Oscillatory tribometry: transition from static to kinetic friction

Oscillatory tribometry is presented as a new method for investigations on the transition from static friction to kinetic friction. The method distinguishes between purely elastic deformations and plastic frictional motions. Friction moduli are defined which are directly proportional to the stored and dissipated energy during an oscillation cycle. The elastic deformations are represented through the elastic friction modulus, which is directly proportional to the average elastic energy over an oscillation cycle, while the frictional motions yield the dissipation friction modulus, which is directly proportional to the average dissipated energy over an oscillation cycle. Energy dissipation in the static and boundary regime can be attributed to local dissolution of contact bonds or intermolecular adhesion bonds at the contact points, and thus to local slip. The ability to measure at very small sliding distances in oscillatory tribometer tests is a crucial prerequisite for such investigations. The methodology provides access to local friction phenomena at the intermolecular level while simultaneously measuring macroscopic samples. The application examples presented show that oscillatory tribometry on a highly sensitive rheometer is particularly valuable for applications such as dry contacts between polymers, elastomers or soft materials.