High-pressure regulated phase transition enables in situ synthesis of high-performance dual-phase Si3N4 ceramics
Dense dual-phase Si3N4 ceramics combining high hardness and toughness were synthesized via high-pressure spark plasma sintering (SPS) at 1500–1560 °C. Using only 5 wt% liquid-phase additives, full densification was achieved at 150 and 200 MPa, whereas 50 MPa SPS at 1600 °C failed. The 200 MPa specimen exhibited stress-induced α→β phase transformation via coherent interface migration, yielding elongated β-columnar grains with embedded α particles. At 1560 °C and 200 MPa, the material achieved a hardness of 21.3±0.3 GPa and fracture toughness of 6.3±0.3 MPa·m1/2, surpassing the 150 MPa baseline which underwent grain coarsening and reduced toughness. The mechanism overcomes the thermodynamic disparity between α and β phases, enabling uniform microstructures with equiaxed and elongated grains. This pressure-regulated approach offers a scalable route for advanced structural ceramics in extreme environments.