Fig. 2
Quantum-chemical quasi-static pressing simulations of model lubricant molecules confined between ta-C surfaces. a Normal pressure PN as a function of the separation h between two rigid outer C layers for five ta-C samples. The Lewis structures of the model lubricant molecules are shown in the top row. The order of the five ta-C samples corresponds to the number of reactive surface atoms Nr (see Methods section). Different symbols are employed for five ta-C samples: circle (sample 1), square (sample 2), triangle (sample 3), diamond (sample 4), and star (sample 5). The colour coding distinguishes interfacial phenomena and structures shown in panels b and c. b, c Representative snapshots for three types of interfacial chemistry: non-bonded state (green, panel b), anchoring (yellow, panel c) to one surface, and cross-linking (red, panel d) of both surfaces. Grey spheres represent carbon atoms from the ta-C. Green and orange spheres depict initially single- and double-bonded C lubricant atoms, respectively. O and H atoms are in red and cyan, respectively. e, f Critical pressures \(P_N^c\) for the anchoring of lubricant molecules on one ta-C surface (e) and for the formation of chemical bonds across two ta-C surfaces (f, cross-linking). Panel g shows means of the critical pressures in panels c and d. Error bars represent standard errors of the means. In all panels, the same colour and symbol coding is used
