Microstructures, microhardness, and crystallization behaviors of amorphous Al₂O₃-YSZ coatings prepared by air plasma spraying

Three types of amorphous Al₂O₃-yttria-stabilized zirconia (YSZ) coatings were prepared using air plasma spraying (APS). The effects of the APS parameters on the microstructure, porosity, amorphous-phase content, and microhardness of the coatings were investigated using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). The results indicated that the microstructure of the coatings was dependent on the APS parameters used. The porosity of the coatings and the number of unmelted particles deposited increased with the flow rate of the primary plasma gas increasing. The amorphous-phase contents and microhardness values of the coatings exhibited a trend opposite to that of their porosity and the number of unmelted particles present in them. The crystallization behaviors and microhardness values of Al₂O₃-YSZ coatings annealed at different temperatures were also investigated. The results indicated that the coatings with higher amorphous-phase contents possessed homogeneous nanocrystalline structure and exhibited higher microhardness values after the heat treatments.