The electrodeposition and characterisation of low-friction and wear-resistant Co-Ni-P coatings

A new Co-Ni-P coating having superior properties to hard chrome coatings for anti-wear applications has been developed by combining the precipitation hardening of Ni-P alloys with the lubricity of cobalt-rich Co-Ni coatings. The evolution of composition and microstructure, hardness, thermal stability and tribological properties have been investigated. The local pH value near the cathode played an important role in the change of the microstructure from nanocrystalline to amorphous along the growth direction as the phosphorous content increased from 7 at.% to 26 at.%. The highest microhardness (980 HV) and the lowest wear rate (one order of magnitude lower than that of hard chrome coatings under the same dry sliding conditions) were achieved by annealing the coatings at 400 °C facilitating precipitation hardening. Furthermore, the coefficients of friction of both the as-deposited Co-Ni-P coating and the heat-treated samples were approximately 0.3, only half of that of hard chrome coatings. The wear rates of Co-Ni-P coatings annealed up to 400 °C are 4-7 times lower compared to that of hard chrome. The roll-like debris found on the worn surfaces of the coating annealed at 500 °C were oriented perpendicularly to the sliding direction. Such part might be related to the slightly reduced coefficient of friction in the stable stage.