To reveal the tribochemical reactions during fretting wear of TiN-coatings sliding against corundum, the microstructure and the chemical composition of debris have been investigated by analytical TEM. Novel observations on fretting wear debris are presented. These observations were possible thanks to a TEM-sample preparation procedure by ultramicrotomy. Based on friction force measurements, the tribological life of the TiN-corundum tribe-pair, was divided in five periods, namely: a running-in period, a first transition, an intermediate steady state, a second transition, and a final steady state. Fretting experiments were stopped at different periods after which the debris were collected and characterised. The structure of fretting wear debris appeared to be dependent upon the moment of formation. The collected debris consisted of either amorphous or nanocrystalline rutile-like material. For comparison, fretting tests were also performed on pure titanium sliding against corundum. The debris originating from titanium consisted of nanocrystalline rutile. The observed difference in microstructure between the debris from TiN and pure titanium is mainly due to differences in the tribochemical reactions occurring during fretting wear. The evolution from amorphous to nanocrystalline debris on TiN, is proposed to be due to the combined effect of relative humidity, energy input, and continuous oxidation. (C) 1998 Elsevier Science S.A. All rights reserved.