To investigate the effect of microwave heating on grain growth behavior in silicon nitride ceramics, microwave annealing was carried out using a microwave radiation frequency of 28 GHz. A comparative ...study of silicon nitride annealed by microwave and by conventional heating was done using scanning electron microscopy (SEM) observation and image analysis. Results show that microwave annealing is more effective in enhancing the grain growth and in producing the bi-modal microstructure than conventional heating. In particular, the growth along the c-axis was more remarkable due to the local and selective heating of the liquid phase by the microwave.
Layered composites with alternating B-SiAlON and silicon nitride layers were fabricated using a tape-casting and laminating technique. A distinct layered structure was obtained by hot pressing, where ...compositions of yttrium and aluminum at the layer interface were functionally graded in a thickness of about 10 microns. A solid-solution reaction prior to densification in the B-SiAlON layer contributed to the coexistence, in equilibrium, of these two layers. Density and Young's modulus depended on the volume fraction of silicon nitride layers according to the rule of mixtures. The strength of the B-SiAlON layer, estimated by an equivalent strain model, increased when compared to that of the monolithic material, although the fracture origin always was observed within the B-SiAlON layer. The layer thickness in an alternating layered configuration affected the strength of discrete layers. (Author)
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 28GHz Gyrotron source at 1600, 1700 and 1750°C for 2h. Microstructure of the specimens was analyzed by SEM, and flexural strength and fracture toughness measured. All seeded specimens showed complete α-β transformation and a bimodal microstructure with a high proportion of elongated and thick grains. Flexural strength of seeded specimens decreased (from 910 to 800MPa) 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 960MPa. Fracture toughness increased up to 7.0MPa⋅m1/2 in non-seeded specimens, and up to 8.1MPa⋅m1/2 in seeded specimens. Suitable combination of strength and toughness can be obtained applying seeding and microwave heating techniques.
The fracture behavior of a silicon nitride layered composite with alternating dense and porous layers was investigated under varying span-to-depth ratio in bending tests. Although the load-deflection ...curves always indicated a linear behavior, delamination cracks were often observed in central porous layers. During four-point bending, a complicated fracture mode and local stepwise crack extension were observed. In the high span-to-depth ratio region, the fracture stress was almost constant and a fracture origin was always confirmed in the dense layer on the tensile surface. As for three-point bending, the fracture was originated in the dense layer on the tensile surface by tensile stress followed by delamination.
Thermal conductivity of sintered silicon nitride has been improved by the alignment of large β-Si3N4 grains. Beta-Si3N4 containing 0.5mol% Y2O3 and 0.5mol% Nd2O3 was seeded with rodlike β-Si3N4 ...grains and tape-cast to align the seed grains. In-situ composite microstructure of the aligned large grains in smaller matrix grains was developed by the grain growth of seed grains during gas-pressure sintering at 2200°C for 4h. Room-temperature thermal conductivity was anisotropic: 137W·m-1·K-1 in the direction of tape-casting, 97W·m-1·K-1 of seet width, and 72W·m-1·K-1 in the direction of seet stack. The thermal conductivity in the direction parallel to the tape-casting direction was high because of the aligned large grains.
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