Tribological characteristics of diamond-like carbon (DLC) based nanocomposite coatings

DLC/WC/WS2 (WCS) nanocomposite coatings having low friction under various sliding conditions were experimentally investigated using a pin-on-disk test instrument. Characterization results confirm that mixed components dominate the sliding behavior of these composite coatings. A simple lubricating layer of graphitic carbon or WS2 was not found on either the wear tracks or pin surfaces. Instead, both components were present in the surface layers in varying amounts depending upon the load and test environment. Thus, the lubrication mechanisms involved in sliding are more complex than expected. Synergistic effects involving the mixed components account for the extremely low friction coefficients of WCS coatings during dry sliding. Molecular dynamics (MD) calculations were implemented to simulate the sliding behavior of composite materials in a 2D system interacting via Lennard-Jones potentials. Sliding results showed that soft nanoscale particles tended to mix with the base material. The mixed material formed a confined weakened zone at the sliding interface, and this helped to maintain shear localization during steady state sliding. In agreement with experiments, the simulations did not show development of a simple interfacial layer during sliding. Vorticity develops where the strain rate is highest and is responsible for changes in composition and microstructure. Temperature profiles indicate that heat conduction during sliding is consistent with results expected for one-dimensional heat flow from the interface region to distant reservoir regions maintained at a constant low temperature. Hard particles tended to agglomerate.