Stress Domains, Relaxation, and Creep
In this section we consider creep and other slow (thermally induced) relaxation processes which occur at low sliding velocity [11.1]. We first argue that the lubrication film at low sliding velocities has a granular structure, with pinned adsorbate domains accompanied by elastic stress domains in the block and substrate. At zero temperature, the stress domains form a “critical” state, with a continuous distribution P(σ) of local surface stresses σ extending to the critical stress σ a, necessary for fluidization of the pinned adsorbate structure. During sliding adsorbate domains will fluidize and re-freeze. During the time for which an adsorbate domain remains in a fluidized state, the local elastic stresses built up in the elastic bodies during “stick” will be released, partly by emission of elastic wave pulses (sound waves) and partly by shearing the lubrication fluid. The role of temperature-activated processes (relaxation and creep) will be studied and correlated with experimental observations. In particular, the model explains in a natural manner the logarithmic time dependence observed for various relaxation processes; this time dependence follows from the existence of a sharp step-like cut-off at σ = σ a in the distribution P(σ) of surface stresses. A simple experiment is suggested to test directly the theoretical predictions: By registering the elastic wave pulses emitted from the sliding junction, e.g., using a piezoelectric transducer attached to the elastic block, it should be possible to prove whether, during uniform sliding at low velocities, rapid fluidization and refreezing of adsorbate domains occur at the interface. Finally, we present a detailed discussion of plastic creep and relaxation in solids, which forms the basis for calculating how the area of real contact between two solids changes with the time of stationary contact. This topic is, of course, also of central importance in many engineering applications as is illustrated with a few examples.
KeywordsFriction Force Surface Stress Slip Event Thermal Excitation Lubrication Layer
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