Tribology of DLC films under fretting conditions

Abstract

The tribological behaviour of several diamond-like carbon (DLC) coatings of the type a-C:H has been investigated under gross-slip fretting conditions and the effects of the counterbody material, ambient atmosphere, ambient temperature and liquid lubricating medium were assessed. DLC films have a significant friction-and wear-reducing effect under fretting conditions. Specifically, with their uses as thin protective films on tribological surfaces, very low friction coefficients in the range 0.01-0.1 with wear coefficients of about 1·10-7 mm3/Nm were realized. This is not only valid for dry sliding in ambient air, but also under lubricated sliding conditions in both the aqueous (deionized water) and organic media (liquid lubricant). The presence of sodium chloride in aqueous test media does not show any significant effect on altering the friction and wear characteristics of these films. However, when increased ambient temperatures are imposed on DLC as one of the sliding partners, the coating structure begins to degrade. Up to 100°C, their friction coefficients are not affected (in fact, noticeable decreases in friction are observed with increasing temperature). However, transformation processes to graphitic carbon seem to be induced immediately with further increases in temperature leading to increased wear coefficients and a significant reduction of the coating lifetime. This reduction of coating lifetime itself depends on the material of the counterbody. © 2008 Springer Science + Business Media, LLC.