Microstructure and properties of Al2O3-ZrO2 composite coatings prepared by air plasma spraying

Three kinds of Al₂O₃-ZrO₂ composite coatings with different composition were prepared by air plasma spraying on titanium alloy substrates. The compositions of the coatings were Al₂O₃-35wt%ZrO_2, Al₂O₃-40wt%ZrO₂ and Al₂O₃-45wt%ZrO₂. The microstructure, microhardness and toughness of the three Al₂O₃-ZrO₂ composite coatings were characterized by X-ray diffraction, scanning electron microscope, energy dispersive spectrometer and micro-Vickers. The result showed that α-Al₂O₃ transformed to γ-Al₂O₃ during the process of plasma spraying Al₂O₃-ZrO₂ composite powders. Meanwhile, amorphous phase was formed in the Al₂O₃-ZrO₂ composite coatings due to the rapid cooling and solidification of plasma spraying and the proper composition. There was more amorphous phase in the Al₂O₃-40wt%ZrO₂ coating compared with the other two composite coatings. The reason was that the composition of the Al₂O₃-40wt%ZrO₂ composite coating was closer to the eutectic point of Al₂O₃-ZrO₂ pseudo-binary system. The Al₂O₃-40wt%ZrO₂ composite coating with more amorphous phase had denser microstructure and lower porosity. With increasing of the content of ZrO₂ in the Al₂O₃-ZrO₂ composite coatings, the microhardness of the coatings decreased. And the microhardness of the Al₂O₃-ZrO₂ composite coatings was significantly higher than that of single phase Al₂O₃ coating and single phase ZrO₂ coating. Vicker indention results showed that the Al₂O₃-ZrO₂ composite coatings had high toughness.