Running-in process of Si-SiOx/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-SiOx) 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-SiOx/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 Ft appeared to be induced mainly by the reduction of adhesion-related interfacial force between the Si-SiOx/SiO2 pair. During the running-in process, the contact area of the Si-SiOx/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-SiOx/SiO2 pair. A phenomenological model is proposed to explain the running-in process of the Si-SiOx/SiO2 pair in ambient air. The results may help us understand the mechanism of the running-in process of the Si-SiOx/SiO2 pair at nanoscale and reduce wear failure in dynamic microelectromechanical systems (MEMS).