Microstructure and hardness scaling in laser-processed B4C–TiB2 eutectic ceramics

The surface layers of the pseudo-binary eutectic composed of boron carbide (B4C) and titanium diboride (TiB2) were directionally solidified via direct laser irradiation in an argon atmosphere. The resulting surface eutectic layers had highly oriented lamellar microstructures, whose scale (i.e. interlamellar spacing) was controlled directly by the laser scan rate, following an inverse square root dependence for lower solidification velocities. Higher velocities (> about 4.2 mm/s) departed from this relationship, although well-ordered microstructures were still achieved. A concomitant increase in the Vickers hardness with decreasing interlamellar spacing was observed, although the trend did not correspond to traditional Hall-Petch behaviour. The hardness of the eutectic composites became load-independent at indenter loads > 9.81 N, indicating a potential transition from plastic to fractural deformation during indentation. A Vickers hardness of 32 GPa was achieved in the highest solidification velocity samples (42 mm/s) which had interlamellar spacings of 180 nm.