| Nowadays, the micromation trends in the high-technology fields, such as information, aerospace and aviation, advanced manufacturing, biological, and so on, create a batch of high performance microelectricmechanical systems (MEMS). However, tribological problems have still been the major concerns in the applications of silicon based MEMS. For instance, the dynamic parts are easy to fail due to the high friction and serious wear of contact interface in humid conditions. In addition to the traditional coating protection and fluid lubrication, the applications of alcohol vapor lubrication in MEMS device have been identified as an effective way to prevent the silicon wear and extend its lifetime. The method is expected to be widely used in MEMS for its advantages of low cost and without being limited by the matrix components complex structure. The mechanism of alcohol vapor protection is a high enough vapor pressure for gas phase delivery of molecules to the MEMS surface. However, the formation of alcohol protection layer strongly depends on not only the alcohol partial pressure, but also the adsorption of water molecules from humid ambient. Thus, it is essential to understand the nanowear of silicon in various relative humidity (RH) and alcohol partial pressure conditions.In this paper, with an environmentally controlled atom force microscope (AFM) and an outside atmosphere control system, the nanowear of silicon against SiO2 microsphere was investigated under various RH and alcohol partial pressure conditions. The protective mechanism of ethanol vapor in humidity environment was also revealed. The main conclusions were summarized as following:(1) The outside atmosphere control system for AFM was designed and built.The outside atmosphere control system is designed and built based on the Antoine equation experience formulas, where the flow control and the environment temperature are used to adjust the required content of experimental atmosphere. In this system, the scope of relative humidity and ethanol relative pressure can be controlled between 0%～90%, respectively. Futhermore, with linking to AFM environmental chamber, the mixed gas of humidity and ethanol with different partial pressure can be accurately controlled with the acuracy of 3%.(2) The nanowear of Si/SiO2 pair was systematically investigated under various relative humidity (RH) and alcohol partial pressure.The nanowear of Si/SiO2 pair strongly depends on RH and ethanol partial pressure. When the ethanol partial pressure keeps constant in the environment, the wear degree of monocrystalline silicon surface will increase firstly and then decrease with the increase of RH. The most serious wear occur at RH of 60%. Under the fixed humidity environment, the wear of silicon surface will reduce with the increase of ethanol relative partial pressure. At RH below 50%, there exists a critical ethanol partial pressure above which tribochemical reaction is completely suppressed and no wear is generated on silicon surface. Such critical alcohol partial pressure reveals an increase when RH increases from 10 to 50%.(3) The protective mechanism of ethanol vapor with various partial pressure on Si/SiO2 nanowear was revealed in humidity environment.In the case of mixed vapor of water and ethanol, the binary adsorbate layers present in a layered structure, in which ethanol is at the adsorbate/vapor interface and water is inside the adsorbate layer. The results suggested that the coadsorption of water and alcohol on silicon surface would affect the activation energy barrier to cleave the Si-O-Si bonds at the interface of Si/SiO2, and then prevent the silicon surface from tribochenmical wear. Due to the competitive relationship of binary adsorbate layers on silicon surface, the increase of adsorbed water molecule will to some extent reduce the amount of adsorbed ethanol molecule, which leads to the variation of nanowear of Si/SiO2 interface with the environmental humidity and ethanol vapor partial pressure. |
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