Department of Mechanical Engineering and Applied Mechanics , University of Pennsylvania , Philadelphia , Pennsylvania 19104 , United States.
Mechanical Engineering Department , North Carolina Agricultural and Technical State University , Greensboro , North Carolina 27411 , United States.
ACS Appl Mater Interfaces. 2019 Oct 30;11(43):40734-40748. doi: 10.1021/acsami.9b08695. Epub 2019 Oct 16.
Nanoindentation and sliding experiments using single-crystal silicon atomic force microscope probes in contact with diamond substrates in vacuum were carried out in situ with a transmission electron microscope (TEM). After sliding, the experimentally measured works of adhesion were significantly larger than values estimated for pure van der Waals (vdW) interactions. Furthermore, the works of adhesion increased with both the normal stress and speed during the sliding, indicating that applied stress played a central role in the reactivity of the interface. Complementary molecular dynamics (MD) simulations were used to lend insight into the atomic-level processes that occur during these experiments. Simulations using crystalline silicon tips with varying degrees of roughness and diamond substrates with different amounts of hydrogen termination demonstrated two relevant phenomena. First, covalent bonds formed across the interface, where the number of bonds formed was affected by the hydrogen termination of the substrate, the tip roughness, the applied stress, and the stochastic nature of bond formation. Second, for initially rough tips, the sliding motion and the associated application of shear stress produced an increase in irreversible atomic-scale plasticity that tended to smoothen the tips' surfaces, which resulted in a concomitant increase in adhesion. In contrast, for initially smooth tips, sliding roughened some of these tips. In the limit of low applied stress, the experimentally determined works of adhesion match the intrinsic (van der Waals) work of adhesion for an atomically smooth silicon-diamond interface obtained from MD simulations. The results provide mechanistic interpretations of sliding-induced changes and interfacial adhesion and may help inform applications involving adhesive interfaces that are subject to applied shear forces and displacements.
在真空中使用单晶硅原子力显微镜探针与金刚石基底进行纳米压痕和滑动实验，并在透射电子显微镜（TEM）中进行原位观察。滑动后，实验测量的粘附功明显大于纯范德华（vdW）相互作用估计的值。此外，粘附功随滑动过程中的法向应力和速度的增加而增加，表明施加的应力在界面反应性中起着核心作用。补充的分子动力学（MD）模拟用于深入了解这些实验中发生的原子级过程。使用具有不同粗糙度的晶体硅尖端和具有不同数量氢终止的金刚石基底进行的模拟表明了两个相关现象。首先，在界面上形成了共价键，键的形成数量受基底的氢终止、尖端粗糙度、施加的应力和键形成的随机性的影响。其次，对于初始粗糙的尖端，滑动运动和相关的剪切应力施加导致不可逆的原子尺度塑性增加，这倾向于使尖端表面变平滑，从而导致粘附力的相应增加。相比之下，对于初始光滑的尖端，滑动会使其中一些尖端变粗糙。在低施加应力的极限下，实验确定的粘附功与从 MD 模拟获得的原子光滑硅-金刚石界面的固有（范德华）粘附功匹配。结果提供了对滑动诱导变化和界面粘附的机械解释，可能有助于为涉及受剪切力和位移作用的粘附界面的应用提供信息。
