Friction characteristics between metal contacting surfaces from antiwear additives with application to metal V-belt type continuously variable transmission lubricants

Abstract

This study has focused on the application of lubricating oil additives in metal V-belt pushing type continuously variable transmission fluids (B-CVTFs) achieve higher friction coefficient and more stable lubricant film formation between the metal-metal contacting surfaces. The metal-metal tribological characteristics of various kinds of antiwear additives and a detergent were experimentally investigated using a ball on plate reciprocating tribometer (Plint TE77), which enables lubricant film formation to be monitored during a rubbing test. Zinc-dithio phosphates (ZnDTPs), a significant difference in instantaneous film formation was observed between primary and secondary type ZnDTP at the moment of increasing step load, due to the variation of the antiwear activity of different ZnDTPs. ZnDTPs in the presence of over-based calcium sulphonate detergent demonstrated slightly less tribofilm formationcompared with ZnDTP alone due to the competition for the rubbing surface. Very interestingly, in the case of phosphonic ester with calcium sulphonate, synergism was observed with more stable film formation and 8 per cent higher friction coefficient than without calcium sulphonate, a positive outcome for a B-CVTF. From surface analysis, it is hypothesized that the tribofilms species composed of iron phosphate, calcium phosphate, and calcium carbonate were generated by the reaction between phosphonic ester and calcium sulphonate, and that the resulting tribofilm plays a dual role in preventingwear and inhibiting the entrainment of lubricant into the interface, resulting in higher friction.