Tribological behavior in dry air and nitrogen of pulsed gas dynamic sprayed Cu-MoS2 composite coatings

Abstract

Using the pulsed gas dynamic spray (PGDS) process, a cold-sprayed Cu coating and two Cu-MoS2 composite coatings containing 11 and 48 vol% MoS2 were deposited. Comparing to the open literature, the retention of MoS2 achieved here was the highest yet reported for a cold-sprayed coating. Tribological properties in dry air and nitrogen were studied in sliding wear with a ball-on-disk tribometer. Material transfer and chemical composition of third bodies in the contact were examined by scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy, as well as Raman and X-ray photoelectron spectroscopy. The Cu-11 vol% MoS2 coating (Cu-SM) showed lower coefficient of friction than Cu coating in both dry air and nitrogen, but due to its relatively low MoS2 content, the friction-reducing effect was reduced compared to the Cu-48 vol% MoS2 coating (Cu-FM). The two composite coatings (Cu-SM and Cu-FM) exhibited higher wear rate than the Cu coating in dry air while the wear rates of the three coatings were comparable in nitrogen. Low hardness of the composite coatings, strong adhesion between Al2O3 ball and Cu oxides grown on worn surfaces as well as breakage of unstable tribolayers led to high wear rate for Cu-SM and Cu-FM in dry air. High amount of MoS2 in Cu-FM was able to provide lubrication initially, but oxidization of MoS2 at contact interfaces caused an increase in friction and ejection of metallic particles. Tribolayers formed on wear track resulted in less MoS2 being replenished to the contact during sliding steady state of Cu-FM. In nitrogen, weak adhesion between first bodies resulted in lower wear rates of all coatings than in dry air. Relatively high amount of MoS2 detected in third bodies of Cu-FM contributed to lowest friction. Under the condition of no oxidation, the friction and wear behavior mainly depended on the MoS2 content in the coating.