Autolubrication of diamond coatings at high sliding speed

We investigated the friction and wear properties of diamond coatings at high sliding speed (V=30 m/s). Various types of diamond coatings were deposited on tungsten carbide pins. These coatings were produced by a combustion flame method under different deposition conditions. The surface properties and morphology of the coatings have been characterised by surface analysis methods, such as scanning electron microscopy (SEM) and Raman spectrometry (RS). The coated pins were tested by sliding against a rotating steel XC30 disc in ambient atmosphere. The hardness of the coated pin and the disc after heat treatment is 70 and 5 GPa, respectively. The experimental results show that at low sliding speed (V=1 m/s) the diamond coatings exhibit a very high friction coefficient of about 0.9-1.1. At high sliding speed (V=30 m/s) the friction coefficient of these films decreases to 0.09-0.15, the wear of the diamond films also decreases. This tribological behaviour of diamond coatings can be explained by the transformation of sp(3) to sp(2) carbon sites on the rubbing surface during friction at high sliding speed. A model of the friction is proposed to explain this phenomenon. (C) 1998 Elsevier Science S.A. All rights reserved.