Formation mechanism of Al2O3-YAG amorphous ceramic coating deposited via atmospheric plasma spraying

Al₂O₃-YAG (Al₅Y₃O₁₂) amorphous ceramic coatings exhibit excellent crack propagation resistance under harsh wear services due to the amorphous phase contributing to the plastic deformation performance of the coating. However, the formation mechanism of the amorphous phase is ambiguous. This study mainly investigated the formation mechanism of Al₂O₃-YAG amorphous coating prepared by atmospheric plasma spraying from the perspective of crystallization chemistry. Nano and microsized powders with low eutectic point ratio were selected as feedstock for comparison. X-ray diffraction, scanning electron microscope, and electron backscattered diffraction were used to analyze the phase composition, morphologies, phase distribution, and structure of the coating. It is concluded that the significant thermodynamically stable structure of polycompound with high coordination numbers of cations prioritized crystallizing in the Al₂O₃-YAG melt, but it needed more time to crystallize and hardly crystallized in the limited time during plasma spraying. Therefore, the selection of as-sprayable powder should also be considered the critical factor for preparing amorphous coatings. The nanoscale or submicro scale powder distributed uniformly with low eutectic point ratio was chosen as the feedstock to ensure the powder droplets diffuse sufficiently during deposition.