What role does 'fretting wear' play in the failure of retaining rings in turbine engines?

In turbine engines, high-frequency vibrations cause the retaining ring to rub against the groove walls and the retained component. This 'fretting' wears away the material, reducing the effective thickness $T$ of the ring and the depth $d$ of the groove. As these dimensions decrease, the thrust capacity drops according to $P_r \propto T$. The debris from fretting (metal oxides) also acts as an abrasive, accelerating the process. Failure is usually caught during overhaul by checking the 'axial play' of the assembly. Prevention involves using anti-fretting coatings like silver plating or using materials with higher surface hardness like 17-7PH.