Studies of elastohydrodynamic lubrication (EHL) frequently utilize the phenomenon of optical interference which is exhibited when an illuminated steel ball is in contact with a partially Cr-coated glass disc in conjunction with a lubricant layer of known refractive index. The use of the steel and chromium surfaces enables lubricants having similar refractive indices to glass to be examined. The EHL contact is composed of two microscopically thin layers: the Cr coating and the lubricant film, bounded by the glass disc and the steel ball. That the two-beam interference method frequently used in optical EHL research may induce significant errors in the measurement of film thickness is reported in this paper. A theoretical analysis which includes consideration of the effects of both multi-beam interference and optical absorption within the Cr film and the steel surface is presented. The analysis is used to devise a multi-beam intensity-based test technique which has been validated for a wide range of lubricant film thickness measurements at a resolution of 1 nm and offers a practical minimum detectable film thickness as thin as 1 nm.

The apparent viscosities of liquid lubricants at high pressures measured using an impact viscometer have been reported in previous research to be time-dependent. In the work reported here, the impact microviscometer technique has been further developed to give measurements of true shear stress and true shear rate, allowing studies of the rheological behavior of liquid without any initial assumptions about flow properties. The temporal variation of viscosity is shown to be a shear-thinning effect. Under high pressures, the lubricants examined exhibited non-Newtonian behavior and a limiting shear stress. Their characteristics may be described by a shear-thinning power law model. A new viscosity-pressure relationship has been developed, which takes the form of a Barus Equation at low pressures and reaches a limiting viscosity at high pressures.