Intuitively Understanding Solid Friction

Friction phenomena tend to be complicated since any practical surface contains unexpected geometric non-uniformity and chemical heterogeneity, such as surface roughness and oxidation films. Although one can attribute the truth that the friction coefficient is not material constant to the unexpected aspects, even if we could remove all of them, it would not become material constant. This article discusses how to understand solid friction intuitively for beginners in tribology. Starting with the friction of plastic materials, we move on to that of viscoelastic materials. Viscoelastic foundation modeling is helpful as a thinking tool for intuitively understanding viscoelastic solid friction, which tells us how essential the boundary condition of sliding systems is. For sliding between a rigid probe and a viscoelastic foundation, the probe lifts upward in the middle- and high-speed regions when it can move vertically, leading to bell-shaped velocity-dependent friction even without bell-shaped rheology. The theoretical analysis provides the dimensionless master curves of indentation depth and friction coefficient, which can be fundamental building blocks to challenge more realistic friction phenomena.