Identify the root cause of 'fretting wear' at the contact points of a multi-turn wave spring and suggest mitigation strategies.

Fretting wear occurs at the wave peaks (the interfaces between turns) due to microscopic relative motion (oscillatory slip) during high-frequency vibration or cycling. This wear removes the protective oxide layer of the stainless steel, leading to localized pitting and stress concentrators that initiate fatigue cracks. Root causes include insufficient axial preload (allowing excessive movement) or resonance with system frequencies. Mitigation involves: 1) Increasing the initial preload to 'lock' the turns; 2) Applying a dry-film lubricant like $MoS_2$ or PTFE to reduce the coefficient of friction; or 3) Using a spring material with higher surface hardness, such as an aged Inconel X-750 or nitrided steel.