Enhanced strength of nano-polycrystalline diamond by introducing boron carbide interlayers at the grain boundaries

Polycrystalline diamond with high mechanical properties and excellent thermal stability plays an important role in industry and materials science. However, the increased inherent brittle strength with the increase of hardness has severely limited its further widespread application. In this work, we produced well-sintered nano-polycrystalline (np) diamond by directly sintering fine diamond powders with the boron carbide (B₄C) additive at high pressure and high temperatures. The highest hardness value of up to ∼90 GPa was observed in the np-diamond (consisting of fine grains with a size of 16 nm) by adding 5 wt% B₄C at 18 GPa and 2237 K. Moreover, our results reveal that the produced samples have shown noticeably enhanced strength and toughness (18.37 MPa m^0.5) with the assistance of the soft phase at the grain boundaries, higher than that of the hardest known nano-twined diamond by ∼24% and a little greater than that of the toughest CVD diamond (18 MPa m^0.5). This study offers a novel functional approach in improving and controlling the hardness and stiffness of polycrystalline diamond.