Friction- and time-dependent effects in contact closure

Contact closure is a time-dependent process and the establishment of equilibrium can take several hours. In general, with near-instantaneous single-step mechanical loading, it is characterised by an initial delay and by two time constants associated with slipping and with seizure at the surface, respectively. There is an optimum load, or narrow range of load, for least resistance. Larger (and smaller) loads result in increased resistance. The least resistance can, but need not, be associated with film-penetration and metal-metal contact. When a suitably sharp and unlubricated asperity or fragment is pushed into the flat surface of a semi-infinite mass of the same material, it is embedded. The shape of the embedded material is a cap of a sphere with an entry angle in the surface of approximately 45° for `soft¿ materials.