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)00133-8Materials 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 reinforce￾ments, 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 tough￾ness, self-healing, and so on. Furthermore, these biomateri￾als have very fine and special structures rather than com￾plicated compositions, which are distinctly different from q This research work was supported by the National Science Founda￾tion 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 bam￾boos 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 magni￾tude 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 biomateri￾als 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 paral￾lel, 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