How Third-Body Processes Affect Friction and Wear

Materials designed for rolling or sliding contact, like corrosion-resistant materials, can provide great cost savings to industry. So why can't such “tribomaterials” be designed based on materials properties that control friction and wear? In recent years, it has become clear that the properties we need to understand are not only those of the starting materials—whether bulk solids or engineered surfaces—but also those of the materials generated within the sliding (or rolling) contact, the so-called “third-body” materials. This article reviews third-body processes and their role in controlling friction and wear of practical surface treatments. It may seem odd that properties like slipperiness and wear resistance can be easily described but are not scientifically understood. We can feel that polytetrafluoroethylene is slippery but know that it is not very wear-resistant because we can scratch it with a fork. We can sense low friction when we rub talcum powder between our fingers but know it cannot lubricate indefinitely because it gets ejected as we rub. Is it contradictory to ask for materials that are both slippery and wear-resistant? Slipperiness is associated with low shear strength whereas wear resistance is modeled on high hardness—that is, high shear strength. Before we can answer this seeming contradiction, it is useful to review some aspects of friction and wear. More than 50 years ago, Bowden and Tabor explained how a low shearstrength film can reduce the friction coefficient between two higher shearstrength materials in sliding contact.