Considerations for Defining the Mechanisms of Friction

The ultimate goal of joints research is to develop predictive models of the interfacial behavior, in terms of the stiffness and friction (energy dissipation). The concept of friction, commonly thought of in terms of Coulomb’s model, is based on an empirical constant, the coefficient of restitution that must be experimentally measured for different geometries, material pairings, surface finishes and chemistries, load histories, etc. Thus, the model of friction is unable to be predictive in an a priori sense. In order to address this shortcoming, predictive models of the stiffness and dissipation characteristics of an interface are needed that can be parameterized in terms of material, geometric, and load history associated properties (in order to capture effects associated with the evolution over time). This necessitates answering the question: what mechanisms are involved in frictional energy dissipation? Some potential mechanisms include heat generation, acoustics, material damping, and plasticity, though other mechanisms cannot be entirely ruled out in all applications, e.g., electromagnetic radiation (such as in a coupled media), electrical resistance, fluid/air resistance, etc. Even in considering dissipation through elastic and plastic means, issues related to the bond and atomistic properties that the elastic properties are derived from must be taken into account. In addition, the crystal orientation and grain boundaries that drive the plastic properties must also be considered in order to develop an a priori predictive model. Thus, the development of a coherent model that can bridge length scales from the atomistic level to the meso-scale is paramount.