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 SiO_(2)microsphere was investigated under various normal loads and displacement amplitudes in ambient air.As the number of sliding cycles increased,both the friction force Ft of the Si-SiO_(x)/SiO_(2)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-SiO_(x)/SiO_(2)pair.During the running-in process,the contact area of the Si-SiO_(x)/SiO_(2)pair might become hydrophobic due to removal of the hydrophilic oxide layer on the silicon surface and the surface change of the SiO_(2)tip,which caused the reduction of friction force and the wear rate of the Si-SiO_(x)/SiO_(2)pair.A phenomenological model is proposed to explain the running-in process of the Si-SiO_(x)/SiO_(2)pair in ambient air.The results may help us understand the mechanism of the running-in process of the Si-SiO_(x)/SiO_(2)pair at nanoscale and reduce wear failure in dynamic microelectromechanical systems(MEMS).
