TiN is used industrially as a coating resistant to sliding wear. However, its importance as an anti-fretting wear coating is less certain. In this paper, we report on the fretting wear behaviour of a TIN coating and the effect substrate properties have on it. The substrate materials used were three ferrous alloys (188 to 674 HV), a titanium alloy (Ti6Al4V) and an aluminium alloy (ASCM20). A TiN coating, thickness 1.6 mu m, was applied using the are-ion plating process. Crossed-cylinder fretting tests were carried out on both coated and uncoated specimens, without external lubrication, at a frequency of 10 Hz. Relative strokes from 15 to 138 mu m and normal forces from 1.4 to 22.7 N, that is, initial Hertzian contact stresses from 301 to 1160 MPa, were used. Each test involved a single substrate material. Substrate hardness had a marked effect on the number of cycles at which breakdown of the TiN coating occurred, breakdown occurring earlier on the lower hardness substrates. More unexpectedly, the effect of the coating, after breakdown, was also substrate dependent. For the ferrous substrates, the coating had almost no effect on post-breakdown wear, and neither did the substrate hardness. In contrast, for both the titanium and aluminium alloy substrates, the coating affected beneficially post-breakdown fretting wear. This behaviour is interpreted qualitatively in terms of the stresses in the TiN coating and substrate, and from EPMA analysis of the wear scars. (C) 1999 Elsevier Science S.A. All rights reserved.