Surface-dependent tribochemistry of cyclopropanecarboxylic acid enabling ultralow friction on TiN coating and steel

The addition of CPCa to PAO4 oil significantly reduced friction and wear in TiN/steel tribopair. Under 0.9 GPa and 120 mm/s, PAO4 containing 2.5 wt% CPCa achieved an ultralow COF (∼0.008) and wear rate (7.0 × 10 −8  mm 3 /N·m). Although CPCa improved the tribological properties of steel/steel tribopair relative to pure PAO4, it still yielded COF and wear rate values that were one and two orders of magnitude higher, respectively, than those of the TiN/steel tribopair. To elucidate this disparity, we conducted a systematic comparison of both tribopairs under varied conditions. Advanced surface analysis (XPS, Raman, FTIR) revealed the formation of carbon-based tribofilms exhibiting exceptional lubricating characteristics, particularly on TiN surfaces. The results indicate that the tribochemical transformation of CPCa is governed by stress-induced ring opening. The kinetics of carbon-based tribofilm formation show a pronounced dependence on the counterface, with the TiN surface facilitating faster film formation and sustaining ultralow friction.