Influence of additives on microstructure, mechanical and tribological properties of nanocrystalline Zn–Ni coatings in a novel alkaline bath
The effects of additives on the microstructure, mechanical and tribological behaviors of nanocrystalline Zn–Ni alloys were investigated in a new alkaline bath with 5,5′-dimethylhydantoin (DMH) as the complexing agent. Results show that Ni content in deposits decreases with the increase of additives. Ni content of coatings has a significant effect on the surface morphology, roughness, phase structure and grain size of Zn–Ni coatings. The grain size and roughness of deposits decrease with the decrease of Ni content in deposits. Furthermore, a new phase γ-NiZn3 along (1300) plane becomes obvious and γ-Ni5Zn21 (411) plane is inhibited by decreasing Ni content from 14.08 wt% to 10.57 wt%. It is observed that mirror-like bright Zn–Ni alloys with excellent wear resistance are obtained with the addition of additives. The bright Zn–Ni coatings exhibit a much lower and more stable value of the friction coefficient. The wear rate of bright Zn–Ni alloys is about 7.95% of that of dull Zn–Ni alloys. The wear resistance of deposits is enhanced with the decrease of hardness, indicating that the improvement of wear resistance does not fit the Archard's law but it is strongly associated with the surface properties and grain size of Zn–Ni alloys. The wear mechanisms of dull and bright Zn–Ni alloys both belong to adhesive, abrasive and oxidative wears. However, it transfers from a severe wear to a slight wear with the addition of additives due to the self-lubricating effect of bright Zn–Ni alloys. Therefore, the addition of the introduced additives is a simple but effective way to improve the wear resistance of Zn–Ni coatings. The excellent friction-reduction effect and better anti-wear performance are obtained in bright nanocrystalline Zn–Ni alloys.