The tin atoms distort the copper matrix enough to help inhibit slip of crystal layers and the intercrystalline phase contributes to hardness. The tin bronzes do develop a low friction deposit of bronze on the mating steel parts. For this reason, tin bronze is the alloy of choice for nonferrous gears. Asperities from the aluminum and manganese bronzes, deposited on the steel, are not as beneficial as with tin bronze. The aluminum bronze asperities eventually develop aluminum oxide content, which is very abrasive.
Zinc oxide is abrasive and zinc in the copper matrix promotes galling when mated with steel.
Tin has a much lower affinity for oxygen than either aluminum or zinc. When a sufficient amount of the bronze asperities become embedded on the surface, wear decreases and friction forces decrease. The basic copper - tin alloy, used for gears, has developed into a series of five popular gear bronzes. 427-93A. Variations are not large. This allows for good machinability and the lead particles will embed any abrasive media during the wearing in period.