Wear resistance and solid lubricity of molybdenum-containing nitride coatings deposited by cathodic arc evaporation

Molybdenum-containing MoTiN, MoAlTiN, MoCrN and MoZrN coatings were deposited on the 17-4 PH stainless steel substrate by the cathodic arc evaporation technique. Pin-on-disc dry sliding tests were performed to investigate their wear resistance and the solid lubricity of the coatings. All of these coatings, with MoTiN and MoAlTiN in particular, demonstrated superior wear resistance and a significantly lower coefficient of friction against WC-6Co ball material when compared to their corresponding Mo-free TiN, AlTiN, CrN and ZrN monolithic coatings. For example, the specific wear rate of the MoAlTiN coating is only 0.14% of the wear rate of the AlTiN coating, and its coefficient of friction is only 0.28 compared to 0.60 of AlTiN. The excellent tribological performance of the Mo-containing coatings is attributed to the formation of a MoO3 surface layer on the wear tracks through the tribo-oxidation process. Further wear tests of the MoTiN and MoAlTiN coatings against Al2O3 ball material revealed less improvement in wear resistance and reduction of the coefficient of friction. When tested against Si3N4 ball material, both coatings, though still showing noticeably better wear resistance than their corresponding Mo-free coatings, did not demonstrate any beneficial effect of Mo on lowing the coefficient of friction. The scanning electron microscopy and energy dispersive X-ray spectroscopy analyses of the wear track surfaces illustrated the importance of retaining a stable and sufficiently thick MoO3 surface layer in order to maintain the beneficial effect of Mo on the tribological performance of the coatings.