Interface Engineering and Controlling the Friction and Wear of Ultrathin Carbon Films: High sp3 Versus High sp2 Carbons

Understanding and engineering interfaces, and controlling the friction and wear of materials, are extremely important for many technological applications, particularly for magnetic storage technologies and micro- and nanoelectromechanical systems (MEMS and NEMS), where one sliding/moving surface comes into contact with another. Ultrathin carbon films are generally employed in most of these technologies. However, their wear and friction mechanisms are not well understood, especially the role of the film–substrate (FS) interface has not been deeply explored and discussed to date. This limits further developments in this field. Through experimental and theoretical experiments, we are able to report on the engineering of a FS interface consisting of high sp³- and high sp²-bonded ultrathin carbon films on Al₂O₃–TiC substrates by introducing a silicon nitride (SiN_x) interlayer and tuning the carbon ion energy. All carbon-based overcoats show a low coefficient of friction (COF) in the range of 0.08–0.16; however, the high sp³-bonded C/SiN_x bilayer overcoat reveals the lowest and most stable friction. The friction mechanism is explained using an integrated framework of surface passivation, rehybridization, material transfer, tribolayer formation, and interfaces. We discover that FS interface engineering substantially reduces the wear of ultrathin carbon films while maintaining/reducing the friction. In general, this approach can be applied to control the friction and wear of ultrathin films of diverse materials.