Experimental Investigation of the Correlation Between Adhesion and Friction Forces

In this study, the effect of adhesion on evolution of friction during the transition of the contact from pre-sliding into full sliding was investigated. In order to achieve the objectives, a micro optical friction (MOF) apparatus was developed to conduct dry sliding friction experiments and to allow for in situ visualization of the contact area for a sphere-on-flat configuration. MOF apparatus was used to measure friction under various load and speed combinations. The friction results exhibit the commonly observed behavior in friction (i.e., static friction is larger than dynamic friction). The results also demonstrated that the difference between static and dynamic friction forces increased with an increase in the applied normal load. We hypothesize and demonstrate that the difference between the measured maximum friction force commonly referred to as static friction force and the steady state or dynamic friction force divided by the dynamic coefficient of friction is the force of adhesion. The adhesion force results obtained from our experimental investigation corroborate well with the force of adhesion described by the DMT model. The reduction in friction force is attributed to the diminishing of adhesion force during full sliding of the contact.