| With the rapid development of nanotechnology, silicon based microelectromechanical systems (MEMS) have been widely used in human life, medicine, advanced manufacture, military defense and so on. However, the nanowear of MEMS has become one of the critical factors to limit the long-term reliable work of MEMS. In addition, nanofabrication is the foundation to realize the transition from nanoscience and nanotechnology to application. There are many nanowear issues in the typical nanofabrications, such as chemical-mechanical polishing (CMP). Therefore, the nanowear not only becomes the key problem in the application of MEMS, but also turns into the basic issues in the nanofabrications. However, lots of previous studies focused on the nanowear of silicon at low relative humid (RH<65%). There was few research focused on the nanowear of silicon at high RHs or in water. Therefore, in order to reveal the nanowear mechanism of silicon, it is very essential to research the nanowear of silicon in the full-range humid conditions and in water. The nanowear research of silicon will enrich the nanotribology theory, optimize the process of CMP and promote the application of MEMS.Based on the pair of Si/SiO2, the nanowear behaviors of monocrystalline silicon (100) against SiO2 tip were investigated by an atomic force microscopy (AFM) at different RH conditions and in water. Firstly, in order to make a reference for the following research in nanoscale, the wear of silicon in macroscale was investigated by a servo-hydraulic reciprocating sliding apparatus. Secondly, the nanowear of silicon was carried out at different RH conditions. Subsequently, the effect of normal load and wear cycle on nanowear of silicon in water was investigated. Finally, the wear mechanism of silicon at various RHs and in water was revealed by the transmission electron microscope (TEM) tests and the wear tests of different friction pairs. Based on these systematical investigations, the main conclusions can be summarized as following:(1) The contributions of oxygen and absorbed water film to the tribochemical wear of silicon in macroscale were revealed.By using a reequipped servo-hydraulic reciprocating sliding apparatus, the macrowear of silicon was investigated in the dry conditions with different oxygen content. It was indicated that the wear of silicon was slight in nitrogen, but serious in dry air and oxygen. The effect of absorbed water film on the wear of silicon was researched by using silicon samples with different hydrophilicity. It was found that the wear of silicon would become serious with the increase of the thickness of absorbed water under the contact pressure around 278 MPa. Further analysis suggested that both of oxygen and absorbed water would facilitate the tribochemical wear of silicon. The research of macrowear on silicon surface provides a reference for the following research in nanoscale.(2) The variation of nanowear on silicon surface at full-range RHs (RH=0%-90%) was demonstrated.By using a reequipped AFM, the nanowear of silicon was investigated at full-range RHs (RH=0%-90%). In nanoscale, a hillock-like wear scar would form on the silicon surface at RH< 10%. With the increase of RH (<50%), the hillock-like scar would turn into the groove-like wear scar and the groove-like wear scar was the most serious at RH=50%. With the further increase of RH, the wear of silicon would be gradually restrained and became wearless at RH>85% and in water. It was clear that the nanowear of silicon would become serious firstly, and then become slight. The nanowear research of silicon at different RHs would offer a new approach to protect the MEMS from serious nanowear.(3) The effect of electrical double layer (EDL) and the elastic hydrodynamic lubrication on the nanowear of silicon in water was negligible. Meanwhile, the tribochemical wear mechanism of silicon in water was revealed.To understand on the nanowear of silicon in water, the nanowear tests under different normal loads and wear cycles were carried out both at 50%RH and in water. It was indicated that the nanowear on silicon surface would become serious with the increase of normal load and the number of wear cycles at RH=50%. However, the nanowear on silicon surface would be close to wearless with the increase of normal load and the number of wear cycles in water. Since the nanowear on silicon surface was slight both in NaCl solution and in water with different sliding speeds, the electrical double layer and the elastic hydrodynamic lubrication were not the main roles causing the wearless of silicon in water. It was demonstrated that the tribochemistry played an important role in the nanowear of silicon in water.(4) The varying thickness of water film, different water structure and the number of "Si-O-Si" bonding bridge between contact surfaces were found to be the nanowear mechanism of silicon.At RH< 10%, there was less than 1 monolayer ice-like water would form on the silicon sample surface (with native oxide layer) and the "Si-O-Si" bonding bridge was hard to form. Then the mechanical interaction dominated the wear of silicon during the wear process. At 10%< RH<50%,1-3 monolayer ice-like water would form on the silicon sample surface and the tribochemistry would be facilitated to take place due to the increased number of "Si-O-Si" bonding bridge. At RH> 50%, the liquid water would grow on the surface of ice-like water. The tribochemistry would be gradually restrained with the decreased number of "Si-O-Si" bonding bridge. This study reveals the removal mechanism of silicon in nanoscale and enriches the nanotribology theory.In summary, the varying thickness of water film, different water structure and the number of "Si-O-Si" bonding bridge between contact surfaces would affect the tribochemical nanowear on silicon surface at different RHs and in water. The research of nanowear on silicon surface at different RHs and in water would not only enrich the nanotribology theory, but also provide the theoretical basis for the tribological design of MEMS and the optimization of the CMP process of silicon. |
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