Polymer wear - Experimental study. Contact mechanics

Experience shows that, in order for two solid bodies to slide over each other, a force must be exerted in the direction of relative movement, known as the friction force. Moreover, observation of the bodies after a significant relative movement usually reveals that their volume has decreased in the contact zone: this is the phenomenon of wear. Wear and friction are therefore two closely related phenomena, studied by tribology, but their mutual relationships are far from being fully understood at present, and depend to a large extent - and often not very intuitively - on contact conditions (pressure, relative speed, etc.), the environment (temperature, humidity, etc.) and the materials involved: thus, changing one of the two materials may lead to a drop in friction, but an increase in wear. These two phenomena are very important in practical terms. In the case of a dry bearing, for example, friction determines the energy dissipated in the contact and therefore the energy lost in the bearing; wear determines the bearing's service life.

Polymers are increasingly used in applications involving their tribological properties: bearings, gears, brakes, seals, orthopedic prostheses, ski soles... In addition, a large number of products made of polymers or coated with thin polymer films need to offer good resistance to the formation of scratches produced by contact with "free" particles or objects: car bodies, spectacle lenses, household appliance bodies... The design of these parts therefore requires a good understanding of their tribological performance. Friction problems with polymers are dealt with in the article Friction of plastics ; moreover, article Rheological and thermal testing presents the various standardized tribological tests.

This study, which comprises two parts, is therefore mainly devoted to the scientific aspects of polymer wear, the discussion of friction aspects being carried out only when it proves necessary for a better understanding of the phenomena. In this first part, we present the essential experimental approach. Then, after a brief review of polymer contact mechanics, we discuss the relationships between the various mechanical properties (rheology, fracture properties) of polymers and their wear under various contact conditions.

The second part...