Multi channel interferometry for measurement of thethickness of very thin EHL lubricant films

Abstract

The possibilities for studying thin film EHL using optical interferometry has improved considerably with the introduction of spacer layer techniques. Film thicknesses down to zero can now be measured using different interferometric evaluation techniques. One technique is based on image analysis and a two-dimensional film thickness map can be obtained for the whole contact between a ball and a transparent disc. The image analysis techniques that have been employed normally requires a calibration table which couples separation with the measured colour parameters. There are a number of disadvantages with such a calibration procedure and the proposed Multi channel method makes it possible to avoid calibration in the same time as the evaluation time is much reduced. This paper presents a first attempt to apply the Multi channel method on the measurement of very thin film and a comparison with a calibration method (the HSI colour calibration method). A spacer layer is used in order to obtain high resolution even for very thin films. It is seen from the results that both methods correspond well at thicker films (> 30 nm) while there are large differences for very thin films. The Multi channel results are promising and the method has a potential to be even further improved.