Tribological properties of MoS2/WC-Co duplex coatings in low viscosity hydrocarbons

A duplex architecture MoS2/WC-Co coating was developed as a wear protective coating for AISI 52100 steel surfaces in mechanical contacts of fuel-handling systems that operate with low-viscosity hydrocarbon fuels. The duplex coating was prepared by a subsequent application of thermal sprayed WC17 wt% Co (WC-17Co) layer and solution sprayed MoS2 layer. The coating surface morphology and architecture were investigated with scanning electron microscopy and a focused ion beam cross-section of the duplex coating. Ethanol and dodecane were used in wear tests as representatives of fuel surrogates with low viscosity. Reciprocating sliding friction and wear tests were performed in unlubricated, ethanol-lubricated, and dodecane-lubricated conditions in a dry nitrogen atmosphere. Significant differences in tribological behavior were observed between MoS2 coatings in ethanol versus dodecane. In dodecane, the coefficient of friction values were considerably lower for both WC17Co and steel coated with MoS2 compared to uncoated baseline tests. The wear of the MoS2/WC-17Co and bare WC-17Co was much lower than that of bare steel and MoS2/steel when sliding in dodecane. In ethanol, removal of the MoS2 top layer and increased wear of the steel counterbody were observed under small contact loads, which was associated with the formation of abrasive MoO3 particles in the sliding contact. However, when testing in ethanol at higher contact loads, friction decreased substantially and a transfer of MoS2 to the steel counterface was observed and mapped with Raman spectroscopy. A tribological mechanism was suggested that at higher contact pressures the fluid film thickness of ethanol decreased, allowing for an easier transfer of solid lubricant to the sliding counterpart and the subsequent formation of a thick and lubricious tribolayer which remained for the duration of the test. This study highlights the benefits of combining MoS2 and a hard WC-Co underlayer in a duplex architecture for use as a protective coating on steel parts in low-viscosity fuel environments. The study also contributes to the understanding of tribological processes that occur at MoS2/WC-17Co sliding surfaces operated in both an alcohol-based and an alkane-based low viscosity hydrocarbon environment.