Influence of tribofilm on superlubricity of highly-hydrogenated amorphous carbon films in inert gaseous environments

In this study, we mainly focus on the structural morphology and inter-atomic bonding state of tribofilms resulting from a highly-hydrogenated amorphous carbon (a-C:H) film in order to ascertain the underlying mechanisms for its superlubric behavior (i.e., less than 0.01 friction coefficient). Specifically, we achieved superlubricity (i.e., friction coefficients of down to 0.003) with this film in dry nitrogen and argon atmospheres especially when the tribo-pair is made of an a-C:H coated Si disk sliding against an a-C:H coated steel ball, while the a-C:H coated disk against uncoated ball does not provide superlubricity. We also found that the state of superlubricity is more stable in argon than in nitrogen and the formation of a smooth and uniformly-thick carbonaceous tribofilm appears to be one of the key factors for the realization of such superlubricity. Besides, the interfacial morphology of sliding test pairs and the atomic-scale bond structure of the carbon-based tribofilms also play an important role in the observed superlubric behavior of a-C:H films. Using Raman spectroscopy and high resolution transmission electron microscopy, we have compared the structural differences of the tribofilms produced on bare and a-C:H coated steel balls. For the a-C:H coated ball as mating material which provided superlow friction in argon, structural morphology of the tribofilm was similar or comparable to that of the original a-C:H coating; while for the bare steel ball, the sp(2)-bonded C fraction in the tribofilm increased and a fingerprint-like nanocrystalline structure was detected by high resolution transmission electron microscopy (HRTEM). We also calculated the shear stresses for different tribofilms, and established a relationship between the magnitude of the shear stresses and the extent of sp(3)-sp(2) phase transformation.