MIAERAL EE ENNEERIINC C ELSEVIER brittleness of ceramics in naturer Biomimetic structure design -a possible approac to change the ng ong Hu Qingfeng Zan, Hai guo, Shengyou Cai The State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Bejing 100084, People's Republic of China Accepted 17 September 1999 Based on the analysis on structure of natural biomaterials, two kinds of ceramic composites with high toughness have been designed and prepared: one is fibrous monolithic Si3 N4/BN composite imitating bamboos or trees in structure, the other is laminated Si composite imitating nacre in structure. Plastic forming methods, including extrusion and roll compaction, respectively, followed by lot-pressed sintering are used to prepare these two materials with particular structures. Both of the two composites have high values of fracture toughness and work of fracture: fracture toughness are 24 MPa m/2 and 28 MPa m/, respectively, for fibrous monolithic and laminated Si3 N4/BN composites, and works of fracture are both more than 4000 J/m. The load-displacement curves reveal that these two materials with biomimetic structure exhibit non-brittle feature when applied load to fracture. Through analysis on fractographs of the materials, it is revealed that high toughness comes from the synergistic toughening among multi-level toughening mechanisms in different scales: weak interfaces, whiskers and elongated grains toughening in ceramic matrix cells. C 2000 Published by Elsevier Science S.A Keywords: Biomimetic structure, Fibrous monolithic; Laminated; Si, N4/BN composites 1. Introduction what we seek for ceramic materials with high toughness through composition control. For example, trees and bam A major problem in the service of ceramics as structural boos are typical long, fiber-reinforced composites. Their materials is their brittleness. Even though many attempt fibers have different sizes and arranged modes in structure have been used to increase their toughness, including so that they can display the optimal behaviors under incorporation of fibers, whiskers or particles reinforce- tensile, bending, compressing stress and other applied load ments,and zro2 phase transformation reinforcing, etc, up Another typical biomaterial is nacre, the structure of which to date the brittleness of ceramics has not been overcome is laminated with brick wall structure It consists of more in nature. It seems that it is impossible for conventional than 99 vol% inorganic phase, aragonite wafers, and less ways to solve this problem than 1 vol. organic phase, mortar of proteins. This On the other hand, in the research on the structure of particular configuration imparts over one order of magnI- natural biomaterials, such as bamboos, trees and nacres, it tude higher bending strength and toughness than those of has been found that these natural biomaterials have very aragonite single crystals. The work of fracture of nacre is reasonable structures which gives them many excellent 3000 times higher than that of pure aragonite [1].So,the properties, such as good carrying capacity, good tough- complicated and reasonable structure of natural biomateri- ess, self-healing, and so on. Furthermore, these biomaterI- als can give us an important insight into making better als have very fine and special structures rather than com- structure materials through biomimetic design plicated compositions, which are distinctly different from Coblenz [2] in 1988 put forward a fibrous monolithic structure imitating the structure of trees or bamboos. In the This research work was supported by the National Science Founda structure, fibrous polycrystalline cells are arranged paral tion of China(grant No. 59632090) lel, and separated and combined by very thin interfacial Corresponding author. Tel: +86-1062785488: fax: +86-1062771160 phase. Baskaran et al. [] in 1993 firstly prepared SiC/C E-mailaddress:wangsa@263.net(C.Wang fibrous monolithic structure ceramics according to the 0928-4931/00/Ssee front matter o 2000 Published by Elsevier Science S.A PI:S0928-4931(00)00133-8
Materials Science and Engineering C 11 2000 9–12 Ž . www.elsevier.comrlocatermsec Biomimetic structure design — a possible approach to change the brittleness of ceramics in natureq Chang-an Wang ), Yong Huang, Qingfeng Zan, Hai Guo, Shengyou Cai The State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua UniÕersity, Beijing 100084, People’s Republic of China Accepted 17 September 1999 Abstract Based on the analysis on structure of natural biomaterials, two kinds of ceramic composites with high toughness have been designed and prepared: one is fibrous monolithic Si N3 4rBN composite imitating bamboos or trees in structure, the other is laminated Si N3 4rBN composite imitating nacre in structure. Plastic forming methods, including extrusion and roll compaction, respectively, followed by hot-pressed sintering are used to prepare these two materials with particular structures. Both of the two composites have high values of fracture toughness and work of fracture: fracture toughness are 24 MPa m1r2 and 28 MPa m1r2, respectively, for fibrous monolithic and laminated Si N rBN composites, and works of fracture are both more than 4000 Jrm2 . The load-displacement curves reveal that these 3 4 two materials with biomimetic structure exhibit non-brittle feature when applied load to fracture. Through analysis on fractographs of the materials, it is revealed that high toughness comes from the synergistic toughening among multi-level toughening mechanisms in different scales: weak interfaces, whiskers and elongated grains toughening in ceramic matrix cells. q 2000 Published by Elsevier Science S.A. Keywords: Biomimetic structure; Fibrous monolithic; Laminated; Si N rBN composites 3 4 1. Introduction A major problem in the service of ceramics as structural materials is their brittleness. Even though many attempts have been used to increase their toughness, including incorporation of fibers, whiskers or particles reinforcements, and ZrO phase transformation reinforcing, etc., up 2 to date the brittleness of ceramics has not been overcome in nature. It seems that it is impossible for conventional ways to solve this problem. On the other hand, in the research on the structure of natural biomaterials, such as bamboos, trees and nacres, it has been found that these natural biomaterials have very reasonable structures which gives them many excellent properties, such as good carrying capacity, good toughness, self-healing, and so on. Furthermore, these biomaterials have very fine and special structures rather than complicated compositions, which are distinctly different from q This research work was supported by the National Science Foundation of China grant No. 59632090 . Ž . ) Corresponding author. Tel.: q86-1062785488; fax: q86-1062771160. E-mail address: wangca@263.net C. Wang . Ž . what we seek for ceramic materials with high toughness through composition control. For example, trees and bamboos are typical long, fiber-reinforced composites. Their fibers have different sizes and arranged modes in structure so that they can display the optimal behaviors under tensile, bending, compressing stress and other applied load. Another typical biomaterial is nacre, the structure of which is laminated with brick wall structure. It consists of more than 99 vol.% inorganic phase, aragonite wafers, and less than 1 vol.% organic phase, mortar of proteins. This particular configuration imparts over one order of magnitude higher bending strength and toughness than those of aragonite single crystals. The work of fracture of nacre is 3000 times higher than that of pure aragonite 1 . So, the w x complicated and reasonable structure of natural biomaterials can give us an important insight into making better structure materials through biomimetic design. Coblenz 2 in 1988 put forward a fibrous monolithic w x structure imitating the structure of trees or bamboos. In the structure, fibrous polycrystalline cells are arranged parallel, and separated and combined by very thin interfacial phase. Baskaran et al. 3 in 1993 firstly prepared SiC w x rC fibrous monolithic structure ceramics according to the 0928-4931r00r$ - see front matter q 2000 Published by Elsevier Science S.A. PII: S0928- 4931 00 00133-8 Ž
C. Wang et al./ Materials Science and Engineering C 11(2000)9-1 above design mode. Clegg et al. [4] in 1990 reported a Table 2 pioneer work of preparing a laminated SiC composite Mechanical properties of laminated Sin N4/BN composites separated by graphite layers, so that very high toughness Average i verage or (MPa) KIc (MPa m and flaw tolerance were obtained: 15 MPa m/2 of fracture thickness of thickness of toughness and 4625 J/m of work of fracture. In recent (mm)ayers matrix layer after years, much attention has been paid to biomimetic struc intering(mm) ture ceramics and many good results and opinions have2 0.087 0951±89.612890±4.14 been obtained [5-7] 740.64±78942840±449 51893±112.9418.75±4.41 In the present paper, according to the structures of trees 70409±127.199.55±1.49 bamboos and nacres, we designed and prepared 1.31 572.50±73.0211.67±3.53 kinds of ceramic composites with high toughness, fibrous monolithic and laminated composites, and a lot of active results were achieved A-2000 Shimadzu universal materials testing machine with a crosshead speed of 0.05 mm/min. The microstructure of 2. Experimental procedure specimens was observed with SEM Ceramic raw powders (Si3 N4, B-SiC whiskers and ring aids)were mixed with organic binder(pva), plasti 3. Results and discussion cizing agents(glycerine) and lubricant(liquid paraffin)and hen repeatedly rolled, turning into a well-distributed plas- 3.1. Mechanical properties tic mud pie. Fibrous green bodies (green fibers) were obtained through extrusion. The mud pie was extruded Table I summarizes the mechanical properties of fi- through orifices with different diameters (1.0.0.7.0.5 and brous monolithic Si,N/BN It can be found th 0.3 mm)to form green fibers. Laminated green bodies the fibrous monolithIc SI N4/BN ceramics have very high (green sheets)were obtained through roll compaction. The toughness compared with conventional monolithic Sia na green bodies(fibers or sheets)were then coated by dipping ceramics. Table 2 shows the mechanical properties of in slurry containing mainly BN. The coated green bodies laminated Sig N4/BN ceramics with different thickness of were dried in air and arranged in a certain order into matrix layer in which 20 wt. SiC whiskers were added. It graphite die and hot-pressed at 1800%C/1.5 h/22 MPa can be seen that the laminated SiNa/BN ceramics can pressure under flow N, atmosphere. In addition, rod MPa β-Si3N4 seeds(3wt%)orβ- SiC whiskers(20w% In Tables 1 and 2. it can be seen that the size of the dded into Si, N4 matrix cells for further toughening matrix cells(fiber or layer)obviously has an effect on the Al2O, or Si, N, was added into the BN interfacial phase properties of the composites. With the decrease in the size for the purpose of adjusting the interfacial bonding state of matrix cell, the fracture toughness of the composites is between Si3 N4 matrix cells markedly improved while the bending strength has no Bending strength was determined by three point bend obvious change. Moreover, in literature [4, 7] the bending testing (test bars 4 x3 x 36 mm ) The tensile surface of ngth of this kind of ceramic with weak interfacial the samples was polished with diamond paste down to 1 P phase at room temperature was considerably reduced due um and the long edges of the tensile surface were rounded Fracture toughness was measured by SENB method(test bars 4 x 30 mm) and the width of the notch was less than 0.25 mm. The curve of load-displacement and work of fracture of the specimen were determined using an 乙 Table I Mechanical properties of in-situ fibrous monolithic Si3 N4/BN compos- Average Add seeds diameter of MPa/2) Or(MPa) MPa) Kic (MPa m /2) 0.5mm 1.0 898±1.047054±712001±1.17 11.52±0.986781±6222.56±1.01 0.5 14.ll±1.00639.7±602296±0.88 0.3 1716±1.02619.8±4723.95±0.92 Fig 1. Typical load-displacement curve of fibrous monolithic Si3 N4/BN
10 C. Wang et al.rMaterials Science and Engineering C 11 2000 9–12 ( ) above design mode. Clegg et al. 4 in 1990 reported a w x pioneer work of preparing a laminated SiC composite separated by graphite layers, so that very high toughness and flaw tolerance were obtained: 15 MPa m1r2 of fracture toughness and 4625 Jrm2 of work of fracture. In recent years, much attention has been paid to biomimetic structure ceramics and many good results and opinions have been obtained 5–7 . w x In the present paper, according to the structures of trees or bamboos and nacres, we designed and prepared two kinds of ceramic composites with high toughness, fibrous monolithic and laminated composites, and a lot of active results were achieved. 2. Experimental procedure Ceramic raw powders Si N , Ž b-SiC whiskers and sin- 3 4 tering aids were mixed with organic binder PVA , plasti- . Ž. cizing agents glycerine and lubricant liquid paraffin and Ž. Ž . then repeatedly rolled, turning into a well-distributed plastic mud pie. Fibrous green bodies green fibers were Ž . obtained through extrusion. The mud pie was extruded through orifices with different diameters 1.0, 0.7, 0.5 and Ž 0.3 mm to form green fibers. Laminated green bodies . Ž . green sheets were obtained through roll compaction. The green bodies fibers or sheets were then coated by dipping Ž . in slurry containing mainly BN. The coated green bodies were dried in air and arranged in a certain order into a graphite die and hot-pressed at 18008Cr1.5 hr22 MPa pressure under flow N atmosphere. In addition, rod-like 2 b-Si N seeds 3 wt.% or Ž. Ž . b-SiC whiskers 20 wt.% were 3 4 added into Si N matrix cells for further toughening. 3 4 Al O or Si N was added into the BN interfacial phase 23 34 for the purpose of adjusting the interfacial bonding state between Si N matrix cells. 3 4 Bending strength was determined by three point bend Ž 3 testing test bars 4=3=36 mm . The tensile surface of . the samples was polished with diamond paste down to 1 mm and the long edges of the tensile surface were rounded. Fracture toughness was measured by SENB method test Ž 3 bars 4=6=30 mm , and the width of the notch was less . than 0.25 mm. The curve of load-displacement and work of fracture of the specimen were determined using an Table 1 Mechanical properties of in-situ fibrous monolithic Si N rBN compos- 3 4 ites Average Add seeds Add whiskers diameter of sf IC f IC Ž. Ž. MPa K s MPa K green fiber 1r2 1r2 Ž. Ž. MPa m MPa m Ž . mm 1.0 689.3"68 8.98"1.04 705.4"71 20.01"1.17 0.7 602.1"62 11.52"0.98 678.1"62 22.56"1.01 0.5 562.4"51 14.11"1.00 639.7"60 22.96"0.88 0.3 530.6"42 17.16"1.02 619.8"47 23.95"0.92 Table 2 Mechanical properties of laminated Si N rBN composites 3 4 1r2 Average Average sf IC Ž. Ž . MPa K MPa m thickness of thickness of green layers matrix layer after Ž. Ž. mm sintering mm 0.2 0.087 709.51"89.61 28.90"4.14 0.4 0.13 740.64"78.94 28.40"4.49 0.8 0.36 518.93"112.94 18.75"4.41 1.6 0.61 704.09"127.19 9.55"1.49 3.2 1.31 572.50"73.02 11.67"3.53 A-2000 Shimadzu universal materials testing machine with a crosshead speed of 0.05 mmrmin. The microstructure of specimens was observed with SEM. 3. Results and discussion 3.1. Mechanical properties Table 1 summarizes the mechanical properties of fibrous monolithic Si N rBN ceramics. It can be found that 3 4 the fibrous monolithic Si N3 4rBN ceramics have very high toughness compared with conventional monolithic Si N3 4 ceramics. Table 2 shows the mechanical properties of laminated Si N rBN ceramics with different thickness of 3 4 matrix layer in which 20 wt.% SiC whiskers were added. It can be seen that the laminated Si N3 4rBN ceramics can reach more than 28 MPa m1r2 of fracture toughness. In Tables 1 and 2, it can be seen that the size of the matrix cells fiber or layer obviously has an effect on the Ž . properties of the composites. With the decrease in the size of matrix cell, the fracture toughness of the composites is markedly improved while the bending strength has no obvious change. Moreover, in literature 4,7 the bending w x strength of this kind of ceramic with weak interfacial phase at room temperature was considerably reduced due Fig. 1. Typical load-displacement curve of fibrous monolithic Si N3 4 rBN composites
C. Wang et al./ Materials Science and Engineering C 11(2000)9-1 400}HP Displacement (um) Fig. 4. Microstructure of laminated Si3N4/BN composites. Fig. 2. Typical load-displacement curve of laminated Si3N4/BN com- inferred that the mechanical performance of ceramic mate- rials can be substantially improved by special biomimetic to the presence of weak separating interfacial structural design However, in the present paper, the mechanical pr can be improved obviously by adding whiskers or 3.3. Microstructure and multilevel toughening mechanisms into the matrix phase Fig. 3 displays the microstructure of Si3 N4/BN fibrous 3. 2. Curve of load-displacement and work of fracture monolithic ceramics. On the two sides planes of the speci- men fibers, which are arranged in uniaxial direction regu- Fig. I and Fig. 2 are curves of load-displacement for larly, can be observed(Fig. 3a). At two end planes of the brous monolithic Si, N4/BN ceramics and laminated specimen, it can be seen that the cross-section of the cells Si,N4/BN ceramics, respectively. Compared with mon are more like an hexagon, just like Coblenz's design mode lithic Si,N, ceramics, fibrous monolithic Si, N4/BN ce [2]. Fig. 4 shows a photograph of crack propagation of ramics and laminated Si, N4/BN ceramics exhibit a non- laminated Si3 N4/BN ceramics, which exhibits a laminated brittle failure manner while the conventional monolithic structure and crack deflection. Note that the cells(fibers or Si,na fractures catastrophically. Furthermore, the size layers)are not single crystals but domains of polycrys the matrix cell has an obvious influence on the mechanical talline Si3 N4. The BN interfacial phase acts as cell bound- performance as shown in Fig. 1. According to the area ary phase to separate the cells covered by the curves of load-displacement, work of frac The common characteristic of the two biomimetic com- ture for two biomimetic materials can be calculated posites is that the strong matrix cells and the relative weak more than 4000 J/m2 in general, while that of monolithic interfacial phases are arranged alternately with each other Si3 N ceramics is only 100 J/'m or so. Hence, it may be There are multilevel toughening mechanisms in different (a) Fig 3. Microstructure of fibrous monolithic Si3 N4/BN composites
C. Wang et al.rMaterials Science and Engineering C 11 2000 9–12 ( ) 11 Fig. 2. Typical load-displacement curve of laminated Si N rBN com- 3 4 posites. to the presence of weak separating interfacial phases. However, in the present paper, the mechanical properties can be improved obviously by adding whiskers or seeds into the matrix phase. 3.2. CurÕe of load-displacement and work of fracture Fig. 1 and Fig. 2 are curves of load-displacement for fibrous monolithic Si N3 4rBN ceramics and laminated Si N rBN ceramics, respectively. Compared with mono- 3 4 lithic Si N ceramics, fibrous monolithic Si N rBN ce- 34 34 ramics and laminated Si N rBN ceramics exhibit a non- 3 4 brittle failure manner while the conventional monolithic Si N fractures catastrophically. Furthermore, the size of 3 4 the matrix cell has an obvious influence on the mechanical performance as shown in Fig. 1. According to the area covered by the curves of load-displacement, work of fracture for two biomimetic materials can be calculated as more than 4000 Jrm2 in general, while that of monolithic Si N ceramics is only 100 Jrm2 or so. Hence, it may be 3 4 Fig. 4. Microstructure of laminated Si N rBN composites. 3 4 inferred that the mechanical performance of ceramic materials can be substantially improved by special biomimetic structural design. 3.3. Microstructure and multileÕel toughening mechanisms Fig. 3 displays the microstructure of Si N rBN fibrous 3 4 monolithic ceramics. On the two sides planes of the specimen fibers, which are arranged in uniaxial direction regularly, can be observed Fig. 3a . At two end planes of the Ž . specimen, it can be seen that the cross-section of the cells are more like an hexagon, just like Coblenz’s design mode w x 2 . Fig. 4 shows a photograph of crack propagation of laminated Si N3 4rBN ceramics, which exhibits a laminated structure and crack deflection. Note that the cells fibers or Ž layers are not single crystals but domains of polycrys- . talline Si N . The BN interfacial phase acts as cell bound- 3 4 ary phase to separate the cells. The common characteristic of the two biomimetic composites is that the strong matrix cells and the relative weak interfacial phases are arranged alternately with each other. There are multilevel toughening mechanisms in different Fig. 3. Microstructure of fibrous monolithic Si N rBN composites. 3 4
C Wang et al./ Materials Science and Engineering C 11(2000)9-1 scales in these kinds of composites: the lst-grade toughen- toughening and elongated matrix grain toughening leads to ing mechanism is weak interfacial layer toughening, which fairly high toughness and work of fracture of the compos- is considered as the main reason for the very high tough ness of the composites; the 2nd-grade toughening mecha- nism is whisker toughening in the matrix cells; and the 3rd-grade toughening mechanism is elongated Si N grain References ghening toughening mechanisms leads to fairly high toughness and I1 R.Z. Wang, Biomimetic preparation and research of perlite and natural bone, Report of postdoctoral research, Tsinghua University work of fracture of the composites [8] China, 1995. [2] w.S. Coblenz, Fibrous monolithic ceramic and method for produc tion, US Patent 4772524, 1988 4. Conclusion [3]S. Baskaran, S D. Nunn, D. PoPovic, J W. Halloran, Fibrous mono- ithic ceramics: I. Fabrication, microstructure and indentation behav- ior,J.Am. Ceram.Soc.76(9)(1993)2217-2224 According to the structures of trees and nacres, two [4] w.J. Clegg, K. Kendall, N.M. Alford, A simple way to make tough kinds of biomimetic structure ceramic composites, fibro eramics, Nature 347(10)(1990)445-44 monolithic and laminated structure Si, N4/BN composi [5] Z. Benlian, The biomimetic design of worst bonding interface for have been successfully prepared with more than 20 ceramic matrix composites, Comp. Eng. 5(10)(1995)1261-1273 m'/2 of high toughness and 4000 J/m2 of work of 16] H Guo, Structure and preparation of Si N, matrix composites with high toughness, Dissertation for PhD, Tsinghua University, China, fracture. The load-displacement curves show that both of 1997 the composites exhibit non-brittle fracture properties. The [] H. Liu, M.S. Hsu, Fracture behavior of multilayer silicon nitride/ size of the structure cell (fiber or layer)is an important (8)Y. Huang, H. Guo, S.Y. Cai, CA.Wang, ZP.Xie, The biomimetic boron nitride ceramic. J. Am. Ceram Soc. 79(9)(1996)2452-2457 structure parameter influencing the mechanical behavior of structure design and preparation of Si, Na matrix composites with the composites. The synergy of multilevel toughening high toughness, In: Ninth Cimtec World Ceramic Congres mechanisms, weak interfacial phase toughening, whisker (CIMTEC 98), Italy, 1998
12 C. Wang et al.rMaterials Science and Engineering C 11 2000 9–12 ( ) scales in these kinds of composites: the 1st-grade toughening mechanism is weak interfacial layer toughening, which is considered as the main reason for the very high toughness of the composites; the 2nd-grade toughening mechanism is whisker toughening in the matrix cells; and the 3rd-grade toughening mechanism is elongated Si N grain 3 4 toughening in the matrix cells. The synergy of multilevel toughening mechanisms leads to fairly high toughness and work of fracture of the composites 8 . w x 4. Conclusion According to the structures of trees and nacres, two kinds of biomimetic structure ceramic composites, fibrous monolithic and laminated structure Si N rBN composites, 3 4 have been successfully prepared with more than 20 MPa m1r2 of high toughness and 4000 Jrm2 of work of fracture. The load-displacement curves show that both of the composites exhibit non-brittle fracture properties. The size of the structure cell fiber or layer is an important Ž . structure parameter influencing the mechanical behavior of the composites. The synergy of multilevel toughening mechanisms, weak interfacial phase toughening, whisker toughening and elongated matrix grain toughening leads to fairly high toughness and work of fracture of the composites. References w x 1 R.Z. Wang, Biomimetic preparation and research of perlite and natural bone, Report of postdoctoral research, Tsinghua University, China, 1995. w x 2 W.S. Coblenz, Fibrous monolithic ceramic and method for production, US Patent 4772524, 1988. w x 3 S. Baskaran, S.D. Nunn, D. PoPovic, J.W. Halloran, Fibrous monolithic ceramics: I. Fabrication, microstructure and indentation behavior, J. Am. Ceram. Soc. 76 9 1993 2217–2224. Ž .Ž . w x 4 W.J. Clegg, K. Kendall, N.M. Alford, A simple way to make tough ceramics, Nature 347 10 1990 445–447. Ž .Ž . w x 5 Z. Benlian, The biomimetic design of worst bonding interface for ceramic matrix composites, Comp. Eng. 5 10 1995 1261–1273. Ž .Ž . w x 6 H. Guo, Structure and preparation of Si N matrix composites with 3 4 high toughness, Dissertation for PhD, Tsinghua University, China, 1997. w x 7 H. Liu, M.S. Hsu, Fracture behavior of multilayer silicon nitrider boron nitride ceramic, J. Am. Ceram. Soc. 79 9 1996 2452–2457. Ž .Ž . w x 8 Y. Huang, H. Guo, S.Y. Cai, C.A. Wang, Z.P. Xie, The biomimetic structure design and preparation of Si N matrix composites with 3 4 high toughness, In: Ninth Cimtec World Ceramic Congress Ž . CIMTEC’98 , Italy, 1998