Georgiy A. Voronin

Enhancement of fracture toughness in nanostructured diamond–SiC composites

We synthesized diamond–SiC nanocomposites with superhardness and greatly enhanced fracture toughness through a synthetic approach based on high-energy ball milling to form amorphous Si precursors followed by rapid reactive sintering at high pressure (P) and high temperature (T). We show how the simultaneous P–T application allows for better control of the reactive sintering of a nanocrystalline SiC matrix in which diamond crystals are embedded. The measured fracture toughness KIC of the synthesized composites has been enhanced greatly, as much as 50% from 8.2 to 12.0 MPa m1/2, as the crystal size of the SiC matrix decreases from 10 μm to 20 nm. Our result contradicts a commonly held belief of an inverse correlation between hardness and fracture toughness. We demonstrate the importance of nanostructure for the enhancement of mechanical properties of the composite materials.

Diamond–SiC nanocomposites sintered from a mixture of diamond and silicon nanopowders

Abstract A novel method of reactive sintering of diamond–SiC nanocomposites based on thorough mixing of diamond and silicon nanosize powders was applied to produce large specimens. For comparison purposes we also sintered pure nanocrystalline diamond compacts and micron-sized diamond–SiC composites by the infiltration method. Structure of these materials was studied by scanning electron microscopy, Raman scattering and X-ray diffraction. Diamond–SiC nanocomposites have remarkably high fracture toughness and are significantly harder than the sintered pure nanocrystalline diamond compacts. Silicon is shown to hinder graphitization of diamond. Broadening of X-ray lines is explained in terms of plastic deformations and size-effect in diamond and silicon carbide crystals.

Partial graphitization of diamond crystals under high-pressure and high-temperature conditions

Diamond powders of different sizes were compressed at pressures up to 2.5 GPa and heated up to 1700 K. Extent of partial graphitization was estimated from x-ray diffraction and Raman scattering. For example, in the presence of water, at p=2.0 GPa and T=1473 K about 22% of diamond was converted into graphite. The rate of this transformation decreases in time and becomes negligibly small after about 20 min of treatment at high-pressure, high-temperature conditions. (Graphitization starts at the surface of the crystals and then graphite crystals grow in the direction perpendicular to the surface and along the surface.) Distribution of graphite on the surface of diamond crystals was obtained from Raman microimaging.

High pressure study of graphitization of diamond crystals

Graphitization of {100} and {111} faces of diamond crystals at pressures of 0.1 and 2 GPa and various temperatures was studied by Raman spectroscopy, x-ray single crystal diffractometry, and scanning electron microscopy. Different primary mechanisms of graphitization are discussed: (a) normal growth of graphite layer by detachment of single atoms from {100} and {111} diamond surfaces and (b) lateral growth of graphite on the {111} surface by breaking off groups of atoms followed by their rearrangement into planar graphitic structures. The growth of oriented graphite crystallites was observed only on the {111} diamond faces and at p=2 GPa. In the case of synthetic diamonds, internal graphitization was observed and explained in terms of catalytic effects on the metal inclusions.

SiC–CNT nanocomposites: high pressure reaction synthesis and characterization

Silicon carbide–carbon nanotube composite was fabricated using the high pressure reactive sintering technique. Samples were synthesized at high pressures, 2 and 8 GPa, and temperatures, 1770 and 1970 K. Their structures were studied using x-ray diffraction, x-ray photoelectron spectroscopy, transmission electron microscopy, and Raman scattering techniques. The composites produced at high pressure have pronounced nanocrystalline structure (the mean crystallite size of the SiC matrix was 32–37 nm) and very promising mechanical properties: fracture toughness of 6.8–7.1 MPa m0.5 and Vickers hardness of 20–21 GPa.

In situ X-ray diffraction study of germanium at pressures up to 11 GPa and temperatures up to 950 K

Abstract In situ X-ray diffraction measurements on germanium were conducted in the pressure range of 5–11 GPa and temperatures up to 950 K. Using our data a better defined P–T diagram for germanium is presented. The coordinates of the triple point between GeI–GeII–GeL have been determined to a better degree of precision. The onsets of the GeI–GeII transition were found both under hydrostatic and non-hydrostatic conditions. Anisotropy of thermal expansion coefficient for the GeII is characterized from the c/a ratios in the temperature interval 473–823 K. Phases GeIII and GeIV are shown to be metastable forms of germanium.

Oriented growth of β-SiC on diamond crystals at high pressure

Interaction between diamond crystals and liquid silicon at pressures of 2 and 9 GPa and various temperatures was studied by Raman spectroscopy, x-ray single crystal diffractometry, and scanning electron microscopy. The mechanism of growth of silicon carbide (SiC) film on diamond crystals depended on the magnitude of applied pressure. At low pressures, in the graphite stable region, only disoriented, fine grain β-SiC crystals were formed inside silicon surrounding diamond crystals. At higher pressures corresponding to the diamond stable region, oriented growth of β-SiC film on both {111} and {100} faces of diamond crystals was observed, even at high growth rates.

Kinetics of the reaction between diamond and silicon at high pressure and temperature

Diamond-silicon carbide composites were sintered from diamond powder and liquid silicon at high pressure-high temperature (HPHT) conditions. Experiments were conducted in the diamond-stable region and then repeated in the graphite-stable region. X-ray diffractograms of the specimens sintered for different time periods provided information on the SiC formation rate and activation energy. Only the late stage of the reaction was investigated, and in the diamond-stable region it was shown that SiC growth was controlled by the diffusion rate of silicon and carbon atoms though the existing layer of SiC. This process is characterized by an activation energy of 264kJ∕mol. At 2GPa, where graphite is the stable form of carbon, in addition to the direct reaction, diamond may first spontaneously transform into graphite, which next reacts with silicon. A combination of these two processes results in a higher activation energy of 410kJ∕mol.