Tribochemical Formation of Sulphide Tribofilms from a Ti–C–S Coating Sliding Against Different Counter Surfaces

Tribochemically active Ti–C–S coatings are nanocomposite coatings containing a S-doped titanium carbide, from which S can be released in a tribological contact. This work studies tribochemical reactions between a Ti–C–S coating and various counter surface materials, and their effect on the tribological performance. Tribological tests were performed in a ball-on-disc set-up, using balls of five different materials as sliding partners for the coating: 100Cr6 steel, pure W, WC, 316-L steel and Al₂O₃. For W balls, a WS₂ tribofilm was formed, leading to low friction (down to µ = 0.06). Furthermore, increasing normal load on the W balls was found to lead to a strong decrease in µ and earlier formation of the low-friction WS₂ tribofilm. Similar WS₂ and MoS₂ tribofilms were, however, not formed from WC- and Mo-containing 316-L balls. The performance when using WC and Al₂O₃ balls was significantly worse than for the two steel balls. It is suggested that this is due to sulphide formation from Fe, analogous to formation of anti-seizure tribofilms from S-containing extreme pressure additives and steel surfaces. The tribochemical activity of Ti–C–S coatings, with the possibility of S release, is thus beneficial not only for pure W counter surfaces, but also for Fe-based sliding partners.