Surface morphologies, tribological properties, and formation mechanism of the Ni–CeO2 nanocrystalline coatings on the modified surface of TA2 substrate

In order to remedy the poor tribological properties and low micro-hardness for pure titanium (TA2) materials, the Ni–CeO2 nanocrystalline coating was deposited from a Watts–Nickel electrolyte on the modified surface of TA2 substrate. In the present study, an available method of chemical activation was innovatively employed to make surface modification for TA2 substrate, in which the modified surface was characteristic of a rough surface and fully covered by the Ce-rich conducting phase for better electrodeposition. In order to study the effect of surface pretreatments on the modified TA2 surface, the surface roughness profiles and average roughness values were carried out using optical profilometry on the measured surfaces. A predictive modeling of TA2 surfaces before and after surface pretreatments was compared and successfully validated for better understanding of the formation mechanisms of electrodeposited Ni coatings reinforced with CeO2 nanoparticles. To clarify the beneficial effects of CeO2 addition on surface morphologies, phase composition, and textural evolution, the as-received nanocrystalline coatings were evaluated using various analytical techniques such as XRD, FE-SEM/EDX, and TEM. In addition, the scratch tests performed with an acoustic emission (AE) detector were conducted for the determination of interfacial adhesion between the as-deposited coating and the modified surface of TA2 substrate. Tribological properties and micro-hardness of investigated specimens were also examined. Experimental results revealed that this innovatively chemical activation was more favorable for increasing interfacial adhesion. The existence of well-distributed CeO2 phase that precipitated along the defective grain boundaries in the coating was contributed to make particulate reinforcement, thereby achieving better structural densification for Ni–CeO2 coatings. Meanwhile it exhibited superior tribological properties of the Ni–CeO2 coatings as compared to those of pure nickel and TA2 substrate, which was mainly attributed to the Ce-rich abrasive products that acted as a self-lubricating phase to make the effect of solid lubricants on worn surfaces.