Molecular dynamics examination of sliding history-dependent adhesion in Si-Si nanocontacts: Connecting friction, wear, bond formation, and interfacial adhesion

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.