Microstructure and Phase Composition of Cold Isostatically Pressed and Pressureless Sintered Silicon Nitride

The microstructure and physical properties of new Y2O3 and Al2O3 oxide-doped silicon nitride ceramics fabricated by cold isostatic pressing and free sintering were investigated. The phase composition of produced material was also studied by X-ray diffraction at room and elevated temperature. The fabricated ceramics featured a microstructure of Si5AlON7 grains with a fine-grained α-Si3N4 with a small amount of Y2SiAlON5. Described ceramics is attractive for many high-temperature structural applications due to beneficial combination of fine-grained structure with improved mechanical properties and small weight loss.


Background
Silicon nitride is one of the most promising structural materials for high-temperature applications because of its excellent strength and toughness at elevated temperatures, good thermal shock resistance, low coefficient of thermal expansion, and chemical stability [1][2][3]. Silicon nitride ceramics are frequently used as structural materials especially for high-temperature engineering applications. The high-temperature properties of Si 3 N 4 -based ceramics strongly depend on the oxide additives used for the densification. However, its densification is rather difficult by classical sintering process due to the strongly covalent in Si-N bonds which results in low self diffusivity [4]. Such oxide additives as MgO, Al 2 O 3 , Y 2 O 3 , and Al 2 O 3 + Y 2 O 3 combination are considered as the most commonly used and ideal additives for Si 3 N 4 ceramics due to their high melting point and because of the possibility to control the α→β phase transformation rates of the Si 3 N 4 , the aspect ratio of the β-Si 3 N 4 , and the grain growth anisotropy. These additives also lead to high mechanical properties at room temperature as well as at elevated temperatures [3]. The use of rare-earth oxide additives to provide a liquid phase for sintering is therefore required for obtaining high-density Si 3 N 4 [5,6]. The control of the grain size and its distribution is one of the most important issues in the processing of Si 3 N 4 ceramics. Particularly, the design of duplex microstructure containing a few large elongated β-Si 3 N 4 grains is quite important to ensure high fracture toughness [7][8][9]. One of the major problems of the silicon nitride processing is the high cost of such commercial methods as hot isostatic pressing (HIP), spark plasma sintering (SPS), and gas pressure sintering (GPS). Thus, it is important to find new and cheaper alternative processing methods while a relatively high strength can be acquired. To overcome this problem, we have developed a novel process involving cold isostatic pressing (CIP) and pressureless sintering in a nitrogen atmosphere.
The purpose of this work is to study the features of the microstructure and phase composition of new CIPed and pressureless sintered silicon nitride ceramics with Al 2 O 3 and Y 2 O 3 oxide.

Methods
Commercial α-Si 3 N 4 powder (Starck, grade M11) was used as the starting powder, and Y 2 O 3 (grade B) and Al 2 O 3 (A 16 SG Grade, Alcoa) were used as the sintering additives. The powder mixture was composed of 95 wt% α-Si 3 N 4 powder and 15 wt% sintering additives ( Table 1). The powder mixture was milled in an attritor mill for 20 min. After milling, the powder mixture was cold isostatically pressed to rectangular bars at 200 MPa (EPSI CIP 400 B-9140 press). The green bodies were sintered in a vacuum furnace (Nabertherm VHT 8/22-GR) under 0.1 MPa in a nitrogen atmosphere at 1650°C ( Table 1). For more details and properties, see [10][11][12]. Figure 1 shows the agglomerate of the initial commercial silicon nitride powder observed by TEM.
Phase composition of the sintered samples was determined by the X-ray diffraction method (XRD, Rigaku Ultima IV diffractometer; Cu Kα-emission (radiation), Ni-filter). A scan rate of 10°C/min was used to record the diffraction patterns in 2θ range between 10 and 60°C. XRD analyses were carried out using a Rigaku Ultima IV automated diffractometer. The sintered material was analyzed in the solid form.
The microstructure was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Structural characterization was performed using an FEI Quanta 200-3D (FEI Company, Hillsboro, OR) and Quanta 600 FEG (FEI Company, Hillsboro, OR) scanning electron microscopes. Since the silicon nitride material under investigation was non-conducting, it was necessary to coat it with a thin layer of carbon to prevent surface charging during examination. One specimen from batch was examined in a transmission electron microscope (JEOL-2100) at 200 kV. Foils for TEM were prepared from slices hand ground and finally ion-polished until perforation.
Thermogravimetric analysis (TGA)/differential thermal analysis (DTA) experiments were performed using an equipment STA 503 BÄHR model at a heating rate of 10°C/min.
For surface examination, specimens were mechanically polished.

Results and Discussion
A lot of researches and developments indicated that high fracture toughness has been attributed to the elongated β-Si 3 N 4 grains and weak grain boundary, which favors crack bridging and deflection. It was reported that the materials having microstructure with the finest and coarsest elongated β-grains showed the high value of fracture toughness, whereas materials with the microstructure with the mixture of elongated thin and thick β-grains had lower facture toughness [13]. Figure 2 shows a microstructure of a polished specimen. Produced ceramics is characterized by the bimodal character of microstructure with a grain size distribution from 300 to 800 nm. The microstructure generally consisted of large and small equiaxed grains.
The density of our material was 2.94 g/cm 3 , the flexural strength was 280 MPa, the Young's modulus was 214 GPa, the microhardness was 1375 HV, and open porosity was 0.1 % as it described in our previous work [10]. Abe et.al described the correlation of CIP condition with structure and properties of both green and sintered bodies [5]. Hotta et al. described the microstructure and fracture strength of CIPed silicon nitride [6]. Sintered in a nitrogen atmosphere for 3 h, materials exhibit high mechanical properties, in particular, a high bending strength 607 and 686 MPa, respectively, and fracture toughness 5.6 MPa/m −1/2 [5,6]. Furthermore, it should be noted that reported high strength ceramics have the same type of microstructure comparing with our ceramics. Figure 3 presents the results of thermal analysis. The sample firstly starts losing weight at about 200°C and ends at about 400°C. The weight loss of the present ceramics was 0.003 % ( Table 2).
Matovic and co-authors investigated pressureless sintered Si 3 N 4 with the Li 2 O-Y 2 O 3 additives. The density of this ceramics ranged from 83.8 to 98.0 % of the theoretical density depending on the amount and type of oxide additives. The weight loss was 3.7 and 4.1 %, respectively [8]. Cinibulk and Thomas described the same type of structure for sintered at 1850 and 1900°C silicon nitride with a YSiAlON glass. β-Y 2 Si 2 O as a secondary phase was observed after the heat treatment at 1350°C, while at a higher temperature of 1450°C, primarily YSiO 2 N and Y 4 Si 2 O 7 N 2 in addition to small amounts of Y 2 SiOs were found [9].
High-temperature XRD patterns of the sintered samples are shown in Fig. 4. The XRD analyses revealed β-Si 5 AlON 7 as a major phase, α-Si 3 N 4 phase, and Yang et al. described porous silicon nitride with 5 wt% different rare-earth oxides sintering additives (Y, La, Lu, and Eu) fabricated by pressureless sintering. A β-Si 3 N 4 phase was only found by XRD analysis. This ceramics showed a typical bimodal microstructure with a large elongated β-Si 3 N 4 grains and small β-Si 3 N 4 matrix grains. The porosity of the described material varied from 48 to 54 %, the bending strength ranged from 150 to 188 MPa, the fracture toughness varied from 1.6 to 3.1 MPa/m −1/2 , and the weight loss varied from 4 to 7 % [7]. Chen et al. also described silicon nitride ceramics with controlled porosity from 42 to 63 % and relatively high flexural strength from 50 to 120 MPa [14]. Luo and co-authors described 90 and 95 % dense Si 2 N 2 O/β-Si 3 N 4 composites fabricated by cold isostatic pressing and Table 2 Physical properties Mass loss, % Flexural strength, MPa [10] ρ, g/cm 3 [10] Open porosity, % [10] 0.003 280 2.94 0.01 Fig. 4 High-temperature XRD sintering at 1700 and 1800°C. This material consists of Si 2 N 2 O and β-Si 3 N 4 phases and has a grain size of 2 and 0.5 μm, respectively, depending on the sintering temperature [15]. Sillapasa and others investigated uniaxially pressed and further cold isostatically pressed presintered at argon atmosphere and then finally sintered in a nitrogen atmosphere silicon nitride. The bulk density of described material ranged from 2.18 to 2.61 g/cm 3 , the porosity varied from 5.78 to 21.30 %, the Young's modulus changed from 121.5 to 198.6 GPa, the bending strength ranged from 104.0 to 250.7 MPa, and the microhardness varied from 403 to 971 HV [16].
The content of all phases changes insignificantly with an increase in the temperature from 20 to 1550°C (Table 3). In particular, the content of Si 5 AlON 7 varied from 64.1 to 72.7 %, the content of α-Si 3 N 4 ranged from 25.2 to 33.4 %, and the amount of the Y 2 SiAlON 5 varied from 2.1 to 2.5 %. Gonon with co-authors described in detail characteristics of the Y 2 SiAlON 5 [17]. Further increase in the testing temperature up to 1500°C is accompanied with a decrease in the β-SiAlON content up to 68.4 %. Some physical properties of the free sintered magnesium oxide doped ceramics were described in Sirota et al. [18].
Tables 2 to 4 summarize the variation of density, mass loss, phase content, and lattice parameter of the specimen after sintering. The data for the specimens referred from the previous paper are also listed in Table 2.

Conclusions
The fabrication of a fine-grained silicon nitride ceramics by cold isostatic pressing and pressureless sintering was reviewed. Commercial submicronic powders were used as raw materials. Al 2 O 3 and Y 2 O 3 additives lead to moderately high density and low weight loss.
The results can be summarized as follows: 1. The obtained ceramic has a fine-grained microstructure with equiaxed grains with the average grain size that ranged from 0.3 to 0.8 μm. 2. Investigated ceramics mainly consists of α-Si 3 N 4 and Si 5 AlON 7 phases and a small amount of the Y 2 SiAlON 5 phase. Phase composition does not change in a wide temperature range up to 1500°C. 3. Relatively high mechanical properties are successfully combined with the absence of porosity and low weight loss. Therefore, the proposed ceramics can be effectively used in many areas of high temperature applications.