SHOCK-INDUCED MARTENSITIC PHASE-TRANSFORMATION OF ORIENTED GRAPHITE TO DIAMOND

ONE important method of diamond synthesis is shock compression of graphite and other forms of carbon to high pressures and temperatures, and subsequent quenching to yield metastable diamond. This process, which occurs in microseconds, happens naturally in the impact of meteors 1,2, within products of explosives 3,4, and by explosive compression of powders 5,6. A major unresolved issue is whether the shock-induced phase transition of graphite to diamond is martensitic of diffusive. The relation between the crystal structures of graphite and hexagonal diamond suggests that the phase transition should be fast and martensitic if shock pressure is applied parallel to the c axis (normal to the basal planes) of the graphite crystal structure. Here we report measurements of shock-wave histories for this transition which show that it occurs in approximately 10 ns. These results imply that the transformation from graphite to diamond is martensitic for temperatures substantially lower than the melting temperature. We observe an unexpectedly large sensitivity of kinetics to sample morphology. As well as answering questions concerning the physical nature of the transformation, our results are relevant to optimization of diamond yield in industrial synthetic methods.