Tin strengthens copper and hardens it sufficiently to be compatible with unhardened steel. The protective Organo-tin films formed with oils have a high desorption temperature. Hence the copper-tin bronzes result in better bearings than the copper-zinc brasses. Copper alloys usually have a face-centered-cubic structure while the mating surface is usually a ferrous alloy with a body-centered-cubic structure. This difference contributes to the anti-seizure characteristics of copper alloys against iron alloys.