Microstructure and mechanical properties of α/β-Si3N4 composite ceramics with novel ternary additives prepared via spark plasma sintering

In this study, alpha/beta-Si3N4 composite ceramics with high hardness and toughness were fabricated by adopting two different novel ternary additives, ZrN-AlN-Al2O3/Y2O3, and spark plasma sintering at 1550 degrees C under 40 MPa. The phase composition, microstructure, grain distribution, crack propagation process and mechanical properties of sintered bulk were investigated. Results demonstrated that the sintered alpha/beta-Si3N4 composite ceramics with ZrN-AlN-Al2O3 contained the most alpha phase, which resulted in a maximum Vickers hardness of 18.41 +/- 0.31 GPa. In the alpha/beta-Si3N4 composite ceramics with ZrN-AlN-Y2O3 additives, Zr3AlN MAX-phase and ZrO phase were found and their formation mechanisms were explained. The fracture appearance presented coarser elongated beta-Si3N4 grains and denser microstructure when 20 wt% TiC particles were mixed into Si3N4 matrix, meanwhile, exhibited maximum mean grain diameter of 0.98 +/- 0.24 mu m. As a result, the compact alpha/beta-Si3N4 composite ceramics containing ZrN-AlN-Y2O3 additives and TiC particles displayed the optimal bending strength and fracture toughness of 822.63 +/- 28.75 MPa and 8.53 +/- 0.21 MPa.m(1/2), respectively. Moreover, the synergistic toughening of rod-like beta-Si3N4 grains and TiC reinforced particles revealed the beneficial effect on the enhanced fracture toughness of Si3N4 ceramic matrix.