Tangential Nanofretting Behavior Of Monocrystalline Silicon

Nanofretting refers to cyclic movements of contact interfaces with the relative displacement amplitude in nanometer scale, where the contact area and normal load are usually much smaller than those in fretting. Due to the mechanical vibration, heat exchange, fluid motion and electromagnetic vibration, nanofretting may exist between the contact interfaces of microdevices in microelectromechanical systems (MEMS). Nanofretting may cause the looseness, bite, signal distortion and noise in MEMS, as well as induce the initiation and propagation of crack on the contact surfaces and shorten the lifetime of MEMS. Therefore, with the development of MEMS, nanofretting damage may become a key tribological concern besides microwear and adhesion.In this paper, the tangential nanofretting behaviors of monocrystalline silicon (100) against diamond and SiO2 spheric tips were investigated by an atomic force microscopy (AFM). During the investigation, an improved wedge calibration method was proposed to accurately calibrate the friction measurement system of AFM. Subsequently, the transition process of nanofretting damage of Si(100)/diamond pairs was enucleated and the critical transition condition was suggested. Based on these results, the effect of interfacial adhesion, environment ambiance and surface hydrophilicity of Si(100) on the nanofretting behaviors of Si(100)/SiO2 pairs was clarified. Eventually, the nanowear mechanism of Si(100)/SiO2 pairs was systematically analyzed, where the role of tribochemical reaction in the nanowear of Si(100)/SiO2 pairs was emphasized. Based on the above systemic investigation, the main conclusions can be summarized as following:(1) Based on the traditional wedge calibration method, an improved wedge calibration method was proposed to accurately calibrate the friction measurement system of AFM. Based on the investigation on the variation of the calibration factors with the increase in loads, this improved method selected the relative stable calibration factor under high load as the actual calibration factor. Since it avoided the error under low load, the calibration result is more reasonable than the traditional one. This method laid an experimental foundation for the following nanofretting research.(2) The nanofretting damage of Si(100) strongly depends on the normal load and nanofretting cycles. With the increase in normal load or number of cycles under a certain load, nanofretting damage on silicon may experience three processes, namely as the generation of hillocks, the formation of depression and material removal. Analysis results indicate that, accompanying the transition of the nanofretting damage from hillock to depression, the critical Hertzian contact pressure is closed to the hardness of Si(100).(3) With the increase in adhesion force between the Si(100)/SiO2 interfaces, the stick regime of nanofretting will expand towards higher displacement amplitude. In vacuum, the friction force during nanofretting is lower, and the nanofretting damage of Si(100) is dominated by mechanical deformation. However, in atmosphere, the friction force during nanofretting is higher and experiences an obvious variation, and the nanofretting damage of Si(100) is dominated by tribochemical reaction. As a result, the nanofretting damage of Si(100) surface was presented as the generation of hillock in vacuum and identified as the formation of groove in atmosphere.(4) The increase in the hydrophilicity of Si(100) surface will induce a higher adhesion force between Si(100)/SiO2 interfaces, and expand the stick regime of Si(100)/SiO2 pairs into a higher value of displacement amplitude. Due to the tribochemical modification of SiO2 tip during nanofretting, the adhesion force and friction force between Si(100)/SiO2 pairs will decrease after nanofretting. The nanofretting damage of Si(100) is weak in vacuum, which is characterized as low hillock on hydrophobic silicon and original silicon and the shallow depression on hydrophilic silicon. However, the nanofretting damage in atmosphere is much more serious, which is identified as very deep grooves on three Si(100) surfaces, and with the increase in surface hydrophilicity, the groove is deeper.(5) In atmosphere, the nanofretting wear of Si(100)/SiO2 pairs is the combined results of the mechanical interaction and tribochemical reaction. However, compared with the mechanical interaction, the tribochemical reaction plays a dominated role. The adsorbed water, SiO2 counter-surface and mechanical shearing act collectively to cause tribochemical wear of system to destroy Si(100) surface. The shearing stress during nanofretting can weaken and break the Si-Si bonds in the silicon substrate, which facilitates the occurrence of sustaining tribochemical reaction. Compared with the oxidation caused by oxygen, the hydrolysis reaction induced by vapor plays a more critical role to the nanofretting wear of silicon. In addition, with the help of water molecules, the SiO2 tip can chemically link with Si(100) substrate through the formation of Si-O-Si bridges, which facilitates the Si atoms to break off the Si(100) substrate.