The Control of Friction by Molecular Fractionation of Base Fluid Mixtures at Metal Surfaces

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Abstract
The elastohydrodynamic film forming and friction properties of ester/hydrocarbon lubricant base fluid mixtures have been measured down to the very thin film regime. The film thickness results show that the near-surface viscosity, and thus elastohydrodynamic (EHD) film formation in the thin film region, is dominated by the Theological properties of the ester even when this has only low bulk concentration. This suggests that the ester in a blend, being the more polar component, concentrates close to the rubbing surfaces. The friction measurements show that this fractionation of ester close to the surface has a direct influence on the friction coefficient in the mixed lubrication regime. When a lubricant is blended from a higher viscosity ester and a lower viscosity hydrocarbon, the resultant enhancement of viscosity close to the surfaces, and thus of the EHD film thickness, produces a transition from boundary to EHD lubrication at lower speeds than would otherwise be the case. The converse is seen when a lower viscosity ester is blended in a higher viscosity hydrocarbon. Mixing appears to have little effect on the thick film EHD traction properties. The findings have implications for the design of energy-efficient liquid lubricants.

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