Synthesis of Si3N4-celsian composite materials

The synthesis of Si3N4-celsian composite materials was studied in this paper. To obtain celsian, two different types of additives were used. In the first case the appropriate mixture of BaCO3, Al2O3 and SiO2 was added to Si3N4, while in the second case a Ba-A-zeolite, previously heated at 850 degrees C, was used as an additive. Mixtures of Si3N4 and the sintering aids were homogenized in a vibratory mill in the presence of isopropanol. The green bodies were pressurelessly sintered in nitrogen atmosphere at temperatures of 1650-1750 degrees C. Densification degree was followed, as well as microstructural changes, as a function of the sintering conditions and the additive used. Reaction of BAS formation was followed by X-ray analysis during the heating up period. It was found that with 10% zeolite as a sintering aid, full densification was achieved at 1750 degrees C but with the same amount of oxide mixture, only 96% TD was attained. Phase changes were followed also during cooling down the samples from sintering temperature. Two crystalline phases, beta-Si3N4 and hexacelsian, were present in both types of sintered samples. (C) 1999 Elsevier Science Ltd and Techna S.r.l. All rights reserved.