Diamond-to-graphite transformation under hypersonic impact

Consolidation of diamond particles into high-strength bulk materials and composites is extremely challenging as it requires high sintering temperatures, at which diamond transforms into graphite, making it impossible to retain the sintered diamond phase. Here we report a synthetic approach based on spark plasma sintering that allows the stabilization of micron-scale diamond grains in composites, using cubic boron nitride (cBN) as the matrix and cobalt (Co) particles as a stabilizer, resulting in a hard-to-machine, tough material. We performed hypersonic speed impact tests on the composites using metal projectiles of different sizes. The composite withstands impact from ∅1 mm metal projectiles (speed Mach 7.5) but breaks apart during the impact from a larger ∅4 mm metal projectile (speed Mach 8.45). Interestingly, during this microsecond-scale impact and fracture event, embedded diamond particles in the composite undergo near-complete phase transformation to graphite. In-depth micro-structural characterizations of the fractured composite, supported by the molecular dynamics simulations reveal details on the impact-induced phase transformation and the creation of diamond-graphite interfaces, suggesting that the energy absorption is primarily enabled via phase change of diamond. ... mehrOur findings provide a pathway to stabilize the diamond phase and a new understanding of phase transformation of diamond under extreme conditions.