【作者】 陈磊；

【导师】 钱林茂；

【作者基本信息】 西南交通大学 ， 机械设计及理论， 2013， 博士

【摘要】 纳米科技开创了21世纪人类生活的新时代,其发展不仅将促使人类认知的革命,也是人类实现可持续发展的重要保证。目前,信息、生物、先进制造、航空航天等高新技术领域的微型化趋势极大地促进了微／纳机电系统(M/NEMS)的发展,催生出批高性能微纳机电系统。然而,由于尺寸和表面效应,黏着和磨损等纳米摩擦学问题成为困扰M/NEMS成功研制和长期可靠服役的关键因素。磨损按照材料去除机理的不同,大致可分为机械磨损和摩擦诱导的化学磨损两大类。相对应纯机械磨损,摩擦化学磨损在大气环境中更容易发生且对材料造成的损伤程度更严重,对M/NEMS的危害也更大。因此,本文针对硅基M/NEMS制造和运行中的纳米磨损问题,首先利用二氧化硅球形探针深入探讨了单晶硅材料的纳米磨损行为和机理,通过改变针尖滑动速度和环境湿度实现了单晶硅对水诱导摩擦化学磨损的屏蔽；最后使用原子力显微镜和纳米压痕/划痕仪系统考察了超薄DLC涂层对单晶硅(100)纳米磨损和纳动磨损的防护能力。本论文通过对单晶硅纳米磨损及其防护的系统研究,得到的主要结论如下：(1)发现了微观摩擦过程中的磨合现象,揭示了其摩擦磨损机制。纳米尺度下,单晶硅／二氧化硅摩擦副在大气环境下表现为典型的磨合过程,摩擦系数和磨损率随循环次数的增加呈先急剧降低而后保持平稳的变化趋势。与宏观条件下机械作用对粗糙表面的平滑机制不同,纳米尺度下硅／二氧化硅摩擦副的磨合过程主要归因于单晶硅表面氧化层的去除以及二氧化硅针尖表面的疏水化。(2)揭示出针尖滑动速度和湿度对单晶硅纳米磨损的影响规律。单晶硅表面的纳米磨损与二氧化硅针尖的滑动速度以及环境湿度密切相关。尽管单晶硅在干燥空气下具有优异的机械磨损抵抗力,但水分子参与的摩擦化学反应导致单晶硅表面极易发生严重磨损。在不同湿度的空气环境中,单晶硅的磨损率随针尖滑动速度的增加呈指数级降低直至稳定；在不同滑动速度下,磨损率随湿度增加表现为先逐渐增大,而当湿度30%后保持平稳。分析表明,单晶硅磨损体积随滑动速度和环境湿度的变化主要归因于接触界面间吸附水膜厚度和结构的改变。(3)大气环境中实现了单晶硅对摩擦化学磨损的屏蔽,并阐明其原因。潮湿空气中单晶硅／二氧化硅配副在滑动过程中的摩擦化学反应并不总会发生。在高速度／低湿度或低速度/高湿度条件下,单晶硅基体在滑动过程中表现为无磨损或轻微磨损现象。在低湿度/高速度环境下(<30%),单晶硅与二氧化硅接触界面在速度足够快时来不及形成吸附液桥,导致摩擦化学反应难以发生。在其它实验条件下,二氧化硅针尖与单晶硅接触界面间主要发生以下两类化学反应：①单晶硅/二氧化硅配副间相连硅烷醇的脱水反应,在这个过程中单晶硅表面出现持续磨损；②单晶硅样品或二氧化硅针尖接触表面相邻硅烷醇的脱水反应,其结果导致单晶硅表面在100次循环滑动后只出现轻微损伤或无损伤。(4)揭示出超薄DLC涂层对单晶硅纳米磨损和纳动磨损的防护机制。通过系统研究不同厚度DLC涂层对单晶硅机械磨损和摩擦化学磨损的影响规律,证明厚度仅为2nm的超薄DLC涂层能够有效防护单晶硅基体的纳米磨损。当使用金刚石针尖时,超薄DLC涂层由于良好的机械性能能够有效阻止单晶硅表面隆起结构的产生。当使用二氧化硅针尖时,DLC涂层因优异的化学稳定性能够有效阻隔单晶硅/二氧化硅配副与水分子接触,防止单晶硅表面摩擦化学磨损的发生。另外,在纳动运行中,超薄DLC涂层因表面能较低,能够显著降低潮湿环境中单晶硅/二氧化硅配副间的黏着效应,有效缓解纳动中的高摩擦磨损,同时使纳动分区向低位移幅值方向移动。更多还原

【Abstract】 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.更多还原