TABLE OF CONTENTS Before discussing the types of wear mechanisms in detail, it will be useful to define the characteristics of an engineering surface that are relevant to these processes. (Material strength and hardness will also be factors in wear.) Most solid surfaces that are subject to wear in machinery will be either machined, ground, or EDM'd, though some will be as-cast or as-forged. In any case, the surface will have some degree of roughness that is concomitant with its finishing process. Its degree of roughness or smoothness will have an effect on both the type and degree of wear that it will experience.
Even an apparently smooth surface will have microscopic irregularities. These can be measured by any of several methods. A profilometer passes a lightly loaded, hard (e.g., diamond) stylus over the surface at controlled (low) velocity and records its undulations. The stylus has a very small (about 0.5 ?m) radius tip that acts, in effect, as a low-pass filter, since contours smaller than its radius are not sensed. Nevertheless, it gives a reasonably accurate profile of the surface with a resolution of 0.125 ?m or better. Figure 12-1 shows the profiles and SEM [1] photographs (100x) of both ( a) ground and ( b) machined surfaces of hardened steel cams. The profiles were measured with a Hommel T-20 profilometer that digitizes 8 000 data points over the sample length (here 2.5 mm). The microscopic "mountain peaks" on the surfaces are called asperities.
