| Nanotechnology has created a new era of human life in21st century. The development of nanotechnology can not only promote the revolution of human cognition, but also be an important guarantee of achievement for human sustainable development. Presently, the miniaturization trend in information, biotechnology, advanced manufacturing, aerospace and other high-tech fields has greatly promoted the development of micro/nano-electromechanical system (M/NEMS), which motivated the appearance of a larger number of high-performance M/NEMS. Due to the surface and size effects in nanoscale, the nanotribology problems, such as adhesion and wear, have become the critical factors to limit the manufacture and long-term reliable work of M/NEMS.According to the different mechanism of material removal, the wear can be divided into two main kinds, including mechanical wear and tribochemical wear. Compared with the pure mechanical wear, the tribochemical wear is more prone to occur in air and is easier to induce material loss and device failure in M/NEMS applications. Based on these problems, the nanowear of monocrystalline silicon (100), the main material used in M/NEMS, as well as its protection were investigated by diamond tip and SiO2microspheric tips with an atom force microscope (AFM). Firstly, the nanowear phenomena and mechanism of silicon in humid air were intensively studied. Combining these results, the self-protection against the tribochemical wear of silicon was achieved by optimizing the sliding speed and the relative humidity (RH). Subsequently, the wear resistance of ultrathin diamond-like carbon (DLC) coating on silicon substrate was verified by using AFM and nano hardness/scratch tester (NHT/NST). Based on these systematical investigations, the main conclusions can be summarized as following:(1) The running-in phenomenon during micro friction process was discovered and the related mechanism was proposed. The nanowear of the Si(100)/SiO2pair exhibited a typical running-in process in humid air. During running-in process, both the friction force of the Si(100)/SiO2pair and the wear rate of silicon rapidly reduced to constant as the sliding cycle increased. Different from the macroscale mechanism of flattening rough asperities on counter surfaces by mechanical actions, the running-in process of the Si(100)/SiO2pair in nanoscale was dominated by the removal of the native oxide layer on silicon substrate as well as the dehydroxylation of the SiO2contact surface.(2) The sliding speed and related humidity (RH) dependent nanowear of Si(100) was revealed. The nanowear of Si(100) strongly depends on the sliding speed of SiO2tip and RH. Although the Si(100) surface was mechanically robust in dry conditions, the water-induced tribochemical reactions made it susceptible to wear in humid air. Normally, the wear volume of silicon exponentially decreased to constant at a critical speed with the increase of sliding speed, and increased with RH to the stable value at the transition RH of30%. The results indicated that the transformation of wear rate on silicon surface was mainly attributed to the variation of the thickness and structure of adsorbed water layer between SiO2tip and silicon interface as the sliding speed or RH varied.(3) The self-protection of Si(100) surface against tribochemical wear was achieved in humidity and the related mechanism was clarified. The nanowear of Si(100) rubbed with SiO2was not ubiquitous, which was highly sensitive to sliding speed and RH. The tribochemical reaction of Si/SiO2pair was restrained and there was almost wearless on silicon substrate surface under the conditions of relatively high v/low RH or low v/high RH. At low RH (<30%), since the water meniscus (bond bridges) had no enough time to be formed between SiO2tip and silicon interface, the tribochemical reaction could not happen when the sliding speed was large enough. Under other conditions, two chemical reactions could take place at sliding interface in humid conditions:(1) dehydration reaction between silanol groups on Si substrate and those on counter-surface, which led to wear;(2) dehydration reactions between adjacent silanol groups on each solid surface, which led to low-friction and low-wear state.(4) The wear resistance mechanism of ultrathin DLC coating on Si(100) was revealed. DLC coating with only2nm in thickness was enough to protect the silicon substrate against mechanical wear and tribochemical wear in nanoscale. When the nanowear test was conducted by a diamond tip, the ultrathin DLC coating could completely prevent the formation of hillock on silicon substrate. When the tests were preformed by SiO2tips, the ultrathin DLC coating with high chemical inertness isolated the direct contact between Si(100)/SiO2pair and water so that it could restrain the water-induced tribochemical wear of silicon substrate. Furthermore, in nanofretting, owing to its excellent surface property, the ultrathin DLC coating could greatly decrease the capillary effect between silicon substrate and SiO2tip in humid air. As a result, it could effectively alleviate the friction and wear in nanoscale and expand the stick regime of Si/SiO2pair into a lower value of displacement amplitude. |
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