Microstructures of carbon polymorphs formed in shocked compressed diamond powder utilizing an interaction of oblique shock waves

Abstract

Microstructures of carbon polymorphs formed in diamond-powder compact obtained by shock compaction were investigated using high-resolution electron microscopy. Shock compaction was performed by impacting iron-flyer 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 an initial powder density of 60% of the bulk density. The particle-size reduction resulted from the pulverization effects due to cracking, friction rubbing between particles, and a tensile wave. The local heating of particle surfaces and plastic deformation promoted the phase transformation from diamond to low-density phases of carbon: turbostratic carbon, i-carbon, and α-carbyne. The higher the shock pressure, the more the effect of particle-size reduction and the local heating were enhanced. Chain-like and ribbon-like turbostratic carbon, and spherical “γ-carbon” developed from gaseous carbon generated by jetting from the diamond-particle surface were found at the 56 GPa region in the compact. The formation of “γ-carbon” was assumed to proceed through vapor → liquid → solid sequences.