On the humidity dependence of highly loaded graphite contacts

Graphite is a well-known solid lubricant that has been studied for decades. At low loads, graphite’s lubricity depends on humidity. Classical models like e.g. the adsorption model explains this by molecular water films on graphite leading to defect passivation and easy sliding of counter bodies [1]. To explore the humidity dependence and validate the adsorption model for high loads (loads around 1 GPa as typically found in rolling bearing elements), a commercial graphite solid lubricant was studied using microtribometry [2]. Even at very high contact pressures, a high and low friction regime is observed - depending on humidity. Transmission electron microscopy (TEM) and Electron Energy Loss spectroscopy (EELS) reveal transformation of the polycrystalline graphite lubricant into turbostratic carbon after high but also after low load (50 MPa) sliding. Quantum molecular dynamics simulations relate high friction and wear to cold welding and shear-induced formation of turbostratic carbon, while low friction originates in molecular water films on surfaces. The combined experiments and simulations lead to a novel, generalized adsorption model including turbostratic carbon formation.
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[1] Savage, R. H. Graphite lubrication. J. Appl. Phys. 19, 1–10 (1948).
[2] C. Morstein, A. Klemenz, M. Dienwiebel, M. Moseler, Nature Comm. 13, 2022