What causes 'Fretting Fatigue' in wave springs, and how can it be identified during inspection?

Fretting fatigue occurs when there is minute oscillatory motion between the wave spring and its mating surfaces (the 'points of contact'). This motion, often caused by vibration, breaks down the protective oxide layer of the metal, leading to micro-pits. These pits act as stress risers ($K_t$), where fatigue cracks initiate. During failure analysis, fretting is identified by the presence of fine reddish or black debris (depending on the material) and a 'pitted' appearance at the wave crests. Increasing the preload or applying a solid-film lubricant can mitigate this by preventing the relative motion.