Oscillatory Tribometry: A New Concept for Gaining More Insight into the Crossover from Static to Kinetic Friction
Oscillatory tribometry is introduced as a new method for investigations on the crossover from static friction to kinetic friction. The method allows distinguishing between purely elastic deformations and plastic frictional movements. 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 movements give rise to the dissipation friction modulus, which is directly proportional to the average dissipated energy over an oscillation cycle, respectively. The dissipation of energy in the static and boundary regime can be attributed to local break-up of contact bonding or intermolecular adhesion bonds at the contact points and thus to local slip. The ability to measure at very small sliding distances in oscillatory tribometric testing is a crucial requirement for such investigations. The methodology gives access to local friction phenomena on intermolecular scale while measuring macroscopic samples. The presented application examples show that tribometric oscillatory testing on a highly sensitive rheometer is particularly valuable for applications such as dry contacts between polymers, elastomers or soft materials.
Friction coefficients in sliding conditions can be accessed by standard equipment. Stribeck curves over various loading conditions are often the basis for the calculation of the coefficient of friction from the measurements of the friction forces. In the mixed and hydrodynamic regimes of lubricated tribological contact where sliding is prevalent the coefficient of friction mainly depends on the viscosity of the lubricant. In the static and boundary regimes, however, the tribological behavior is governed by surface interactions between the two contacting bodies for dry conditions and between the tribopair and the lubricant for lubricated systems.
Critical aspects of tribological systems like start-stop motions or stick-slip properties are characterized by mechanisms which take place both in static and dynamic friction. However, the properties in static friction are not easily measureable. Measurements at extremely low speeds and/or very small deflections are necessary to look at these phenomena in detail and to investigate the nature of the phenomena. Different approaches have been employed to measure static friction behavior starting from tilted surfaces and weight and pulley setups to more sophisticated apparatuses such as a centrifugal device, linear friction testers[2,3], and rotational tribometers operated at very small speeds or in torque control. The coefficient of friction at the break-away point corresponds to the limiting friction of the system and marks the crossover point from static to kinetic (or dynamic) friction.
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