The Influence of Chemical Alloying on the High Temperature Wear Resistance of H-Free DLC Coatings

A commercial RF-sputtering deposition rig was employed to deposit H-free diamond-like carbon (DLC) coatings. The influence of alloying elements such as Ti and Si on the structure, mechanical and tribological properties of the coatings was investigated. The coating was observed in cross section and in plan view with SEM, TEM and AFM. Because of the highly-ionized plasma generated by the RF-powered glow discharge, ion bombardment suppresses the formation of a columnar structure regardless of the composition of the coatings. The method produces featureless microstructures and smooth surfaces. TEM investigations confirm that no crystalline phases form in the coatings regardless of the presence of considerable concentrations of Ti and Si. Tribological tests were performed with a high-temperature tribometer in a ball-on-disk configuration, using coated disks and different materials for the ball countepart. At ambient T the sliding friction coefficient decreases as the concentration of alloying elements increases. Nevertheless, high-T tribotests with a constant thermal load showed that the presence of alloying elements decreases the thermal stability of the coatings. For each coating a temperature exists above which a sudden increase of friction coefficient is observed, with subsequent detachment and failure of the coatings. The mechanism of disruption of the self-lubrication effects is identified and the influence of the alloying elements on the thermal degradation of tribological performance of the coatings is discussed.