Tribological behaviour of the nitrided layer sliding against self-mating under dry sliding and grease-lubricated conditions in vacuum was investigated on a pin-on-disk type tribometer, with the interaction among the nitrided layer and grease being focused on. The morphologies of the worn surfaces were observed by scanning electron microscopy (SEM). The chemical states of typical elements on the worn surfaces of the nitrided disks and pins were examined by means of X-ray photoelectron spectroscopy (XPS). The chemical compositions of grease samples taken from worn surfaces on the nitrided disks were analysed by Fourier transform infrared spectroscopy ( FTIR). Experimental results show that under grease-lubricated conditions, the nitrided layer exhibits lower average friction coefficient and higher wear resistance than those for the dry sliding. In the former case, the wear of nitrided layer shows a transition from the mild adhesive to the severe adhesive wear and even to the delamination wear, with increasing normal load from 10 to 90 N. In the latter case, a transition takes place from the mild abrasive wear to the micro-fatigue type one with increasing the normal load. Tribochemical reactions occur between the nitrided layer and perfluoropolyalkyl ether (PFPE) grease during the lubricated sliding of the nitrided disks against self-mating in vacuum. (C) 2010 Elsevier B.V. All rights reserved.