Tribological behavior of WC/DLC/WS2 nanocomposite coatings

The sliding behavior of amorphous DLC coatings, with or without hydrogen (i.e., a-C or a-C:H), depends on environment. Nanocomposite coatings developed at the Air Force Research Laboratory are designed for operation in environments ranging from humid air to the vacuum of space, as required for aerospace applications. A magnetron sputter-assisted pulsed laser deposition process was used to produce coatings with a matrix of DLC and nanoparticles of both WC and WS2. Coatings were tested on a pin/disk tribometer equipped with an environmental chamber and an in situ Kelvin probe. Low friction was achieved in both air and vacuum. Post-test characterization of disk wear tracks, pin wear scars and debris involved the use of scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), focused ion beam/transmission electron microscopy (FIB/TEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Surface material with mixed components dominates the tribological behavior of these coatings in both air and vacuum. Molecular dynamics (MD) is being used to model an amorphous material containing crystalline nanoparticles. Results suggest that soft particles (e.g., WS2) tend to mix with the local matrix material rather than spreading on the surface as a separate layer. This is consistent with experimental observations.