Crystallization and texturing in graphite-diamond transformation

Abstract

Here I introduce the crystallization and texturing mechanism of nano-polycrystalline diamond (NPD) in the graphite-diamond direct transformation in laboratory and in nature. Two types of transformation processes, diffusion-controlled (nucleation and growth) process and diffusion-less martensitic process are involved in the diamond formation depending on the crystallinity of the initial starting material used. The former process is dominated when using low crystalline graphite/carbon materials and produces a granular texture, while the latter process is favorable when using high crystalline graphite and produces lamellar (layered) texture. Understanding the transformation and texturing mechanism has enabled the texture control of NPD and even opened a window to the production of new varieties of NPDs with novel microtextures. It is also helpful to understand the formation process and origin of the unique microtexture of natural NPD (impact diamond), which was recently identified from the large impact crater in Siberia, Russia. Stress-induced local fragmentation of the source graphite and subsequent rapid transformation to diamond in the limited time scale are the key factors for accelerating the multiple diamond nucleation and suppressing the overall grain growth to produce the unique nanocrystalline texture of natural NPD.