Molecular probes reveal deviations from Amontons’ law in multi-asperity frictional contacts

• Published in: Nature communications Vol. 9; no. 1; pp. 888 - 7
• Main Authors: Weber, B., Suhina, T., Junge, T., Pastewka, L., Brouwer, A. M., Bonn, D.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.03.2018; Nature Publishing Group; Nature Portfolio
• Subjects: 132/124; 147/3; 639/301/119/544; 639/301/357/537; Asperity; Coefficient of friction; Computer simulation; Friction; Humanities and Social Sciences; Legal issues; multidisciplinary; Plasticity; Science; Science (multidisciplinary); Tribology; Ultrasonic testing
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-018-02981-y

Amontons’ law defines the friction coefficient as the ratio between friction force and normal force, and assumes that both these forces depend linearly on the real contact area between the two sliding surfaces. However, experimental testing of frictional contact models has proven difficult, because few in situ experiments are able to resolve this real contact area. Here, we present a contact detection method with molecular-level sensitivity. We find that while the friction force is proportional to the real contact area, the real contact area does not increase linearly with normal force. Contact simulations show that this is due to both elastic interactions between asperities on the surface and contact plasticity of the asperities. We reproduce the contact area and fine details of the measured contact geometry by including plastic hardening into the simulations. These new insights will pave the way for a quantitative microscopic understanding of contact mechanics and tribology. Amontons’ law assumes that friction and normal forces depend linearly on the contact area. Here, the authors use a new contact detection method to show that the law is broken because asperities interact and deform in the contact area to change it, thereby also changing the friction force.