Nanofretting Behavior of Monocrystalline Silicon (100) Against SiO2 Microsphere in Vacuum

With an atomic force microscopy, the tangential nanofretting between spherical SiO2 tips and monocrystalline Si(100) surface was carried out at various displacement amplitudes (0.5–250 nm) under vacuum condition. Similar to fretting, the nanofretting of Si(100)/SiO2 pair could also be divided into stick regime and slip regime upon the transition criterion. However, it was found that the energy ratio corresponding to the transition between two nanofretting regimes varied between 0.32 and 0.64, which was higher than the normal value of 0.2 for the transition criterion to determine the partial slip and gross slip regimes in fretting. One of the reasons may be attributed to the effect of adhesion force, since whose magnitude is at the same scale to the value of the applied normal load in nanofretting. During the nanofretting process of Si(100)/SiO2 pair, the adhesion force may induce the increase in the maximum static friction force and prevent the contact pair from slipping. The higher the applied load, or the higher the adhesion force, the larger will be the transition displacement amplitude between two regimes in nanofretting. Different from fretting wear, the generation of hillocks was observed on Si(100) surface under the given conditions in nanofretting process. With the increase in the displacement amplitude in slip regime of nanofretting, the height of hillocks first increased and then attained a constant value. Compared to chemical reaction, the mechanical interaction may be the main reason responsible for the formation of silicon hillocks during the nanofretting in vacuum. The results in the research may be helpful to understand the nanofretting failures of components in MEMS/NEMS.