Passivation of free carbon bonds in the a-C diamond-like coatings by chemical bonds with sulfur and hydrogen atoms can have a significant impact on the tribological properties of this type of coatings. The possibilities of controlled doping of the a-C : H : S diamond-like coatings with S and H elements are investigated using the method of reactional pulsed laser deposition from a carbon target in hydrogen sulfide. It is found that increasing the H2S pressure from 2 to 20 Pa makes it possible to increase the S concentration from 10 to 50 at %. At the same time, the H concentration changes to a lesser extent (from 10 to 18 at %). A decrease in the coating growth rate with increasing H2S pressure indicates the interaction of carbon laser plasma with gas molecules causing both the activation of coating doping processes and the formation of volatile hydrocarbons. The study of coatings using the Raman spectroscopy shows a significant influence of S atoms on the local packing of C atoms in the coatings. This effect also has a significant impact on the tribological properties of the a-C : H : S coatings deposited on steel substrates. In tribotests using the reciprocating sliding method of a steel ball, the best properties are demonstrated by coatings containing ∼21 at % of S and ∼14 at % of H. In humid air, the friction coefficient for these coatings is ∼0.17, but tribo-induced graphitization of the coating causes relatively rapid wear in the friction pair. In an inert gas (argon), the friction coefficient is reduced to a very low value of 0.02 with slight wear of the coating and counterbody. Under these conditions, graphitization of the a-C : H : S coating proceeds only in a thin surface layer (tribofilm), and no noticeable change in the structural state of the coating itself is detected in the wear track. Further increase in sulfur concentration has a negative impact on the tribological properties of the a-C : H : S coatings.
Grigoriev et al. (Wed,) studied this question.