Fig. 5

Characterization of the contact area produced on a-C:H:Si film-coated SUJ2 steel ball surface. The corresponding superlubricity test for self-mated a-C:H:Si (9.3 at% Si, ACF-6) surfaces is shown in Supplementary Fig. 8. The transfer of all the film material from the ball surface to the wafer side and the simultaneous re-generation of tribolayers on both the surfaces demonstrate the reconstruction of the sliding interface (Supplementary Figs 8 and 9). a BF-STEM image showing a mainly C-Fe intermixed tribolayer of ~ 5 nm thickness generated at the ball wear scar center. Scale bar, 5 nm. b, c BF-STEM and IFFT (or FFT) images specifying a highly-crystalized Fe₃C shear band (b) and an amorphous-like local structure (c) in the tribolayer as marked in a. Scale bars in b, 0.5 nm. Scale bars of BF and FFT images in c are 0.5 nm and 20 nm⁻¹, respectively. d HAADF-STEM image across the tribolayer as marked in a. Scale bar, 2 nm. e Intensity profile along the dashed line as marked in d confirming the atomic structure of the tribolayer. f BF-STEM image showing a tribolayer of ~ 20 nm thickness generated on the ball wear scar edge. Scale bar, 10 nm. The tribolayer internally evolved into three individual sublayers (marked as g, h and i): a well-crystalized Fe-rich sublayer near the sliding interface, a C-rich amorphous sublayer in the middle and a highly oxidized Fe-based sublayer near the steel ball surface. g–i BF-, HAADF-, false-color displayed BF-STEM and local IFFT (or FFT) images of the three sublayers as marked in f. Scale bars of BF-, HAADF-, false-color displayed BF-STEM images in g–i are all 2 nm, while scale bars of IFFT (FFT) images in g–i are 1 nm, 20 nm⁻¹ and 1 nm, respectively. j Evolution of Si-L, C-K, O-K and Fe-L EELS core-edge spectra recorded across the tribolayer as marked in f