Effects of dopants Ti and Al on microstructure, mechanical and tribological behaviors of ZnO/MoS2 coating deposited by magnetron co-sputtering

This paper investigates the underlying mechanisms of Ti and Al doped into ZnO/MoS2 coatings to develop better tribological properties for MoS2-based lubrication coatings. To this end, Ti-ZnO/MoS2 and Al-ZnO/MoS2 coatings with gradient processing were successfully prepared using magnetron co-sputtering with a subsequent annealing treatment. The effects of Ti and Al on the structures and properties of ZnO/MoS2 coatings were analyzed using various analytical methods. The results showed that Ti-ZnO/MoS2 coatings annealed at 300 °C displayed a minimum friction coefficient of 0.07, and Al-ZnO/MoS2 coatings annealed at 300 °C possessed the best oxidation resistance during wear process. Similarly, the highest nano-hardnesses of Ti and Al doped coatings were 1.9 GPa and 2.0 GPa after annealing at 300 °C. The growth of ZnO grains in Ti-ZnO/MoS2 coatings were different from the other coatings based on the Raman analysis, and Al-doped coating showed better fracture toughness and binding forces from indentation and scratch testing. These results suggest that the respective underlying mechanisms of these metals in ZnO/MoS2 coatings were different. As a result, this study shows that select metals could improve the different properties of metal-ZnO/MoS2 coating by utilizing different mechanisms. Formation of ZnS might account for the synergistic lubrication of ZnO and MoS2 co-doped coatings, and ZnO/MoS2 coatings had moisture resistance because of no sulfate radical appeared by XPS analysis. Therefore, metal-ZnO/MoS2 will be potential lubrication coatings to be widely used under natural ambient conditions.