Electrodeposited Co–Ni–Al2O3 composite coatings

Composite coatings of Co–Ni–Al2O3 were studied by electrolytic codeposition of Co–Ni alloys and Al2O3 from a sulfamate electrolyte containing Al2O3 particles. The influence of plating parameters on the content of the co-deposited Al2O3 particles in Co–Ni alloys was investigated. The maximum value of co-deposited Al2O3 can be achieved at a particle content of 80 g l−1 in bath, a current density of 3 A dm−2, a pH of 4.5, and a stirring rate of 100 rpm. In the process of codeposition, cathodic polarization increases with the increase of Al2O3 concentration in bath and cobalt ions in electrolyte caused the reduction of polarization while nickel ions do not change the polarization behavior. Surface morphology and microstructure of Co–Ni–Al2O3 coatings were determined by means of scanning electron microscopy, atomic force microscopy and X-ray diffraction. It was found that the phase structure of solid solution cannot be varied by codeposition of Al2O3 particles in Co–Ni alloys, and it only influences the growth and orientation of crystal planes. It was shown that the presence of Al2O3 particles in deposit greatly improves the hardness and the wear resistance of composite coatings. However, the codeposition of Al2O3 increases the tensile internal stress of Co–Ni–Al2O3 deposit. The coefficient of thermal expansion and the thermal conductivity of Co–Ni–Al2O3 composite coatings are varied with the increase of Co contents and the temperature.