The borderline of elastohydrodynamic and boundary lubrication

There is a growing trend for lubricated systems to operate for much of their operating life with very thin lubricating films. This paper reviews our current understanding of such films, at the borderline between elastohydrodynamic and boundary lubrication. The nature and properties of these films are very complex, since the proximity of the solid surfaces influences the structure and rheology of thin liquid layers, while boundary films can, themselves, possess rheological characteristics that vary with thickness. Novel experimental tools, such as atomic force microscopy and ultra-thin-film interferometry have greatly accelerated our understanding of this area in the last few years and it has recently become possible to map lubricant film thickness within rough surface contacts. These tools are beginning to provide the level of understanding of thin-film rough surface behaviour required to develop accurate numerical and simulation models. The next 5 years should see a very rapid progression of our understanding of this important regime.