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 µm thick for the Cu–MoS₂ coating, and 10–30 µm thick for the Cu coating.