Both fretting wear and fretting fatigue suffer from particle detachment and cracking induced damage. Wear induced by fretting is related to a three-stage phenomenon: (1) accommodation of the displacement in the upper layers of the two counterbodies; (2) detachment of particles from material with a modified or transformed structure; and (3) third-body behaviour, i.e. accommodation of the velocity in the powder bed. The specific transformed structure from which debris is made is called the tribologically transformed structure or TTS. TTS has been shown to form in the first accommodation stage within a very few initial fretting cycles. Understanding of its formation and degradation are required to control and predict wear generated by fretting. Extensive studies focussed on the nature of TTSs depending on several metallic contacts (steels, aluminum alloys, α-, β-, or α+β-titanium alloys, …) and different testing conditions (load, sliding amplitude, number of cycles, environment). Powerful analytical tools were utilized to determine the TTS composition and structure. TTS appeared as a nanocrystalline structure, corresponding to the chemical composition of the initial material and made of the more stable structure in accordance with the equilibrium diagram. No specific effects of oxygen or hydrogen was detected. In the present article, experimental results will be discussed to point out possible mechanisms of the formation of the TTS. An analogy with butterflies which form under rolling fatigue will be outlined. At last, an energy approach to explain formation will be presented for the case of low-alloy steel. It is demonstrated that TTS formation is related to a critical cumulative plastic deformation associated with a specific threshold dissipated energy.
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