Abstract
Consolidated bodies of polycrystalline diamond with grain sizes less than 100 nm, nano-polycrystalline diamond (NPD), has been experimentally produced by direct conversion of graphite at high pressure and high temperature. NPD has superior hardness, toughness and wear resistance to single-crystalline diamonds because of its peculiar nano-textures, and has been successfully used for industrial and scientific applications. Such sintered nanodiamonds have, however, not been found in natural mantle diamonds. Here we identified natural pure NPD, which was produced by a large meteoritic impact about 35 Ma ago in Russia. The impact diamonds consist of well-sintered equigranular nanocrystals (5–50 nm), similar to synthetic NPD, but with distinct [111] preferred orientation. They formed through the martensitic transformation from single-crystal graphite. Stress-induced local fragmentation of the source graphite and subsequent rapid transformation to diamond in the limited time scale result in multiple diamond nucleation and suppression of the overall grain growth, producing the unique nanocrystalline texture of natural NPD. A huge amount of natural NPD is expected to be present in the Popigai crater, which is potentially important for applications as novel ultra-hard material.
Highlights
Are polycrystalline aggregates of a micron to submicron diamond crystals
The present study reveals the nanocrystalline nature of the Popigai impact diamonds through careful transmission electron microscopy (TEM) observations on a number of oriented cross-sections prepared using focused ion beam (FIB) and discusses the unique transformation and texturing process of the natural Nano-polycrystalline diamond (NPD)
Our experiments demonstrated that the kinetics for graphite-diamond transformation via intermediate lonsdaleite depends largely on temperature at a fixed pressure: graphite and lonsdaleite are clearly dominant in the products from lower temperatures
Summary
Are polycrystalline aggregates of a micron to submicron diamond crystals. The occurrence of apographitic diamond grains (pseudomorphic after single-crystal graphite) and the presence of lonsdaleite, a hexagonal polymorph of diamond (up to 25% of the whole17) in most of those grains imply their martensitic formation from well-crystalline graphite. In spite of these previous studies, the details of the microtexture and crystallographic features of Popigai diamonds have not been clearly identified (i.e. the overall textural characteristics remained unclear). Earlier observations by transmission electron microscopy (TEM) observations were made only at local scales on crushed or ion-thinned samples. The present study reveals the nanocrystalline nature of the Popigai impact diamonds through careful TEM observations on a number of oriented cross-sections prepared using focused ion beam (FIB) and discusses the unique transformation and texturing process of the natural NPD
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