HOT-PRESS SINTERING STUDIES OF AMORPHOUS-SILICON NITRIDE POWDERS

The hot-press sintering behaviour of amorphous Si3N4 powders prepared from the ammonia pyrolysis of polycarbosilane or hydridopolysilazane polymers was studied. In the presence of yttria and alumina, the amorphous powders sintered to > 98% of theoretical density at 2023 K. Both the microstructure and average four-point MOR bend strengths, of test bars machined from the sintered compacts were comparable to those obtained from UBE SN-E-10 Si3N4 powder processed under the same conditions. However, in contrast to commercial crystalline powders, between approximately 1690 and 1700 K the amorphous Si3N4 powders underwent a rapid shrinkage corresponding to 50-60% of the total densification. In this narrow temperature regime, a radical change in morphology and phase composition of the amorphous powder occurred. Prior to 1690 K, the Si3N4 powders were totally amorphous as determined by X-ray diffraction analysis and consisted of angular shards with an average particle size of 2-3-mu-m. Samples quickly cooled after heating to 1700 K, consisted of a 53/47 mixture of equiaxed alpha and beta Si3N4 crystallites with an average particle size of 0.1-0.3-mu-m. Thus, the rapid densification at approximately 1700 K is identified with the amorphous to (alpha + beta) transition. Beyond approximately 1700 K, these samples gradually densified to the maximum density as mentioned. The fully densified samples consisted of 100% beta-Si3N, phase.