GROWTH PRESSURE-TEMPERATURE REGION OF CUBIC BN IN THE SYSTEM BN-MG

The growth pressure-temperature region of cBN in the system BN-Mg was determined under the conditions up to 8 GPa and 2300° C. Hexagonal BN with different oxygen contents (1.9 wt % for R-type and 7.9 wt % for N1-type) was used as a starting material. The lower temperature limit of the cBN region obtained from the R-type is about 1380° C under pressures of 6 to 8 GPa. This limit can be compared with the eutectic point in the system hBN-Mg3B2N4. The data suggest that cBN crystals grow through the dissolution and precipitation process from a eutectic liquid. The cBN region obtained from the N1-type is located at higher temperatures than that of the R-type, the lower limit of which is reached at about 1700° C at 6 GPa. MgO and/or Mg3 (BO3)2 are formed as by-products in such a system. The finding implies that Mg3B2N4, a solvent of BN, reacts with oxide impurities (especially B2O3) by the following reaction; Mg3B2N4+3/2O2=3MgO+2BN+N2 or Mg3B2N4+3O2=Mg3 (BO3)2+2N2. It is deduced that the cBN growth region shifts towards higher temperatures depending on the effect of oxygen. © 1979 Chapman and Hall Ltd.