A multi-beam intensity-based approach for lubricant film measurements in non-conformal contacts

Abstract

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.