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Protective coatings are important for enhancing tribological behavior, preventing surface degradation, and reducing friction-induced energy losses during the operation of mechanical systems. Recently, tribocatalytically driven formation of protective carbon films at the contact interface has been demonstrated as a viable approach for repairing and extending the lifetime of protective coatings. Here, we study the effect of catalytic metals, specifically their composition and amount, on the tribocatalysis process. To achieve this, we test the tribological performance of electro-deposited amorphous CoNiP and CoCuP coatings in different hydrocarbon-rich environments. Our results indicate that the tribocatalytic repair of wear-induced damage is optimal when Ni and Cu are included in the Co-P matrix at 5 wt% Ni and 7 wt% Cu, respectively. Characterization of the wear tracks suggests that among the considered samples, the tribofilms formed on the surface of Co7CuP have the highest concentration of graphitic carbon, leading to a more significant reduction in the COF and wear rate. The carbon tribofilm formation was more pronounced in decane and synthetic oil than in ethanol, which is attributed to the difference in the length of the hydrocarbon molecules affecting viscosity and the lubricant film thickness during boundary lubrication sliding.

 

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.

 

Abstract Minimizing the wear of the surfaces exposed to mechanical shear stresses is a critical challenge for maximizing the lifespan of rotary mechanical parts. In this study, we have discovered the anti-wear capability of a series of metal nitride-copper nanocomposite coatings tested in a liquid hydrocarbon environment. The results indicate substantial reduction of the wear in comparison to the uncoated steel substrate. Analysis of the wear tracks indicates the formation of carbon-based protective films directly at the sliding interface during the tribological tests. Raman spectroscopy mapping of the wear track suggests the amorphous carbon (a-C) nature of the formed tribofilm. Further analysis of the tribocatalytic activity of the best coating candidate, MoN-Cu, as a function of load (0.25–1 N) and temperature (25 °C and 50 °C) was performed in three alkane solutions, decane, dodecane, and hexadecane. Results indicated that elevated temperature and high contact pressure lead to different tribological characteristics of the coating tested in different environments. The elemental energy dispersive x-ray spectroscopy analysis and Raman analysis revealed formation of the amorphous carbon film that facilitates easy shearing at the contact interface thus enabling more stable friction behavior and lower wear of the tribocatalytic coating. These findings provide new insights into the tribocatalysis mechanism that enables the formation of zero-wear coatings. 

Here, we report the high-temperature superlubricity phenomenon accomplished in coatings produced by burnishing powders of antimony trioxide (Sb 2 O 3 ) and magnesium silicate hydroxide coated with carbon (MSH/C) onto the nickel superalloy substrate. The tribological analysis performed in an open-air experimental setup revealed that with the increase of testing temperature, the coefficient of friction (COF) of the coating gradually decreases, finally reaching the superlubricity regime (the COF of 0.008) at 300°C. The analysis of worn surfaces using in-situ Raman spectroscopy suggested the synergistic effect of the inner Sb 2 O 3 adhesion layer and the top MSH/C layer, which do not only isolate the substrate from the direct exposure to sliding but also protect it from oxidation. The cross-sectional transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) results indicated the tribochemically-activated formation of an amorphous carbon layer on the surface of the coating during sliding. Formation of the film enables the high-temperature macroscale superlubricity behavior of the material system.