When ultrathin carbon layer system chemistry dictates the tribo-interface: Origin of slippery and wear-resistant surfaces

Overcoats are predominantly employed to tackle tribological challenges in numerous moving mechanical systems. However, when overcoats are thinned down to sub-10 nm levels, their performance gets significantly compromised due to the dominance of surface and interface effects. Here, we discovered the efficacy of the chemistry of sub-10 nm thick carbon-based overcoats in regulating friction and wear of rough ceramic surfaces, particularly Al₂O₃+TiC (AlTiC). Carbon overcoats up to 4 nm in thickness grown with low energy (~4-5 eV) atoms/ions caused no significant changes in the tribological performance of AlTiC. However, carbon overcoats grown at a moderate energy of 90 eV saw an exceptional reduction in friction and wear of AlTiC at similar thickness levels up to 4 nm. The addition of a 6 nm thick RF-sputtered carbon layer on top of these carbon overcoats caused no significant improvement in the tribological performance. However, the addition of a multilayer graphene overlayer was found to slightly reduce the friction further for the thicker carbon overcoats grown at 90 eV. Chemical bonding and carbon microstructural analysis, along with ion interaction simulations, were performed to elucidate the fundamental mechanisms behind the observed friction and wear performances. We discovered that atomic mixing and high sp³ bonding caused by the 90 eV growth process primarily dictated the friction and wear control at £ 10 nm overcoat thicknesses. Thus, by adopting suitable carbon overcoat technology the excellent tribological properties can be attained at even sub-5 nm overcoat thickness levels which is critical for numerous applications.