The indentation characterisation of the mechanical properties of various carbon materials: Glassy carbon, coke and pyrolytic graphite

The force-displacement response of pyrolytic graphite, glassy carbon and coke with spherical tipped indenters is examined. The force-displacement behaviour for all these materials exhibits almost complete recovery with significant hysteresis between loading and unloading, the extent of which varies with material. Methods are discussed whereby the hysteretic indentation results may be interpreted to reveal evidence of deformation mechanisms. All three materials are shown to be elastic at very small strains with a secondary mechanism contributing to the inelastic displacement above a critical mean pressure. Indentation then proceeds with power law hardening. Unlike the behaviour of metals, which it resembles on loading, the secondary mechanism is reversible, with almost complete recovery on unloading. The secondary mechanism and hysteresis are discussed and rationalised in terms of the microstructure of the materials. Copyright (C) 1996 Elsevier Science Ltd.