Influencing mechanisms of deposition bias voltage on superlubricious a-C:H films: Key role of nanoclustering structures in controlling structural evolution of transfer film

Hydrogenated amorphous carbon (a-C:H) films exhibit super-low friction coefficient but high wear rate in vacuum, while adjusting the deposition bias voltage is a practical method to enhance their anti-wear abilities. However, the inherent mechanisms are still no well comprehended. Here, systematic characterizations were conducted to unveil the intrinsic relationship between the superlubricious transfer films and their corresponding initial ion-energy induced bonding structures. The results indicated that the establishment of hydrogen-rich, graphite-like transfer films was the key factor for the ultra-low friction. The elevated deposition ion energy can cause structural changes of the initial film from layered-like nanoclustering structures to disordered bonding network, which could hamper the graphitization of transfer films and lead to the failure of superlubricity after the depletion of the sp2-rich, highly hydrogenated surface nanolayers. For the sustainable structural evolution from the inherent nanoclusters to superlubricious transfer films, sufficient hydrogen and mechanical stiffness of the film bulk were necessary, which can be optimized by balancing the growth process of subplantaion and chemical adsorption via controlling the carbon ion energy to approach the theoretical surface penetration threshold of 30 eV. Under this condition, the films can achieve an ultra-low wear rate of 6.6 x 10-8 mm3N-1m-1 with a lowest friction coefficient of 0.007 in vacuum. These findings can provide guidance for the design of superlubricious carbon coatings for aerospace applications.