Extreme mechanical anisotropy in diamond with preferentially oriented nanotwin bundles

Significance Diamond is an essential material for industrial and scientific applications due to its exceptional properties. The regulation of diamond performance through microstructural design is a challenging and hot topic. Here, we report the synthesis of diamond with preferentially oriented nanotwin bundles via direct phase transformation of superaligned multiwalled carbon nanotube films under high temperature and pressure. The preferentially oriented nanotwin bundles endow diamond with the highest hardness anisotropy ever recorded and a record hardness value in the direction perpendicular to the oriented nanotwin bundles. Current findings provide valuable insights on the microstructural design and modification of diamond and related materials, contributing to the further tuning of properties in the future.