CHANGE OF CRYSTAL PHASES AND MICROSTRUCTURE OF AMORPHOUS SI-C-N POWDER BY HOT-PRESSING

An amorphous Si-C-N powder with Y2O3 and Al2O3 powder as sintering add itives was hot-pressed at 1900 degrees C for 120 min in a nitrogen atm osphere. Changes in the crystalline phases and microstructure of the a morphous Si-C-N powder during sintering were investigated by X-ray dif fractometry (XRD) and transmission electron microscopy (TEM). The defe cts at the fracture origins of the sintered bodies after bending tests also were investigated by scanning electron microscopy (SEM) and elec tron probe microanalysis (EPMA). XRD showed that alpha-Si3N4 was forme d initially from the amorphous Si-C-N by 1530 degrees C, which then tr ansformed to beta-Si3N4 at 1600 degrees C. Also, a slight formation of crystalline SIC occurred during the transformation from alpha- to bet a-Si3N4, and it increased after the transformation was completed at 19 00 degrees C. TEM revealed that many SiC nanoparticles were incorporat ed into beta-Si3N4 grains after the transformation from alpha- to beta -Si3N4 at 1600 degrees C. They were located at the triple points of th e grain boundaries of beta-Si3N4 after continued Si3N4 grain growth at 1900 degrees C. Besides the SiC nanoparticles, large agglomerations o f carbon or SiC particles of 20-60 mu m size were observed by SEM and EPMA at the fracture origins of the sintered bodies after the bending tests.