Tribological performance of ammonium thiomolybdate as water-soluble lubricant additive for steel-steel contacts

The friction and wear reducing properties of ammonium thiomolybdate as a water-soluble lubricant additive were evaluated using a pin-on-disck tribometer on a steel-steel contact. The tribological performance of aqueous solutions of thiomolybdate prepared at concentrations between 0.1 and 0.3 wt% Mo were evaluated at the same load (10 N), entrainment speed, sliding distance and temperature. Although there is a reduction of friction and wear compared with pure water, a significant difference is observed, depending on the additive concentration. SEM/EDAX and Raman spectroscopy analyses of the wear tracks of specimens tested at concentrations below 0.3 wt% Mo reveal the formation of FeS which is responsible for gradually reducing the coefficient of friction from values between 0.30 and 0.5 down to ~0.11. At 0.3 wt% Mo, the analyses indicated the formation of a MoS₂ which keeps the coefficient of friction in ~0.05, the lowest recorded value.