Effect of grain boundaries on iron distribution in polycrystalline cubic boron nitride

The influence of grain boundaries on local electronic states and the distribution of Fe-57 ions in cubic boron nitride (c-BN) polycrystals has been investigated as a function of technological processing. The study was performed on two sets of c-BN samples: (1) prepared by catalytic synthesis and (2) synthesized by direct phase transformation. It has been established that under catalytic synthesis iron ions are predominantly incorporated into the c-BN grain bulk in the Fe3+ charge state. In contrast, after direct phase transformation it-on ions are distributed mainly along the grain boundaries in the Fe2+ state, probably due to the enhanced grain boundary diffusion of iron. Suppression of grain boundary diffusion of iron (and its incorporation mainly into the grain bulk) under catalytic synthesis can be attributed to interlocking of the movement of iron ions by precipitation of nitrides and borides of catalytic impurities (Al and Si) formed along the grain boundaries during c-BN synthesis.