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 Si₃ N₄ 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 Si₃ N₄ 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 Si₃ N₄ 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/Si₃ N₄ 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 Si₃ N₄ 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 Si₃ N₄ 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.