Friction Coefficient, Lubricated Steel on Steel

A film of ordinary mineral oil cuts steel-on-steel friction by roughly a factor of six, to 0.05–0.15. That range is boundary and mixed lubrication — the regime where asperities still touch through a molecular film — which is what you get at start-up, at low speed, and under high load. It is the value behind the standard bolt-torque relation T = K d F with a nut factor K near 0.20 for lightly oiled threads against 0.30 dry, which is why lubricating a fastener and applying the dry torque overloads it by about 50 %.

Full hydrodynamic lubrication is a different regime entirely: once a converging film separates the surfaces completely, the coefficient falls to 0.001–0.01 and depends on viscosity and speed rather than on the materials. The Stribeck curve maps the transition, and the minimum friction sits right at the boundary between them — which is also where a bearing is most vulnerable, because any drop in speed or rise in load pushes it back into metal-to-metal contact. Extreme-pressure additives, phosphate coatings and solid lubricants exist to make the boundary regime survivable.