Effect of carbon concentration and argon flow rate on the microstructure and triboperformance of magnetron sputtered WS2/a-C coatings

WS2/a-C coatings with various carbon contents (0-65 at.%) were deposited on silicon wafers by magnetron co-sputtering under different Ar flow rates. Increasing the argon flow rate increases the chemical stoichiometric S/W ratio, but the coating gradually becomes porous and columnar-like. The S/W stoichiometry is less influenced by the variation in carbon concentration. TEM and XRD confirm well crystallized basal (002) planes in the sputter deposited pure WS2 or low-carbon WS2/a-C coatings. The hardness of the composite coatings increases with increasing carbon content or decreasing Ar flow rate. The coating at 40 at.% C exhibits the highest hardness (10.6 GPa). Tribotests show that, together with a ultralow wear rate of 10(-7) mm(3) m(-1) N-1, the coefficient of friction can be as low as 0.02 in dry air (5% R-H) and around 0.15 in moisture (55% R-H) and remains stable within a sliding distance of 1000 rn. The wear resistance is strongly affected by the carbon concentration, deposition pressure and the testing atmospheres.