MICROSTRUCTURES OF CARBON POLYMORPHS FORMED IN SHOCKED COMPRESSED DIAMOND POWDER UTILIZING AN INTERACTION OF OBLIQUE SHOCK-WAVES

Microstructures of carbon polymorphs formed in diamond-powder compact obtained by shock compaction were investigated using high-resolution e lectron microscopy. Shock compaction was performed by impacting iron-f lyer plate at impact velocities of 1.6 and 2.3 km/s. The maximum shock pressure was estimated as 18 and 56 GPa by numerical simulation for a n initial powder density of 60% of the bulk density. The particle-size reduction resulted from the pulverization effects due to cracking, fr iction rubbing between particles, and a tensile wave. The local heatin g of particle surfaces and plastic deformation promoted the phase tran sformation from diamond to low-density phases of carbon: turbostratic carbon, i-carbon, and alpha-carbyne. The higher the shock pressure, th e more the effect of particle-size reduction and the local heating wer e enhanced. Chain-like and ribbon-like turbostratic carbon, and spheri cal ''gamma-carbon'' developed from gaseous carbon generated by jettin g from the diamond-particle surface were found al the 56 GPa region in the compact. The formation of ''gamma-carbon'' was assumed to proceed through vapor --> liquid --> solid sequences.