Visualization of Tribological Phenomena: What Happens at the Frictional Interface?

Tribological phenomena (friction, wear, and lubrication) are not limited to the interaction between two friction surfaces, but are also affected by foreign matter and transfer particles at the frictional interface, as well as by surrounding gases and lubricants. Especially for polymer materials, including rubber materials, the mechanism of friction and wear becomes a more complex many-body problem due to the effects of energy loss and heat generation caused by the inherent viscoelasticity. Since the frictional interface is usually hidden, it is necessary to stop the mechanical system, remove the relevant parts, and separate the frictional interface for observation and analysis. However, when the external force is released, the frictional interface changes completely from its actual state. In rubber materials, which are mainly elastically deformed, the deformation and damage caused by friction may return to its original shape and appear as if nothing had happened. Therefore, if tribological phenomena can be visualized without opening the frictional interface, it is possible to accurately understand the actual behavior of the material. This article describes visualization techniques for tribological phenomena and presents examples of studies using visualization techniques in order to deepen our understanding of the complex phenomena that occur at frictional interfaces.