High-pressure synthesis and characterization of thermal-stable boron-doped diamond single crystals

A high-pressure and high-temperature synthesis route from a Fe-Ni-C-B catalytic system to boron-doped diamond single crystals with gray color and about 300-500 μm in diameter is reported in this paper. The Fe-Ni-C-B catalytic system was made by powder metallurgy method using pure iron, nickel, and graphite powders as main raw materials and hexagonal boron nitride (h-BN) as the boron source. It is showed that from Raman spectra, the first-order Raman scattering peak is shifted to low frequency by 43 cm−1 and the full width at half maximum is increased by 7 cm−1. The Raman spectra provide some indirect evidences for the boron atoms being incorporated into diamond lattice. Especially, the covalent B-C bond in diamond with an absorption band at 2842 cm−1 is directly detected in infrared absorption spectrum. The thermal stability and main mechanical properties of boron-doped diamond are greatly improved comparing with that of undoped diamond on base of the data of thermal etching rate, static compressive strength and impact toughness, which were obtained at different testing temperatures in air and argon, respectively. In addition, the differential thermal analysis reveals that the oxidation resistance of boron-doped diamond is advanced by 168.3 °C. A boron coated atomic model is introduced to illustrate the origin of the excellent thermal stability of boron-doped diamond.