Tribological Behavior of a Cold-Sprayed Cu-MoS2 Composite Coating During Dry Sliding Wear

Two cold spray coatings, one pure Cu and the other a Cu-MoS2 composite coating, were studied for their tribology performance in dry air. It was demonstrated that a small amount of MoS2 (1.8 +/- 0.99 wt%) could significantly decrease coefficient of friction (CoF) from around 0.7 (Cu coating) to 0.14-0.15. MoS2 patches on the wear track exhibited a lower local CoF, and the main velocity accommodation mechanism was shearing MoS2-containing debris. Even though the coating wear rates were high in the early sliding (8.61-12.8 nm/cycle in penetration depth during the first 100 cycles), slow wear (0.12-0.22 nm/cycle) over the subsequent sliding was observed. It was also found that the presence of MoS2 helped to achieve high endurance of the first steady-state CoF. The dynamics of the process, material transfer, and phase transformation were examined using scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy. The MoS2 patches developed on the wear track and the counterface served as reservoirs to replenish MoS2 in the contact and became depleted with sliding. Cross-sectional microstructure revealed by electron channeling contrast imaging technique showed a layer of sliding-induced microstructure, 3-5 mu m thick for the Cu-MoS2 coating, and 10-30 mu m thick for the Cu coating.