Superhard B–C–N materials synthesized in nanostructured bulks

Abstract

We report here the high-pressure synthesis of well-sintered millimeter-sized bulks of superhard BC 2 N and BC 4 N materials in the form of a nanocrystalline composite with diamond-like amorphous carbon grain boundaries. The nanostructured superhard B-C-N material bulks were synthesized under high P-T conditions from amorphous phases of the ball-milled molar mixtures. The synthetic B-C-N samples were characterized by synchrotron x-ray diffraction, high-resolution transmission electron microscope, electron energy-loss spectra, and indentation hardness measurements. These new high-pressure phases of B-C-N compound have extreme hardnesses, second only to diamond. Comparative studies of the high P-T synthetic products of BC 2 N, BC 4 N, and segregated phases of diamond + cBN composite confirm the existence of the single B-C-N ternary phases.

Evidence for a low-compressibility carbon nitride polymorph elaborated at ambient pressure and mild temperature

“…Thus, the bulk modulus of the cubic carbon nitride appears remarkably high with a compressibility equivalent to 73% and 91% of diamond (this work) and c-BN [2] compressibilities, respectively. Moreover, its performances appear better than the low-density cubic BC 2 N phase and comparable with this heterodiamond high-density polymorphs recently claimed (all BC 2 N polymorphs being elaborated under high pressure and temperature) [3,[30][31]. Finally, despite the fact that c-CN x exhibits a bulk modulus lower than theoretical values calculated for dense C 3 N 4 polymorphs, its very low compressibility remains impressive and of the same order than values predicted for CN phases involving sp 2 bonds [9].…”

Amorphous Diamond: A High-Pressure Superhard Carbon Allotrope

“…We further observed evidence for extraordinary hardness in this phase based on the large pressure difference generated by using a glassy carbon ball as a spherical indenter [41,42] against diamond anvils. Glassy carbon spheres of various sizes together with a tiny ruby ball were placed in soft KI pressure transmitting medium in a sample chamber created by drilling a hole in a Re gasket, and compressed between two 400 μm culet diamond anvils (Fig.…”

Phase transition in BCx system under high-pressure and high-temperature: Synthesis of cubic dense BC3 nanostructured phase

“…However, confirmation of the cubic atomic chemistry, chemical composition, and nanostructure of the BC 2 N phase was obtained later by Zhao et al 22 from a TEM/EELS study of a single BC 2 N nano-grain, and then by Langenhorst and Solozhenko 23 , who characterized B-C-N samples synthesized under HPHT conditions using a HRTEM, ED, and EELS. achieved by ion cleaning of the FIB-generated damaged surface layer on the TEM specimen, which indicates that the remnant * peak may be due entirely to FIB sample preparation.…”

“…The stoichiometry, nanostructure, chemical purity, and homogeneity of our c-BC 3 can only be unambiguously established by using TEM in conjunction with EELS 22,23 . These allow measurement of the elemental composition and determination of the nanocrystalline structure and nature of the chemical bonds from an area of few nanometers.…”

“…For instance, if the starting BCx phase is decomposed under HPHT conditions on a nano-grain structure consisting of cubic carbon and amorphous boron nanograins then the x-ray of the obtained phase would be similar to that of diamond. Zhao et al 22 conducted ED and EELS measurements from a single nano-grain to determine the structure of the BC 2 N phase synthesized under HPHT conditions, and they found that c-BC 2 N nano-grains were surrounded by an amorphous matrix in the quenched specimen. Therefore the stoichiometry, nanostructure, chemical purity, and homogeneity of BC x phases obtained under HPHT conditions can only be unambiguously established by using TEM in conjunction with ED and EELS techniques 22,23 .…”