Densification and strengthening mechanism in spark plasma sintering of B4C-VB2 ceramics via carbide boronizing

To reduce the negative effects of the long-time and B2O3 phase on the traditional sintering process for B4C-based composite ceramics, nearly fully dense B4C-VB2 composite ceramics were prepared by reactive spark plasma sintering (SPS) technology at 2000 degrees C with B and V8C7 powders as raw materials in this paper. The effects of the degassing time during SPS on the microstructure and the mechanical properties of the final products were investigated in detail. The results revealed that the proper degassing time was beneficial for the vent of B2O3 during the sintering process, which refined the grain size, promoted densification and improved the mechanical properties of the composite ceramic. However, the redundant degassing time increased the holding time at high temperature, resulting in abnormal grain growth and mechanical performance deterioration. In the present work, the optimal degassing time was 6 min, and the final product prepared under the above conditions exhibited excellent comprehensive performance with a relative density of 99.2%, Vickers hardness of 31.2 GPa, bending strength of 654 MPa and fracture toughness of 5.7 MPa m(1/2). In addition, the strengthening and toughening mechanisms of the products were mainly attributed to the residual thermal stresses and bridging structure caused by the fine B4C and VB2 grains distributed uniformly.