Boundary Lubrication

Abstract

It is very hard experimentally to directly probe the nature of the rapid processes which occur at a sliding interface. But some information can be inferred indirectly by performing sliding friction measurements on well-defined systems, and registering the macroscopic (e.g., center-of-mass) motion of the block as a function of time. For lubricated surfaces, such measurements have been performed during the past few years, using the Surface Forces Apparatus (Chap. 3) [9.1–5]. These studies usually use mica surfaces which can be produced atomically smooth (without a single step) over macroscopic areas. A spring is connected to the mica block and the “free” end of the spring is moved with some velocity v _s , which typically is kept constant but sometimes is allowed to change in time. The force in the spring is registered as a function of time and is the basic quantity measured in most of these friction studies. It is obvious that the time dependence of the spring force (and its dependence on v _s ) contains information about the nature of the processes occurring at the sliding interface, but this information is very indirect.