High toughness Si₃N₄ ceramic composites synergistically toughened by multilayer graphene/β-Si₃N₄ whisker: Preparation and toughening mechanism investigation

This work aimed at investigating the superiorities of multilayer graphene (MLG) and β-Si₃N₄ whisker (β-Si₃N_4w) in synergistically enhancing the toughness of Si₃N₄ ceramic composites. First, Si₃N₄ ceramic composites with hybrid 1 wt% MLG and 0 – 5 wt% β-Si₃N_4w were sintered by spark plasma sintering. Phase constitutions and microstructures of Si₃N₄-based ceramic composites were characterized by X-rays diffraction, Raman spectra, and scanning electron microscope. Then, the optimal β-Si₃N_4w content and sintering process of MLG/β-Si₃N_4w-toughened Si₃N₄ ceramic composites were comprehensively optimized by orthogonal experiments. Results indicated that in the case of maintaining the hardness of 15.87 – 18.84 GPa, the fracture toughness of Si₃N₄ ceramic composites incorporated with 1 wt% MLG and 3 wt% β-Si₃N_4w was advanced to 11.04 MPa·m^1/2, which was 11.29% and 87.12% higher than that of 1 wt% MLG- and 3 wt% β-Si₃N_4w-reinforced Si₃N₄ ceramic composites, respectively. Orthogonal experiments demonstrated that 1 wt% MLG/3 wt% β-Si₃N_4w-toughened Si₃N₄ ceramic composites sintered at 1600 ℃, 40 MPa, and holding time for 6 min had optimum comprehensive mechanical properties. MLG wrapping and the synergistic effects of pull out, crack deflection, crack branching, and bridging resulted by MLG and β-Si₃N_4w were the main responsible for the enhancement of toughness.