Theory 12/Tribology & Wear /Surface science/3 min
Running-In Is a Critical Transition
New parts are said to wear in. They may be finding a fixed point.
Running-in is a self-organised critical process in which surfaces converge on a characteristic roughness rather than simply smoothing, so the steady state should be set by load and material rather than by initial finish, and parts should converge to it from either direction.
01The evidence
- The running-in behaviour of metallic tribo-components has been reviewed as a distinct phase with its own mechanisms, not merely as the early part of steady wear.Wear
02The tension
Running-in is described as wear until the surfaces fit together, which predicts that the final state depends on the initial one: a rougher surface should end rougher. In practice, surfaces run in from very different starting points and arrive at similar topographies.
03The theory
A contact under load redistributes stress until the real contact area can carry it. The process creates and destroys asperities in a feedback loop: high spots wear faster, which loads their neighbours, which wear next.
That feedback produces a self-organised state in which the topography is set by the load and the material rather than by the starting finish. The state is an attractor, and both rough and smooth surfaces converge to it.
The prediction is that final roughness should be independent of initial roughness within a range, and that the running-in duration should depend on the distance from the attractor rather than on the initial roughness itself.
The practical consequence is that specifying an extremely fine initial finish may be wasted, because the surface will roughen to the attractor, and that running-in protocols should target the attractor rather than the initial finish.
04What this is not
Those models predict that the final state inherits the initial one. This predicts convergence to an attractor set by load and material, which is why different starting finishes end similarly.
05Predictions
- P1Final roughness should be independent of initial roughness within a range.
- P2Running-in duration should scale with distance from the attractor.
- P3Smoother-than-attractor surfaces should roughen during running-in.
06What would refute it
Final topography inheriting initial finish across a range would leave wear-until-fit adequate.
07What it would change
It would change finish specifications and running-in protocols.
08Evidence base
- On the running-in nature of metallic tribo-components: a review — Wearhttps://www.sciencedirect.com/science/article/abs/pii/S004316482100260X