Effect of oxygen in surface layers formed during sliding wear of Ni-ZrO2 coatings

Nickel-based coatings with different content of zirconia were deposited on steel by short-pulse laser melting of powder mixtures of pure components. High temperatures during laser short-pulse melting lead to dissolution of the zirconia in the nickel-zirconia melt. Ultrafast cooling of the melt fixes in the solid state the supersaturated solid solution of zirconia in the nickel matrix. The short-pulse laser processing allows the precise amount of zirconia to be dissolved in the metal matrix, therefore the main objective of this research is to study the effect of the oxide content on tribological properties of the coatings. Nickel-zirconia coatings with low zirconia content show a steep decrease in the coefficient of friction (COF) down to 0.03 during wear tests in boundary lubrication conditions. The structure and composition of tribolayers formed both on coated disks and counterfaces, made of cast iron, have been determined and described as mechanically alloyed layers. The main feature of the tribolayers is their almost unoxidized state after wear tests, which was verified by XPS characterization. The results reveal the influence of the dissolved zirconia in the coatings on resistance to oxidation and adhesion of the rubbed surfaces in contact during severe sliding friction.