Boundary Film Formation by Lubricant Base Fluids

Abstract

The measurement of lubricant film thicknesses in a rolling steel ball on glass flat contact, down to one nanometer is now possible by using ultrathin film interferometry. This technique has been used to study the film-forming properties of a range of synthetic base fluids in the mixed elastohydrodynamic and boundary lubrication regimes. For a very highly purified sample of hexadecane, it was found that classical EHD theory was obeyed down to less than one nanometer, indicating that any boundary film formed by this fluid was less than I nm thick. Most other synthetic fluids tested gave thicker films than predicted from EHD theory under very thin film conditions. This deviation from theory occurred at between 1 and 10 nm for different fluids. The effect can be interpreted as resulting from the presence of boundary layers, one or two molecular layers thick of the fluid on each solid surface, which are more viscous than the bulk lubricant.