Abstract
Classical laws of friction suggest that friction force is proportional to the normal load and independent of the nominal contact area. As a great improvement in this subject, it is now widely accepted that friction force is proportional to the real contact area, and much work has been conducted based on this hypothesis. In present study, this hypothesis will be carefully revisited by measuring the friction force and real contact area in-site and real-time at both normal loading and unloading stages. Our experiments reveal that the linear relation always holds between friction force and normal load. However, for the relation between friction force and real contact area, the linearity holds only at the loading stage while fails at the unloading stage. This study may improve our understanding of the origin of friction.
Highlights
Classical laws of friction suggest that friction force is proportional to the normal load and independent of the nominal contact area
Approximate 20 percent of the global energy consumption is attributed to harmful friction[1], and it is of practical significance to improve the fundamental understanding of friction
Back in 1493, Leonardo da Vinci observed that the friction force between two solids is proportional to the applied normal load and independent of the nominal contact area[2]
Summary
Classical laws of friction suggest that friction force is proportional to the normal load and independent of the nominal contact area. As a great advance in understanding of friction, Bowden and T abor[7] argued that the tangential force required to slide each contact junction is proportional to the junction area by a critical shear strength, which leads to the linearity between friction force and the real contact area. There are multi-asperities models[8,9,10], fractal models[11,12,13], Persson theory[14,15], etc Almost all these theories showed approximately linear load-area relationships within a certain contact fraction[16]. The real contact area decreases monotonously but nonlinearly with respect to normal load, which may provide a scenario to test whether friction force exclusively depends linearly on contact area or normal load
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