Microstructure and tribological property of HVOF-sprayed adaptive NiMoAl–Cr3C2–Ag composite coating from 20 °C to 800 °C

Adaptive NiMoAl–Cr3C2–Ag composite coating was deposited by high velocity oxy-fuel (HVOF) spraying and its tribological properties from 20 °C to 800 °C under unlubricated conditions were evaluated. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy were used to characterize the coating and corresponding wear tracks to determine the lubrication mechanisms. The results showed that HVOF-sprayed composite coating exhibited an extremely dense structure, high bonding strength, and excellent friction reduction and wear resistance. When the applied load is 15 N, friction coefficients of the composite coating were below 0.3 and wear rates maintained at the order of 10− 6 mm3/N·m except 400 °C. Post-wear testing structural and chemical characterization revealed that silver with low shear strength provided lubrication below 400 °C. Subsequently, the composite coating suffered serious adhesion wear at 400 °C, nevertheless, the adhesive degree could be greatly alleviated at a lower applied load. At higher temperatures, silver molybdate that was formed through tribo-chemistry reactions acted as high temperature lubricants.