Structure-mechanical property relations of non-graphitizing pyrolytic carbon synthesized at low temperatures

Polymer-derived carbon ceramics are highly desirable for lightweight, high strength, extreme environment material architectures, but their mechanical performance as a function of structure and processing is not currently understood and cannot be predicted. In this study, the mechanical behavior for bulk-scale pyrolytic carbons (PyCs) made via polymer pyrolysis of phenol-formaldehyde precursors is established as a function of heat treatment temperature and the resulting average nano- and meso-scale order and disorder via   X-ray diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy. The PyCs exhibit crystallite evolution on both the atomic- and meso-scale for pyrolysis temperatures of 600 °C to 1000 °C, whereas only atomic-scale crystallite evolution is observed for pyrolysis temperatures of 1000 °C to 1400 °C. The measured Vickers hardness of the PyCs is observed to scale non-monotonically as a function of the pyrolysis temperature reaching a peak at ∼4∼4 GPa for samples prepared at 1000 °C. New modeling results, based on the elastic constants of disordered graphite, indicate that this counter-intuitive Vickers hardness scaling, which is a decades-old open question, originates from the PyC inter-layer shear elastic constant and the crystallite aspect ratio evolution with processing temperature. PyCs studied here are shown to be the lightest super-hard materials, having Vickers hardness-to-density ratios that are comparable to super-hard carbides, oxides, nitrides, and phosphides.

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