Formation of inter-splat bonding and intra-splat microstructure during plasma spraying of ceramic coating

Plasma-sprayed ceramic coatings are formed by successive deposition of a stream of molten and partially molten particles through their impact following by flattening, rapid cooling and solidification. They have been widely applied to different industrial fields to enhance the performances of metal materials employed at high temperature environments. The unique deposition process of thermal spraying makes the coatings have unique microstructure features and properties. In this paper, the coating microstructure features will be presented in terms of intra-splat microstructure and interface microstructure of inter-splats based on the recent advances of splat microstructure studies. It is known that metastable phases are usually present in splats. The recent investigation assisted by FIB-HR-TEM analysis revealed that the intra-splat microstructure much depends on the deposition temperature and inter-splat bonding. The amorphous phase is present in plasma-sprayed Al₂O ₃ splats as far as the bonding is formed at low deposition temperature and then phase changes from amorphous, γ-Al₂O ₃ to α-Al₂O₃ with the change of deposition temperature. Moreover, the inter-splat bonding can be controlled by the deposition temperature. A microstructure map is proposed to control the phase structure of thermal spray splats and subsequent coating based on the deposition temperature and inter-splat bonding.