NANOTRIBOLOGICAL RESPONSES OF A MOLECULARLY THIN LUBRICANT CONFINED BETWEEN ROUGH SURFACES

Recent innovations and developments in the field of nanotechnology and subsequent advent of miniaturized moving components have led scientists to extensively investigate the atomic-scale origins of friction and lubrication. According to the classical Stribeck relation, the minimum value of friction occurs in the mixed lubrication regime where sliding surfaces are mainly separated by a thin layer of a lubricant which is highly demanded for many applications such as magnetic storage devices and nano/micro electromechanical systems. In this regime, the amount of space between surfaces rapidly decreases as surface asperities move toward each others which causes a highly nonuniform pressure between sliding surfaces. To optimally design an efficient lubricant at the nanoscale, a fundamental understanding of lubrication mechanisms is required. In this study, first we examine the Stribeck relation at the nanoscale. More specifically, we investigate frictional responses of a molecularly thin lubricant (MTL) confined between two idealized rough surfaces in terms of various parameters such as applied force, sliding velocity and inter-facial adhesion. Figure 1a and b show sliding responses of a polymeric network lubricant under different sliding velocities. It can be seen that the sliding velocity is a key factor which governs the lubrication mechanism. For the smaller velocity, sliding occurs mainly in a thin central layer of the lubricant where most of chains are reoriented to the sliding direction (figure 1a). While for the case with a higher velocity, two surface layers are dominantly responsible for the lubrication (figure 1v). This result highlights the importance of the interaction between different characteristic length scales involve in the system such as the surface roughness, lubricant chain length and its overall thickness.