Tribocatalytically-activated formation of protective friction and wear reducing carbon coatings from alkane environment

Tribocatalytically-active nickel/cobalt phosphorous films for universal protection in a hydrocarbon-rich environment

High-contact stresses generated at the sliding interfaces during their relative movement provide a unique combination of local heating and shear- and load-induced compression conditions. These conditions, when involving the sliding of surfaces with certain material characteristics, may facilitate tribochemical reactions with the environment, leading to the formation of a protective, damage-suppressing tribofilm directly at the contact. Here, we employ the electrodeposition process to design a coating composed of a hard cobalt-phosphorous matrix with the inclusion of tribocatalytically-active nickel clusters. The coating is optimized in terms of its relative composition and mechanical characteristics. We demonstrate the excellent tribological performance of the coating in the presence of a hydrocarbon environment, both in the form of a liquid lubricant and as a hydrocarbon-saturated vapor. Characterization of the wear track indicates that the origin of such performance lies in the formation of a protective carbon-based tribofilm on the surface of the coating during sliding. These results contribute to the advancement of knowledge on material transformations in the contact, thus providing a robust and versatile approach to addressing tribological challenges in mechanical systems.

MoVN-Cu Coatings for In Situ Tribocatalytic Formation of Carbon-Rich Tribofilms in Low-Viscosity Fuels

Inhibiting the tribological failure of mechanical assemblies which rely on fuels for lubrication is an obstacle to maintaining the lifetime of these systems with low-viscosity and low-lubricity fuels. In the present study, a MoVN-Cu nanocomposite coating was tribologically evaluated for durability in high- and low-viscosity fuels as a function of temperature, load, and sliding velocity conditions. The results indicate that the MoVN-Cu coating is effective in decreasing wear and friction relative to an uncoated steel surface. Raman spectroscopy, transmission electron microscopy, and electron-dispersive spectroscopy analysis of the MoVN-Cu worn surfaces confirmed the presence of an amorphous carbon-rich tribofilm which provides easy shearing and low friction during sliding. Further, the characterization of the formed tribofilm revealed the presence of nanoscale copper clusters overlapping with the carbon peak intensities supporting the tribocatalytic origin of the surface protection. The tribological assessment of the MoVN-Cu coating reveals that the coefficient of friction decreased with increasing material wear and initial contact pressure. These findings suggest that MoVN-Cu is a promising protective coating for fuel-lubricated assemblies due to its adaptive ability to replenish lubricious tribofilms from hydrocarbon environments.

Achieving Ultralow Friction and Wear by Tribocatalysis: Enabled by In-Operando Formation of Nanocarbon Films

Under the high-contact-pressure and shear conditions of tribological interfaces lubricated by gaseous, liquid, and solid forms of carbon precursors, a variety of highly favorable tribocatalytic processes may take place and result in the in situ formation of nanocarbon-based tribofilms providing ultralow friction and wear even under extreme test conditions. Structurally, these tribofilms are rather complex and may consist of all known forms of nanocarbon including amorphous or disordered carbon, graphite, graphene, nano-onion, nanotube, etc. Tribologically, they shear readily to provide ultralow friction and protection against wear. In this paper, we review some of the latest developments in catalyst-enabled tribochemical films resulting from gaseous, liquid, and solid sources of carbon. Particular focus is given to the nature and lubrication mechanisms of such in situ derived tribofilms with the hope that future tribological surfaces can be designed in such a way to exploit the beneficial impact of catalysis in friction and wear control.