Origin of unique microstructures in nano-polycrystalline diamond synthesized by direct conversion of graphite at static high pressure

The influence of the local stress state and crystallinity of graphite on the graphite-diamond trans formation process was investigated using a laser-heated diamond anvil cell and two types of graphite samples with contrasting crystalline characteristics. The samples recovered from the experiments using highly oriented graphite (HOG) have layered structures composed of lonsdaleite and diamond arranged in a coaxial relation, indicative of their martensitic formation from graphite. The compression of HOG perpendicular to the graphite c axis results in a partial fragmentation of the original layered structure, but no clear evidence of diamond formation by the diffusion process is found even in the fragmented regions. On the other hand, the sample recovered from the experiment using mechanically milled graphite powder consists of euhedral diamond nanocrystals (size: 20-50 nm), indicating that the diamond is formed by the diffusion process. The results of the present study suggest that the graphite-diamond transformation process and the resulting microstructures are controlled mainly by the initial structure characteristics (such as crystallinity and crystallite size) of the graphite, while the influence of the local stress state in the graphite under compression is likely less significant for the formation process and the microtexture.