Structure, mechanical and tribological properties of Mo-S-N solid lubricant coatings

Mo–S–C self-lubricating coatings were deposited by d.c. magnetron sputtering from carbon and molybdenum disulphide targets. The power ratio of the targets was varied in order to prepare films with carbon content in the range 0–55 at.%. Whatever the carbon content, the S/Mo ratio was higher than 1.25. The hardness of the films increased almost linearly with the carbon content. X-ray photoelectron spectroscopy showed evidence of Mo–C bonds; nevertheless, the size of molybdenum carbide grains was expected to be very small, since X-ray diffraction did not reveal any peaks related to any Mo–C phase. The coatings tested by pin-on-disc exhibited low friction, decreasing with increasing carbon content, when humid air was present. In nitrogen, the friction of all films was lower than 0.02 except for the reference MoS2 (0.04). Mo–S–C outperformed the wear resistance of MoS2; on the other hand, the results were in some cases hindered by the low adhesion of the coatings. The films were very sensitive to air exposure leading to surface oxidation.