Viscoelastic sliding and diffusive relaxation along grain boundaries in polycrystalline boron nitride

Abstract

Dense hexagonal boron nitride (BN) materials were prepared via two different processing routes: (i) hot-pressing with the addition of a Ca/B-containing glass and (ii) chemical vapor deposition (CVD). The resulting microstructure of both materials was studied by scanning and transmission electron microscopy. While the hot-pressed BN material shows, apart from large BN matrix grains, an inhomogeneous distribution of residual glass at room temperature, the CVD deposition yields a homogeneous fine grained microstructure with no amorphous residue detectable. Internal-friction experiments were performed to study the micromechanical response of the materials when exposed to high temperatures. The CVD material revealed no relaxation peak during testing up to 2300°C, while the glass-doped sample showed a pronounced relaxation peak at a peak-top temperature of about 600°C. This temperature corresponds to the softening temperature known for bulk Ca/B-glasses and it is, therefore, concluded that the glass homogeneously wets the BN grains at elevated temperatures. The results presented are seen as the first clear evidence that the internal friction peak monitored for various glass-containing ceramics is indeed related to a viscous sliding process along grain boundaries.