Preparation And Performance Study Of New Type Superhard Materials In B-C-N System

In this paper, the possible superhard materials existed in the B-C-N system were studied by the structural prediction and experimental synthesis. We proposed the six kinds of new stable B-C-N structures and O-BN structure, whose hardness and electric conductivity were predicted, by means of first principles calculation. New ultrahardness materials of nano-twined diamond were synthesized using high temperature and high pressure technique. A new carbon allotropes of M-diamond was found in some nano-twined diamond samples. Mechanical properties and thermal stability of nano-twined diamond bulk materials were systematically measured and researched.We predicted three new superhard BC4 N and three superhard BC6 N structures useing first-principles calculations. o-BC4N-1 structure has the largest vickers hardness of 64 GPa in the predicted B-C-N structures. Basing on the band structure calculation, we found that D-BC6N-4 structure have electrical conductivity, R-BC4 N is insulator and other B-C-N structures predicted are semiconductor. A novel superhard and semiconductive O-BN compound with orthogonal structure was proposed by first-principles prediction. We proposed the formation mechanism of O-BN through the "chair" and "boat" deformation in h-BN monolayer and predicted a possible route obtaining O-BN structure by directly compressing h-BN.The B-C-N precursor was prepared using h-BN and carbon black as raw materials by the impinging-streams technology. On the basis of structural predication, we try to synthesize the new superhard B-C-N structures using the precursor under condition of 15~20GPa and 1600~1850oC in T25 high temperature and high pressure system. In range of experimental temperature and pressure, we did not found the ternary B-C-N phase in the recovered samples, instead the mixture of c-BN nanocrystal and diamond nanocrystal. During hardness testing, we found some area with ultrahardness(vickers hardness up to 271.6GPa) in the mixed crystal bulk material. High Resolution Transmission Electron Microscopy(HRTEM) analysis revealed that there are a lot of nano-twined diamond structure in the ultrahard area.The carbon material with nano onion structure with a size from 20 to 50 nm was successfully prepared with high purity carbon black as raw material by the impinging-streams technology. Transparent nano-twins diamond(nt-diamond) bulk material with extremely hardness are successfully synthesized using the nano-onion carbons as the raw material under 18~25GPa and 1850~2000oC in the T25 system. HRTEM revealed that the material contains a large number of nano-twined structure with an average thickness of 5nm. The nano-twin structure greatly improve the mechanical performance and thermal stability of nt-diamond bulk materials. The hardness and fracture toughness of nt-diamond material are nearly 2 times and 2~3 times higher than the natural diamond, respectively. Its onset oxidation temperature is above 200 oC higher than the natural diamond.Black nt-diamond bulk materials with extremely hardness and tiny twin structure are successfully synthesized using the prepared nano-onion carbons as the raw material under 10~15GPa and 1600~2000oC. The average twin thickness is only 2.5 nm in the sample recovered from 15 GPa and 1850 oC. In this sample, we found a new carbon allotropes named M-diamond with monoclinic structure. The measuring results present the sample vicker hardness was up to 320-340 GPa and its fracture toughness is 6.5-11.9 MPa·m0.5. We consider that ultrahigh hardness for the sample is due to reasonable distribution of M-diamond phase between nt-diamond grains.