Abstract
Superhard boron-carbon materials are of prime interest due to their non-oxidizing properties at high temperatures compared to diamond-based materials and their non-reactivity with ferrous metals under extreme conditions. In this work, evolutionary algorithms combined with density functional theory have been utilized to predict stable structures and properties for the boron-carbon system, including the elusive superhard BC5 compound. We report on the microwave plasma chemical vapor deposition on a silicon substrate of a series of composite materials containing amorphous boron-doped graphitic carbon, boron-doped diamond, and a cubic hard-phase with a boron-content as high as 7.7 at%. The nanoindentation hardness of these composite materials can be tailored from 8 GPa to as high as 62 GPa depending on the growth conditions. These materials have been characterized by electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, X-ray diffraction, and nanoindentation hardness, and the experimental results are compared with theoretical predictions. Our studies show that a significant amount of boron up to 7.7 at% can be accommodated in the cubic phase of diamond and its phonon modes and mechanical properties can be accurately modeled by theory. This cubic hard-phase can be incorporated into amorphous boron-carbon matrices to yield superhard materials with tunable hardness values.
Highlights
The first row of elemental solids (C, N, O, and B—jointly referred to as CNOB) form dense covalent solids in three-dimensional (3D) network structures that are extremely hard, have a high-energy density content, and exhibit unique electronic and optical properties
We focus on a sub-set of superhard materials based on the boron-carbon system, where the synthesis of superhard BC5 material has previously been claimed using the high-pressure high-temperature technique at a pressure of 24 GPa and temperature of about 2200 K [1]
Aided by evolutionary algorithm predictions, we have synthesized a novel series of boron-carbon materials using a microwave plasma chemical vapor deposition technique employing hydrogen/methane/diborane gas-phase chemistry
Summary
The first row of elemental solids (C, N, O, and B—jointly referred to as CNOB) form dense covalent solids in three-dimensional (3D) network structures that are extremely hard, have a high-energy density content, and exhibit unique electronic and optical properties. 45 GPa) have long been established as the cornerstone of a multi-billion dollar abrasives industry, there is considerable scientific and technological interest in novel superhard materials (hardness greater than 40 GPa) based on CNOB. We focus on a sub-set of superhard materials based on the boron-carbon system, where the synthesis of superhard BC5 material has previously been claimed using the high-pressure high-temperature technique at a pressure of 24 GPa and temperature of about 2200 K [1]. The synthesized BC5 material had a measured hardness value of 71 GPa and high thermal stability up to 1900 K [1]. A significant amount of theoretical work has suggested various
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