Running-in process of Si–SiO_x/SiO₂ pair at nanoscale—Sharp drops in friction and wear rate during initial cycles

Using an atomic force microscope, the running-in process of a single crystalline silicon wafer coated with native oxide layer (Si–SiO_x) against a SiO₂ microsphere was investigated under various normal loads and displacement amplitudes in ambient air. As the number of sliding cycles increased, both the friction force F_t of the Si–SiO_x/SiO₂ pair and the wear rate of the silicon surface showed sharp drops during the initial 50 cycles and then leveled off in the remaining cycles. The sharp drop in F_t appeared to be induced mainly by the reduction of adhesion-related interfacial force between the Si–SiO_x/SiO₂ pair. During the running-in process, the contact area of the Si–SiO_x/SiO₂ pair might become hydrophobic due to removal of the hydrophilic oxide layer on the silicon surface and the surface change of the SiO₂ tip, which caused the reduction of friction force and the wear rate of the Si–SiO_x/SiO₂ pair. A phenomenological model is proposed to explain the running-in process of the Si–SiO_x/SiO₂ pair in ambient air. The results may help us understand the mechanism of the running-in process of the Si–SiO_x/SiO₂ pair at nanoscale and reduce wear failure in dynamic microelectromechanical systems (MEMS).