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.