The friction coefficients of different metal pairs are determined by the difference in the interatomic distance between each metal material under low load conditions. Here, eight types of metal pins were rubbed against Si (111) substrates that were covered with a native oxide film. Results showed that the friction coefficient increased when the interatomic distance of the metal approached 3.1 Å, which corresponds to the interatomic distance of SiO_2. The same eight types of metal pins were rubbed against bare Si (111), and the friction coefficient increased with the smaller difference in the interatomic distance between the pin and Si (2.35 Å). A similar relationship was found between pairs of Ag/Cu/Ni and Au/Pt/Si (100).
