Amorphous carbon films have attracted substantial interest due to their exceptional mechanical and tribological properties. Previous studies revealed that the amorphous carbon films exhibited lower coefficient of friction (COF) because of the transformation in bond structure from sp³-C to sp²-C during friction processes. However, the mechanism for such a transformation during friction is not well understood. This study is conducted to get an insight into the metastable transformation in amorphous carbon film during friction by means of experiments and molecular dynamics (MD) simulation. Relevant wear tests showed that wear of the film changed from an abrasive wear mode to a mixture of abrasion and adhesive wear, resulting in a decrease in growth rate of the wear rate after the running-in stage. It is worth noting that the sp³-C atoms were increased during the running-in stage when the films contained lower sp³/sp² ratios. However, the formed sp³-C atoms could only be short-lived and gradually transformed to sp²-C atoms with the graphitization generated on the wearing surface of the films. The radial distribution function and translational order parameter indicated that the films' high sp³/sp² ratio led to an increased sp²-C proportion on the wear scar after friction, which caused an increased structural ordering.

It is imperative to develop a novel matching of metallic substrate and self-lubricating coating for aircraft spherical plain bearing in a wide range of service conditions. As a new type of superelastic material, 60NiTi alloy meets the performance requirements of aerospace bearing materials, but exhibits poor tribological performance, especially under the conditions of dry sliding friction. A Hydrogenated Diamond-Like Carbon (H-DLC) coating was deposited on the 60NiTi alloy to improve its tribological performance. The microstructure and mechanical behavior of the 60NiTi alloy and its H-DLC coating were explored. Results show that improvement of friction and wear performance of the H-DLC coating deposited on the 60NiTi substrate is mainly achieved by graphitization at the friction interface and the transfer film produced on the counterpart ball. The increased friction load leads to intensification of graphitization at the friction interface and formation of continuous and compact transfer film on the surface of the counterpart ball.