On the nature of running-in

The terms run-in, break-in, and wear-in are related but not identical. All of them concern special cases within the general subject of tribological transitions. Tribological transitions can be induced by imposed changes in operating conditions or they may occur naturally as the system ages, without external intervention. Transitions can occur as an inherent consequence of design, as in the case of a piston ring and cylinder system. The attributes of frictional running-in include the overall trend in friction force with time, the duration of characteristic features in the friction/time curve, and the instantaneous level of frictional fluctuations superimposed on the general trend. Changes in friction and wear that occur during running-in are more than a consequence of surface roughness alterations alone. Depending on the tribosystem, they can also include changes in surface composition, microstructure, and third-body distribution. Examples of how factors like contact alignment and surface pre-conditioning affect the form of friction/time curves are given. Friction and wear relationships during running-in are discussed, as are scale effects whose relative influence also changes with time. Initial running-in behavior can be subject to the influence of nanometer-sized films and progress to the scale of micrometer-sized asperities and larger-sized surface structures as sliding proceeds. Piecewise models for friction during running-in should include the effects of wear since wear affects the surface topography as well as the formation of transfer films, mechanically mixed layers, and third-body agglomerates.