Abstract:
Si3N4-Al2O3-CaO composites, taking silicon nitride, reactive alumina micropowder and pure calcium aluminate cement as the raw materials, are fired in coke powder bed at 1500℃,1600℃and 1650℃. Their sintering properties and phase transformations are investigated, and their microstructure and reaction process are observed and analyzed by means of SEM, EDX and XRD. The results show that their sintering properties are associated with the compositions and the sintering temperature of the tested samples. The bulk density of the composites increases and their apparent porosity decreases when the temperature is raised from 1500℃ to 1600℃. However, when the temperature is raised to 1650℃, in contrast, the bulk density of the composites decreases, and the apparent porosity increases. Also, the bulk density of composites decreases with the increment of Si3N4 content at the same sintering temperature. The mass changes are found with all fired composites: at 1500℃, increase of masses are recorded with all fired composites; however, at1600℃and 1650℃, the masses of the fired composites become to decrease. According to thermodynamic analysis, there are complicated chemical reactions taking place during the firing process. The reaction Si3N4(s)+3/2CO(g)=3/2Si2N2O(s)+1/2N2(g)+3/2C(s) leads to the mass gain of all composites at 1500℃, and the reaction Si3N4(s)+3/2CO(g)=3/2SiC(s)+3/2SiO(s)+2N2(g) leads to the weight loss at temperature higher than 1500℃. In addition, the phase transformations are found with all composites examined by XRD as the sintering temperature rises up, besides corundum, Si3N4, or Si3N4 as the main phases. At 1500℃, anorthite is produced in the system, and at 1600℃, anorthite is dissolved into liquid phase and at the same time, Ca-α-Sialon and β-Sialon are created. At 1650℃, Ca-α-Sialon disappear and a great deal ofβ-Sialon are produced. According to above experiment results, we believe that anorthite is a product created by the reaction of CaO (in the calcium aluminate cement), SiO2 (on the surface of Si3N4) and Al2O3. When the temperature rises up, it further reacts with Si3N4 to produce Ca-α-Sialon; at 1650℃, the disappearance of Ca-α-Sialon might be the result of deviation of composites from the Ca-α-Sialon phase caused by its internal chemical reaction at that temperature; β-Sialon is a product of the reaction between Si3N4 with Al2O3 in solid solution, and its content depends on the content and firing temperature of Si3N4 and Al2O3 in the composites.