The lubricity of graphite flake inclusions in sintered polyimides affected by chemical reactions at high temperatures

The internal lubrication of 15 vol% graphite flakes added to polyimide was investigated during sliding at 50–200 N normal loads, 0.3 m/s and 23–260 °C, showing that chemical reactions in the polyimide bulk have an important effect on the sliding efficiency of the graphite. The latter are illustrated by Raman spectroscopy, thermo-analytical measurements and mass spectroscopy. At low temperatures (23–100 °C), transfer consists of pure graphite and mechanical shear prevails. At intermediate temperatures (120–180 °C), transfer consists of coexistent polyimide and graphite. A regime with decreasing friction is related to hydrolysis reactions in the bulk polyimide, lowering the effective water content in the sliding interface. At higher temperatures (180–260 °C), transfer consists of mixed polyimide–graphite. A low friction regime is related to imidisation of the bulk polymer and interfacial water supply, favouring the graphite lubricating properties. The graphite debris morphology occurs as flakes (low temperature) or powders (intermediate temperatures), likely resulting from cleavage between the basal planes or embrittlement across the basal planes.