Silicon nitride is featured by its high elevated temperature strength, good thermal shock resistance, small elevated temperature creep, ideal wear resistance, excellent inoxidizability and high chemical stability, and is regarded as one of the most high performance structural ceramic materials with wide applications in mechanical, chemical, electronic and military fields [1]. The history of applying silicon nitride as a refractory dates back to the 70’s of last century. It was mixed with carborundum material to prepare Si3N4 – SiC bricks, which were used to build the blast furnace lining. Till now, this type of bricks is still widely used in tuyere area, bosh area, belly area, middle and lower stack and the stockline abrasive surface of many blast furnaces. With excellent performance of alkali resistance, thermal shock resistance and were resistance, this material has already been developed as a high-grade long-life refractory for blast furnaces of middle and large sizes [2]. In 80’s of last century, materials containing Si3N4 was used to produce the horizontal continuous casting separation rings, which had already been successfully applied in many steel ingot continuous casting facilities [3]. In recent years, this material has been introduced into the blast furnace stemming, meeting the on-site application demands of the blast furnace workplace [4]. It is also shown in recent initial research results that: introduction of certain amount of nitrides into the unshaped refractory of traditional oxides system can evidently improve the performance of slag permeation resistance and slag corrosion resistance of the material [5].

In this experiment, we chose as the raw material the pure calcium aluminate cement and the reactive alumina micro powder which are widely used in unshaped refractories, then mixed and shaped them with Si3N4. By adopting the traditional refractory firing method of burying the material in coke powder bed, we investigated the sintering properties and phase transformations of the Si3N4-Al2O3-CaO composites, and observed and analyzed their microstructure and phase composition using measuring means of SEM, EDX and XRD. Their reaction process is discussed in this article, in order to provide theoretic basis for further applications.

Experiment
The materials used in this experiment include: reactive alumina micro powder (the mass fraction of Al2O3 at 99%, produced in Kaifen, Helan Province, P.R.China), calcium aluminate cement (the mass fraction of Al2O3 at 70.47%, and the mass fraction of CaO at 28.08%, produced by Lafarge Calcium Aluminate Cement Company, Secar A[7]), and the Si3N4 powder (the mass fraction of N at 36.3%, and the mass fraction of free Si at 0.34%) prepared in our lab.

Take 5 shares of Si3N4-Al2O3 composites (each share in different proportions), and the total mass fraction of the content of Si3N4 & Al2O3 is at 86%. In these 5 shares, the mass fractions of the Si3N4 contents are 29%, 43%, 57%, 72% and 86% respectively. A share of calcium aluminate cement with mass fraction at 14% is introduced into these composites, which are then put into the ball grinder (alumina balls as the grinding media) for further treatment. After 40 minutes of mixing, the composites are rolled together with conjugation agent, and then machine-moulded to prepare cylinder specimens of φ20mm x 200mm. After being dried and put into the corundum-mullite sagger, the specimens are heated to 1500℃, 1600℃ and 1650℃ respectively for 3 hours with protection of compacted coke powder (the temperature rise speed at 4℃.min-1).

The changes on masses (before and after the firing process) and the bulk densities and apparent porosities of the specimens (after the firing) are investigated. The phase composition of the specimens after firing are analyzed by using the Rotary Target X-Ray Diffraction Instrument from the Rigaku Co.,Ltd, Japan; The microstructures of the specimens after firing are observed by using the scanning electronic microscope (Model: XL-30TMP) from Philip Corporation; The microzone compositions are also analyzed with the Energy Spectrometer.