The development and application of the spacer layer imaging method for measuring lubricant film thickness

This paper reviews the historical development of optical interferometry as applied to the study of lubricant films. The technique was first applied to lubricated contacts in the 1960s, when it played an important role in the validation of the elastohydrodynamic theory of lubrication. Initially the m ethod was not suited to the study of mixed and boundary lubrication because it could not measure film thicknesses of less than about 50 mn. In the 197 0s, however, this limitation was partially overcome by the use of a spacer layer and this, coupled in the early 1990s with spectrometric analysis of t he interfered light, enabled films down to just 1 nm thick to be measured i n lubricated contacts, well within the boundary lubrication regime. Recentl y a number of workers have applied colorimetric image analysis to optical i nterference images to enable accurate three-dimensional maps of film distri bution in lubricated contacts to be determined. This approach, coupled with the use of a spacer layer, has led to the spacer layer imaging method, whi ch can map film thickness in boundary and mixed lubricated contact. Some re cent applications of this technique are described.