Morphology of Micropitting

Printed with permission of the copyright holder, the American Gear Manufacturers Association, 1001 N. Fairfax Street, Fifth Floor, Alexandria, VA 22314-1587. Statements presented in this paper are those of the author(s) and may not represent the position or opinion of the American Gear Manufacturers Association. General Morphology To the unaided eye, micropitting appears dull, etched or stained, with patches of gray. Micropitting is difficult to see under diffuse fluorescent lighting and is best observed with intense directional lighting. A flashlight with a concentrated beam held in the proper direction effectively illuminates micropitting. With intense lighting, micropitting might sparkle or appear speckled. Figure 1 is a scanning electron microscopy (SEM) image that shows the floor of a micropit crater sloping gently downward from its origin at the tooth surface. The floor has a rough surface typical of that caused by ductile-fatiguecrack propagation. A feather-edge forms at the back of the crater due to plastic flow of material over the crater rim. The feather-edge appears white in SEM when it becomes charged with electrons. Material surrounding a micropit generally appears smooth and featureless, unless abraded.