Atomic simulations of effects of contact size and interfacial interaction strength on superlubricity in incommensurate sliding interface

Understanding the effects of contact size and interfacial interaction strength on superlubricity in incommensurate sliding interface is critically needed for the design and development of nanoscale ultra-low friction devices. This study uses molecular dynamics simulations to explore the sliding friction behaviors of an incommensurate interface consisting of a diamond slider and a silver substrate. The instantaneous relative lattice constant is proposed to quantitatively describe the commensurability of contacting surfaces in the sliding process. It is found that when the contact size is large, the slider exhibits ultra-low friction force. While for small contact size, superlubricity behavior breaks down,which is due to the transition of incommensurate-commensurate interfacial configuration in the local contact region. It is also found that when the interfacial interaction strength is reduced below a critical value, the obvious stick-slip motion observed for the small slider with large interfacial interaction strength disappears and superlubricity behavior occurs, which results from the incommensurate interfacial configuration in the contact region maintained during the sliding process. These results provide a first demonstration that the instantaneous incommensurate-commensurate transition in the local contact region can result in the breakdown of superlubricity in a realistic three-dimensional sliding system. The obtained results not only may guide the design of nanoscale ultra-low friction devices, but also provide some insights into the origins of friction at macroscopic interfaces which usually consists of many small nanoscale contacts.