Synergistic effect of diisopropyl phosphite and triphenyl thiophosphate additives on tribological properties of T8 steel/Al2O3 ceramics

The effect of friction-reducing and antiwear of lubricant additives on T8 steel/Al2O3 ceramics was investigated using a pin-on-disk tribometer at a room temperature. Diisopropyl phosphite (T451) and triphenyl thiophosphate (T309 or TPPT) with various mass concentrations were blended in 500SN base oil to produce the lubricants, and their friction-reducing and antiwear were examined under the given applied normal loads of 220 N (mainly for assessment of friction-reducing) and 400 N (for antiwear), respectively. The results showed that base oil containing T451 and T309 can be more essential for improving the friction-reducing and antiwear behaviors, rather than those under the pure base oil, or the single additive. In detail, at a load of 220 N, the synergistic effect of friction-reducing increased with addition of T451 and T309 within a suitable amount. At a load of 400 N, the synergistic effect of antiwear prominently related with the content of T451, and the appropriate composition was 2%T451 + 1%T309. SEM analysis of worn surfaces and AES spectra of typical elements indicated that the synergistic friction-reducing and antiwear effect is owing to the thick sulfur-rich and phosphorus-rich protective film formed on the worn surface T8 steel pin. The wear mechanisms were characterized by fracture and scuffing under the lubricants.