Synergistic effect of Ag and MoS2 on high-temperature tribology of self-lubricating NiCrBSi composite coatings by laser metal deposition

Abstract

To increase the performance of the tool materials operating at high temperatures (HT), the protection of the surface from wear is usually needed. Conventional lubricants pose problems due to their degradation and harmful effects on human health; therefore, self-lubricating materials are of great importance. In the current work, self-lubricating NiCrSiB millimeter-thick composite coatings with the addition of solid lubricants Ag and MoS2 were fabricated on stainless steel substrates using laser metal deposition. Upon microstructural analysis, it was revealed that the addition of MoS2 resulted in a uniform distribution of Ag throughout the coating thickness owing to a unique phenomenon of silver encapsulation. Further, the coatings were subjected to sliding wear tests against an alumina counterbody at a load of 5 N, a speed of 0.1 m s−1, and a distance of 500 m under a unidirectional condition in a ball-on-plate configuration. The tests were performed from 20 to 800 °C. The results show that self-lubricating composite coatings demonstrate an exceptional performance up to 800 °C. Significant friction (best at 800 °C ⁓0.25) and wear (best at 600 °C ⁓4.3 times less) reduction for Ag + MoS2 added coating is seen. It was revealed that the presence of CrxSy and Ag on the wear track delivered lubrication at temperatures ≤400 °C while, a glazed tribolayer rich in silver molybdate phase (along with CrxSy, MoO3) was an effective lubricant at 600 and 800 °C. The results are reported in comparison to the unmodified Ni-based alloy.