Interfacial Microstructure and Bonding Properties of Plasma-Sprayed Multilayer Ceramic Coating (Al2O3/BaTiO3/Al2O3-40 wt.% TiO2)

In multilayer coatings, the bonding mechanisms between the different layers determine the cohesive strength of the coatings, which in turn to a large extent controls the mechanical properties of coatings under different loading conditions. In this study, the interfacial bonding state and adhesive strength of a plasma-sprayed single-layer coating (Al2O3), a double-layer coating (Al2O3/BaTiO3), and a three-layer coating (Al2O3/BaTiO3/Al2O3-40 wt.% TiO2) were investigated, as well as their phase composition and microstructure. Scanning electron microscopy observation showed that the microstructure of the three ceramic coatings was relatively dense with a good interface bonding state. However, micro-cracks were observed in the smooth region of the double-layer coating interface, while pores were observed at the BaTiO3/Al2O3-40 wt.% TiO2 interface in the three-layer coating. Transmission electron microscopy observation revealed that element diffusion occurred at the interface. The diffusion depth at the Al2O3/BaTiO3 and BaTiO3/Al2O3-40 wt.% TiO2 interfaces reached 12 nm and 10 nm, respectively. Therefore, both mechanical interlocking (the dominant mechanism) and limited chemical diffusion contributed to interface adhesion in the multilayer coatings. The adhesion strengths of the double-layer, single-layer, and three-layer coatings were 40.1, 21.8, and 15.3 MPa, respectively. The latter exhibited the lowest adhesion strength mainly because of the relatively weak Al2O3/BaTiO3 interface.