Change of Crystal Phases and Microstructure of Amorphous Si-C-N Powder by Hot Pressing

Abstract

An amorphous Si-C-N powder with Y2O3 and Al2O3 powder as sintering additives was hot-pressed at 1900°C for 120 min in a nitrogen atmosphere. Changes in the crystalline phases and microstructure of the amorphous Si-C-N powder during sintering were investigated by X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The defects at the fracture origins of the sintered bodies after bending tests also were investigated by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). XRD showed that alpha-Si3N4 was formed initially from the amorphous Si-C-N by 1530°C, which then transformed to s-Si3N4 at 1600°C. Also, a slight formation of crystalline SiC occurred during the transformation from alpha- to s-Si3N4, and it increased after the transformation was completed at 1900°C. TEM revealed that many SiC nanoparticles were incorporated into s-Si3N4 grains after the transformation from alpha- to s-Si3N4 at 1600°C. They were located at the triple points of the grain boundaries of s-Si3N4 after continued Si3N4 grain growth at 1900°C. Besides the SiC nanoparticles, large agglomerations of carbon or SiC particles of 20-60 µm size were observed by SEM and EPMA at the fracture origins of the sintered bodies after the bending tests.