Electrodeposited Co–Ni–Al 2O 3 composite coatings

Abstract

Composite coatings of Co–Ni–Al 2O 3 were studied by electrolytic codeposition of Co–Ni alloys and Al 2O 3 from a sulfamate electrolyte containing Al 2O 3 particles. The influence of plating parameters on the content of the co-deposited Al 2O 3 particles in Co–Ni alloys was investigated. The maximum value of co-deposited Al 2O 3 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 Al 2O 3 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–Al 2O 3 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 Al 2O 3 particles in Co–Ni alloys, and it only influences the growth and orientation of crystal planes. It was shown that the presence of Al 2O 3 particles in deposit greatly improves the hardness and the wear resistance of composite coatings. However, the codeposition of Al 2O 3 increases the tensile internal stress of Co–Ni–Al 2O 3 deposit. The coefficient of thermal expansion and the thermal conductivity of Co–Ni–Al 2O 3 composite coatings are varied with the increase of Co contents and the temperature.