Enhancing the Tribological Performance of MoS\(_2\) Coatings in Humid Environments with the Addition of Bi\(_2\)S\(_3\)

The present work investigates the friction and wear properties of composite Bi\(_2\)S\(_3\)–MoS\(_2\) soft coatings under different humidity levels. To achieve this goal, suspensions containing different proportions of Bi\(_2\)S\(_3\) and MoS\(_2\) were sprayed onto polished SAE 1010 steel substrates to generate soft lubricating coatings. The coatings were subjected to reciprocating sliding tribological tests in air at different humidity levels, between 20 and 80%. Additionally, the formation of tribofilms in dry conditions was evaluated by means of reciprocating tests with increasing total sliding distances. The resulting wear surfaces were analyzed using SEM-EDS, Raman microspectroscopy, and laser confocal microscopy. The frictional behavior of the composite coatings was more stable than that of neat ones. The addition of Bi\(_2\)S\(_3\) to MoS\(_2\) greatly enhanced the durability of the coatings at all the tested humidity levels, reaching up to a 760% increase in their durabilities. This effect depended on the humidity level and the ratio between Bi\(_2\)S\(_3\) and MoS\(_2\).