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Humidity-dependent lubrication of highly loaded contacts by graphite and a structural transition to turbostratic carbon
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Humidity-dependent lubrication of highly loaded contacts by graphite and a structural transition to turbostratic carbon
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Journal Article
Humidity-dependent lubrication of highly loaded contacts by graphite and a structural transition to turbostratic carbon
2022
Overview
Graphite represents a promising material for solid lubrication of highly loaded tribological contacts under extreme environmental conditions. At low loads, graphite’s lubricity depends on humidity. The adsorption model explains this by molecular water films on graphite leading to defect passivation and easy sliding of counter bodies. To explore the humidity dependence and validate the adsorption model for high loads, a commercial graphite solid lubricant is studied using microtribometry. Even at 1 GPa contact pressure, a high and low friction regime is observed - depending on humidity. Transmission electron microscopy reveals transformation of the polycrystalline graphite lubricant into turbostratic carbon after high and even 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. In this work, a generalized adsorption model including turbostratic carbon formation is suggested.
Graphite, one of the oldest known dry lubricants, loses its friction-reducing properties in dry environments. Here, the authors show that this effect is associated with both chemical modifications of the surfaces and a structural transformation of the graphite to turbostratic carbon.
Publisher
Nature Publishing Group UK,Nature Publishing Group,Nature Portfolio
Subject
