Mechanical performance of friction-induced protrusive nanostructures on monocrystalline silicon and quartz

Through nanoindentation and nanoscratch tests, it is demonstrated that friction-induced protrusive nanostructures (or hillocks) showed good mechanical behaviour. At the maximum indentation depth of 2 nm, the elastic modulus of the hillocks on the silicon/quartz surface was only 5.3/14.8 lower than that measured on their substrates. After scratching under a contact pressure of 10.3 GPa on silicon hillocks or 7.2 GPa on quartz hillocks, no surface damage was observed on the scratched area. Hence, the friction-induced hillocks can withstand typical contact in dynamic MEMS. Even though the scratch depths were much larger than the height of the detected hillocks, no peeling-off scar or surface crack could be detected beside the grooves on the hillocks, which indicated that the hillocks bonded strongly to their substrates. Further analysis revealed that the strength of the friction-induced hillocks was strongly dependent on the friction-induced process. The hillock produced at high sliding speed can lead to a high elastic modulus. Transmission electron microscope detection showed that the deformed silicon matrix formed chiefly at high sliding speed can enhance the elastic modulus of the hillocks. As a comparison, the amorphous silicon layer formed mainly at low sliding speed can reduce the elastic modulus of the hillocks.