Effect of surface hydrophilicity on the nanofretting behavior of Si(100) in atmosphere and vacuum

Abstract

With an atomic force microscopy, the effect of surface hydrophilicity on the nanofretting behavior of Si(100) against SiO2 microsphere was investigated under vacuum and atmosphere conditions, respectively. The surface hydrophilicity revealed a strong effect on the motion behavior, adhesion force, friction force, and nanofretting damage of Si(100)/SiO2 pairs. The increase in the hydrophilicity of Si(100) surface could expand the stick regime of Si(100)/SiO2 pairs into a higher value of displacement amplitude. While the nanofretting ran in atmosphere, both adhesion and friction forces in the initial cycle would be larger when the Si(100) surface was more hydrophilic. However, because of the in situ chemical modification of SiO2 tip in nanofretting, they might reveal a decrease with increasing nanofretting cycles. Either in vacuum or in atmosphere, the nanofretting damage was weaker when the Si(100) surface was more hydrophobic. Because of the lack of oxygen and vapor in vacuum, the nanofretting damage on the Si(100) surface was dominated by mechanical interaction. The damage was characterized as the depression of 0.1–0.2 nm in depth on hydrophilic Si and the hillocks of 0.8–0.9 nm in height on hydrophobic Si and original Si. However, the nanofretting damage in atmosphere was much more serious, which was identified as the grooves of 8–11 nm in depth on Si(100) surfaces. Analysis indicated that even if the nanofretting damage in atmosphere was the coupled results of mechanical interaction and tribochemical reaction, the tribochemical reaction played a dominated role. These results will help us to understand the effect of surface properties on nanofretting of silicon and optimize the surface treatment technology to minimize the potential nanofretting failure of microdevices in microelectromechanical system (MEMS).