Electrodeposited nickel-cobalt composite coating containing nano-sized Si3N4

Ni-Co/Si3N4 composite coatings with various contents of Si3N4 nano-particulates were prepared by electrodeposition in a Ni-Co plating bath containing Si3N4 nano-particulates to be co-deposited. The shape and size of the Si3N4 nano-particulates were observed and determined on a transmission electron microscope. The polarization behavior of the composite plating bath was examined on a PAR-273A potentiostat/galvanostat device. The friction and wear behaviors of the Ni-Co/Si3N4 composite coatings were evaluated on a ball-on-disk UMT-2MT test rig. The worn surface morphologies of the Ni-Co/Si3N4 composite coatings wear observed using a scanning electron microscope. It was found that the Si3N4 nano-particulates appeared as microspheres of a diameter about 20 nm. The cathodic polarization potential of the composite electrolyte increased with increasing Si3N4 concentration in the plating bath, and the addition of Si3N4 particulates led to changes in the morphologies of the composite coatings. Namely, the Ni-Co alloy coating was composed of needle-like micro-crystallites, while the Ni-Co/Si3N4 composite coatings were characterized by particulate-like structure and had more compact and fine granular morphologies. At the same time, the morphologies of the composite coatings changed from fibril to granular, which implied that the co-deposited Si3N4 nano-particulates at a larger content could agglomerate to some extent. The co-deposited Si3N4 nano-particulates were uniformly distributed in the Ni-Co matrix and contributed to greatly increasing the microhardness and tribological properties of the Ni-Co alloy. The microhardness of the composite coatings increased and the friction coefficients and wear rates decreased with increasing content of the nano-Si3N4 in the composite coatings. This was attributed to the grain fining and dispersive strengthening effects of the co-deposited hard Si3N4 nano-particulates. At the same time, the hydroxylated silicon oxide formed by the tribochemical reaction between the nano-Si3N4 of high reactivity and water vapor in the air also contributed to decreasing the friction coefficient.