Extraordinary control of friction and wear enabled by ultrathin carbon and its atomic-scale interface and thickness manipulation

R Kumar and A Neogi and RJ Yeo and MS Zhang and SKRS Sankaranarayanan and C Dhand and N Dwivedi, CARBON, 238, 120262 (2025).

DOI: 10.1016/j.carbon.2025.120262

Overcoats are widely used to engineer the tribo-interface of moving mechanical systems for tailoring friction and wear. However, when tribological control is required with sub-3 nm thick overcoats, the structural and protective properties of overcoats degrade as the tribologically-inferior substrates dominate the tribo-interface, thus hindering their practical applications. Here, we demonstrate an extraordinary tribo-engineering approach using carbon-containing overcoats. By introducing a similar to 1.7 nm-thick amorphous carbon (C) overcoat at the sliding Si-Al2O3 tribo-interface, a low and stable coefficient of friction (COF) of similar to 0.23 and high wear- resistance was attained, as compared to an unmodified Si-Al2O3 tribo- interface (COF similar to 0.45). Furthermore, by adding a nanometer- thick silicon nitride (SiNx) interlayer between the Si substrate and the carbon overcoat, the wear protection was further enhanced, although replacing the SiNx interlayer with NiFe failed to achieve the same result. Nonetheless, thermal annealing of the C/NiFe bilayer overcoat improved its tribological behavior. Slightly increasing the carbon layer thickness to 2.2 nm for both the C/SiNx and C/NiFe bilayer overcoats further boosted their friction and wear control, as evident from the drop in the average COF to similar to 0.16-0.17. These results display a classic example of "materials engineering to achieve outstanding tribo- engineering" which is further supported by large-scale atomistic simulations.

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