Effect of Mo and TiC addition on the microstructure and mechanical properties of spark plasma sintered Si3N4 composites

Abstract

In this study, to improve the fracture toughness of Si3N4 ceramics and overcome the reduction in the hardness caused by toughening, the Si3N4-based composite ceramics were prepared by spark plasma sintering. Fixed amounts of Y2O3 (7 wt%) and TiC (8 wt%) together with Mo adjusted in the range of 0-8 wt% were incorporated into the Si3N4 matrix. Besides, Si3N4 composites were also prepared as control groups by excluding TiC and Mo. The results reveal that the addition of TiC improves the hardness of the Si3N4 composites by inhibiting the transformation from α-Si3N4 to β-Si3N4 and grain refinement. Meanwhile, the high hardness of the composites is maintained even after toughening with Mo. The addition of Mo promotes the nucleation and phase transformation of α-Si3N4 by promoting the transfer of the Si and N atoms. At the same time, molybdenum silicide is formed in-situ, improving fracture toughness through various toughening mechanisms. The Si3N4 composites with 8 wt% TiC and 8 wt% Mo sintered at 1700 °C exhibit the optimal properties, with Vickers hardness and fracture toughness determined to be 18.25 GPa and 10.2 MPa•m1/2, respectively.