FRICTION CHARACTERISTICS AND TOPOGRAPHY OF TRIBOFILMS FROM ANTI-WEAR ADDITIVES APPLIED TO METAL V-BELT TYPE CVT FLUIDS

摘要

This study has investigated the metal-meal friction properties and the topography of tribochemical films derived from anti-wear 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 B-CVTF. 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.