Effects of water and oxygen on the tribochemical wear of monocrystalline Si(100) against SiO_2 sphere by simulating the contact conditions in MEMS

摘要

By using a servo-hydraulic reciprocating sliding apparatus, the wear tests of Si(100) surface against SiO_2 sphere (Si/SiO_2) were performed under four kinds of atmosphere conditions, namely as pure nitrogen, dry and humid air, and pure oxygen. Three kinds of silicon samples with different surface hydrophilicity were prepared for wear tests. To simulate the contact in microelectromechanical systems (MEMS), the contact pressure was set as 278-390 MPa and the displacement amplitude was 100 μm. The experimental results indicated that the effect of water molecule on the wear of Si/SiO_2 pairs was prominent under a low load but unobvious under a high load. Since the tribochemical reaction dominated the wear of Si/SiO_2 pairs under a low load, the increase in hydrophilicity of Si(100) surface will induce an obvious increase of the friction coefficient of Si/SiO_2 pairs and the wear depth of silicon surface. However, when the surface hydrophilicity was destroyed by serious wear under a high load, the mechanical interaction will dominate the wear process of Si/SiO_2 pairs. As a result, the friction coefficient of Si/SiO_2 pairs and the wear depth of silicon samples with different surface hydrophilicity were almost same. Moreover, oxygen played a very important role in the wear behavior of Si/SiO_2 pairs. Since no oxidation reaction occurred in pure nitrogen, the wear depth on silicon surface was smallest. The wear debris was silicon in sheet shape. However, while the wear tests were conducted in pure oxygen, the wear depth on silicon surface was highest due to the strong oxidation reaction. The wear debris was silicon oxide in powder shape. Finally, the wear depth on silicon surface in dry air was between that in pure nitrogen and pure oxygen. The results may help us to understand the tribochemical mechanism of silicon and optimize the surface treatment technology to minimize the wear failure of microdevices in MEMS.