Comparative aspects of the homogeneous degradation of c-BN and diamond

Although cubic boron nitride (c-BN) and diamond reveal quite similar materi al properties, there is a considerable difference in their synthesis, chemi cal behaviour and reactivity. Growth of coarse crystalline c-BN is so far o nly possible in a high-pressure high-temperature (HPHT) melt synthesis (rev ersed homogeneous degradation) with flux precursors dissolving boron nitrid e, but it cannot yet be achieved in a mere CVD reaction similar to the diam ond CVD process from the gas phase. The influence of agents leading to a ho mogeneous decay (straightforward formation of an intermediate phase by eith er flux precursors or reactive gases) was studied on diamond and c-BN HPHT samples. Carbon-dissolving metals were used to react with diamond crystals, whereas c-BN crystals were treated with various flux precursors, as well a s reactive gases, leading to a homogeneous degradation at elevated temperat ures. The morphological features and quality of the remaining crystallites were studied by SEM and Raman spectroscopy, indicating straightforward homo geneous degradation for diamond and c-BN. Diamond crystals exhibit a signif i cantly different reaction behaviour, since the degradation starts on all crystal facets ([111] as well as [100] planes), In the c-BN case, degradati on patterns could only be found on [111] planes. For this reason, a specifi c surface reconstruction and topology play a dominant role for growth and d issolution and an atomic model for c-BN degradation are presented. The reac tion mechanism is discussed for the c-BN and diamond case. The detailed stu dy of the homogeneous decay of c-BN resembles the reversed growth and gives information on the molecular mechanisms involved in formation of the cryst als. The study of the molecular mechanisms establishes new growth routes fo r c-BN in a CVD process by altering the gas-phase homogeneous degradation i nto a growth reaction. (C) 2001 Elsevier Science B.V. All rights reserved.