Synergistic Effect of MoS2 and Ag on High-Temperature Tribology of Self-Lubricating Ni-Based Composite Coatings by Cold Spray Deposition

As technology advances, tribo-coupling components, viz. gears, seals, bearings etc., frequently function under severe conditions, and this enhanced the demand of effective lubrication for reduced friction and wear of contacting interfaces. The optimal content of solid lubricants in the matrix is a viable alternative to acquire the desired lubrication in extended regime of temperatures. The present work investigates the tribological characteristics of cold sprayed Ni-based self-lubricating composite coatings with varying content of Ag (5, 10, 12.5 and 15 wt.%) and fixed concentration of MoS₂ (10 wt.%). The mechanical, microstructural and the tribological properties were evaluated from room temperature (RT) to 800 °C. The results revealed that the coating having 12.5 wt.% of Ag along with 10 wt.% of MoS₂ has delivered superior lubricity among all compositions in terms of lower coefficient of friction COF (0.18) and wear rate 5 × 10^-5 mm³/Nm at 800 °C. The average COF, without the use of Ag and MoS₂, has attained (0.49) at 800 °C. However, the coating containing 12.5 wt.% of Ag has attained increased COF (0.38) and wear rate 7.1 × 10^-5 mm³/Nm at 400 °C; thereafter, a declining trend was observed. The improved frictional properties were accredited to the synergistic effects of impregnated solid lubricants and novel lubricious phases (Ag₂MoO₄, Ag₂Mo₂O₇, Ag₂Mo₄O₁₃ NiO, etc.) formed on the worn surface. The observed wear mechanisms were correlated with the morphologies and tribo-chemical reaction between the contacting interfaces.