Fig. 5

(a–c) Friction force variations for three models of 0 Å, 4 Å and 6 Å under different normal loads, (d) Average friction force variations under different normal loads. (e–g) Friction force variation of three models of 0 Å, 4 Å and 6 Å under different sliding velocities, (h) Average friction force variation at different sliding velocities.