《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_7Mechanical properties and microstructure of laminated-whisker

MATERIALS ELSEVIER Materials Letters 57(2002)336-342 www.elsevier.com/locate/matlet Mechanical properties and microstructure of laminated Si3N4+SiCw/BN+AL,O3 ceramics densified b 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 Science and engineering Received 18 September 2001: received in revised form 20 February 2002; accepted 21 February 2002 Abstract A laminated Si3 N4+ SiCwBN+ 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/m. The arrangement of the hard Si3 N4+ SiCw layers and the soft BN+Al2O3 layers was the main reason for improving toughness of this ceramic. The Sic whisker and B-Si N4 had strong preferred orientation as indicated by X-ray diffraction(XRD) and scanning electron microscopy (SEM) C 2002 Elsevier Science B. V. All rights reserved. Keywords: SPS; Laminated ceramics; Si,N4; Toughness; Preferred orientation; SiC whisker 1. Introduction extremely rapid densification can be accomplished by this process [1, 2]. Spark plasma sintering(SPS)is a newly developed The SPS process has been applied to compacts of technique that enables ceramic powder to be fully various types of materials, e.g. SiC, Al2O3 and Si3N4 densified at comparatively low temperature and in etc. [2-5]. Si3 N4-based ceramics possess excellent very short time similar to conventional hot properties, which make them ideal candidate mate pressing, which is carried out in a graphite die, but rials for high-temperature applications. However, the the heating is by means of spark discharge in voids brittleness is one of the important problems in their between particles. Due to these discharges, the par- applications. A laminated microstructure design has ticle surface is activated and purified, and self-heating been substantiated to be an effective way to in- phenomena are generated among these particles, crease the toughness of those materials. Recently, leading to heating transfer and mass transfer to be laminated Si3N4/BN ceramics have been widely completed in an extremely short time. Therefor studied [5]. To combine the advantages of sPs techniqu and laminated ceramics, laminated Si3N4+ SiCw/ BN+ Al2O3 ceramics were rapidly densified by the E-mailaddress:licuiwei99@mails.tsinghua.edu.cn(C.Li). SPS process. The microstructure, mechanical proper 0167-577X/02/S- see front matter c 2002 Elsevier Science B v. All rights reserved PI:s0167577X(02)00787-5

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 par￾ticle 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, 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 mate￾rials 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 in￾crease 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 Si3N4 + SiCw/ 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: S0167-577X(02)00787-5 * Corresponding author. E-mail address: licuiwei99@mails.tsinghua.edu.cn (C. Li). www.elsevier.com/locate/matlet December 2002 Materials Letters 57 (2002) 336 – 342

C. Li et al. Materials Letters 57(2002)336-342 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 apan), 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 Fig. 2. Schen howing the orientation of a lamina After drying, the milled ceramic powders were mixed SiCwBN+A ramics bar in a three-point bending with 10 wt. organic polymer binders(PVA)and 1 wt% plasticizing agent (glycerine). After repeated applied pressure was kept constant at 22 MPa from rolling, thin sheets of 0. 2 mm thick were formed. the start to the end of dwell time. The sintered samples Subsequently, the sheets were dried and cut int were approximately 35 X 35 squares with dimensions of 35 x 35 mm. To intro Densities of sintered samples were measured by the duce weak interfaces between the si3 N4+ Sicw Archimedes'method. For bending strength and work both sides of the cut sheets were coated with a of fracture measurement, sintered samples were cut of Bn containing 36 vol. Al2O3. After the coated and ground to test bars with a dimension of 4 X 3 X 35 sheets were dried, they were stacked and pressed (5 mm, and then each bar was polished with diamond kg) in a graphite die. After degreasing (400C), the pastes to 3.5 um finish on the side that would experi green body was densified by SPS(SPS-1050, Sumi- ence tension stress during testing. The corners were mo Coal Mining, Japan)in a vacuum(0.6x 10-2 rounded with a 15-mm diamond-grinding wheel. The Pa)by heating to the maximum temperature(1650 bending strength measurement was carried out using a C)in 13 min, with a 15-min dwell time and cooling three-point bending method with a span length of 30 in about 40 min to room temperature. The temper- mm and a crosshead speed of 0.5 mm/min. The work atures of the samples during sintering were measured of fracture measurement was conducted with a y means of an optical pyrometer focused on to the mechanical tester(Astron 2000A), using a three-point sintered sample through a small hole in the die. The bending method with a span length of 30 mm and a SisN+SiC+additiv drying Roll compac Binder+Plasticizing agent Roll compacting BN+ALOn+suspending agent Cutting Coating Stacking Degreaser SPS process Fig. I Flow chart for fabrication of a laminated Si, N4+ SiCw/BN+ AlO, ceramics

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 intro￾duce 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, Sumi￾tomo 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 temper￾atures 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 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 experi￾ence 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. Fig. 2. Schematic showing the orientation of a laminated Si3N4 + SiCw/BN + Al2O3 ceramics bar in a three-point bending test. C. Li et al. / Materials Letters 57 (2002) 336–342 337

338 C. Li et al. Materials Letters 57(2002)336-342 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 th orientation of the laminated Si3 N4+ SiCw/BN+ AlO3 ceramics bars are shown schematically in Fig. 2.2 Microstructure analysis by scanning electron micro- 8 scopy(SEM) was carried out on test bars polished with diamond pastes to 1 um finish and placed in a platinum crucible with melting NaoH at 400C for 1.5 min and cleaned with boiling water repeatedly. X-ray diffrac- tion(XRD)and scanning electron microscopy(SEM) were used to analyze the main phases, microstructure Displacement(um) and crack deflection, and also propagation of the laminated Si3N4+ SICw/BN+AlO3 ceramics Fig. 4. Load-displacement curve of a laminated Si3N4+ SiCw BN+ AlO3 ceramics. 3. Results and discussion 120 Hm, and the thickness of each BN+ Al,O3 inter- 3. 1. Mechanical properties layer is about 20 um. The density of the sintered materials is 3. 18 g/cm, which is much higher than Fig 3 shows the structure of the laminated Si3N4+ that (3.07 g/cm) sintered by hot pressing [10]. The SiCw/BN+AlzO3 ceramics SPSed. The Si3 N4+ SiCw high density is achieved by the SPs technique in runs layers show grey and the Bn+ Al2O3 interlayers of short duration(<30 min), compared to hot pressing appear as thin and bright layers eparatl g the (2 h), and at comparatively lower temp perature (1650 Si3N4+ SiCw layers. The layers in these materials), i.e. 170C lower than that of hot pressing [10] are aligned evenly in the horizontal direction. The The bending strength of the laminated Si3N4+ thickness of each Si3 N4+ SiCw matrix layer is about SiCw/BN+ Al2O3 ceramics is 600 MPa, which is higher than that (550 MPa) of laminated Si3 N4+ SiCw/BN+ Al2O3 ceramics densified by hot pressin [10]. Fig. 4 shows the load-displacement curve for one bend bar of these materials. a nominal stress of 588 MPa is achieved from the peak load. After the drop of the peak load, the load- 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 trough thickness crack is initialed on the tensile surface. then it deflects along the weak interface. As the crack advances, the load drops or to the specimen compliance. As the displacement further increases, the load and stress increase. Trough-thick- ness crack propagation is assumed to occur when the load on the intact portion of the beam gives rise to a 125μm stress equal to a critical failure stress [6,7]. The for men are 4000 and 3500 J/m, respectively. Thus Fig. 3. SEM micrograph showing polished cross section of a laminated ceramics possess"plastic"characteristic laminated Si3 N4+ SiCwBN+ Al2O3 ceramics. and show a non-catastrophic failure

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 micro￾scopy (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 diffrac￾tion (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. 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 120 Am, and the thickness of each BN + Al2O3 inter￾layer 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 ( 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 trough￾thickness 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-thick￾ness 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 speci￾men are 4000 and 3500 J/m2 , respectively. Thus, laminated ceramics possess ‘‘plastic’’ characteristic and show a non-catastrophic failure. Fig. 3. SEM micrograph showing polished cross section of a laminated Si3N4 + SiCw/BN + Al2O3 ceramics. Fig. 4. Load – displacement curve of a laminated Si3N4 + SiCw/ BN + Al2O3 ceramics. 338 C. Li et al. / Materials Letters 57 (2002) 336–342

C. Li et al. Materials Letters 57(2002)336-342 300um Fig 5 SEM micrographs showing a side surface of a laminated Si N4+ SiCw/BN+AlO3 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 maximum shear stress should exceed the shear bridging in this specimen. As shown in Fig. 5(a), both strength of the BN+ Al2O3 layer the tensile failure of the surface Si3N4+ SiCw layer and the delamination crack along the bn+Al,o 3. 2. XRD analysis crack still propagate through the specimen, although it is deflected by Fig 6 shows the XRD patten of this sample, where almost every BN+Al2O3 layer. One remarkable fea- only three phases, B-Si3N4, SiC and hexagonal BN, ture observed in Fig. 5(b)and (c)is the interlocking of are present. No peaks are visible for a-Si3N4 because toothlike, debond layers. These indicate that the sPs is performed above the a-B; transformation b: hex-BN 阝 50 60 (deg) Fig. 6. XRD pattern of a laminated Si3 N4+ SiCw/BN+ Al2O3 ceramics by SPs

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 fea￾ture 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. 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. 6. XRD pattern of a laminated Si3N4 + SiCw/BN + Al2O3 ceramics by SPS. C. Li et al. / Materials Letters 57 (2002) 336–342 339

C. Li et al. Materials Letters 57(2002)336-342 Table I different from the former since the axis direction of The XRD data of SiC whisker phase in the sintered sample the long rod-like B-Si3N4 is(001)direction, in these hkl d(A) INt(cps) 20(deg) o(%) o in samples, the c-axis of B-Si3N4 grains incline to be JCPDS ard(%)ml parallel to the pressing surface due to the strong pressure during the SPs processing. According to 1112.51571094 35.660 41400 the above discussions, the B-Si3N4 grains and Sic 2201.54011262 35 whiskers both have the inclination to align parallel to 3111.3140385 71.78019 the pressing surface, which will enhance the bending strength and toughness of the samples. temperature and the transformation of a-Si3N4 to B- 3.3. Microstructure Si3N4 has been completely achieved XRD data of Sic whisker and B-Si3N4 phases fo Fig 6 are listed in Tables 1 and 2. In this work, FCC Fig. 7 shows the microstructure of the Si3N4+ Sic whisker is used, and its axis direction is [1111 SiCw matrix layer. According to Fig. 7(a), long rod direction. Hence, the planes parallel to the [1111 like B-Si3N4 is visible in the matrix layer and grows direction of Sic whisker have the inclination to be very well. SiC whisker intersperses amor increased, and those normal to [111] direction(such as rod-like B-Si3N4 equally. The Si3 N4+ Sic matrix many planes in family)to be decreased. possesses preferred orientation caused by pressure Therefore, the variable Z is used to characterize the Most long rod-like B-Si3N4 and Sic whisker are texture of the Sic whisker [8]. 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 R°/R Z=1+R/Ra (1) bending strength and work of fracture of this material It is seen in Fig. 7(b) that the broken long rod-like B- Si3n4 grains and Sic whiskers are observed, and the where R is the ratio of the intensities of (111)and traces after the whisker pulled-out are observed. Thus, (220)plane in oriented samples and R is that in non oriented samples. Z ranges from 0 to 1. The larger the Z value is, the more strongly the Sic whisker incline Table n da SI3N4 phase in the sintered sample to parallel to the pressing surface. From Table 1, the ratio of(111)intensity to(220)intensity is about hk d(a) Jh(cps)20(deg)I/o(%)o in 2.857 in JCPDS card [ll], while it is about 0.876 in JCPDS this sample, which is quite different from the former. Hence. Z value is more than 0.5. which indicates that 1006.5826 13.44031 1103.8017677 23.38033 iC whisker is significantly textured 2003.2924 pressure exists, which will rotate the whisker to be 8920 parallel to the pressing surface of laminated Si3N4+ 220 19020 223 47. 11 Cw/BN+ Al,O3 ceramics. It is also not difficult to 3101.8267 49.880 understand that B-Si3N4 grains will have strong pre-301 1.7528723 2.140 37 ferred orientation for the rod-like grain morphology 2211.5918 57880 According to the work of Lee and Bowman [9], the 59.700 ratio of the(210)and (101)intensity is used as an 3201.5110383 61.300 63.940 indication of the degree of preferred orientation of B- 410 1.4368 64.840 Si3N4. From Table 2, the ratio of (210)intensity to 321 1.3413 (101)intensity is about 0.939 in JCPDS card [12], 411 1.2892 387 73380 while it is about 1. 811 in this sample, which is quite 3301.2679142 74.820

temperature and the transformation of a-Si3N4 to h￾Si3N4 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 family) to be decreased. Therefore, the variable Z is used to characterize the texture of the SiC whisker [8]. Z ¼ 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 non￾oriented 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 pre￾ferred 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 h￾Si3N4. 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 rod￾like 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 h￾Si3N4 grains and SiC whiskers are observed, and the traces after the whisker pulled-out are observed. Thus, Table 1 The XRD data of SiC whisker phase in the sintered sample hkl d (A˚ ) Ihkl 0 (cps) 2h (deg) I/I0 (%) I/I0 in JCPDS card (%) [11] 111 2.5157 1094 35.660 53 100 200 2.1792 514 41.400 25 20 220 1.5401 1262 60.020 61 35 311 1.3140 385 71.780 19 25 Table 2 The XRD data of h-Si3N4 phase in the sintered sample hkl d (A˚ ) Ihkl 0 (cps) 2h (deg) I/I0 (%) I/I0 in JCPDS card(%) [12] 100 6.5826 638 13.440 31 34 110 3.8017 677 23.380 33 35 200 3.2924 2086 27.060 100 100 101 2.6620 1102 33.640 53 99 210 2.4887 1999 36.060 96 93 111 2.3121 121 38.920 6 9 300 2.1964 149 41.060 8 10 220 1.9020 223 47.780 11 8 310 1.8267 304 49.880 15 12 301 1.7528 723 52.140 35 37 221 1.5918 205 57.880 10 12 311 1.5476 170 59.700 9 6 320 1.5110 383 61.300 19 15 002 1.4534 184 63.940 9 15 410 1.4368 199 64.840 10 8 321 1.3413 843 70.100 41 39 411 1.2892 387 73.380 19 18 330 1.2679 142 74.820 7 7 340 C. Li et al. / Materials Letters 57 (2002) 336–342

C. Li et al. Materials Letters 57(2002)336-342 transgranular fracture is the main mode of the frac- ture, 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 la- minated Si3N4+ SiCwBN+ Al,O3 ceramics are close- ly related to its microstructure. 3.4. Toughening mechanism The high work of fracture of laminated ceramics is aused by its special structure, which consists of hard Si3N4+ SiC and soft BN+ Al2O3 arranging alter- nately. When a propagating crack reaches an inte layer, 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 inter- layer 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 um long rod-like B-Si3N4 grains are contributing to the increase of work of fracture 4. Conclusion Laminated Si3N4+ SiCw/BN+ AlO3 ceramics have been prepared by SPS technique, at compara- tively low temperature(1650C)and in a very short time(28 min) with a density of 3. 18 g/cm, which is much higher than that of hot-pressed samples. B- Si3N4, SiC and hexagonal BN are the main phases and the first two are significantly The bending strength of laminated Si3N4+ SiCw BN+ Al,O3 ceramics reaches 600 MPa, which is higher than that of laminated Si3 N4+ SiCw/ BN+Al2O3 ceramics densified by hot pressing. The 2um average work of fracture is 3500 J/m. The load displacement curve of the material appears nonlinear Fig. 7. SEM micrographs showing matrix layer:(a)etched surface nd this material shows a non-catastrophic failure sic whisker, 2-long rod-like B-Si3N4; (b) fracture surface: 1- Crack deflection and propagation caused by the spe- race after the whisker pullout; 2--broken whisker; (c) platelet like cial structure are the main reason of toughening. In BN in the interlayers. addition, pullout and break of Sic whiskers and break

transgranular fracture is the main mode of the frac￾ture, 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 la￾minated Si3N4 + SiCw/BN + Al2O3 ceramics are close￾ly 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 alter￾nately. When a propagating crack reaches an inter￾layer, 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 inter￾layer 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 Laminated Si3N4 + SiCw/BN + Al2O3 ceramics have been prepared by SPS technique, at compara￾tively 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. h￾Si3N4, 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 Si3N4 + SiCw/ 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 spe￾cial structure are the main reason of toughening. In addition, pullout and break of SiC whiskers and break 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. C. Li et al. / Materials Letters 57 (2002) 336–342 341

C. Li et al. Materials Letters 57(2002)336-342 of long rod-like B-Si3N4 grains are also contributing 3] D.S. Perera, M. Tokita, S. Moricca, J. Eur. Ceram Soc. 18 to the toughness of the materials. Therefore, the (1998)401. samples obtained by SPS technique have advantages [4]J.Hong, L Gao, S D.D. L. Torre, H. Miyamoto, K. Miyamoto Mater.Let43(2000)27. such as high density and excellent mechanical proper- [5]P Wei, L Chen, A Okubo, THirai, Mater. Lett. 49(2001)239 ties, showing that SPS is a novel-sintering technique 6]D. Kover, M D. Thouless, J W. Halloran, J. Am. Ceram Soc. with a wide range of potential applications 81(1998)1004 []H Liu, S.M. Hsu, J. Am. Ceram Soc. 79(1996)2452. C.A. Wang, Y. Huang, J. Chin. Ceram. Soc. 25(1997)54 References 9]F. Lee, K.J. Bowman, J Am Ceram Soc. 75(1992)1748 O S.Y. Cai, Y. Huang, PhD Thesis, Tsinghua University, 199 [1]Z Shen, M. Nygren, J Mater. Chem. 11(2001)204 (in Chinese [2]L. Gao, J.S. Hong, H. Miyamoto, S.D. D L. Torre, J. Eur. [11] JCPDS card #29-1129 Ceram Soc.20(2000)2149 [12]JCPDS card #33-1160

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 proper￾ties, showing that SPS is a novel-sintering technique with a wide range of potential applications. References [1] Z. Shen, M. Nygren, J. Mater. Chem. 11 (2001) 204. [2] L. Gao, J.S. Hong, H. Miyamoto, S.D.D.L. Torre, J. Eur. Ceram. Soc. 20 (2000) 2149. [3] D.S. Perera, M. Tokita, S. Moricca, J. Eur. Ceram. Soc. 18 (1998) 401. [4] J. Hong, L. Gao, S.D.D.L. Torre, H. Miyamoto, K. Miyamoto, Mater. Lett. 43 (2000) 27. [5] P. Wei, L. Chen, A. Okubo, T. Hirai, Mater. Lett. 49 (2001) 239. [6] D. Kover, M.D. Thouless, J.W. Halloran, J. Am. Ceram. Soc. 81 (1998) 1004. [7] H. Liu, S.M. Hsu, J. Am. Ceram. Soc. 79 (1996) 2452. [8] C.A. Wang, Y. Huang, J. Chin. Ceram. Soc. 25 (1997) 54 (in Chinese). [9] F. Lee, K.J. Bowman, J. Am. Ceram. Soc. 75 (1992) 1748. [10] S.Y. Cai, Y. Huang, PhD Thesis, Tsinghua University, 1998 (in Chinese). [11] JCPDS card #29-1129. [12] JCPDS card #33-1160. 342 C. Li et al. / Materials Letters 57 (2002) 336–342