Microstructure and mechanical properties of microwave sintered silicon nitride

Use of microwave heating for sintering of silicon nitride ceramic is being actively investigated, Sintering of seeded and non-seeded Silicon Nitride with Y2O3, Al2O3 and MgO as sintering aids was carried out in an applicator fed by a 28 GHz Gyrotron source at 1600, 1700 and 1750 degrees C for 2h, Microstructure of the specimens was analyzed by SEM, and flexural strength and fracture toughness measured. All seeded specimens showed complete alpha-beta transformation and a bimodal microstructure with a high proportion of elongated and thick grains. Flexural strength of seeded specimens decreased (from 910 to 800 MPa) as the sintering temperature increased, due to the increase in size of elongated grains, as well as to interlocking of these grains. In non-seeded samples, strength increases as it does the sintering temperature, achieving a level of 960 MPa. Fracture toughness increased up to 7.0 MPa.m(1/2) in non-seeded specimens, and up to 8.1 MPa.m(1/2) in seeded specimens. Suitable combination of strength and toughness can be obtained applying seeding and microwave heating techniques.