Tribo-pair dependence of friction and wear moisture sensitivity for a-C:Si:Al carbon-based coating

Providing a low, predictable and stable friction as well as low wear for mechanical components across a wide range from dry to wet atmosphere is of great importance in many engineering applications, the urgent challenge is to control tribological moisture sensitivity of their protective coatings. In this paper, the a-C:Si:Al carbon-based coating was fabricated successfully using multifunctional magnetron sputtering system. The microstructure and mechanical properties of as-fabricated coating were investigated; especially the friction and wear of coating sliding against Si3N4, SiO2, GCr15, brass and aluminum counterparts were evaluated under different relative humidity atmospheres. Results showed that the as-fabricated a-C:Si:Al carbon-based coating presented typical amorphous microstructure and good combined mechanical properties. Under low relative humidity conditions, the graphitized carbon tribo-layers on the contact surfaces were responsible for low friction; under high relative humidity conditions, the low shear strength colloidal silica on the contact surfaces induced by tribo-chemical reaction were mainly responsible for low friction. In particular, the lowest tribological moisture sensitivity can be achieved by the a-C:Si:Al/SiO2 pair, which was mainly attributed to the formation of tribo-layers on the contact surfaces induced by the graphitization and tribo-chemical reaction under different relative humidity conditions. These indicate that the a-C:Si:Al carbon-based coating might be a good candidate as low tribological moisture sensitivity material in engineering applications.