Lecture 7: Bearing Metals

The most important properties of a bearing metal are that it should be hard and wear-resistant, and have a low coefficient of friction. At the same time, however, it must be tough, shock-resistant, and sufficiently ductile to allow for running in processes made necessary by slight misalignments. Such a contrasting set of properties is almost impossible to obtain in single metallic phase. Thus, whilst pure metals and solid solutions are soft, tough and ductile, they invariably have a high coefficient of friction, and consequently a poor resistance to wear. Conversely, intermetallic compounds are hard and wear-resistant, but are also brittle, so that they have a negligible resistance to mechanical shock. For these reasons, bearing metals are generally compounded so as to give a suitable blend of phases, and generally contain small particles of a hard compound embedded in the tough, ductile background of a solid solution. During service, the latter tend to wear away slightly, thus providing channels through which lubricants can flow, whilst the particles of intermetallic compound are left standing proud , so that the load is carried with a minimum of frictional loss. cheap