PERFORMANCE OF VISCOSITY INDEX IMPROVERS IN LUBRICATED CONTACTS

The boundary film forming properties of polymeric viscosity index impr overs are examined in an elastohydrodynamic (EHD) contact. Ultrathin i nterferometry is used, a recent development of conventional optical. i nterferometry, which allows the film thickness generated in a point co ntact to be measured down to a value of 1 nm. The ability of polymers to adsorb on the solid/liquid interface is shown to depend strongly on the polymer type, those without a polar group showing little or no bo undary film forming properties, while those with a polar group are abl e to generate a boundary film of approximately 20 nm. These boundary f ilms appear to be viscous sublayers with a viscosity up to 30 times hi gher than that of the solvent base stock, At very slow speeds, this en hanced viscosity layer generates an EHD type film many times that of t he bulk fluid. In the higher temperature ranges studied, this adsorbed layer is observed to eventually fall off to the lowest measurable val ue of 1 nm.