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

Although cubic boron nitride (c-BN) and diamond reveal quite similar material properties, there is a considerable difference in their synthesis, chemical behaviour and reactivity. Growth of coarse crystalline c-BN is so far only possible in a high-pressure high-temperature (HPHT) melt synthesis (reversed homogeneous degradation) with flux precursors dissolving boron nitride, but it cannot yet be achieved in a mere CVD reaction similar to the diamond CVD process from the gas phase. The influence of agents leading to a homogeneous decay (straightforward formation of an intermediate phase by either 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 as reactive gases, leading to a homogeneous degradation at elevated temperatures. The morphological features and quality of the remaining crystallites were studied by SEM and Raman spectroscopy, indicating straightforward homogeneous degradation for diamond and c-BN. Diamond crystals exhibit a signifi cantly different reaction behaviour, since the degradation starts on all crystal facets ([111] as well as [100] planes), In the c-BN case, degradation patterns could only be found on [111] planes. For this reason, a specific surface reconstruction and topology play a dominant role for growth and dissolution and an atomic model for c-BN degradation are presented. The reaction mechanism is discussed for the c-BN and diamond case. The detailed study of the homogeneous decay of c-BN resembles the reversed growth and gives information on the molecular mechanisms involved in formation of the crystals. The study of the molecular mechanisms establishes new growth routes for c-BN in a CVD process by altering the gas-phase homogeneous degradation into a growth reaction. (C) 2001 Elsevier Science B.V. All rights reserved.