Microstructure and vacuum tribology of TiC–Ag composite coatings deposited by magnetron sputtering-pulsed laser deposition

Composite titanium carbide–silver films have been co-deposited by magnetron sputtering-pulsed laser deposition (MSPLD) to study their friction and wear properties in vacuum. The films deposited were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The silver content in the films ranged from 6 to 46 at.%. Structural characterization of the films revealed that Ag had a nano-crystalline structure when dispersed in the film, but larger crystallites of Ag (∼50–200 nm) also formed on the surface. Films with higher Ag contents showed evidence of higher diffusion rates, leading to a coarser structure and greater surface coverage. Pin-on-disk friction tests were performed under vacuum to observe the friction and wear behavior of these films. Friction was lower with higher Ag content, but wear was higher; the optimal Ag concentration was found to be 15%. SEM images from the surface of the films and wear tracks were obtained to understand the morphology of this type of composite ceramic coating, and revealed Ag layers in the wear track elongated in the direction of wear. Based on these results, the significance of silver as a friction-reducing agent in vacuum environments was demonstrated.