BN ceramics

Materials Letters 57 (2003) 3473 – 3478 www.elsevier.com/locate/matlet

Effects of sintering aids on the microstructure and mechanical properties of laminated Si3N4/BN ceramics Cuiwei Li *, Chang-an Wang, Yong Huang, Qingfeng Zan, Shike Zhao State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China Received 4 December 2002; accepted 29 January 2003

Abstract Laminated Si3N4/BN ceramics with two types of sintering aids, MgO – Y2O3 – Al2O3 (MYA) and La2O3 – Y2O3 – Al2O3 (LYA), were fabricated through roll compaction and hot-pressing. Sintering aids influence evidently the microstructure and mechanical properties of laminated Si3N4/BN ceramics. In comparison with La2O3 – Y2O3 – Al2O3, MgO – Y2O3 – Al2O3 sintering aid is easier to form a glassy phase with lower viscosity and lower eutectic temperature, which is much easier to migrate into BN interlayers. This results in the denser interlayer microstructure and good bending strength of laminated Si3N4/ BN ceramics at room temperature, but poor work of fracture (WOF) at room temperature, low strength and work of fracture at elevated temperature. In addition, the LYA sintering aid is good for forming elongated and interlocked h-Si3N4 grains and beneficial to the mechanical properties of the laminated Si3N4/BN ceramics. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Laminated Si3N4/BN ceramics; Sintering aid; Mechanical properties

1. Introduction To improve the toughness and the resistance to damage of ceramics, the laminated ceramics, usually consisting of strong matrix layers and weaker interlayers arranged alternately, have been extensively studied [1 – 5]. The strong matrix layers include Si3N4, SiC, Al2O3, etc., and the interlayers include BN, C, etc. The unique structure makes the laminated ceramics with high toughness and non-brittle fractural behavior [2,4,5]. In laminated Si3N4/BN ceramics, Si3N4 matrix layers occupy f 80 vol.% of laminated ceramics, therefore, these layers play an important * Corresponding author. E-mail address: [email protected] (C. Li).

role on the mechanical properties of the materials. Improvements on the microstructure and properties of these layers should improve the microstructure and properties of the whole materials. As it is well known, some oxides are often used as sintering aids to process Si3N4 monolith ceramics profoundly. The sintering aids react with each other as well as with the thin oxidized layer that surrounds each individual particle of Si3N4, and form an oxynitride liquid, which, upon cooling, forms a grain boundary phase that is typically amorphous. In the process of laminated Si3N4/BN ceramics, some of the oxynitride liquid migrates into the BN interlayers because these layers are loose and not dense enough during hot-pressing [5,6]. The viscosity of oxynitride liquid during sintering, which is mainly determined by

0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00101-0

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Fig. 1. Flow chart for fabrication of the laminated Si3N4/BN ceramics.

the composition of sintering aids, may affect its migration to BN interlayers, and furthermore influence the microstructure and mechanical properties of Si3N4/BN laminated ceramics, especially the properties at the elevated temperature. To increase the viscosity and refractory of oxynitride liquid, some rare earth oxides (such as, Y2O3, La2O3, etc.) have been used as sintering aids [7,8]. Based on this understanding, it was hypothesized that the mechanical properties of Si3N4/BN laminated ceramics could be improved by using La2O3 as a sintering aid in Si3N4 matrix layers. In the paper, two kinds of sintering aids are used to aid the densification of the laminate ceramics: one is 11.3 wt.% La2O3 + 8 wt.% Y2O3 + 2.5 wt.% Al2O3 (labeled as LYA-LCs), and the other is 1.5 wt.% MgO + 8 wt.% Y2O3 + 2.5 wt.% Al2O3 (labeled as MYA-LCs). The effects of varying the composition of sintering aids on microstructure and mechanical properties are discussed.

sheets of f 0.2 mm thick were formed. Subsequently, the sheets were dried and cut into squares with dimensions of 36  48 mm2. The cut sheets were coated with a slurry composed of 64 vol.% BN and 36 vol.% Al2O3. After dried, the coated sheets were stacked in a graphite die that was coated by BN slurry to prevent the sample from reacting with and sticking to the die wall. The coated green sheets were heated slowly to 500 jC in air to remove the organic binders. Then, the green bodies were hot-pressed at 1800 jC and 23 MPa using a graphite die in nitrogen for 1.5 h. In order to compare the effects of sintering aid, Si3N4/ BN laminated ceramics with a sintering aid of 8 wt.% Y2O3 (purity>99.9%, Hokke chemicals), 2.5 wt.% Al2O3 (purity>99.9%, commercial powder) and 1.5 wt.% MgO (purity>99.9%, commercial powder) had been prepared using the same fabrication processing. Sintered samples were cut and ground into test bars with a dimension of 4  3  36 mm3, and then each bar was polished with diamond pastes down to 1 Am

2. Experimental procedure The fabrication process of the Si3N4/BN laminated ceramics is illustrated schematically in Fig. 1. Si3N4 (a-Si3N4>92 wt.%, from General Steel Research Institute, China) powder with 11.3 wt.% La 2O 3 (purity>99.9%, commercial powder), 8 wt.% Y2O3 (purity>99.9%, Hokke chemicals, Japan), and 2.5 wt.% Al2O3 (purity>99.9%, commercial powder) were ball-milled with agate balls and ethanol in a nylon pot for 24 h to achieve a homogenous mixture. After dried, the mixture was mixed with 10 wt.% organic polymer binders (PVA) and 1 wt.% plasticizing agent (glycerine). After repeatedly rolled, the thin

Fig. 2. Schematic diagram showing the orientation of a laminated Si3N4/BN ceramics bar in a three-point bending test.

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Table 1 Mechanical properties of the laminated Si3N4/BN ceramics Materials

Bending strength (MPa), RT

Work of fracture (J/m2), RT

Bending strength (MPa), 1300 jC

Work of fracture (J/m2), 1300 jC

MYA-LCs LYA-LCs

700.46 F 25.62 650.57 F 46.28

2100 F 44 3500 F 100

126.56 F 33.75 325.85 F 48.76

1300 F 140 2400 F 220

on the side, which was normal to the hot-pressing direction and would experience tension stress during testing. The two edges near the tensile surface were rounded with a 15-mm diamond grinding wheel. The mechanical properties of the samples were determined with the mechanical test machine (Astron AG2000A). The test direction and the orientation of the laminated Si3N4/BN ceramics bars are shown schematically in Fig. 2. The strength measurement was carried out using a three-point bending method with a span of 30 mm and a crosshead speed of 0.5 mm/min. Also, work of fracture (WOF) measurement was conducted using a three-point bending method with a span of 30 mm and a crosshead speed of 0.05 mm/min. Crack deflection and polished surface parallel to the hot-pressing direction were examined with scanning electron microscopy (SEM). Microstructure analysis with scanning electron microscopy (SEM) was carried out on test bars polished with diamond pastes to 1 Am and etched in a platinum crucible with melting NaOH at 400 jC for 1.5 min and cleaned with boiling water repeatedly.

thirds of that of LYA-LCs. However, the cases are different at elevated temperature. At 1300 jC, the bending strength and WOF of the LYA-LCs are much higher that those of MYA-LCs, twice and once higher than those of MYA-LCs, respectively. This can be explained based on the feature of the glass phases resulted from the sintering aids. It is well known that

3. Results and discussion 3.1. Mechanical properties In this work, the bending strength of the specimen was calculated using three-point bending stress equation, whereas the WOF was calculated by dividing the area under the load –displacement curve by twice the cross-sectional area of the specimen. The mechanical properties of testing materials at room temperature and at 1300 jC are given in Table 1. From Table 1, it can be seen that the type of sintering aids remarkably affects the mechanical properties of the laminated Si3N4/BN ceramics. At room temperature, the bending strength of MYA-LCs is f 50 MPa higher than that of LYA-LCs, even though the work of fracture (WOF) of MYA-LCs is only two-

Fig. 3. Load – displacement curves of both materials (a) at room temperature and (b) at 1300 jC.

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the eutectic temperature for the MYA type of sintering aid is obviously lower than that for the LYA type of sintering aid, [9] therefore, the viscosity of the liquid phase in the MYA-LCs at the sintering temperature is lower than that in the LYA-LCs. This brings about two effects. One is that the lower viscosity of the liquid phase is in favor of the migration of the liquid phase from the Si3N4 matrix layers to the BN interlayers, which is good for the densification of the BN interlayers. This is the reason why the bending strength of the MYA-LCs at the room temperature is a little bit higher that that of the LYA-LCs, however, the denser interlayers are not good for the interface fracture resistance to the fracture, so the work of fracture is lower in the MYA-LCs than in the LYA-LCs. At 1300 jC, the grain boundary glassy phase starts to melt and dominates the mechanical properties of the laminated ceramics. Since the melting point of the glassy phase in MYA-LCs is lower than that in LYA-LCs [9], the bending strength and work of fracture in LYA-LCs are much higher than those in MYA-LCs.

The load –displacement curves of both materials at room temperature and at 1300 jC are shown in Fig. 3. From Fig. 3(a), both of MYA-LCs and LYA-LCs exhibit nonlinear fracture characteristics at room temperature, however, MYA-LCs has a higher load level but smaller area under the curve than those in LYALCs. These agree well to the mechanical properties of the two laminated ceramics, as mentioned above. The load increases linearly until a crack is initiated from the tensile surface and propagates into the nearby BN-containing interphase, where the crack is deflected and arrested. Subsequent specimen deflection causes the delamination cracks to propagate in the interphase at an almost constant load. As displacement of the specimen is continued, the load again begins to increase linearly to a lower level until the uncracked portion of the beam cannot support the applied load anymore. A crack then initiates in the Si3N4 layer close to the delamination crack and propagates until it is deflected in the next BN-containing interphase. Much energy is dissipated during this process. The

Fig. 4. SEM micrographs showing zigzag crack path of two materials: (a) MYA-LCs and (b) LYA-LCs.

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much lower load level and much smaller area under load –displacement curve, compared to those in the LYA-LCs sample. These also agree well to the discussion above. More refractory glassy phase leads to stronger material at the elevated temperature. 3.2. Microstructure Fig. 5. shows the microstructure of the polished surface parallel to the hot-pressing direction in Si3N4/ BN laminated ceramics with MYA and LYA as sintering aids, respectively. Comparing Fig. 5 (a) with (b), there are fewer voids along the interface between

Fig. 5. SEM micrographs showing polished surface parallel to the hot-pressing direction of two materials: (a) MYA-LCs and (b) LYALCs.

process is repeated again and again until the thickness cracks propagate completely through the specimen. Large load and long displacement as well as large area under the load – displacement curve lead to high energy absorption, which results in high WOF and high toughness if the dimension of the specimen is the same, so the WOF value of LYA-LCs is much higher than that of MYA-LCs. Fig. 4. shows the zigzag crack path of both materials tested at room temperature. It can be easy to find that the total length of crack propagation path in LYA-LCs is longer than that in MYA-LCs, which is consistent with the results above. From Fig. 3(b), even though both of the samples exhibit non-brittle fractural behaviors, the load –displacement curve in the MYA-LCs sample shows a

Fig. 6. SEM micrographs showing Si3N4 matrix layer of two materials: (a) MYA-LCs and (b) LYA-LCs.

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BN interlayers and Si3N4 matrix layers in MYA-LCs than those in LYA-LCs, and there also are fewer defects in BN interlayer of MYA-LCs than those in LYA-LCs. So the structure of MYA-LCs is denser than that of LYA-LCs. This also agrees well to the discussion above. Fig. 6. shows the different microstructural characteristics of Si3N4 matrix layer in MYA-LCs and LYALCs, respectively. It is very clear to see the elongated and interlocked h-Si3N4 grains in the Si3N4 matrix layers, however, the aspect ratios of h-Si3N4 grains in LYA-LCs (Fig. 6(b)) are much higher that those in MYA-LCs (Fig. 6(a)). As mentioned above, the Si3N4 raw material consists of >92 wt.% a-Si3N4, so the h-Si3N4 grains are derived from the phase transformation of the a-Si3N4 because of liquid phase sintering mechanism [10,11]. In the beginning of sintering, a-Si3N4 grains dissolve in the liquid phase and then precipitate as needle like h-Si3N4 grains. The growth of h-Si3N4 grains is controlled by diffusion of silicon and nitrogen through the liquid phase [12], so change of liquid phase composition, modifying silicon and nitrogen diffusion rates, mostly affect the phase transformation and the h-Si3N4 grains’ growth rate. For MYA-LCs, the viscosity of the sintering aid liquid phase formed at high temperature is low because of the existence of MgO, which results in the good fluidity of the liquid phase. So during hot-pressing, the liquid phase is easy to migrate into the BN interlayers under the ambient pressure and the concentration gradient. Thus, there is not enough liquid phase left for h-Si3N4 grains to grow fully. However, for LYA-LCs, the viscosity of the sintering aid liquid phase is much higher than that of MYA-LCs due to the addition of La2O3, so there is enough liquid phase for h-Si3N4 to grow well. Thus, the aspect ratio of h-Si3N4 grains in LYA-LCs is much higher than that of MYA-LCs.

4. Conclusions Sintering aids influence evidently the microstructure and mechanical properties of laminated Si3N4/BN

ceramics. Laminated Si3N4/BN ceramics with MgO – Y2O3 –Al2O3 as sintering aid possesses good bending strength at room temperature, but has poor work of fracture at room temperature, low strength and work of fracture at elevated temperature. As a comparison, laminated Si3N4/BN ceramics with La2O3 – Y2O3 – Al2O3 as sintering aid has lower bending strength at room temperature. However, the work of fracture at room temperature and bending strength and work of fracture at elevated temperature of LYA-LCs are much higher than those of MYA-LCs. In addition, the LYA sintering aid is good for forming of elongated and interlocked h-Si3N4 grains and beneficial to the mechanical properties of the laminated Si3N4/BN ceramics.

Acknowledgements This work was supported by the National Science Foundation of China.

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