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A study has been made of the lubricant film-forming properties of viscosity index improver-containing oils in non-steady state, high-pressure contacts. Two types of non-steady speed condition have been investigated, sudden halting of motion and cyclically-varying entrainment speed. Film thickness has been measured in a ball on flat contact using ultra-thin film interferometry. It has been shown that viscosity index improver polymers in solution exhibit an enhanced squeeze behavior during halting and a viscoelastic response to acceleration/deceleration.

Recent work by the authors has indicated that some types of viscosity index improver polymers can form thick boundary films in lubricated contacts. These films appear to result from the adsorption of molecules of polymer on metal surfaces to produce layers, about 20 nm thick, having higher polymer concentration and thus higher viscosity than the bulk solution. In the current paper it is shown that these VII boundary films are able to separate rubbing surfaces in both rolling and sliding contacts and that they make a significant contribution towards reducing friction and wear at temperatures up to at least 140°C. The mechanism by which these polymers reduce friction and wear is elucidated.