Theory of friction: Stress domains, relaxation, and creep

I discuss the microscopic nature of a block-substrate interface during sliding of an elastic block on a substrate with a lubrication film of molecular thickness (boundary lubrication). Arguments are given 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 ${\mathrm{\sigma }}_{\mathit{a}}$, necessary for fluidization of the pinned adsorbate structure. During sliding adsorbate domains will fluidize and refreeze. During the time period that an adsorbate domain remains in a fluidized state, the local elastic stresses built-up in the elastic bodies during ‘‘sticking’’ 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) is 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 occurrence of a sharp steplike cutoff at σ=${\mathrm{\sigma }}_{\mathit{a}}$ in the distribution P(σ) of surface stresses. Finally, I suggest a simple experiment to test directly the theoretical predictions: by registering the elastic wave pulses emitted from the sliding junction, e.g., by 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.