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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.

Fuel economy has become the dominant criterion in the design of new automobiles. The globally enacted targets for fleet average emissions pose true challenges to automobile manufacturers. Increasing fuel economy requires enhancements both in hardware as well as in lubricant performance. As a key component of the lubricant, poly(alkyl methacrylate) PAMA viscosity index improvers have been identified as crucial design element due to their multiple modes of action. In their original application, they serve the well-known mechanism of polymer coil expansion at high temperatures and collapse at low temperatures. They help to flatten the viscosity/temperature relationship of the lubricant and allow for reduced low temperature viscosities and reduced internal friction, which directly translates into fuel economy. In addition to this bulk application, interfacial tribological phenomena contribute significantly to efficiency and fuel economy.

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.