Effect of Different Sintering Additives on the Microstructure, Phase Compositions and Mechanical Properties of Si3N4/SiC Ceramics

Y 2 O 3 and CeO 2 were chosen as additives to investigate the effect of different additives on the microstructure, composition of phases and mechanical properties of Si 3 N 4 /SiC ceramics using pressureless sintering. Si 3 N 4 /SiC ceramic without additives had a high density, while after adding Y 2 O 3 and CeO 2 , the density and exural strength of Si 3 N 4 /SiC ceramics were signicantly decreased due to the increase of porosity. The main phase compositions of samples were β-Si 3 N 4 and SiC. Moreover, the liquid phases Y-Si-O-N and Ce-Si-O-N were observed after adding Y 2 O 3 and CeO 2 respectively. It also indicated that for Si 3 N 4 /SiC composite ceramics, the high aspect ratio β-Si 3 N 4 overlapped with each other and closely bonded with glass phase could improve exure strength effectively. Besides, the SiC crystal grains mainly existed in grain boundary, which could inhibit the secondary recrystallization to avoid that the decrease of exural strength caused by the overgrowth of β-Si 3 N 4 grains.


Introduction
Silicon nitride (Si 3 N 4 ) ceramics have been widely applied in modern industry due to its excellent properties including thermal shock resistance, outstanding anti-corrosion and good wear resistance [1][2][3].
However, the shortcomings of the ceramics such as brittleness, unworkableness and high sensitivity to internal aws of materials are the main hindrances to its applications [1,3,4]. It has been revealed that adding second phase (TiC, TiO 2 , SiC, BN, etc.) with high hardness and chemical resistance, as the reinforcing phase, could improve the properties of ceramics, such as exural strength, fracture toughness and creep resistance. For example, silicon carbide (SiC) as reinforcing phase includes whiskers and particles [1,8,9]. Kai and Yang, et al. [1] obtained the Si 3 N 4 /SiC composite ceramics with high density and excellent mechanical properties by using the SiC whiskers as a reinforcing phase in Si 3 N 4 matrix.
Moreover, many researchers have reported that the SiC nanoparticles were widespread on the grain boundary. The phenonmen has caused interlocking of neighboring Si 3 N 4 grains and impede grainboundary sliding, therefore, the hardness, strength and high-temperature properties such as creep behavior of ceramics had been improved [9][10][11]. In order to industrilise Si 3 N 4 /SiC ceramics, pressureless sintering was selected as a possible method because it was practical to control the size and shape of the product along with temperature schedule [12]. Si 3 N 4 and SiC belong to covalent compounds. Both the covalent bonding Si-N and Si-C are so strong, and it is di cult to sinter for Si 3 N 4 /SiC ceramics since the sintering temperature is very high [12,13]. The oxides are always added to ceramics as sintering additives to promote the process of liquid-phase sintering by forming eutectic liquids at the low sintering temperature. The liquid-phase nally could be reserved as glassy or partially crystalline after cooling, and it usually locates in the junctions of three or two grains [14,15]. The common sintering additives including MgO, Y 2 O 3 , Yb 2 O 3 , Al 2 O 3 , CeO 2 and so on have been reported [12][13][14][15][16]. In addition, different types and quantities of additives have different effects on the materials. Lojanová and Tatarko [10,17,18], et al. investigated the effects of different rare-earth oxides RE 2 O 3 (RE = La, Nd, Lu, etc.) on the bending strength and microstructure of Si 3 N 4 -SiC and Si 3 N 4 nanocomposites, and it also analyzed the role of SiC grains inter the matrix. It indicated that the additives and surfaces oxidation layer could form the liquid-phase showing different the viscosity, which in uenced the mechanical properties of ceramics and the location of SiC grains. Y 2 O 3 and CeO 2 have been successfully used as sintering additives for Si 3 N 4 . The density and bending strength of the materials were improved obviously within certain range of contents [13,19].
In this study, in order to investigate the effect of both SiC and rare-earth oxides on mechanical properties of Si 3 N 4 /SiC composite ceramics, using Si 3 N 4 and SiC particles as raw materials, 3 wt.% rare earth oxides Y 2 O 3 and CeO 2 as additives respectively to prepare samples by pressureless sintering method.
Furthermore, the microstructure and phase compositions were also analyzed, which might provide evidences to reveal the relationships between the sintering additives and the mechanical properties.

Sample Preparation
Si 3 N 4 powders (α-phase 92%, β-phase 8%, Shanghai Shuitian Technology Co., LTD., Shanghai, China, Chemical Composition and Content of Si 3 N 4 were shown in Table 1) were used as the matrix materials and SiC powders (average particle size 5 µm, purity > 99.9%,Shanghai ST-Nano Technology Co. Ltd., China) were used as reinforcement in this study. The proportion of Si 3 N 4 and SiC is 95 : 5. Rare earth oxides Y 2 O 3 (purity > 99.95%, Ganzhou Jin Chengyuan new materials Co., LTD., Jiangxi, China) and CeO 2 (purity > 99.95%) were added as sintering additives respectively, both the content of Y 2 O 3 and CeO 2 is 3 wt. %. The detailed information is shown in Table 2. Figure 1 showed the XRD pattern ( Fig. 1(a)) and SEM micrograph ( Fig. 1

Characterization
The densities (ρ) and apparent porosity (P) of samples were measured by the Archimedes method. The exure strength via the three-point bending test using electromechanical universal testing machine The main properties of the samples were shown in Table 3, and Fig. 5 was drawn to describe the relationship between the performances clearly. From Fig. 5, it was not di cult to see that sample SC without admixture had higher density after sintering, and the porosity of SC was less than 0.5 percent. It was indicating that this sample had been fully sintered. With the sintering process, the pores of SC samples could be moved following the migration of grain boundary and then discharged from the body through the grain boundary, resulting in the densi cation of the material.
However, after adding Y 2 O 3 and CeO 2 , the porosities of SCY and SCC were increased signi cantly resulting in lower densities and mechanical properties, which were contrary to expectations. Generally, sintering additives promote liquid phase sintering through dissolution and precipitation mechanism [17].
Besides, according to previous reports, the type and content of sintering additives affect the amount and viscosity of the liquid, which in turn affect densi cation, grain coarsening and mechanical properties of Si 3 N 4 as well as in uence the stability of SiC [1]. Therefore, the relative density reduction might be due to that the glass phase formed after adding the sintering additives. The glass phase is conducive to the growth of Si 3 N 4 grains and tends to form larger grains, which hinder the ow of liquid phase. When the porosity is high in ceramics, it will become the main factor to reduce the exural strength since the increase of defects. The additive loss (bloating) during prolonged annealing at high temperatures caused the increase of the pores quantity [6,20]. And another possibility was that parts of Si 3 N 4 grains were decomposed at high temperature without pressure, reducing the densi cation of the material and increasing the porosity. Furthermore, the densi cation behavior and mechanical properties of SCY and SCC ceramics could be in uenced by sintering temperature, sintering time and the addition amount of the additives, which are the research targets in the future. It can be seen that the trend of porosity in the materials was converse with the exure strength (Fig. 5), which is consistent with other studies [19,21]. The ceramic with lower porosity indicated that there were fewer internal defects and more grain boundaries, which explained the high strength of the material. Therefore, the exure strength of SC was signi cantly higher than SCY and SCC. It can be concluded that the additives Y 2 O 3 and CeO 2 were bene cial to improve the porosity of materials. The factors in uencing the strength could be analyzed from the internal structure and the crystal phase composition of materials. The action mechanisms for exure strength will be described in more detail below.

SEM and XRD analysis
The microstructure of the fracture surface of Si 3 N 4 /SiC composite materials was shown in Fig. 6. It can be seen that the fracture surface of SC without any additives was very dense and had few pores. Rod-like β-Si 3 N 4 grains and a small amount of glass phase could be observed from Fig. 6 (a) and (b). The interweave of β-Si 3 N 4 grains and glass phase effectively improved the mechanical properties of materials. In addition, previous research has shown that the overlapping structure of β-Si 3 N 4 crystals with high aspect ratio, by bridging and pullout, is bene cial to improve the strength of materials, while the oversized grains with high width have a negative effect on the strength [2]. However, adding SiC dispersion particles could inhibit the secondary recrystallization of β-Si 3 N 4 crystals [9]. Thus, the size of β-Si 3 N 4 grains would not be too large, so as to decrease the mechanical properties. That may aplain why sample SC had the high exure strength. For SCY and SCC, it can be seen from Fig. 6 that the density decreased, the porosity increased obviously, and the structure was loose, which explained why the exure strength decreased after adding the sintering aids. In addition, short columnar β-Si 3 N 4 grains and glass phases were observed in these samples, and the pores were mainly formed by the overlapping of β-Si 3 N 4 .
It could also be found that the exure strength of SCY was slightly higher than SCC. For all the samples, the fracture surface of the samples displayed the intergranular fracture as a whole. The holes left by rodlike β-Si 3 N 4 crystals after fracture could be seen clearly in Fig. 6, particularly in SCC. Besides, the phenomenon that the crystal grains were pulled out from the matrix was also observed in SC and SCY. Therefore, the strengthening mechanism of composites could be attributed to intergranular fracture and grain pullout.
The phase composition of the samples was shown in Fig. 7. The main phase compositions of the composite materials were β-Si 3 N 4 and SiC, which is consistent with the SEM (Fig. 6). Besides, α-Si 3 N 4 crystal phase was not be observed, indicating that the α-β phase transformation was complete at high temperature. For sample SCY and SCC, the sintering additives Y 2 O 3 and CeO 2 will react with the SiO 2 oxide lm formed at the particle surface at high temperature so that the ternary system of Si 3 N 4 -Y 2 O 3 -SiO 2 and Si 3 N 4 -CeO 2 -SiO 2 formed. The liquid phases Y-Si-O-N and Ce-Si-O-N were generated at last as shown in Fig. 7, which could reduce the sintering temperature of the materials and promote the α-β phase transformation as previously stated. Besides, the Y 2 SiO 7 phase was also observed in sample SCY.

TEM analysis
TEM with higher resolution microscope was used on sample SCC to fully analyze the crystal phase composition and structure of it, as shown in Fig. 8. Many pores and β-Si 3 N 4 grains with a low aspect ratio were observed in Fig. 8(a). Moreover, there were many internal defects in sample SCC, which may be formed by the decomposition of Si 3 N 4 at the high temperature without external force, or by the failure of energy release during the rapid cooling. This veri ed the conclusion which mentioned before and further analyzed the reason of why the bending strength decreased after adding CeO 2 . As shown in the Fig. 8(b), the interplanar spacing was 0.62 nm proved that the phase was β-Si 3 N 4 , which was consistent with the results of XRD. The liquid phase (indicated by the arrow in Fig. 8(b)) formed by the SiO 2 oxide lm of Si 3 N 4 surface and sintering aids could be observed clearly, and it was located at the junction of two or more grain phases. Moreover, the SiC grains with irregular shape were identi ed in the grain boundaries.
Many researches have shown that SiC grains have an inhibitory effect on the secondary recrystallization of Si 3 N 4 grains and the inhibition effect increases with the increase of the size [9,11]. Therefore, the β-Si 3 N 4 grains would not grow too large to reduce the strength of materials.

Conclusions
The in uences of Y 2 O 3 and CeO 2 additives on the mechanical and microstructure of Si 3 N 4 /SiC composite ceramics have been investigated, and the impact of SiC on the material structure has also been studied. The sample SC obtained by using pressureless sintering without additives had high bending strength and density, while the exure strength of samples SCY and SCC decreased obviously. It was mainly in uenced by the viscosity of the eutectic mixture formed by Si 3 N 4 , oxides and decomposition of silicon nitride. The main crystal phase of Si 3 N 4 /SiC composite ceramics was β-Si 3 N 4 , which could combine with glass phase closely to improve the exure strength. In addition, SiC particles which located in grain boundary could inhibit the secondary recrystallization of the grains effectively, and avoid adverse effects on the strength of materials due to the oversize β-Si 3 N 4 grains. The in uences of additive content and sintering temperature will be research targets in the future.     Effect of additives on the density, porosity and exure strength of composite materials.