrAlY powders were weighed and mixed in graded layers, followed by blending in a turbula mixer (T2F Bachofen AG) with zirconia balls for 6 hours. INCONEL 625 (Ni 63%, Cr 20.6%, Mo 7.9%, Fe 4.3%, Nb 3.2%, Al 0.4%, Mn 0.2%) was chosen as the substrate material. Substrates with a diameter of 25 mm and a thickness of 2 mm were prepared by cleaning and grit blasting for 4 minutes to improve adhesion. Bond coat powders were applied to the prepared substrate surface using the high-velocity oxy-fuel (HVOF) process (2700 DJHE DJ, Sulzer Metco), achieving a total thickness of 100 ± 30 µm. The ceramic top coat powders were then sprayed onto the bond coat using an APS system (Sulzer Metco, 9MCE Plasma system, and 9MB plasma gun), starting with a 100% MCrAlY layer and transitioning to a 100% CYSZ top layer. For the SPS process, the ceramic top coat powders were layered as a functionally graded 5-layer structure and poured into a graphite die on the substrate (Fig. 1 b). Die-powder contact surface was deposited by h -BN spray to inhibit the carbon diffusion from the die. The ceramic coatings had a total thickness of approximately 250 ± 80 µm. As production parameters for SPS, different temperatures were tested, and a temperature of 1025°C and a pressure of 40 MPa were found to be most suitable. The layer thickness was similar across both methods. Extensive studies have shown that CYSZ outperforms YSZ, making it the preferred choice. Specifically, CYSZ offers significant advantages in thermal barrier coatings, including enhanced thermal shock resistance and hot corrosion resistance, making it a more robust choice for applications demanding high durability under extreme conditions [ 11 – 13 ]. Although hex-BN spray was used after the sintering process, carburization occurred within the ceramic layer. Therefore, based on a previous study [ 21 ], SPS samples were held in a furnace at 1000°C for 2 hours to perform decarburization. In this study, APSed and SPSed functionally graded CYSZ and MCrAlY powders were produced. Coating design and thickness were given in Fig. 1 (a). Wang et al. investigated the design and optimization of coating structures by producing coatings with ceramic top layer thicknesses ranging from 300 to 420 µm for comparison. Their findings highlighted that coating thickness is a critical factor in enhancing thermal insulation, with thicker ceramic layers generally providing better insulation. However, increased thickness also leads to higher residual stress [ 14 ]. Additionally, a comparative study on thermal cyclic performance demonstrated that ceramic top layer thicknesses within a broader tolerance range (600 ± 50 µm) could be effectively used for performance comparisons [ 15 ]. Therefore, spraying and sintering processes were carried out with the ceramic top layer coating thickness set to 400 µm and the bond coat to 100 µm. The spraying parameters for HVOF and APS are given in Table 1 . Microstructure analysis was conducted using a JEOL JSM 7000F field emission scanning electron microscope (FESEM) equipped with an energy-dispersive spectrometer (EDS). Porosity values were measured through backscatter electron microscope images analyzed with ImageJ software. Phase analysis of powder and the produced coating was important to the analysis of inhabited phase transformation. X-ray diffractometer (MiniFlex, Rigaku Corp.) was employed to analyze the samples over a scanning range of 2–90°, utilizing Cu Kα radiation (λ = 1.54 Å). A Leica VMHT microhardness testing machine was utilized with a dwell time of 12 seconds and an applied force of 1000 gf to evaluate the differences in coating hardness based on the production method. Hardness measurements were taken at 10 different points of the substrate, bond coat and ceramic top coat, and the average value in GPa was calculated. Table 1 HVOF and APS Spraying parameters HVOF Parameters NiCoCrAlY APS Parameters CYSZ Propan (scfh) 40 Hydrogen (scfh) 15 Oxygen (scfh) 24 Argon (scfh) 90 Air (scfh) 50 Air (scfh) 13.5 Powder speed (lb/min) 7.4 Amper (A) 400 Spray distance (cm) 25 Volt (V) 64 Platform return speed (Hz) 50 Powder speed (lb/min) 6.5 Spraying angle (°) 90 Spraying distance (cm) 7.5 Pass number 12 Spray angle (°) 90 3. Results and discussion 3.1. Microstructural characterization Cross-sectional microstructures of the APSed and SPSed CYSZ MCrAlY functionally graded 5-layered design are shown in Fig. 2 (a-b). The figure illustrates strong adhesion between the bond coat and the ceramic coat, as well as between the bond coat and the substrate. The average thickness of the bond coat and top coat, measured along ten different thickness lines for each coating, was found to be 100 ± 30 µm and 250 ± 80 µm, respectively. The porosity values measured by the Imagej program for APS and SPS were 12.8% and 8.5%, respectively. In the light of APSed microstructures, the top coat exhibits porosities located between splats of different phases, while the bond coat is free from porosities. A strong adhesion is observed between the bond coat and the top coat. During the production process via air plasma spraying (APS), the thermally grown oxide (TGO) layer does not develop. However, under high-temperature operating conditions, a TGO layer forms at the interface between the bond coat and top coat due to the oxidation of aluminium. When we examined the SPSed structures, excellent adhesion was observed among the top coat (TC), bond coat (BC), and substrate. The structure produced via spark plasma sintering (SPS) exhibited higher density and lower porosity compared to coatings produced by air plasma spraying (APS) (Fig. 2 (c-d)). The composition of the coating was functionally graded, with ceria-yttria-stabilized zirconia (CYSZ) and the bond coat material distributed in a controlled manner. The CYSZ powders were homogeneously dispersed within the metallic NiCoCrAlY powders. Additionally, dark formations were detected at the interfaces between the CYSZ and NiCoCrAlY grains. These dark formations were identified as a-Al₂O₃ (alumina) thermally grown oxide layers with dimensions below 1 µm, as determined by EDS analysis (Fig. 3 (a)). The thickness of the thermally grown oxide (TGO) layer impacts both the temperatures at which cracks initiate and the lengths of these cracks in thermal barrier coatings (TBCs). For TBCs produced via air plasma spraying (APS), the initial TGO thickness plays a crucial role in their lifespan, with a critical threshold of 5 µm. Beyond this thickness, the coatings experience rapid failure. The growth of the thermally grown oxide (TGO) layer increases internal stresses, which can result in coating spallation. Thicker TGO layers are prone to forming multiple cracks at the interface between the TGO and the bond coat, which helps delay the initiation of surface cracks and extends the time to failure of the thermal barrier coating (TBC) [ 16 – 18 ]. Therefore, the TGO alumina formation that developed during sintering did not reach dimensions that would affect the coating's strength harmfully. As seen in Fig. 3 b, the thickness of the formed TGO is approximately 0.5 µm, which will not result in a decrease in the mechanical properties of the coating. Furthermore, it is believed that the TGO formation at these dimensions enhances the coating's thermal insulation, thereby improving its oxidation resistance. Conversely, despite the high oxidation, a reduction in chromium concentration within the coating was anticipated. However, the gray regions exhibit a uniform chromium distribution with no signs of depletion. The inclusion of chromium in bond coatings enhances TBC adherence and environmental resistance, resulting in greater durability and improved protection for the underlying substrate. In the upper regions of the SPSed TBC, the TGO formation occurring around CYSZ has not led to oxidation causing chromium depletion in the lower regions (Fig. 3 (b)) [ 19 , 20 ]. 3.2. Hardness measurement The hardness measurements of the coatings produced by both production methods (APS and SPS) were taken using the Vickers microhardness method. The hardness values were measured at 10 different points on the substrate, bond coat, and 3 different layers of ceramic top coat cross-sections, and the average and standard deviation were calculated (Table 2 and Fig. 4 ). Micro-hardness of APSed and SPSed substrates were 2.1 ± 0.1 GPa and 4.5 ± 0.05 GPa respectively. It has been calculated that the hardness values of the substrates placed in the die increase by up to 2 times due to the effects of high temperature and pressure. Microhardness of bond coat APSed and SPSed 3.15 ± 0.6 GPa and 5.5 ± 0.8 GPa respectively. Although the bonding layer in both samples was produced using the HVOF process, it has been observed that the hardness increased by almost twice due to the pressure and temperature applied during the SPS process. When the microhardness values of the bottom layer of the ceramic top layer, which is the 100% MCrAlY layer, were examined, they were determined to be 2.5 ± 0.2 and 4.2 ± 0.8 GPa, respectively. The microhardness values measured in the 50–50% CYSZ and MCrAlY layers were determined to be 2.4 ± 0.4 and 4.5 ± 0.8 GPa, respectively. It has been determined that in all layers of the ceramic coating, the bonding layer, and the substrate, the microhardness measurement in the top 100% CYSZ layer also increased by up to 2 times, with values similar to those measured in the other layers. Accordingly, for APS and SPS, the top layer hardness values were measured as 3.1 ± 0.1 and 4.4 ± 0.5 GPa, respectively. In a similar study where YSZ was used for the ceramic top layer, parallel to this study [ 21 ], SPS-prepared coatings showed higher hardness than APS coatings, especially in the topmost ceramic layer. The SPS coating's top layer hardness increased by 26% (from 6.08 GPa to 7.65 GPa), while the intermediate layers saw a 10% improvement. This is due to the SPS process achieving full densification and retaining nanostructured grains, enhancing the coating's resistance to indentation. Table 2 Micro-hardness results of APSed and SPSed TBC Microhardness APSed (GPa) SPSed (GPa) 100% CYSZ 3.1 ± 0.1 4.4 ± 0.5 50% CYSZ + 50% MCrAlY 2.4 ± 0.4 4.5 ± 0.8 100% MCrAlY 2.5 ± 0.2 4.2 ± 0.8 Bond Coat 3.15 ± 0.6 5.5 ± 0.8 Substrate 2.1 ± 0.1 4.5 ± 0.05 3.3 Phase Analysis Likely, changes in the phases of the coatings during the APS and SPS processes may vary. Therefore, phase analyses of the coatings were conducted after the powder and production processes. This helped us understand potential phase transformations and phase reductions that could occur post-production. Some studies have shown that zirconia, in particular, undergoes a monoclinic to tetragonal transformation [ 22 ]. This transformation creates a metastable tetragonal phase, which undergoes further reversible transformations at the operational temperatures of the coating, leading to volumetric changes, cracking, and, ultimately, the failure of the coating. After the spraying process of APS, the monoclinic phase was eliminated, indicating that the thermal treatment during spraying likely promoted the transformation of monoclinic zirconia to other stable phases. Additionally, the CeO 2 phase experienced a reduction in its concentration, which suggests that the spraying process caused some decomposition or phase changes in the ceria, potentially affecting its distribution and role within the coating (Fig. 5 (a)). After the sintering process, no phase transformation occurred, and there was no reduction in the monoclinic phase or CeO 2 content. The CeO 2 phase showed an increase, which contributes to enhanced thermal stability in the thermal barrier coating (TBC). The increase in CeO 2 helps to reinforce the coating's ability to withstand high temperatures, promoting better structural integrity and resistance to thermal stresses during operation. This improved thermal stabilization is crucial for the longevity and performance of the TBC, particularly under high-temperature conditions (Fig. 5 (b)). 4. Conclusion -CYSZ and NiCoCrAlY powders were produced using both the APS and SPS processes. -The coatings produced with the commercially used APS process exhibit no oxidation before thermal exposure. However, under high temperatures and pressure, this lack of oxidation leads to spallation. -A comparison of the microstructures from the two processes revealed that the SPS-treated sample demonstrates a more uniform oxide formation in the top coating. This oxide layer, formed during operation, offers better protection to the bond coat and substrate than the newly formed TGO (Thermal Growth Oxide) layer. -The α-Al 2 O 3 TGO layer formed between the ceramic and metal interfaces is continuous, with a thickness of less than 1 µm. -The SPS-coated specimen displayed higher micro-hardness relative to the APS-coated specimen, suggesting better mechanical performance. -Following production using the APS process, the CeO 2 phase showed a reduction, and the monoclinic zirconia phase completely disappeared. -In contrast, after the SPS process, no phase transformation occurred between the powder and the coating, indicating that the SPS process preserved the original phase composition without any significant changes. Declarations Acknowledgement The author thanks Prof. Dr. Gultekin Goller for his permission to use Biomaterials Research and Characterization Laboratory of Istanbul Technical University Fundings There is no funding for this study. 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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-6399970","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[{"code":1,"date":"2025-04-09 05:02:32","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":456272994,"identity":"7c45ae1b-22c8-4ad7-805d-4725eee7d26e","order_by":0,"name":"FATIH KIRBIYIK","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA4ElEQVRIiWNgGAWjYBACCWYGNoYEBgkD9vbGBpAAYwPRWnjOHATrAWphJqCFAagFCAx4biQwEqdFsp332YOHbRbGPJKP2x/zMNjIbjjAf+wDPi3SzOzmBoltEmY80omNzTwMacYbDjAzz8CnRY6ZjU0CqMXGHqLlcCJIC16HwbXwSB4EaflPWIs0VIsZjwQjSMsBwlokm9nYDRLOSRjz8CQ2zpxjkGw88zCzMV4tEuePsT38UVZn2MN+/MGHNxV2sn3HGx/j1QIGjGwwlgEQE4gWKPhDlKpRMApGwSgYqQAAnn0/f/6XgyoAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0002-3654-5315","institution":"Adiyaman University: Adiyaman Universitesi","correspondingAuthor":true,"prefix":"","firstName":"FATIH","middleName":"","lastName":"KIRBIYIK","suffix":""}],"badges":[],"createdAt":"2025-04-08 06:40:54","currentVersionCode":2,"declarations":{"humanSubjects":false,"vertebrateSubjects":false,"conflictsOfInterestStatement":false,"humanSubjectEthicalGuidelines":false,"humanSubjectConsent":false,"humanSubjectClinicalTrial":false,"humanSubjectCaseReport":false,"vertebrateSubjectEthicalGuidelines":false},"doi":"10.21203/rs.3.rs-6399970/v2","doiUrl":"https://doi.org/10.21203/rs.3.rs-6399970/v2","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s41779-025-01281-1","type":"published","date":"2025-09-19T15:57:05+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":82694292,"identity":"6264bb9a-09cd-4095-a801-76d27d5f5a90","added_by":"auto","created_at":"2025-05-14 08:26:31","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":4571543,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea) \u003c/strong\u003eSchematic view 5-layered functionally graded CYSZ/NiCoCrAlY TBC \u003cstrong\u003eb) \u003c/strong\u003eschematic view SPS device \u003cstrong\u003ec) \u003c/strong\u003emicrostructure image of NiCoCrAlY powder \u003cstrong\u003ed) \u003c/strong\u003emicrostructure image of CYSZ powder.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6399970/v2/11717404d7ceacc0c4d7c0d3.png"},{"id":82694295,"identity":"e0fefd3c-cc94-4fa5-aa50-a2f9c3e35379","added_by":"auto","created_at":"2025-05-14 08:26:31","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":6753003,"visible":true,"origin":"","legend":"\u003cp\u003eCross section microstructure image of each specimen; a) APSed, b) SPSed, c) APSed, d) SPSed.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6399970/v2/48647f4a754f44f8d805b4f6.png"},{"id":82694694,"identity":"ef046526-9a8b-401b-b1cf-815a1597df1b","added_by":"auto","created_at":"2025-05-14 08:34:31","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":6693174,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea) \u003c/strong\u003eEDS analysis of SPSed TBC \u003cstrong\u003eb) \u003c/strong\u003eTGO formation of SPSed TBC\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6399970/v2/e1c67e79e1e73dc0f508cf16.png"},{"id":82694297,"identity":"83b142f7-acc9-4f2c-8c1d-d921f1c3080c","added_by":"auto","created_at":"2025-05-14 08:26:31","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":686437,"visible":true,"origin":"","legend":"\u003cp\u003eMicrohardness result of APSed and SPSed samples\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6399970/v2/5ebcf559484b794cc6fdbe68.png"},{"id":82695832,"identity":"9bd8d21c-b195-459b-b82f-50e3010f3307","added_by":"auto","created_at":"2025-05-14 08:42:31","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":896172,"visible":true,"origin":"","legend":"\u003cp\u003ePhase analysis of \u003cstrong\u003ea) \u003c/strong\u003eAPSed and \u003cstrong\u003eb) \u003c/strong\u003eSPSed TBC\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6399970/v2/ef2d82bedb1e81425c174248.png"},{"id":91889777,"identity":"b38a081a-22d1-44bf-8a41-2294eced6b36","added_by":"auto","created_at":"2025-09-22 16:01:35","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":19149732,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6399970/v2/82cf4734-60d8-4149-8793-f18d78bf33d0.pdf"}],"financialInterests":"","formattedTitle":"Microstructural and Mechanical Investigation of CYSZ/MCrAlY Thermal Barrier Coatings: A Comparison of APS and SPS Processes","fulltext":[{"header":"Highlights","content":"\u003cp\u003e1. SPS TBCs showed higher microhardness than APS coatings.\u003c/p\u003e\u003cp\u003e2. SPS preserved the CeO₂ phase, ensuring thermal stability.\u003c/p\u003e\u003cp\u003e3. SPS coatings had lower porosity and better layer adhesion.\u003c/p\u003e\u003cp\u003e4. SPS TBCs formed a uniform TGO layer for better oxidation resistance.\u003c/p\u003e\u003cp\u003e5. SPS offered superior performance for high-temperature applications.\u003c/p\u003e"},{"header":"1. Introduction","content":"\u003cp\u003eThermal barrier coatings (TBCs) are crucial for improving the performance and longevity of components operating in high-temperature environments, especially in aerospace and automotive industries. The production of TBCs can be achieved through various methods. The most used two production methods are atmospheric plasma spraying (APS) and electron beam physical vapour deposition (EB-PVD). While APS coatings offer better thermal insulation due to higher porosity, EB-PVD coatings provide higher thermal diffusivity and structural integrity. Spark plasma sintering (SPS) is an advanced technique used to fabricate TBCs. The method offers several advantages over traditional techniques, including rapid processing times and the ability to create dense, multi-layered structures. SPS TBCs demonstrate better sintering resistance compared to APS and EBPVD coatings. This is crucial for maintaining mechanical properties and thermal conductivity at high temperatures. [\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. This study will investigate the comparison of APS and SPS methods for fabricating TBC and its microstructure and mechanical properties.\u003c/p\u003e \u003cp\u003eTBCs are primarily composed of ceramic materials, such as Yttria-Stabilized Zirconia (YSZ), which provide excellent thermal insulation and protect underlying substrates from thermal fatigue and oxidation [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. These coatings are commonly applied to turbine blades in gas turbines, where they help maintain efficiency by allowing for higher operating temperatures [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. The effectiveness of TBCs is influenced by their microstructure, which can be tailored through various deposition techniques, including plasma spraying and electron beam physical vapour deposition (EB-PVD) [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. However, these traditional methods often face challenges related to coating porosity and adhesion, which can compromise performance.\u003c/p\u003e \u003cp\u003eSpark plasma sintering has gained attention as a promising alternative for producing TBCs due to its unique advantages. SPS allows for the simultaneous densification of the coating and the substrate, leading to improved adhesion and reduced porosity compared to conventional methods [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. The process involves applying a pulsed electric current to the powder material, which facilitates rapid heating and densification at lower temperatures [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. This method has been shown to produce TBCs with enhanced mechanical properties and thermal stability, making them suitable for high-temperature applications [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. For instance, Nozahic et al. demonstrated that self-healing TBC systems fabricated by SPS exhibited superior performance under thermal cycling conditions, highlighting the potential of this technique for advanced applications [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eDespite its advantages, the application of SPS in TBC production is not without challenges. The control of microstructural features during the sintering process is critical, as variations can lead to differences in thermal conductivity and mechanical properties [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Additionally, the cost of SPS equipment and the need for specialized knowledge can limit its widespread adoption in industrial settings [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Nevertheless, ongoing research continues to explore the optimization of SPS parameters to enhance the quality and performance of TBCs, making it a viable option for future advancements in thermal protection technologies.\u003c/p\u003e \u003cp\u003eIn this study, commonly used thermal barrier coating material Ceria-Yttria-Stabilized Zirconia (CYSZ) and NiCoCrAlY bond coat material was produced functionally graded by APS and SPS methods to investigate for microstructure and mechanical properties.\u003c/p\u003e"},{"header":"2. Experimental Procedures","content":"\u003cp\u003eCeria-yttria stabilized zirconia (CYSZ, Metco 205NS: ZrO₂, 24CeO₂, 2.5Y₂O₃, particle size range \u0026minus;\u0026thinsp;90\u0026thinsp;\u0026plusmn;\u0026thinsp;16 \u0026micro;m) and NiCoCrAlY (Sulzer Metco, Amdry 9951: Ni 23Co 20Cr 8.5Al 4Ta 0.6Y, particle size range\u0026thinsp;\u0026plusmn;\u0026thinsp;37 \u0026micro;m) were utilized as ceramic top coat powders (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec-d). The ceramic top coats were produced in a functionally graded, 5-layered design using both APS and SPS methods (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a)). NiCoCrAlY was also applied as the bond coat layer. The CYSZ and MCrAlY powders were weighed and mixed in graded layers, followed by blending in a turbula mixer (T2F Bachofen AG) with zirconia balls for 6 hours. INCONEL 625 (Ni 63%, Cr 20.6%, Mo 7.9%, Fe 4.3%, Nb 3.2%, Al 0.4%, Mn 0.2%) was chosen as the substrate material. Substrates with a diameter of 25 mm and a thickness of 2 mm were prepared by cleaning and grit blasting for 4 minutes to improve adhesion. Bond coat powders were applied to the prepared substrate surface using the high-velocity oxy-fuel (HVOF) process (2700 DJHE DJ, Sulzer Metco), achieving a total thickness of 100\u0026thinsp;\u0026plusmn;\u0026thinsp;30 \u0026micro;m. The ceramic top coat powders were then sprayed onto the bond coat using an APS system (Sulzer Metco, 9MCE Plasma system, and 9MB plasma gun), starting with a 100% MCrAlY layer and transitioning to a 100% CYSZ top layer. For the SPS process, the ceramic top coat powders were layered as a functionally graded 5-layer structure and poured into a graphite die on the substrate (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). Die-powder contact surface was deposited by \u003cem\u003eh\u003c/em\u003e-BN spray to inhibit the carbon diffusion from the die. The ceramic coatings had a total thickness of approximately 250\u0026thinsp;\u0026plusmn;\u0026thinsp;80 \u0026micro;m. As production parameters for SPS, different temperatures were tested, and a temperature of 1025\u0026deg;C and a pressure of 40 MPa were found to be most suitable. The layer thickness was similar across both methods. Extensive studies have shown that CYSZ outperforms YSZ, making it the preferred choice. Specifically, CYSZ offers significant advantages in thermal barrier coatings, including enhanced thermal shock resistance and hot corrosion resistance, making it a more robust choice for applications demanding high durability under extreme conditions [\u003cspan additionalcitationids=\"CR12\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Although hex-BN spray was used after the sintering process, carburization occurred within the ceramic layer. Therefore, based on a previous study [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e], SPS samples were held in a furnace at 1000\u0026deg;C for 2 hours to perform decarburization.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn this study, APSed and SPSed functionally graded CYSZ and MCrAlY powders were produced. Coating design and thickness were given in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a). Wang et al. investigated the design and optimization of coating structures by producing coatings with ceramic top layer thicknesses ranging from 300 to 420 \u0026micro;m for comparison. Their findings highlighted that coating thickness is a critical factor in enhancing thermal insulation, with thicker ceramic layers generally providing better insulation. However, increased thickness also leads to higher residual stress [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Additionally, a comparative study on thermal cyclic performance demonstrated that ceramic top layer thicknesses within a broader tolerance range (600\u0026thinsp;\u0026plusmn;\u0026thinsp;50 \u0026micro;m) could be effectively used for performance comparisons [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Therefore, spraying and sintering processes were carried out with the ceramic top layer coating thickness set to 400 \u0026micro;m and the bond coat to 100 \u0026micro;m. The spraying parameters for HVOF and APS are given in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Microstructure analysis was conducted using a JEOL JSM 7000F field emission scanning electron microscope (FESEM) equipped with an energy-dispersive spectrometer (EDS). Porosity values were measured through backscatter electron microscope images analyzed with ImageJ software. Phase analysis of powder and the produced coating was important to the analysis of inhabited phase transformation. X-ray diffractometer (MiniFlex, Rigaku Corp.) was employed to analyze the samples over a scanning range of 2\u0026ndash;90\u0026deg;, utilizing Cu Kα radiation (λ\u0026thinsp;=\u0026thinsp;1.54 \u0026Aring;). A Leica VMHT microhardness testing machine was utilized with a dwell time of 12 seconds and an applied force of 1000 gf to evaluate the differences in coating hardness based on the production method. Hardness measurements were taken at 10 different points of the substrate, bond coat and ceramic top coat, and the average value in GPa was calculated.\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\u003eHVOF and APS Spraying parameters\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\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 \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHVOF Parameters\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNiCoCrAlY\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAPS Parameters\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCYSZ\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePropan (scfh)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eHydrogen (scfh)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eOxygen (scfh)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eArgon (scfh)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e90\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAir (scfh)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAir (scfh)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e13.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePowder speed (lb/min)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e7.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAmper (A)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e400\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSpray distance (cm)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eVolt (V)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e64\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePlatform return speed (Hz)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePowder speed (lb/min)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSpraying angle (\u0026deg;)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSpraying distance (cm)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e7.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePass number\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSpray angle (\u0026deg;)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e90\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Microstructural characterization\u003c/h2\u003e \u003cp\u003eCross-sectional microstructures of the APSed and SPSed CYSZ MCrAlY functionally graded 5-layered design are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a-b). The figure illustrates strong adhesion between the bond coat and the ceramic coat, as well as between the bond coat and the substrate. The average thickness of the bond coat and top coat, measured along ten different thickness lines for each coating, was found to be 100\u0026thinsp;\u0026plusmn;\u0026thinsp;30 \u0026micro;m and 250\u0026thinsp;\u0026plusmn;\u0026thinsp;80 \u0026micro;m, respectively. The porosity values measured by the Imagej program for APS and SPS were 12.8% and 8.5%, respectively. In the light of APSed microstructures, the top coat exhibits porosities located between splats of different phases, while the bond coat is free from porosities. A strong adhesion is observed between the bond coat and the top coat. During the production process via air plasma spraying (APS), the thermally grown oxide (TGO) layer does not develop. However, under high-temperature operating conditions, a TGO layer forms at the interface between the bond coat and top coat due to the oxidation of aluminium. When we examined the SPSed structures, excellent adhesion was observed among the top coat (TC), bond coat (BC), and substrate. The structure produced via spark plasma sintering (SPS) exhibited higher density and lower porosity compared to coatings produced by air plasma spraying (APS) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(c-d)). The composition of the coating was functionally graded, with ceria-yttria-stabilized zirconia (CYSZ) and the bond coat material distributed in a controlled manner. The CYSZ powders were homogeneously dispersed within the metallic NiCoCrAlY powders. Additionally, dark formations were detected at the interfaces between the CYSZ and NiCoCrAlY grains. These dark formations were identified as a-Al₂O₃ (alumina) thermally grown oxide layers with dimensions below 1 \u0026micro;m, as determined by EDS analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a)). The thickness of the thermally grown oxide (TGO) layer impacts both the temperatures at which cracks initiate and the lengths of these cracks in thermal barrier coatings (TBCs). For TBCs produced via air plasma spraying (APS), the initial TGO thickness plays a crucial role in their lifespan, with a critical threshold of 5 \u0026micro;m. Beyond this thickness, the coatings experience rapid failure. The growth of the thermally grown oxide (TGO) layer increases internal stresses, which can result in coating spallation. Thicker TGO layers are prone to forming multiple cracks at the interface between the TGO and the bond coat, which helps delay the initiation of surface cracks and extends the time to failure of the thermal barrier coating (TBC) [\u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Therefore, the TGO alumina formation that developed during sintering did not reach dimensions that would affect the coating's strength harmfully. As seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb, the thickness of the formed TGO is approximately 0.5 \u0026micro;m, which will not result in a decrease in the mechanical properties of the coating. Furthermore, it is believed that the TGO formation at these dimensions enhances the coating's thermal insulation, thereby improving its oxidation resistance. Conversely, despite the high oxidation, a reduction in chromium concentration within the coating was anticipated. However, the gray regions exhibit a uniform chromium distribution with no signs of depletion. The inclusion of chromium in bond coatings enhances TBC adherence and environmental resistance, resulting in greater durability and improved protection for the underlying substrate. In the upper regions of the SPSed TBC, the TGO formation occurring around CYSZ has not led to oxidation causing chromium depletion in the lower regions (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b)) [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Hardness measurement\u003c/h2\u003e \u003cp\u003eThe hardness measurements of the coatings produced by both production methods (APS and SPS) were taken using the Vickers microhardness method. The hardness values were measured at 10 different points on the substrate, bond coat, and 3 different layers of ceramic top coat cross-sections, and the average and standard deviation were calculated (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Micro-hardness of APSed and SPSed substrates were 2.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 GPa and 4.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05 GPa respectively. It has been calculated that the hardness values of the substrates placed in the die increase by up to 2 times due to the effects of high temperature and pressure. Microhardness of bond coat APSed and SPSed 3.15\u0026thinsp;\u0026plusmn;\u0026thinsp;0.6 GPa and 5.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8 GPa respectively. Although the bonding layer in both samples was produced using the HVOF process, it has been observed that the hardness increased by almost twice due to the pressure and temperature applied during the SPS process. When the microhardness values of the bottom layer of the ceramic top layer, which is the 100% MCrAlY layer, were examined, they were determined to be 2.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2 and 4.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8 GPa, respectively. The microhardness values measured in the 50\u0026ndash;50% CYSZ and MCrAlY layers were determined to be 2.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4 and 4.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8 GPa, respectively. It has been determined that in all layers of the ceramic coating, the bonding layer, and the substrate, the microhardness measurement in the top 100% CYSZ layer also increased by up to 2 times, with values similar to those measured in the other layers. Accordingly, for APS and SPS, the top layer hardness values were measured as 3.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 and 4.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5 GPa, respectively. In a similar study where YSZ was used for the ceramic top layer, parallel to this study [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e], SPS-prepared coatings showed higher hardness than APS coatings, especially in the topmost ceramic layer. The SPS coating's top layer hardness increased by 26% (from 6.08 GPa to 7.65 GPa), while the intermediate layers saw a 10% improvement. This is due to the SPS process achieving full densification and retaining nanostructured grains, enhancing the coating's resistance to indentation.\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\u003eMicro-hardness results of APSed and SPSed TBC\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMicrohardness\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAPSed (GPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSPSed\u003c/p\u003e \u003cp\u003e(GPa)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e100% CYSZ\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e3.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e4.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e50% CYSZ\u0026thinsp;+\u0026thinsp;50% MCrAlY\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e2.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e4.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e100% MCrAlY\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e2.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e4.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBond Coat\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e3.15\u0026thinsp;\u0026plusmn;\u0026thinsp;0.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e5.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSubstrate\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e2.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e4.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05\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\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Phase Analysis\u003c/h2\u003e \u003cp\u003eLikely, changes in the phases of the coatings during the APS and SPS processes may vary. Therefore, phase analyses of the coatings were conducted after the powder and production processes. This helped us understand potential phase transformations and phase reductions that could occur post-production. Some studies have shown that zirconia, in particular, undergoes a monoclinic to tetragonal transformation [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. This transformation creates a metastable tetragonal phase, which undergoes further reversible transformations at the operational temperatures of the coating, leading to volumetric changes, cracking, and, ultimately, the failure of the coating. After the spraying process of APS, the monoclinic phase was eliminated, indicating that the thermal treatment during spraying likely promoted the transformation of monoclinic zirconia to other stable phases. Additionally, the CeO\u003csub\u003e2\u003c/sub\u003e phase experienced a reduction in its concentration, which suggests that the spraying process caused some decomposition or phase changes in the ceria, potentially affecting its distribution and role within the coating (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a)). After the sintering process, no phase transformation occurred, and there was no reduction in the monoclinic phase or CeO\u003csub\u003e2\u003c/sub\u003e content. The CeO\u003csub\u003e2\u003c/sub\u003e phase showed an increase, which contributes to enhanced thermal stability in the thermal barrier coating (TBC). The increase in CeO\u003csub\u003e2\u003c/sub\u003e helps to reinforce the coating's ability to withstand high temperatures, promoting better structural integrity and resistance to thermal stresses during operation. This improved thermal stabilization is crucial for the longevity and performance of the TBC, particularly under high-temperature conditions (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b)).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003e-CYSZ and NiCoCrAlY powders were produced using both the APS and SPS processes.\u003c/p\u003e\n\u003cp\u003e-The coatings produced with the commercially used APS process exhibit no oxidation before thermal exposure. However, under high temperatures and pressure, this lack of oxidation leads to spallation.\u003c/p\u003e\n\u003cp\u003e-A comparison of the microstructures from the two processes revealed that the SPS-treated sample demonstrates a more uniform oxide formation in the top coating. This oxide layer, formed during operation, offers better protection to the bond coat and substrate than the newly formed TGO (Thermal Growth Oxide) layer.\u003c/p\u003e\n\u003cp\u003e-The α-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e TGO layer formed between the ceramic and metal interfaces is continuous, with a thickness of less than 1 µm.\u003c/p\u003e\n\u003cp\u003e-The SPS-coated specimen displayed higher micro-hardness relative to the APS-coated specimen, suggesting better mechanical performance.\u003c/p\u003e\n\u003cp\u003e-Following production using the APS process, the CeO\u003csub\u003e2\u003c/sub\u003e phase showed a reduction, and the monoclinic zirconia phase completely disappeared.\u003c/p\u003e\n\u003cp\u003e-In contrast, after the SPS process, no phase transformation occurred between the powder and the coating, indicating that the SPS process preserved the original phase composition without any significant changes.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe author thanks Prof. Dr. Gultekin Goller for his permission to use Biomaterials Research and Characterization Laboratory of Istanbul Technical University\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFundings\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThere is no funding for this study.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThere is no competing interest for this manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availibility\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data are open to journal and it is all included in manuscript. If there is any required data like raw data for phase analysis it can be shared without hesitation.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eLv, B., M\u0026uuml;cke, R., Fan, X., Wang, T., Guillon, O., \u0026amp; Va\u0026szlig;en, R. (2018). Sintering resistance of advanced plasma-sprayed thermal barrier coatings with strain-tolerant microstructures. Journal of the European Ceramic Society. https://doi.org/10.1016/J.JEURCERAMSOC.2018.07.013.\u003c/li\u003e\n\u003cli\u003eLv, B., Fan, X., Li, D., \u0026amp; Wang, T. (2017). Towards enhanced sintering resistance: Air-plasma-sprayed thermal barrier coating system with porosity gradient. Journal of The European Ceramic Society, 38, 1946-1956. https://doi.org/10.1016/J.JEURCERAMSOC.2017.12.008.\u003c/li\u003e\n\u003cli\u003eLiu, L., Shankar, R., \u0026amp; Howard, P. (2010). High sintering resistance of a novel thermal barrier coating. 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Insights on the High-Temperature Operational Limits of ZrO2-Y2O3 TBCs Manufactured via Air Plasma Spray. Journal of Materials Engineering and Performance, 26, 1272-1282. https://doi.org/10.1007/s11665-017-2562-5.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"Adıyaman University","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"CYSZ, MCrAlY, Thermal barrier coating, Atmospheric plasma spray, Spark plasma sintering","lastPublishedDoi":"10.21203/rs.3.rs-6399970/v2","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6399970/v2","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study investigates the microstructure and mechanical properties of Ceria-Yttria-Stabilized Zirconia (CYSZ) thermal barrier coatings (TBCs) with a NiCoCrAlY bond coat, fabricated using Atmospheric Plasma Spraying (APS) and Spark Plasma Sintering (SPS). TBCs play a vital role in high-temperature applications within the aerospace and automotive industries. APS coatings demonstrated greater porosity, increased thermally grown oxide (TGO) formation, and more significant thermal degradation at elevated temperatures. Conversely, SPS coatings were denser with superior adhesion, lower porosity, and enhanced mechanical properties, including higher microhardness. Phase analysis showed APS reduced CeO₂ content and transformed zirconia phases, while SPS preserved the original phase composition, ensuring better structural integrity and thermal stability. Overall, SPS outperformed APS in mechanical strength, phase stability, and oxidation resistance, making it a superior method for producing high-performance TBCs.\u003c/p\u003e","manuscriptTitle":"Microstructural and Mechanical Investigation of CYSZ/MCrAlY Thermal Barrier Coatings: A Comparison of APS and SPS Processes","msid":"","msnumber":"","nonDraftVersions":[{"code":"","date":"2025-07-09 08:30:27","doi":"","editorialEvents":[],"status":"private","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}},{"code":2,"date":"2025-05-14 08:26:26","doi":"10.21203/rs.3.rs-6399970/v2","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"d08949c2-88c0-4278-81c3-456328a8a25f","owner":[],"postedDate":"May 14th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-09-22T15:58:45+00:00","versionOfRecord":{"articleIdentity":"rs-6399970","link":"https://doi.org/10.1007/s41779-025-01281-1","journal":{"identity":"journal-of-the-australian-ceramic-society","isVorOnly":false,"title":"Journal of the Australian Ceramic Society"},"publishedOn":"2025-09-19 15:57:05","publishedOnDateReadable":"September 19th, 2025"},"versionCreatedAt":"2025-05-14 08:26:26","video":"","vorDoi":"10.1007/s41779-025-01281-1","vorDoiUrl":"https://doi.org/10.1007/s41779-025-01281-1","workflowStages":[]},"version":"v2","identity":"rs-6399970","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6399970","identity":"rs-6399970","version":["v2"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}