Influence of graphite crystallinity on the microtexture of nano-polycrystalline diamond obtained by direct conversion

Abstract

Nano-polycrystalline diamond (NPD) is a super-hard pure polycrystalline aggregate of nano-diamonds and has a characteristic microtexture composed of a mixture of granular and lamellar crystals. We investigated the origin of the unique microtexture and the influence of the crystallinity of initial graphite sources on the resulting microtexture of NPDs. Polycrystalline graphite rods used for NPD synthesis were found to consist of coke-derived relatively large crystals and pitch-derived nanocrystalline particles. Upon conversion to NPD, the former are converted to cubic and hexagonal diamond mixtures by the martensitic transformation and left a lamellar texture behind, while the latter transform to granular nano-diamonds by diffusion-controlled nucleation and subsequent crystal growth, which initiate preferentially at lattice defects and crystal surfaces. A clear correlation between the crystallite size of the initial graphite and the grain size of the granular nano-diamonds in the NPDs was also found. Our results suggest that the average grain size and the relative abundance of lamellar domains in NPD can potentially be controlled by carefully choosing initial graphite sources based on their crystallinity.

Nanolayered Diamond Sintered Compact Obtained by Direct Conversion from Highly Oriented Graphite under High Pressure and High Temperature

“…Such microtexture and coaxial relations indicate that the phase transition from graphite to cubic diamond via hexagonal diamond intermediate occurred essentially by martensite-like mechanism at the studied PT conditions. This is because if the phase transition occur by nucleation and growth mechanism, both the original layered texture of graphite starting material and lattice relations among the three phases would not be preserved, like the case of nanopolycrystalline diamond which consists mostly of randomly oriented granular crystals [7][8][9]11]. The martensite-like transformation of graphite to hexagonal diamond and cubic diamond is thought to occur by the sliding and/or buckling of graphite planes (e.g., [15,16]) and predominantly takes place when well-crystalline graphite is used as a starting material [8,9].…”

“…This is because if the phase transition occur by nucleation and growth mechanism, both the original layered texture of graphite starting material and lattice relations among the three phases would not be preserved, like the case of nanopolycrystalline diamond which consists mostly of randomly oriented granular crystals [7][8][9]11]. The martensite-like transformation of graphite to hexagonal diamond and cubic diamond is thought to occur by the sliding and/or buckling of graphite planes (e.g., [15,16]) and predominantly takes place when well-crystalline graphite is used as a starting material [8,9]. The sample synthesized at 15 GPa and 2300 ∘ C consists mostly of cubic diamond and of a small amount of hexagonal diamond, which are also in the coaxial relation stated above, suggesting that the conversion of graphite to diamonds fully completes above 1800 ∘ C at this pressure.…”

“…NPD is characterized to have a unique microtexture usually composed of a mixture of randomly oriented equigranular crystals of a few to several tens of nanometers and lamellar crystals stacked along [111] direction of diamond [7]. The origin of such two types of microtextures of NPD is derived from the heterogeneous crystalline features of the starting graphite source (polycrystalline graphite compact), where relatively large, well-crystallized graphite flakes (cokederived) are mixed in a nanocrystalline graphite matrix (binder-pitch-derived) [8]. The former graphite crystals transform to diamond by the diffusion-limited (Martensitic) process via metastable hexagonal diamond as an intermediate phase, maintaining the original layered structure (forming lamellas), while the latter matrix converts to granular diamond nanocrystals by the diffusion process in which nucleation and subsequent grain growth of diamond (cubic) take place by atomic diffusion [8][9][10].…”

“…The origin of such two types of microtextures of NPD is derived from the heterogeneous crystalline features of the starting graphite source (polycrystalline graphite compact), where relatively large, well-crystallized graphite flakes (cokederived) are mixed in a nanocrystalline graphite matrix (binder-pitch-derived) [8]. The former graphite crystals transform to diamond by the diffusion-limited (Martensitic) process via metastable hexagonal diamond as an intermediate phase, maintaining the original layered structure (forming lamellas), while the latter matrix converts to granular diamond nanocrystals by the diffusion process in which nucleation and subsequent grain growth of diamond (cubic) take place by atomic diffusion [8][9][10]. Therefore, the transformation pathway and resulting microtexture of diamond produced through direct conversion by high pressure and high temperature (HPHT) method is most likely determined by the crystallinity (crystallite size) of the initial graphite sources.…”

A novel large-volume Kawai-type apparatus and its application to the synthesis of sintered bodies of nano-polycrystalline diamond

“…The starting material is pure consolidated graphite (>99.99), which is made of a mixture of minute (a few micrometers) graphite crystals (flakes derived from "cokes") and poorly crystallized fine graphite powders (derived from "pitches") as described in Ohfuji et al (2012).…”