Boundary Lubrication and Boundary Films

Abstract

Despite many years of research, boundary lubrication remains the least well understood regime in the Tribological pantheon. This is in part because it is a broad church, spanning a wide range of quite different phenomena which are, in practical terms, very difficult to disentangle. Also the processes involved in boundary lubrication are very localised, hidden within the impenetrable walls of a contact, far away from the gaze of the curious scientist. This review paper examines our current understanding of boundary lubrication and boundary films and, in particular, the mechanism and nature of friction-reducing, oiliness films. It shows how modern techniques for looking in-situ at boundary films, either directly within contacts or out of contact but whilst still covered with supernatant oil show great promise in advancing our understanding of such films. Up to the present, these techniques have tended to confirm that there are several different mechanisms by which oiliness films are formed and that each of these may have a role to play in boundary lubrication.