Shock-induced transformations in hexagonal boron nitride by high-velocity thermal spray

Shock synthesis of cubic BN (c-BN) was accomplished using high-velocity thermal spray. Al-8Si-20BN (hexagonal BN, h-BN) composite powders were injected into a high-energy flame where the particles partially melted and accelerated to impact on steel substrates. The shock wave generated by the sudden impact of the droplets propagated through the underlying deposit, which experienced a polymorphic transition to high-pressure forms. Transmission electron microscopy revealed thai the deposits contained platelike c-BN embedded in h-BN. The c-BN formed with a specific orientation relation, with (0001) h-BN parallel to (111) c-BN and [11-20] h-BN nearly parallel to [2-20] c-BN. As a result of shock propagation, a number of half-Frank loops with Burgers vector 1/2[0001] were introduced and thin (∼3 nm) layers of amorphous BN (a-BN), parallel to the (0002) plane, were formed. Significant shearing of the (10-10) plane with respect to the c-axis, delamination, and kink bands were also observed in most of the h-BN grains. The Hugoniot pressure calculation suggested that the impact pressure was sufficient to trigger the nucleation of c-BN.