High Hardness B _a C _b -(B _x O _y /BN) Composites with 3D Mesh-Like Fine Grain-Boundary Structure by Reactive Spark Plasma Sintering

Boron carbide B₄C powders were subject to reactive spark plasma sintering (also known as field assisted sintering, pulsed current sintering or plasma assisted sintering) under nitrogen atmosphere. For an optimum hexagonal BN (h-BN) content estimated from X-ray diffraction measurements at ∼0.4 wt%, the as-prepared B _a C _b -(B _x O _y /BN) ceramic shows values of Berkovich and Vickers hardness of 56.7 ± 3.1 GPa and 39.3 ± 7.6 GPa, respectively. These values are higher than for the vacuum SPS processed B₄C pristine sample and the h-BN -mechanically-added samples. XRD and electronic microscopy data suggest that in the samples produced by reactive SPS in N₂ atmosphere, and containing an estimated amount of 0.3–1.5% h-BN, the crystallite size of the boron carbide grains is decreasing with the increasing amount of N₂, while for the newly formed lamellar h-BN the crystallite size is almost constant (∼30–50 nm). BN is located at the grain boundaries between the boron carbide grains and it is wrapped and intercalated by a thin layer of boron oxide. B _x O _y /BN forms a fine and continuous 3D mesh-like structure that is a possible reason for good mechanical properties.