The sintering mechanism and microstructure evolution in SiC-AlN ceramics studied by EFTEM

The microstructures of three hot-pressed SiC ceramic materials with 5 vol.%, 10 vol.%, and 15 vol.% AlN were investigated by analytical electron microscopy, especially electron energy-loss spectroscopic mapping (EELS mapping). AlN additives dissolve into SiC to form a core/rim structure and to refine the SiC grains. The crystallographic configurations for cores revealed by EELS mapping elucidate the sintering mechanism as liquid-phase sintering in the SiC-AlN composites. The additional 0.5 wt.% Y₂O₃ additives that distribute in the intergranular regions facilitate the formation of liquids to improve the sintering property. With the increase of AlN, the microstructure evolution experienced three stages characterized as solid solution with core/rim structure for a few elongated grains, core/rim for the general equiaxed grains, and the emergence of individual AlN grains with SiC-rich precipitations, respectively. The refined grains and a variety of microstructures contribute to an excellent combination of mechanical properties, with the flexural strength as high as 1000 MPa while the fracture toughness remains moderately high.