WEAR

the removal of material from a solid surface as a result of mechanical action exerted by another solid. Wear chiefly occurs as a progressive loss of material resulting from the mechanical interaction of two sliding surfaces under load. Wear is such a universal phenomenon that rarely do two solid bodies slide over each other or even touch each other without a measurable material transfer or material loss. Thus, coins become worn as a result of continued contact with fabrics and human fingers; pencils become worn after sliding over paper; and rails become worn as a result of the continued rolling of train wheels over them. Only living things (such as bone joints) are in some sense immune to the permanent damage caused by wear, since they have the property of regrowth and healing. There are four basic types of wear: adhesive, abrasive, corrosive, and surface-fatigue. The most common type, adhesive wear, arises from the strong adhesive forces that are generated at the interface of two solid materials. When solid surfaces are pressed together, intimate contact is made over a number of small patches or junctions. During sliding, these junctions continue to be made and broken, and, if a junction does not break along the original interface, a wear particle is formed. These particles eventually break away. Adhesive wear is undesirable for two reasons: first, the loss of material will eventually lead to a deterioration in the performance of the mechanism; and second, the formation of large wear particles in closely fitted sliding members may cause the mechanism to seize at an early stage in its productive life. Adhesive wear is many times greater for unlubricated than for effectively lubricated metal surfaces. Abrasive wear occurs when a hard, rough surface slides over a softer one, producing grooves on the latter. It also can be caused by loose, abrasive particles rolling between two soft sliding surfaces or by particles embedded in one of the opposing surfaces. Abrasive fragments borne by a stream of liquid or gas may wear down a surface if they strike the surface at high speeds. Since abrasive wear takes place when the abrading material is rough and harder than the surface to be abraded, it can be prevented either by eliminating the hard, rough constituent or by making the surface to be protected harder still. Corrosive wear occurs whenever a gas or liquid chemically attacks a surface left exposed by the sliding process. Normally, when a surface corrodes, the products of corrosion (such as patina) tend to stay on the surface, thus slowing down further corrosion. But, if continuous sliding takes place, the sliding action removes the surface deposits that would otherwise protect against further corrosion, which thus takes place more rapidly. A surface that has experienced corrosive wear generally has a matte, relatively smooth appearance. Surface-fatigue wear is produced by repeated high stress attendant on a rolling motion, such as that of metal wheels on tracks or a ball bearing rolling in a machine. The stress causes subsurface cracks to form in either the moving or the stationary component. As these cracks grow, large particles separate from the surface and pitting ensues. Surface-fatigue wear is the most common form of wear affecting rolling elements such as bearings or gears. For sliding surfaces, adhesive wear usually proceeds sufficiently rapidly that there is no time for surface-fatigue wear to occur. Though the wear process is generally thought of as harmful, and in most practical situations is so, it has some practical uses as well. For example, many methods of producing a surface on a manufactured object depend on abrasive wear, among them filing, sanding, lapping, and polishing. Many writing instruments, principally the pencil, crayon, and chalk, depend for their effect on adhesive wear. Another use is seen in the wear of the incisor teeth of rodents. These teeth have a hard enamel covering along the outer curved surface but only soft dentine on the inner surface. Hence, abrasive and adhesive wear, which occurs more rapidly on the softer side, acts to maintain a sharp cutting edge on the teeth.