ABSTRACT
This chapter presents the hierarchy of interactions between two surfaces will start with dry contacts to deliberately applied coatings or solid lubricants, to those surfaces that operate on a combination of fluid film and asperity interaction and ending with the process of elastohydrodynamic lubrication (EHL). Just as boundary and mixed lubrication constitute wings abutting the solid lubricants and the EHL regimes, so, too, we have partial as well as full EHL modes, which can be interposed between the mixed and hydrodynamic lubrication regimes. In a most general way, one can say that the dynamics of an EHL film result from a superposition of hydrodynamic and Hertzian stresses. The extreme edge of mixed lubrication wherein fluid film covers only a small area can be said to coincide with the partial EHL mode, where there is only a small subregion of asperity interaction.
