2. Results & Discussions

2.1 The Sintering Properties of The Si3N4-Al2O3-CaO Composites

Fig. 1 below shows the changes of bulk densities and apparent porosities of Si3N4-Al2O3-CaO composites after being fired at different temperature degrees.

(Fig.1: Bulk density and apparent porosity of Si3N4-Al2O3-CaO composite fired at high temperatures)

We can see from Fig.1: along with the increment of Si3N4 content, the bulk density of the specimen shows a trend of decreasing, which might be because that the density of Si3N4 is lower than that of α. Al2O3 [6]; when the firing temperature is heightened from 1500℃ to 1600℃, the bulk densities of all the specimens see evident increases; when the firing temperature….(text missing out of the left bottom of page 2)….the apparent porosities of all specimens decrease obviously; and when the temperature is heightened to 1650℃, the apparent porosities are increasing in contrast.

Fig. 2 shows the changes on masses of Si3N4-Al2O3-CaO composites before and after being fired at different temperature degrees. From Fig.2 we can see: after being fired at 1500℃, the mass of specimen increases evidently; at 1600℃, along with the increment of Si3N4 content, the mass increment of specimen turns into mass loss (the specimen begins to lose its mass when the Si3N4 content is higher than 57%); at 1650℃, mass losses are found with all the specimens, and under this temperature, the higher the content of Si3N4, the greater the mass loss. This indicates: when firing the Si3N4-Al2O3-CaO composites in coke powder bed under high temperature, there might be complicated chemical reactions taking placing within the specimens, causing the changes on their masses. By comparing Fig.1(b) and Fig.2, we can see that: in temperature range of 1600℃ ~ 1650℃, along with the increment of Si3N4 content, the masses of specimens decrease and the apparent porosities increase after the firing process.

(Fig.2: Mass change rate of Si3N4-Al2O3-CaO composites fired at high temperatures)

2.2 The Changes of Microstructures of Si3N4-Al2O3-CaO Composites

Before being fired, all the specimens are in grayish-white. After all the cylinder specimens are cut open, we can see that, for specimens fired under different temperature degrees, the colors of their cross-sections are different: the surface and internal of the specimens fired at 1500℃ are evenly in black; the color of specimens fired at 1600℃ gradually changes from black into gray; and for all specimens fired at 1650℃, their internal colors are in gray. The changes of these colors may reflect the changes of carbon content levels of the specimens – the carbon contents of specimens fired at 1500℃ is evidently higher than that of specimens fired at 1650℃.

The microstructures of the specimens fired at different temperature degrees are further observed by using SEM. It is disclosed that: at 1500℃, the internal structure of the specimens are rather loose, some tiny pores are found existing around the grains; at 1600℃, the specimens are becoming obviously more compact; when the temperature is heightened to 1650℃, the number of pores within the specimens increases again. Meanwhile, the microstructure of the specimens are associated their compositions: under same firing temperature, along with the increment of the Si3N4 content, the specimens become less compact, the apparent porosities rise higher and the apertures of the pores increase. This tallies with the aforementioned rule of relation between the changes of bulk densities of Si3N4-Al2O3-CaO composites and their temperatures and compositions. Fig.3 shows …. (text missing out of the right bottom of page 2)….

(Fig.3: Microstructures of composites containing 29% and 86% Si3N4 fired at 1500℃, 1600℃ and 1650℃)

2.3 The Phase Transformations of Si3N4-Al2O3-CaO Composites

During the process of the experiment, further phase transformation analysis has been carried out with the specimens that were fired at the temperature degrees of 1500℃, 1600℃ and 1650℃ respectively. In the specimens fired at 1500℃, besides the corundum, β- Si3N4 andα- Al2O3 phases, there is also CaAl2Si2O3 phase observed; along with the increment of the Si3N4 content, the content of corundum phases gradually decreases. In specimens containing 86% Si3N4, there are onlyβ- Si3N4, α- Al2O3 and a small amount of CaAl2Si2O3 phases observed, and no corundum phase is found. In addition, although the silicon nitride material used in this experiment is a direct product of nitridized silicon powder, and its main phase isα- Si3N4 [7], the diffraction peaks of β- Si3N4 are still higher than the diffraction peaks of α- Si3N4 in all specimens. This is because when the temperature is over 1400℃, the lower temperature phase ofα- Si3N4 will be inevitably converted into the higher temperature phase ofβ- Si3N4. If the firing temperature is at 1600℃, Ca-α-Sialon and β-Sialon phases (besides Si3N4 and corundum phases) are found in all specimens, and the CaAl2Si2O3 phase disappears; In specimens containing 86% Si3N4, there are only β- Si3N4, α- Al2O3 and Ca-α-Sialon observed. Regarding the formation process of Ca-α-Sialon, Zhou Yanping [8] and his colleagues believe: when the temperature is around 1250℃, CaO and the SiO2 at the surface of Si3N4 begin to form eutectic point phases; along with the rise of temperature, the reactions in the composite system lead to the formation of cacoclasite; when the temperature is further heightened, the cacoclasite is formed in the reaction….( text missing out of the left bottom of page 3)..

Sialon is a product of Al2O3 reacting in Si3N4 solid solution. When the firing temperature is further heightened to 1650℃, the Ca-α-Sialon disappears from the composition of the specimens. Besides the phases of Si3N4 and corundum, there are still a lot of β-Sialon phase observed, and its content is associated with the content of Si3N4-Al2O3 that was initially introduced: In specimen containing 29% Si3N4, the peak value of β-Sialon is lower than that of specimen containing 57% Si3N4; and in specimen containing 86% Si3N4, there are onlyβ-Sialon and a small amount of α-Sialon observed. Fig. 4 is the XRD pattern of specimens containing 57%α-Sialon after being fired under the temperature degrees at 1500℃, 1600℃ and 1650℃ respectively.

(Fig.4: XRD patterns of Si3N4-Al2O3-CaO composites (57% Si3N4))

2.4 Discussion

When the specimens are fired in the coke powder bed, the main gases in the powder bed are CO and N2, which have the partial pressures close to 105Pa [9]. If the Si3N4-Al2O3-CaO composites are fired in this mentioned powder bed, the surrounding atmosphere may affect the stability of ingredients of the composites, especially for the Si3N4 in condensed phase. Fig.5 is the phase stability diagram displaying the relation between O2 partial pressure and the Si3N4, Si2N2O, SiC and SiO2 in condensed phase (PCO +N2 = 105Pa). There might be chemical reactions taking place between the Si3N4 in condensed phase and the main gas phase CO, involving the following reaction equations:

Si3N4(s)+3/2CO(g)=3/2Si2N2O(s)+1/2N2(g)+3/2C(s)
Lg K1 = 3288/T – 4.31 (1)

Si3N4(s)+3/2CO(g)=3/2SiC(s)+3/2SiO(s)+2N2(g)
Lg K2 = 39052/T +20.90 (2)

The thermodynamic data in above reaction equations and in Fig.5 are quoted from the reference literature [9], and other data are from JANAF Data [10]. Buried in coke powder bed at 1500℃ ~ 1600℃, the Gibbs Free Energy values of both of these two reactions are less than zero, which means these two reactions are ABLE to take place in this coke powder bed. The reaction (1) is a mass increment reaction, while the reaction (2) is a mass-loss reaction. ….( text missing out of the right bottom of page 3)..

…(following the text from page 3).., at 1500℃, reaction (2) is the main reaction taking place. Phases of Si2N2O and SiC are not found in XRD patterns, but at 1500℃, the grayish color of the surface and internal of all specimens before firing have changed into black after firing, indicating that there is some C precipitated out from the internal of the specimens; At 1600℃, along with the increase of Si3N4 content, the mass increment of the fired specimens turns into mass loss. At 1650℃, mass losses are observed with all fired specimens, and the higher the Si3N4 content, the greater the mass loss, and the higher the apparent porosity. This fact provides evidence for the positive relation between the mass loss and Si3N4 content level. Yokoyama K and his colleagues [10] believe: under high temperature and with the presence of C, the significant mass loss in process of sintering the Si3N4 ceramic is associated with reaction (2).

(Fig.5: Phase stability diagram as a function of O2 partial pressures (PCO +N2 ≈ 105Pa) )

In coke powder bed, the partial pressures of N2 and O2 are relatively lower in comparison to that of CO. From below equation:

C(s)+1/2O2(g)=CO(g)
Lgk3=6000/T+4.48 (3)

we can calculate the partial pressures of oxygen (Po2) at 1500℃, 1600℃ and 1650℃ to be 10-11.6 Pa, 10-11.3 Pa and 10-11.1 Pa respectively, corresponding to point a, b and c in Fig.5.

From Fig.5 we can see: when the specimens are buried in the coke powder bed, along with the rise of sintering temperature, the partial pressures of oxygen on the surface of the specimens increase from a lower level to a higher level (a → b → c), and the stability area of condensed phases transits from Si2N2O phase to SiC phase. Actually, the partial pressure of oxygen within Si3N4 is much higher that on its surface. Therefore, we can see from the figure above that it is absolutely possible for the reaction (1) and (2) to take place within the specimens at 1500℃ to1600℃. However, there are different dominant reactions taking place at different temperature degrees – the main reaction is reaction (1) when the temperature is below 1500℃, and on the other hand, reaction (2) dominates the temperature range above 1500℃. In this experiment, small amount of Ca-α-Sialon is found in all specimens fired at 1600℃; however, it can not be detected by X-Ray Diffraction Instrument in specimens fired at 1650℃, which is because the internal chemical reactions deviate its composition point, leading to the decline or even disappear of Ca-α-Sialon phase.