Wear debris -Related indices in tribology

Tribology, the "crossroads science", has been approached from three different angles over the last fifty years. The first approach, which focuses on volume tribology, has led to the determination of friction and wear behavior laws. It is based above all 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 macroscopic. The second approach focuses on the tribology of surfaces, and more specifically on the study of materials. Thanks to the development of ever finer surface analysis tools and their increasingly widespread use (ESCA, XPS, Auger, SIMS, SDL, AFM...), this has led to the physico-chemistry of surfaces and thus to an understanding of tribological behavior on a microscopic or even nanoscopic scale. The third approach focuses on the tribology of interfaces, leading to a complementarity between mechanics and materials, i.e. a "mix" of the first two approaches. It also takes into account the notion of static and dynamic interlayers. The three basic conceptual tools of this latter approach give a decisive role to interfacial elements (third bodies) bordering the two masses (first bodies). These three conceptual tools are :

• the tribological triplet ;

• speed accommodation mechanisms ;

• the tribological circuit.

Interface tribology cannot predict phenomena, but it does provide a conceptual framework, a logical construct in which they can be listed and classified. It is a framework theory, based on experimental data, which provides general principles and opens the door to other, more specific theories, with scope for innovation.

It should also be noted that, whatever the type of wear, when surfaces are in contact and in relative motion, third-body wear debris can be generated more or less immediately. This debris is either ejected from the contact or trapped at the interface of the two opposing parts, potentially causing further damage to the surfaces. The morphology of wear debris is directly related to the destruction of the interacting surfaces, and is therefore indicative of wear processes and their severity. Since the 1970s, image analysis systems have been used to qualify debris. This article lists a number of examples of how these clues can be taken into account to identify "tribological damage" scenarios. It describes the concepts of third body, tribological circuit and the various descriptors often used to characterize wear particles. The analytical ferrography technique applied to wear debris encountered in joint prostheses will illustrate this particular field of tribology.