Fig. 2

Characterization of the contact areas for self-mated a-C:H (ACF-1) surfaces. The corresponding superlubricity tests at various normal loads are shown in Supplementary Fig. 2. a HRTEM image of the as-grown a-C:H film on SUJ2 steel ball. (Inset) FFT image derived from the square bright-blue box (2 nm × 2 nm) shows the typical amorphous character of the film. b–f HRTEM images showing the bonding structures of the sliding interfaces from different contact areas: b ball wear scar center at 2 N, c ball wear scar center at 5 N, d ball wear scar edge at 10 N, e ball wear scar center at 10 N and f wafer wear track center at 10 N. (Insets) FFT images obtained from the local 2 nm × 2 nm bright-blue boxes indicate the structural evolution of the outer-most sliding interfaces. Scale bars in a–e and f are 5 and 10 nm, respectively. The initial peak Hertz contact pressures under loads of 2, 5 and 10 N are 0.68, 0.93 and 1.17 GPa, respectively. g BF-STEM image and EELS results indicating the nearly intact bonding structure of the a-C:H layer near the sliding interface for the scar center at 5 N. h BF-STEM image and EELS results confirming the sp ² clustering and ordering in the outer-most ~ 3 nm region near the sliding interface for the scar center at 10 N. Scale bars in g and h, 5 nm. Error bars in g and h represent s.d. of calculated carbon bond fractions