MICROSTRUCTURE AND PROPERTIES OF Co-Ni-Al₂O₃ COMPOSITE COATINGS AT THE HIGH TEMPERATURE

The metal matrix composite coating that Al₂O₃ particles incorporate into Co-Ni alloy was prepared by the electrodeposition technique from sulfamate electrolyte. It illustrates from the examined results of SEM, AFM and XRD that surface morphology and microstructure of Co-Ni-Al₂O₃ coatings appear to be mainly influenced by variations in cobalt content. The codeposition of Al₂O₃ particles in Co-Ni alloys cannot change the phase structure of solid solution, only affects the growth and orientation of crystal planes and mostly increases the d value of lattice. The high-temperature properties of the Co-Ni-Al₂O₃ coatings, including hardness, wear resistance, oxidation resistance, coefficients of thermal expansion and thermal conductivity at high temperature were also investigated. The results show that Co-Ni-Al₂O₃ composite coatings have good performance of wear resistance and oxidation resistance at the high temperature. Furthermore, the composite coatings with high-cobalt content show lower coefficients of thermal expansion and higher thermal conductivity than coatings with low-cobalt content do.