The Tribological Performances of Multilayer Graphite-Like Carbon (GLC) Coatings Sliding Against Polymers for Mechanical Seals in Water Environments

Abstract

Graphite-like carbon (GLC) coatings are being increasingly used in mechanical seals, especially under the water-lubricated condition, to improve component durability by providing a low friction coefficient and high wear resistance. We have addressed the friction and wear performances of multilayer Cr/CrN/GLC coatings sliding against polyether–ether–ketone (PEEK), polyimide (PI), and polytetrafluoroethylene (PTFE) in distilled water under various applied loads and sliding speeds using a ring-on-block test rig. Stribeck curves were developed to analyze the water lubrication mechanisms. Start–stop tests were also carried out to evaluate the tribological performances of the coatings. Our results showed that the friction coefficients and wear rates of the polymers decreased with increasing sliding speeds. Compared to the applied load, the sliding speed had a major effect on the friction and wear performance of the polymer. Three coating/polymer tribopairs demonstrated different tribological behaviors, which were attributed to their different molecular structures, the different mechanical properties of the polymers, and the different lubrication mechanisms. The transfer film determined the lubrication mechanism of the three tribopairs and also resulted in the low friction coefficient and wear rate. The initial and steady-state friction coefficient also decreased with increases in the repeated start–stop times. Based on our results, we concluded that the coating/PEEK tribopair presented an excellent tribological performance that we ascribed to the chemical properties of PEEK and the hydrodynamic lubrication regime of the coating/PEEK tribopair at high sliding speeds.