Low temperature self-densification of high strength bulk hexagonal boron nitride.

Hexagonal boron nitride (hBN) ceramics are expected to have wide applications at high temperatures as both a structural and functional material. However, because of its flake structure and general inertness, it is currently impossible to sinter hBN powder to a dense bulk (with a relative density of above 96%) even at 2000 °C. Here, we report dense bulk hBN with 97.6% theoretical density achieved at a lower preparation temperature (1700 °C) via a self-densifying mechanism without sintering additives. During the sintering process, cubic boron nitride particles incorporated into the hBN flake powders transform into BN onions with a significant volume increase, thus filling in voids among the hBN flakes and highly densifying the hBN bulks. The resulting dense hBN ceramics possess 2-3 times the strength of traditional hBN ceramics. This phase-transition-induced volume expansion strategy could lead to dense sintered compacts with high performance in other ceramic systems.