Tribologically transformed structure in fretting

Both fretting wear and fretting fatigue suffer from particle detachment and cracking induced damage. Wear induced by fretting is related to a three-st age phenomenon: (1) accommodation of the displacement in the upper layers o f the two counterbodies; (2) detachment of particles from material with a m odified or transformed structure; and (3) third-body behaviour, i.e. accomm odation 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. Un derstanding of its formation and degradation are required to control and pr edict wear generated by fretting. Extensive studies focussed on the nature of TTSs depending on several metallic contacts (steels, aluminium alloys, a lpha-, beta-, or alpha + beta -titanium alloys,...) and different testing c onditions (load, sliding amplitude, number of cycles, environment). Powerfu l analytical tools were utilized to determine the TTS composition and struc ture. TTS appeared as a nanocrystalline structure, corresponding to the che mical composition of the initial material and made of the more stable struc ture in accordance with the equilibrium diagram. No specific effects of oxy gen 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 o utlined. 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 spec ific threshold dissipated energy. (C) 2000 Elsevier Science S.A. All rights reserved.