Structural, mechanical and tribological behavior of fullerene-like and amorphous carbon nitride coatings

Abstract

The mechanical and tribological properties of fullerene-like (FL) and amorphous carbon nitride (CN x) coatings synthesized by reactive unbalanced magnetron sputtering were analyzed and compared to carbon coatings deposited without N 2. Elastic recoil detection analysis (ERDA) was used to determine the amount of incorporated nitrogen while X-ray photoelectron spectroscopy (XPS) was used to study the local bonding environment. Nano-indentation revealed a large spread in hardness, elastic modulus and elastic recovery. The tribological performance of the coatings was tested with a reciprocating sliding tribometer that allowed in situ visualization of the sliding contact. Tests were performed with a sapphire hemisphere sliding at 4 mm/s in dry and ambient humidity air. Friction coefficients of the FL–CN x coatings in both humidities were slightly higher than those of diamond-like carbon coatings (DLC); the values were between 0.1 and 0.25, dropping as the bias voltage increased. FL–CN x coatings had higher wear resistance than amorphous CN x, DLC and graphite coatings in sliding contact although the hardnesses of the coatings were comparable. Wear rates of the FL–CN x coatings in ambient air were lower than in dry air and orders of magnitude lower than the other carbon coatings. Furthermore, the most wear-resistant high-temperature FL–CN x coating had the highest hardness, but the shortest wear life of the three high-temperature FL–CN x coatings. In situ visualization indicated films transferred from the coating to the sapphire hemisphere; the transfer film isolated the hemisphere from the coating, thereby controlling both the friction coefficient and the wear behavior. Effects of deposition conditions, structure and hardness on wear and friction behavior are discussed.