Friction Characteristics and Topography of Tribofilms From Anti-Wear Additives Applied to Metal V-Belt Type CVT Fluids

This study has investigated the metal-meal friction properties and the topography of tribochemical films derived from antiwear additives for application to metal-V-belt pushing type continuously variable transmission fluids (B-CVTFs). The influence of metal-metal tribological properties of various kinds of anti-wear additives were evaluated using a ball on plate tribometer, enabling lubricant film formation to be monitored during a reciprocating sliding test. In addition, contact mode atomic force microscope (AFM) was used to characterize the nature of the tribofilms at the namometre scale. For zinc-dithiophosphate (ZnDTPs), significant differences were observed in the friction coefficients and the topographic images between secondary and aryl type ZnDTP. The films derived from secondary type exhibit large solid pads, 5–30μm in length elongated in the sliding direction while the aryl type forms films in streaks on the ridges, with 8% higher friction coefficient than the secondary ZnDTP. Notably, the combination of over-based calcium sulphonate and hydrogen phosphite demonstrated a synergism with 8% higher friction coefficient and more stable film formation than the individual cases, providing a positive outcome for a BCVTF. Tribofilm species produced by a chemical reaction between hydrogen phosphite and over-based calcium sulphonate were densely deposited on the rubbing tracks, exhibiting rougher surfaces than those observed in the individual cases, consequently with a higher friction coefficient. These results suggest that the friction properties between metal-metal contacting surfaces strongly depend on the morphology of tribofilms derived from lubricant additives.