Nanocomposites and nanomaterials

An evolution of grain structure in polycrystals formed during HPHT sintering of nanostructured diamond powders made using dynamic synthesis
Oleynik G. S.1, Kotko А. V.1, Solonin Ju.M.1, Bochechka O.O.2, Chernienko O.I.2, Kushch O.V.2
1Frantsevich Institute for Problems of Materials Science of NAS of Ukraine, Kyiv, Ukraine E-mail: g.oleynik@hotmail.com
 2Bakul Institute for Superhard Materials of NAS of Ukraine, Kyiv, Ukraine
E-mail: obochechka@gmail.com

Topics discussed: a. substructure of source powder particles of diamond made using dynamic synthesis, obtained from graphite during shockwave stress-loading, and b. grain microstrustructure of polycrystalline samples sintered at p = 7.7 GPa at 1700–2100 0С. The research was conducted using transmission electron microscopy of thin foil and spall particles.
Particles have a shape of sheets. In a developed surface the range of sheet sizes equals 1–5 μm. Generally, particles appear to consist of two phases: cubic and hexagonal diamond, with prevailing presence of cubic diamond. These phases are cryptographically connected. Particles have a four-level structure with the smallest element size of 3–5 nm. As stated for the first time in [1], such elements could form non-porous associates within inner boundary layer.
It is found, that the formation of the grain structure in polycrystals happens based on the smallest substructure elements of the powder particles. Process occurs the following way: self-assembly of particles with the formation of non-porous associates  mono-crystallization of associates  formation of the grain boundaries and grain faceting. Grain size growth occurs at T  20000С.
Grain size at mentioned temperature is under 70–80 nm. Hardness of such polycrystals equals 30–33 GPa (indenter load 9.8 N).

1.Трефилов В.И.,Саввакин Г.И., Скороход В.В, Солонин Ю.М. Особенности структуры ультрадисперсных алмазов, полученных высоко-температурным синтезом в условиях взрыва // ДАНСССР. 1978. Т. 239, №4. С.838-841