Published: Jan 2008
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WHEN TWO SURFACES IN RELATIVE MOTION ARE completely separated by a liquid film that carries the applied load, a preferred state with low friction and without wear is achieved. This state of hydrodynamic or elasto-hydrodynamic lubrication (EHL), however, is not always the case in tribology practices. An increase in load, decrease in velocity or changes of surface roughness can lead to disappearance or discontinuity of hydrodynamic films and transition of the lubrication condition. Boundary lubrication refers to a lubrication regime where hydrodynamic lubrication is no longer effective, and the physics and chemistry of the interfacial substance play a dominant role in protecting the surfaces from direct contact . The transition to boundary lubrication is a progressive process that goes across a region known as mixed lubrication where hydrodynamic films gradually disappear as the film thickness decreases continuously and the “asperity contacts” carry an increasing portion of the applied loads. In this sense, boundary lubrication can be regarded as a lower limit of the mixed lubrication region. The Stribeck curve shown in Fig. 1 provides an illustrative description for the transition from hydrodynamic lubrication to mixed and boundary lubrication. Due to the absence of a hydrodynamic effect, boundary film thickness is expected to be independent of speed of surface movement, as can be observed in the left part of the Stribeck curve. This is a significant criterion that distinguishes boundary lubrication from EHL and mixed lubrication, and provides an opportunity for measuring boundary film thickness using an interferometer, for example. More details will be presented in Section 2. Boundary lubrication is an extremely complex process in which numerous mechanisms, including rheology transition, adsorption, tribochemical reactions, selective transfer, etc., simultaneously participate in the play. In a sense, the term of boundary lubrication may take different meanings to different investigators. Physicists may regard boundary lubrication as a process where adsorbed monolayer plays a primary role in reducing friction; rheology scientists are investigating rheological transitions in confined lubricant films and their response to shearing, and chemists are more interested in tribochemical processes and formation of reaction layers that protect surfaces under severe conditions of wear. The essential function of boundary lubrication, however, is to reduce or eliminate immediate contacts between two solids, via adsorbed or reactive surface layers.
Tsinghua University, Beijing,