Wear Debris. Related Indices in Tribology

Tribology, the "crossroads science," has been approached from three different angles over the past sixty years. The first approach, which focuses in particular on volume tribology, has enabled the determination of laws governing friction and wear behavior. It is based primarily on mechanical concepts, and materials are reduced to a few fundamental intrinsic properties (Young's modulus, Poisson's ratio, hardness, etc.). The scale considered is therefore macroscopic. The second approach focuses on surface tribology and is more specifically aimed at studying materials. Thanks to the development of increasingly sophisticated surface analysis techniques and their increasingly widespread use (ESCA, XPS, Auger, SIMS, SDL, AFM, etc.), this has led to the introduction of surface physicochemistry and thus to an understanding of tribological behavior at the microscopic and even nanoscopic scale. The third approach focuses on interface tribology and involves a mechanical/materials complementarity, i.e., a combination of the first two approaches. It thus takes into account the concept of static and dynamic intercalary screens. The three basic conceptual tools of this approach give a decisive role to interfacial elements (third body) bordering the two solids (first bodies). These three basic concepts are:

• the tribological triplet;

• speed accommodation mechanisms;

• the tribological circuit.

Interface tribology does not enable the prediction of phenomena, but provides a conceptual framework, i.e. a logical construct in which they can be listed and classified. It is a theoretical framework based on experimental data that provides general principles and opens the door to other more specific theories, with opportunities for innovation.

It should also be noted that regardless of the type of wear, when surfaces are in contact and in relative motion, wear debris, i.e., third bodies, can be generated more or less immediately. These are either ejected from the contact or trapped at the interface between the two opposing parts, which can cause even more damage to the surfaces. The morphology of wear debris is directly related to the destruction of interacting surfaces and is therefore indicative of wear processes and their severity. Since the 1970s, image analysis systems have been used to characterize debris. This article lists a number of examples showing how these clues can be used to trace back to "tribological damage" scenarios. It describes the concepts of third bodies and tribological circuits, as well as the various descriptors often used to characterize wear particles. The technique of analytical ferrography applied to wear debris found in joint prostheses...