Microstructural evolution and mechanical properties of atmospheric plasma sprayed Y₂O₃ coating with state of in-flight particle

Y₂O₃ coating on Al₂O₃ substrate was prepared by atmospheric plasma spray (APS). Computational fluid dynamics (CFD) was carried out to predict the state of in-flight Y₂O₃ particles at different powder feeding rates. Microstructure and mechanical properties were found to be affected by the spray distance and powder feeding rate. In this study, the hardness was calculated using a field emission-scanning electron microscope (FE-SEM) because the indentation in the coating is too small to measure using a hardness test machine. The formation of pores causes a decrease in the mechanical property, and the pore length of over 10 μm substantially decreases the hardness. Meanwhile, the solidification behavior is affected by the maximum temperature of the in-flight particles. Based on computational fluid dynamics (CFD) analysis, the maximum temperature of the in-flight particles was found to decrease with increase of the powder feeding rate at the same spray distance. At the powder feeding rate of 60 g/min, a lower adhesion strength was confirmed than that at feeding rate of 30 g/min because splats were insufficiently spread due to the lower maximum temperature of the in-flight particles. The roughness and height of the coating surface were evaluated by confocal microscopy and atomic force microscopy(AFM) analyses. The roughness is the resultant of accumulated splats and the accumulation mechanism of splats is affected by the state of the in-flight particles. Furthermore, there were nano-scale differences of height on the splat surface, on which the nucleation looks like ‘rugged bark’ during solidification of splats when the in-flight particles impact the substrate.