Shock compression of pyrolytic graphite to 18 GPa: Role of orientational order
Shock wave response of highly-oriented pyrolytic graphite (HOPG) compressed to stresses above the phase transformation onset (∼20 GPa) depends strongly on the HOPG orientational order [Erskine and Nellis, Nature 349, 317 (1991)]. To gain insight into this finding, which is not understood, and because corresponding results do not exist at stresses below the transition stress, the shock compression responses of three grades of pyrolytic graphite, differing in their orientational order, were examined at peak stresses below ∼20 GPa. Measured wave profiles and the corresponding end states reveal significant differences in the shock wave response of highly oriented ZYB-grade HOPG, less oriented ZYH-grade HOPG, and as-deposited pyrolytic graphite (PG). For peak stresses above 9 GPa, ZYB-grade HOPG exhibits a two-wave structure (elastic-inelastic response); the large elastic wave amplitudes for ZYB-grade increase linearly with peak stress, reaching 16 GPa for a peak stress of 18 GPa. In contrast, ZYH-grade HOPG and PG exhibit single (overdriven) wave profiles, due to considerably smaller elastic limits, at all peak stresses. Measured peak states show that PG is more compressible than ZYB- and ZYH-grade HOPG. Our results show that orientational order plays a significant role in the shock wave response of pyrolytic graphite over a broad range of stresses below the transition stress (∼20 GPa). This finding agrees well with the orientational order effects observed above the phase transition onset. The ZYB-grade HOPG, the only pyrolytic graphite that showed a clear phase transition response above ∼20 GPa in the work by Erskine and Nellis, is also the only pyrolytic graphite that displayed a large elastic limit which increases with peak stress. The latter finding suggests that the rapid phase change observed in shocked ZYB-grade HOPG may be a consequence of the large elastic compression observed for this orientational order.