CHAPTER 11: The Microstructures of Polycrystalline Diamond, Ballas and Nanocrystalline Diamond

Diamond has unique properties due to its covalent sp³ bonding structure and crystal lattice. With decreasing crystal size, mechanical and physical properties become different from coarse grained types. In the case of CVD diamond there are various types of nanocrystalline materials that have been described, e.g., ballas diamond, NCD (nanocrystalline diamond) and UNCD (ultra-nanocrystalline diamond). From the present point of view the nanocrystalline materials can be described by various microstructures and surface morphologies. Ballas is a polycrystalline, spherically grown diamond having a radial structure containing twin grain boundaries and a non-faceted surface. Several morphological ballas structures have been observed by varying the diamond deposition conditions, i.e., ballas mixed with faceted areas, flat ballas, ballas with graphitic inclusions etc. The NCD and UNCD coatings are grown by decreasing the atomic-hydrogen-to-carbon ratio and, therefore, increasing the amorphous carbon content in the grain boundaries. Finally, there is a continuous changeover between the different deposition types and, because of some freedom in the interpretation of the results of various analytical characterisation methods, it is problematic to distinguish between different diamond types.