We simulate the contact between nanoscale hydrogen-terminated, single-crystal silicon asperities and surfaces using reactive molecular dynamics (MD) simulations. The results are consistent with recent experimental observations of a more than order-of-magnitude sliding-induced increase in interfacial adhesion for silicon-silicon nanocontact experiments obtained using in situ transmission electron microscopy (TEM). In particular, the MD simulations support the hypothesis that the increased adhesion results from sliding-induced removal of passivating species, in this case hydrogen, followed by rapid formation of Si–Si covalent bonds across the interface, with little plastic deformation of the asperities. The MD results concur with the additional hypothesis that subsequent readsorption of passivating species explains the experimental observation that adhesion reverts to low values upon subsequent contact. However, the simulations further reveal that the sliding-induced adhesion increase is only observed when there are a sufficient number of preexisting surface defects in the form of incomplete hydrogen coverage. Increased hydrogen coverage suppresses interfacial bonding, within the time span of the simulations. Furthermore, the relative alignment of the surface crystal axes plays a strong role in affecting the probability of bond formation during sliding and the subsequent adhesive pull-off force. Also, the hydrogen coverage and sliding distance significantly impact friction at low to moderate hydrogen coverages. Atomic-scale wear does occur during the sliding process primarily through Si–Si bond formation across the interface followed by pull-out of Si atoms from the tip. At low hydrogen coverages, wear is far more severe, Archard’s wear law is obeyed, and significant morphological changes of the asperity occur. The bond formation process is highly stochastic, but shows a general trend of greater numbers of bonds with greater sliding distances. Tips wear by losing large clusters of material, then smaller clusters and individual atoms, and eventually enter into a wearless regime as hydrogen termination increases.

Graphical Abstract

A hydrogen-terminated Si tip (green and blue) in sliding contact with a hydrogen-terminated Si substrate (yellow and red). The sliding direction is indicated by the black arrow. At this level of hydrogen termination, wear is initiated by the removal of hydrogen atoms from the tip (blue atoms at left of figure). Continued sliding causes the formation of interfacial Si-Si bonds followed by the transfer of Si and H from the tip to the surface.

中文翻译：

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Si-Si纳米接触中取决于滑动历史的粘附的分子动力学检查：连接摩擦，磨损，键形成和界面粘附

我们使用反应分子动力学（MD）模拟来模拟纳米级氢封端的单晶硅粗糙与表面之间的接触。该结果与最近的实验观察结果一致，该观察结果表明，使用原位透射电子显微镜（TEM）获得的硅-硅纳米接触实验的界面粘附性出现了数量级以上的滑动诱导增加。尤其是，MD模拟支持这样的假设，即附着力增加是由滑动诱导的钝化物质（在这种情况下为氢）的去除引起的，然后在整个界面上迅速形成Si-Si共价键，而细孔的塑性变形很小。MD结果与另外的假设一致，即随后钝化物质的再吸收解释了实验观察结果，即在随后的接触中粘附力恢复为低值。然而，模拟进一步揭示，仅当存在足够数量的以不完整的氢覆盖形式存在的表面缺陷时，才能观察到滑动诱导的粘附性增加。在模拟的时间内，增加的氢覆盖率可抑制界面键合。此外，表面晶轴的相对排列在影响滑动过程中形成键的可能性以及随后的粘合剂剥离力方面起着重要作用。同样，氢覆盖率和滑动距离会显着影响中低氢覆盖率下的摩擦。在滑动过程中，确实会发生原子级磨损，主要是通过在界面上形成Si-Si键，然后从尖端拉出Si原子。在低氢覆盖率下，磨损要严重得多，遵守阿卡德磨损定律，并且会发生粗糙的形态变化。键的形成过程是高度随机的，但是显示出具有更大数量的键并具有更大的滑动距离的总体趋势。尖端磨损会损失大量的材料团，然后损失较小的团簇和单个原子，最终随着氢封端的增加而进入无磨损状态。并且粗糙的形态发生了明显的变化。键的形成过程是高度随机的，但是显示出具有更大数量的键并具有更大的滑动距离的总体趋势。尖端磨损会损失大量的材料团，然后损失较小的团簇和单个原子，最终随着氢封端的增加而进入无磨损状态。并且粗糙的形态发生了明显的变化。键的形成过程是高度随机的，但是显示出具有更大数量的键并具有更大的滑动距离的总体趋势。尖端磨损会损失大量的材料团，然后损失较小的团簇和单个原子，最终随着氢封端的增加而进入无磨损状态。

图形概要

氢终止的硅尖端（绿色和蓝色）与氢终止的硅衬底（黄色和红色）滑动接触。滑动方向由黑色箭头指示。在此氢终止水平下，磨损是通过从尖端去除氢原子（图左侧的蓝色原子）而引发的。持续的滑动会导致形成界面Si-Si键，然后将Si和H从尖端转移到表面。