Fluid Film Interferometry in Lubrication Studies

Abstract

INTERFEROMETRY has been used in both surface chemical investigations¹ and lubrication studies² to measure the thickness of thin hydrocarbon films. Until now experiments have suffered from the disadvantage of being restricted to films thicker than a quarter of a wavelength and this has meant that films of less than about 900 Å thick could not be investigated. Fig. 1A shows the general arrangement of the apparatus used by Gohar and Cameron² in their studies of the elastohydrodynamic lubrication of a hard steel ball. Partial reflexion occurred at the diamond (or high refractive index glass) to oil interface because of the difference in refractive indices. Better fringe definition can be obtained by the use of optical crown glass (µ≃1.5) which has been vacuum coated³ with a quarter wavelength film of titanium dioxide, as is shown in Fig. 1B. The interference pattern gives a direct measure of the lubricant film contours under diverse conditions.