Plasma Spraying Process of YSZ Thermal Barrier Coatings

To enhance the corrosion resistance and thermal cycling life of thermal barrier coatings (TBCs), the optimization of atmospheric plasma spray (APS) processes has emerged as a pivotal research direction. Given the complexity of factors affecting coating quality, this study examined four primary spray parameters: current, main gas flow rate, powder feed rate, and spray distance. Each parameter was set at three distinct levels. Using an orthogonal experimental design, nine YSZ coatings with differing processes were fabricated. A mixed salt of Na2SO4 and V2O5 (with a 1∶1 mass ratio) was applied to the coating surface at 15 g/cm2, after which the samples were placed in a muffle furnace set at 900 ℃ for 6 hours of corrosion. To simulate the real-world operational environment of TBCs, the post-corrosion samples were cooled to room temperature and then placed in a heat-treatment furnace at 1 150 ℃. The samples were quenched in water every 10 minutes. The surface changes in the coating after 20 cycles were observed. The surface roughness, morphology (both surface and cross section), and elemental composition and distribution of the coating were characterized by confocal microscopy, X-ray diffraction (XRD), and energy dispersive spectroscopy (EDS). ImageJ software was employed to analyze the pore size and distribution of the coating. The combined metrics of the coating's porosity and the mass loss rate post combined thermal corrosion and shock tests were utilized as evaluative parameters for the coating's overall performance. The range analysis method was adopted to transform multiple parameter indicators into a single criterion for process optimization. A comparative analysis was conducted between the coating from the optimized process and the nine coatings from the orthogonal tests. The optimized coatings were placed in a muffle furnace set at 1 150 ℃, with each 5-minute hold followed by quenching in water, counting as one cycle. The number of thermal shock failures was recorded, and the failure behavior of the coating was analyzed. Results revealed evident disparities in porosity, molten salt corrosion, and thermal cycle life for coatings prepared under different processes. The range analysis method identified the W3X3Y1Z3 parameter combination as the optimized spray process. For the TBC-1 coating prepared through the orthogonal experiment-optimized process, the porosity stood at 9.65%, with an average pore size of 6.18 μm2. No obvious large pores or cracks were observed. The coating's surface roughness was 3.48 μm, and the powder exhibited a satisfactory melting condition. For post thermal corrosion, the cross-sectional content of the molten salt element V in the coating was 2.03%, with no signs of salt accumulation. After 20 combined thermal corrosion and shock tests, the coating's mass loss rate was only 0.25%, maintaining an intact surface without noticeable spalling. The failure mode of the optimized coating in thermal shock tests was extensive coating delamination, with a failure occurrence at 172 cycles. In conclusion, APS process parameters significantly influence the comprehensive performance of coatings. Larger cracks and pores in the coating can serve as pathways for molten salt permeation, accelerating coating corrosion and diminishing thermal cycling life. Coatings prepared after process optimization, with uniform internal pore distribution and smaller average pore sizes, exhibit superior corrosion resistance and thermal cycling life. The growth stress of the thermally grown oxide (TGO) and the thermal mismatch stress between the ceramic and bond coat layers are crucial factors contributing to crack propagation and coating failure. © 2024 Chongqing Wujiu Periodicals Press. All rights reserved.