SYNTHESIS OF CUBIC BORON-NITRIDE USING MG AND PURE OR M'-DOPED LI3N, CA3N2 AND MG3N2 WITH M'=AL, B, SI, TI

The growth pressure-temperature region of cubic boron nitride (cBN) in the systems Mg-BN and MxNy-BN (MxNy = Li3N, Ca3N2, Mg3N2) has been redetermined using well-crystallized hexagonal BN (hBN) with low oxygen content (0.2%) as the starting material. The data on the MxNy-BN systems are compatible with the existence of two growth regions: a high-temperature region where cBN grows from a liquid phase, and a low-temperature region where cBN forms from solid-solid reactions. Previous data are discussed according to this model and possible solid-state reactions are proposed on the basis of thermodynamic considerations. The results for the Mg-BN system confirm the effect of the O2 content of the starting BN on the cBN growth region. The systems (MxNy + M')-BN (M' = Al, B, Si, Ti) have been shown to produce cBN crystals of increased size and improved morphology (more compact and perfect) compared to those obtained with the MxNy-BN systems. Their colour is dark or black and their size reaches 0.6 mm. The effect of the relative proportions of M' and MxNy on the growth region and yield has been determined and is discussed on the basis of the chemical reactions likely to occur.