BN+Al2O3 ceramics densified by spark plasma sintering

BN+Al2O3 ceramics densified by spark plasma sintering

December 2002 Materials Letters 57 (2002) 336 – 342 www.elsevier.com/locate/matlet Mechanical properties and microstructure of laminated Si3N4+SiCw/...

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December 2002

Materials Letters 57 (2002) 336 – 342 www.elsevier.com/locate/matlet

Mechanical properties and microstructure of laminated Si3N4+SiCw/BN+Al2O3 ceramics densified by spark plasma sintering Cuiwei Li *, Yong Huang, Chang-an Wang, Ke Tang, Shuqin Li, Qingfeng Zan State Key Lab of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China Received 18 September 2001; received in revised form 20 February 2002; accepted 21 February 2002

Abstract A laminated Si3N4 + SiCw/BN + Al2O3 ceramics was prepared by spark plasma sintering (SPS). Bending strength of this sample reached 600 MPa and the work of fracture reached 3500 J/m2. The arrangement of the hard Si3N4 + SiCw layers and the soft BN + Al2O3 layers was the main reason for improving toughness of this ceramic. The SiC whisker and h-Si3N4 had strong preferred orientation as indicated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). D 2002 Elsevier Science B.V. All rights reserved. Keywords: SPS; Laminated ceramics; Si3N4; Toughness; Preferred orientation; SiC whisker

1. Introduction Spark plasma sintering (SPS) is a newly developed technique that enables ceramic powder to be fully densified at comparatively low temperature and in very short time. It is similar to conventional hot pressing, which is carried out in a graphite die, but the heating is by means of spark discharge in voids between particles. Due to these discharges, the particle surface is activated and purified, and self-heating phenomena are generated among these particles, leading to heating transfer and mass transfer to be completed in an extremely short time. Therefore,

*

Corresponding author. E-mail address: [email protected] (C. Li).

extremely rapid densification can be accomplished by this process [1,2]. The SPS process has been applied to compacts of various types of materials, e.g. SiC, Al2O3 and Si3N4, etc. [2– 5]. Si3N4-based ceramics possess excellent properties, which make them ideal candidate materials for high-temperature applications. However, the brittleness is one of the important problems in their applications. A laminated microstructure design has been substantiated to be an effective way to increase the toughness of those materials. Recently, laminated Si3N4/BN ceramics have been widely studied [5]. To combine the advantages of SPS technique and laminated ceramics, laminated Si 3N4 + SiC w/ BN + Al2O3 ceramics were rapidly densified by the SPS process. The microstructure, mechanical proper-

0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 7 8 7 - 5

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ties and the toughening mechanism of these ceramics are discussed.

2. Experimental The fabrication process of the laminated ceramics is illustrated schematically in Fig. 1. Si3N4 (from General Steel Research Institute, China) powder with 8 wt.% Y2O3 (purity>99.9%, Hokke Chemicals, Japan), 2.5 wt.% Al2O3 (purity>99.9%) and 1.5 wt.% MgO (purity>99.9%) were ball-milled with 20 wt.% SiC (Tws-400, Hokke Chemicals) whisker in ethanol for 24 h to achieve a homogenous mixture. After drying, the milled ceramic powders were mixed with 10 wt.% organic polymer binders (PVA) and 1 wt.% plasticizing agent (glycerine). After repeated rolling, thin sheets of 0.2 mm thick were formed. Subsequently, the sheets were dried and cut into squares with dimensions of 35  35 mm2. To introduce weak interfaces between the Si3N4 + SiCw layers, both sides of the cut sheets were coated with a slurry of BN containing 36 vol.% Al2O3. After the coated sheets were dried, they were stacked and pressed (5 kg) in a graphite die. After degreasing (400 jC), the green body was densified by SPS (SPS-1050, Sumitomo Coal Mining, Japan) in a vacuum (0.6  10  2 Pa) by heating to the maximum temperature (1650 jC) in 13 min, with a 15-min dwell time and cooling in about 40 min to room temperature. The temperatures of the samples during sintering were measured by means of an optical pyrometer focused on to the sintered sample through a small hole in the die. The

Fig. 2. Schematic showing the orientation of a laminated Si3N4 + SiCw/BN + Al2O3 ceramics bar in a three-point bending test.

applied pressure was kept constant at 22 MPa from the start to the end of dwell time. The sintered samples were approximately 35  35  5 mm3. Densities of sintered samples were measured by the Archimedes’ method. For bending strength and work of fracture measurement, sintered samples were cut and ground to test bars with a dimension of 4  3  35 mm3, and then each bar was polished with diamond pastes to 3.5 Am finish on the side that would experience tension stress during testing. The corners were rounded with a 15-mm diamond-grinding wheel. The bending strength measurement was carried out using a three-point bending method with a span length of 30 mm and a crosshead speed of 0.5 mm/min. The work of fracture measurement was conducted with a mechanical tester (Astron 2000A), using a three-point bending method with a span length of 30 mm and a

Fig. 1. Flow chart for fabrication of a laminated Si3N4 + SiCw/BN + Al2O3 ceramics.

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crosshead speed of 0.05 mm/min. In this work, four to five specimens were tested for each crosshead speed to get an average value. The test direction and the orientation of the laminated Si3N4 + SiCw/BN + Al2O3 ceramics bars are shown schematically in Fig. 2. Microstructure analysis by scanning electron microscopy (SEM) was carried out on test bars polished with diamond pastes to 1 Am finish and placed in a platinum crucible with melting NaOH at 400 jC for 1.5 min and cleaned with boiling water repeatedly. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to analyze the main phases, microstructure and crack deflection, and also propagation of the laminated Si3N4 + SiCw/BN + Al2O3 ceramics.

Fig. 4. Load – displacement curve of a laminated Si3N4 + SiCw/ BN + Al2O3 ceramics.

3. Results and discussion 3.1. Mechanical properties Fig. 3 shows the structure of the laminated Si3N4 + SiCw/BN + Al2O3 ceramics SPSed. The Si3N4 + SiCw layers show grey and the BN + Al2O3 interlayers appear as thin and bright layers separating the Si3N4 + SiCw layers. The layers in these materials are aligned evenly in the horizontal direction. The thickness of each Si3N4 + SiCw matrix layer is about

Fig. 3. SEM micrograph showing polished cross section of a laminated Si3N4 + SiCw/BN + Al2O3 ceramics.

120 Am, and the thickness of each BN + Al2O3 interlayer is about 20 Am. The density of the sintered materials is 3.18 g/cm3, which is much higher than that (3.07 g/cm3) sintered by hot pressing [10]. The high density is achieved by the SPS technique in runs of short duration ( < 30 min), compared to hot pressing (>2 h), and at comparatively lower temperature (1650 jC), i.e. 170 jC lower than that of hot pressing [10]. The bending strength of the laminated Si3N4 + SiCw/BN + Al2O3 ceramics is 600 MPa, which is higher than that (550 MPa) of laminated Si3N4 + SiCw/BN + Al2O3 ceramics densified by hot pressing [10]. Fig. 4 shows the load –displacement curve for one bend bar of these materials. A maximum nominal stress of 588 MPa is achieved from the peak load. After the drop of the peak load, the load-bearing ability of this specimen is 83% of the peak load. After a second major load drop, the sample retains about 33% of its load-bearing ability. The troughthickness crack is initialed on the tensile surface, then it deflects along the weak interface. As the crack advances, the load drops owing to the increasing specimen compliance. As the displacement further increases, the load and stress increase. Trough-thickness crack propagation is assumed to occur when the load on the intact portion of the beam gives rise to a stress equal to a critical failure stress [6,7]. The apparent and average work of fracture for this specimen are 4000 and 3500 J/m2, respectively. Thus, laminated ceramics possess ‘‘plastic’’ characteristic and show a non-catastrophic failure.

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Fig. 5. SEM micrographs showing a side surface of a laminated Si3N4 + SiCw/BN + Al2O3 ceramics bending bar: (a) propagating of a major crack from tensile surface (top) through the bar via a zigzag path; (b,c) interlocking of toothlike, broken layers.

Fig. 5 shows the zigzag crack path and crack bridging in this specimen. As shown in Fig. 5(a), both the tensile failure of the surface Si3N4 + SiCw layer and the delamination crack along the BN + Al2O3 layer are observed. A major crack still propagates through the specimen, although it is deflected by almost every BN + Al2O3 layer. One remarkable feature observed in Fig. 5(b) and (c) is the interlocking of toothlike, debond layers. These indicate that the

maximum shear stress should exceed the shear strength of the BN + Al2O3 layer. 3.2. XRD analysis Fig. 6 shows the XRD pattern of this sample, where only three phases, h-Si3N4, SiC and hexagonal BN, are present. No peaks are visible for a-Si3N4 because SPS is performed above the a ! h; transformation

Fig. 6. XRD pattern of a laminated Si3N4 + SiCw/BN + Al2O3 ceramics by SPS.

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Table 1 The XRD data of SiC whisker phase in the sintered sample 0 ˚) hkl d (A Ihkl (cps) 2h (deg) I/I0 (%) I/I0 in JCPDS card (%) 111 200 220 311

2.5157 2.1792 1.5401 1.3140

1094 514 1262 385

35.660 41.400 60.020 71.780

53 25 61 19

[11]

100 20 35 25

temperature and the transformation of a-Si3N4 to hSi3N4 has been completely achieved. XRD data of SiC whisker and h-Si3N4 phases for Fig. 6 are listed in Tables 1 and 2. In this work, FCC SiC whisker is used, and its axis direction is [111] direction. Hence, the planes parallel to the [111] direction of SiC whisker have the inclination to be increased, and those normal to [111] direction (such as many planes in < 110> family) to be decreased. Therefore, the variable Z is used to characterize the texture of the SiC whisker [8]. Z¼

1  Ro =Ra 1 þ Ro =Ra

ð1Þ

where Ro is the ratio of the intensities of (111) and (220) plane in oriented samples and Ra is that in nonoriented samples. Z ranges from 0 to 1. The larger the Z value is, the more strongly the SiC whisker incline to parallel to the pressing surface. From Table 1, the ratio of (111) intensity to (220) intensity is about 2.857 in JCPDS card [11], while it is about 0.876 in this sample, which is quite different from the former. Hence, Z value is more than 0.5, which indicates that SiC whisker is significantly textured after SPS, and most SiC whiskers prefer to be paralleled to the pressing surface. In the process by SPS, very strong pressure exists, which will rotate the whisker to be parallel to the pressing surface of laminated Si3N4 + SiCw/BN + Al2O3 ceramics. It is also not difficult to understand that h-Si3N4 grains will have strong preferred orientation for the rod-like grain morphology. According to the work of Lee and Bowman [9], the ratio of the (210) and (101) intensity is used as an indication of the degree of preferred orientation of hSi3N4. From Table 2, the ratio of (210) intensity to (101) intensity is about 0.939 in JCPDS card [12], while it is about 1.811 in this sample, which is quite

different from the former. Since the axis direction of the long rod-like h-Si3N4 is (001) direction, in these samples, the c-axis of h-Si3N4 grains incline to be parallel to the pressing surface due to the strong pressure during the SPS processing. According to the above discussions, the h-Si3N4 grains and SiC whiskers both have the inclination to align parallel to the pressing surface, which will enhance the bending strength and toughness of the samples. 3.3. Microstructure Fig. 7 shows the microstructure of the Si3N4 + SiCw matrix layer. According to Fig. 7(a), long rodlike h-Si3N4 is visible in the matrix layer and grows very well. SiC whisker intersperses among the long rod-like h-Si3N4 equally. The Si3N4 + SiC matrix possesses preferred orientation caused by pressure. Most long rod-like h-Si3N4 and SiC whisker are parallel to the direction of stress and perpendicular to the major crack plane, which is consistent with XRD analysis and contributes to the increase of bending strength and work of fracture of this material. It is seen in Fig. 7(b) that the broken long rod-like hSi3N4 grains and SiC whiskers are observed, and the traces after the whisker pulled-out are observed. Thus, Table 2 The XRD data of h-Si3N4 phase in the sintered sample 0 ˚) hkl d (A Ihkl (cps) 2h (deg) I/I0 (%) I/I0 in JCPDS card(%) 100 110 200 101 210 111 300 220 310 301 221 311 320 002 410 321 411 330

6.5826 3.8017 3.2924 2.6620 2.4887 2.3121 2.1964 1.9020 1.8267 1.7528 1.5918 1.5476 1.5110 1.4534 1.4368 1.3413 1.2892 1.2679

638 677 2086 1102 1999 121 149 223 304 723 205 170 383 184 199 843 387 142

13.440 23.380 27.060 33.640 36.060 38.920 41.060 47.780 49.880 52.140 57.880 59.700 61.300 63.940 64.840 70.100 73.380 74.820

31 33 100 53 96 6 8 11 15 35 10 9 19 9 10 41 19 7

34 35 100 99 93 9 10 8 12 37 12 6 15 15 8 39 18 7

[12]

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transgranular fracture is the main mode of the fracture, which consumes more energy than intragranular fracture. It is seen in Fig. 7(c) that platelet-like hexagonal BN is the main phase, and voids and flaws are easily observed in the interlayers, which make crack to be deflected easily and propagate further in the interlayer. From microstructural analysis, it can be seen that high bending strength and work of fracture of the laminated Si3N4 + SiCw/BN + Al2O3 ceramics are closely related to its microstructure. 3.4. Toughening mechanism The high work of fracture of laminated ceramics is caused by its special structure, which consists of hard Si3N4 + SiC and soft BN + Al2O3 arranging alternately. When a propagating crack reaches an interlayer, the stresses acting on crack-tip will change from three dimensions to two dimensions because of the weak interlayer. Hence, the crack-tip is blunted and deflected, and it will be deflected by the interlayer and become an interfacial crack. Much energy is consumed in this process. In addition, the pullout and breakage of SiC whiskers and the breakage of long rod-like h-Si3N4 grains are contributing to the increase of work of fracture.

4. Conclusion

Fig. 7. SEM micrographs showing matrix layer: (a) etched surface: 1—SiC whisker, 2—long rod-like h-Si3N4; (b) fracture surface: 1— trace after the whisker pullout; 2—broken whisker; (c) platelet like BN in the interlayers.

Laminated Si 3 N 4 + SiC w /BN + Al 2 O 3 ceramics have been prepared by SPS technique, at comparatively low temperature (1650 jC) and in a very short time (28 min) with a density of 3.18 g/cm3, which is much higher than that of hot-pressed samples. hSi3N4, SiC and hexagonal BN are the main phases, and the first two are significantly textured after SPS. The bending strength of laminated Si3N4 + SiCw/ BN + Al2O3 ceramics reaches 600 MPa, which is higher than that of laminated Si 3 N 4 + SiC w / BN + Al2O3 ceramics densified by hot pressing. The average work of fracture is 3500 J/m2. The load – displacement curve of the material appears nonlinear and this material shows a non-catastrophic failure. Crack deflection and propagation caused by the special structure are the main reason of toughening. In addition, pullout and break of SiC whiskers and break

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of long rod-like h-Si3N4 grains are also contributing to the toughness of the materials. Therefore, the samples obtained by SPS technique have advantages such as high density and excellent mechanical properties, showing that SPS is a novel-sintering technique with a wide range of potential applications.

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