ournal J Am Cerum Soc 85 [101 3457-61( Control of Composition and Structure in Laminated Silicon Nitride/Boron Nitride Composites Chang-an Wang, Yong Huang, Qingfeng Zan, Linhua Zou, and Shengyou Cai State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering Tsinghua University. Beijing, 100084, People's Republic of China Based on a biomimetic design, Si, N,/BN composites with MPam. Thereafter, various methods including tape casting and laminated structures have been prepared and investigated slip casting have been applied to make ceramic-matrix composites through composition control and structure design. To further with multilayered structures. The laminated design is considered to improve the mechanical properties of the composites, SiN be a promising method to overcome the brittle nature of ceramics matrix layers were reinforced by SiC whiskers and BN ser pa- However, the bending strength of the laminated materials at room rating layers were modified by adding Si,N, or Al2O3. The temperature is considerably reduced because of the presence of results showed that the addition of Sic whiskers in the SiaNa weak separating layers. Laminated composites consist of matrix layers could greatly improve the apparent fracture ceramic-matrix and interfacial separating layers. The former toughness(reaching 28.1 MPam"), at the same time keeping mainly determines the bulk strength and the latter gives rise to the the higher bending strength (reaching 651.5 MPa) of the high toughness due to the deflection of crack. Therefore, to composites. Additions of 50 wt% AL,O, or 10 wt% Si,N, to BN improve the mechanical properties of these composites, two ways interfacial layers had a beneficial effect on the strength and may be used: adjusting the composition and structure of the toughness of the laminated Si, N,/BN composites. Through interfacial separating layers and/or reinforcing the ceramic matrix observation of microstructure by SEM. multilevel toughening mechanisms contributing to high toughness of the laminated In the current paper, laminated Si, N,/BN composites were Si, N/BN composites were present as the first-level toughening prepared to imitate the structure of mollusk shell: thin Si, N, layers mechanisms from BN interfacial lavers as crack deflection were laminated like the aragonite plates with BN separating layer bifurcation, and pull-out of matrix sheets, and the secondary as the organic joining. To improve the mechanial properties of toughening mechanism from whiskers in matrix layers. laminated Si, N, /BN composites, SiC whiskers were added to Si,N, matrix layers for further reinforcement of matrix layers, and AL, O, or Si N, was incorporated into the BN layers for the I. Introduction purpose of adjusting the bonding strength between Si N, layers S LICON NITRIDE is known as an important high-temperature structural ceramic with excellent mechanical properties from IL. Experimental Procedure ambient to elevated temperatures, but the brittleness limits its applications. Therefore, various approaches have been chosen to Si,N,(Founder High-Tech Ceramic Corp, China) ration of particles and whiskers. - etc but improvement in the AL O,(purity>99.9%) were ball-milled with 20 wt% Sic toughness of the material has been only limited. However, it is well known that some natural biomaterials, such to achieve a homogeneous mixture. The slurry was dried and bamboo, wood, and nacre, have specific composite structures sieved through a 60-mesh screen; then the powder was mixed with and exhibit good mechanical properties in that they have high organic binders and then rolled into a sheet about 200 um thick by toughness and excellent damage resistance, compared with artifi- rolling compaction. The green sheets were subsequently coated formed by aragonite layers about I um thick joined by a kind of at 450C for 2 h in air. the green body was hot-pressed in a mortar of proteins. This particular configuration imparts over 1 graphite resistance furnace in flowing N2 at 1820 C for 1.5 hFor order of magnitude higher bending strength and toughness than comparison, Si, N, monolithic samples were fabricated using the those of aragonite single crystals. These natural composites may same composition and processing of Si, N,/BN laminated compos- give us some insights into making better structural materials with high toughness through biomimetic design The mechanical properties of the laminated Si, N,/BN compos Recently, some researchers-> introduced laminated or multi ites were determined on an A-2000 Shimadzu universal material layered structures to improve the mechanical properties of struc testing machine. Six samples were tested for each property. The tural ceramics. In 1990, Clegg reported preparing a laminated Sic loads were applied perpendicular to the plane of laminates composite separated by graphite layers, and the apparent fracture Bending strength was determined by three-point bend testing (test toughness of the composite was improved from 3.6 to 17.7 crosshead speed of 0.5 mm/min. The tensile surface of the samples was polished with diamond paste down to I um, and the corners were chamfered with a 15 mm diamond grinding wheel N. Claussen--contributing editor As for the fracture toughness of laminated ceramics, even though there are only a very few references mentioned, in which the fracture toughness was measured by SENB. SEVNB, and the indentation method, up to the present, there has not been a nusenipt No. 188266. Received September 14. 2000; approv suitable method to measure the fracture toughness of laminated This work was supported by the National Science Foundation of China ceramics because these methods are suitable only for homoge- neous ceramics while laminated ceramics are not homogeneous ijournal J Am. Cetiiin. S-i . HS |l(l| 24'i7-61 (2002) Control of Composition and Structure in Laminated Silicon Nitride/Boron Nitride Composites Chang-ati Wang,* Yong Huang,* Qingfeng Zan. Linhua Zou, and Shengyou Cai State Key Labonilory of New Ceramics and Fine Processing. Department ol Materials Science ;ind Engineering. Tsinghua University. Beijing. I(X)O84. People's Republic nl" China Based on a biomimetic deNign, Si^N^/BN compasites with luminated structures have been prepared and investigated through i-i)mpo.siti(>n control and structure design. To further improve the mechanical properties of the compo.sites. Si,N4 matrix layers were reinforeed by SiC whiskers and BN sepa￾rating layers were modified hy adding Si^Nj or AUO,. The result.s showed that the addition of SiC whiskers in the SiiN^ matrix layers eould greatly improve the apparent fracture toughness (reaching 28.1 MPa ni"'). at the same time keeping the higher hendiug strength (reaching 651.5 MPal of the composites. Additions of 50 wt% .XUO^ or 10 wt% Si^Nj to BN interfacial layers had a beneficial effecl on the strength and toughness of the laminated Si^Nj/BN composites. Through observation of microslructure by SEM. multilevel toughening mechanisms contributing lo high toughness of the laminated Si^Nj/BN composites were present as the first-level toughening mechanisms from BN interfacial layers as crack deflection, bifurcation, and pull-out of matrix sheets, and tbe secondary lougbening meehanism Irom whiskers in matrix layers. I. introduction S ILICON NiTRiDH is known as an important high-temperature structural ceramic with excellent mechanical propenies from anibienl lo elevated temperatures, but the brittleness limit.s its applications. Therefore, various approaches have been cbosen to increase its toughness including self-reinforcement' and incorpo￾ration of particles and whiskers,'""* etc., but improvement in the toughness of the material has been only limited. However, it is well known that some natural biomaterials, such as bamboo, wood, and nacre, have specific composite structures and exhibit good mechanical properties in that they have high toughness and excellent damage resistance, compared with artifi￾cial composites.^ A typical example of a layered biomaterial is mollusk shell. The hierarchical structure of a mollusk shell is formed by aragonite layers about I fxm thick joined by a kind of mortar of proteins. This particular configuration imparts over I order of magnitude higher bending strength and toughness than those of aragonite single crystals. These natural composites may give us some insights into making better structural materials with high toughness through biomimetic design. Recently, some researchers introduced laminated or multi￾layered structures to improve the mechanical properties of stmc￾tural ceramics. In 1990. Clegg'* reported preparing a laminated SiC compttsite separated by graphite layers, and the apparent fracture toughness of the composite was improved from 3.6 to 17.7 N. Clausseii—conlribuling editor Manuscripl No. I8K266. Received Sepleniber !4. KHXk approved February 12. m. This work was suppined by ihc National Science Foundution of China. 'Member. American Ceramic Society. MPa-m''". Thereafter, various methods including tape casting and slip casting have been applied to make ceramic-matrix composites with multilayered structtircs. The laminated design is considered to be a promising method to overcome the brittle nature of ceramics. However, the bending strength of the laminated materials at mom temperature is considerably reduced because ot the presence of weak separating layers.^'"'^ Laminated composites consist of ceramic-matrix and interfacial separating layers. The former mainly determines the bulk strength and the latter gives rise to the high toughness due to the deflection of crack. Therefore, to improve ihe mechanical properties of these composites, two ways may be used: adjusting the composition and structure of the inierfacial separating layers and/or reinforcing Ihc ceramic matrix layers. In the current paper, laminated SiiNj/BN composites were prepared to imitate the structure of mollusk shell: thin Si,N4 layers were laminated like the aragonite plates with BN separating layers as the organic joining. To improve ihe mechanial properiies of laminated Si^fN^BN composites. SiC whiskers were added to Si ,N^ matrix layers for further reinforcement of matrix layers, and Ai,O, or Si,N4 was Incorptiraied into the BN layers for the purpose of adjusting the bonding strength between S'y^N^ layers. II. Experimental Procedure SiiN4 {Founder High-Tech Ceramic Corp.. China) powders with 7 wi% Y,0, (99.9%. Hokke Chemicals. Japan) and 3 wt% AI,O, (purity > 99.9<7f) were ball-milled with 20 wi9i SiC whiskers (TWS-4()(). Tokai Carbon Co.. Japan) in ethanol for 24 h to achieve a homogeneous mixture. The slurry was dried and sieved through a 60-mesh screen; then the powder was mixed with organic binders and then rolled into a sheet about 200 fini thick by rolling compaction. The green sheets were subsequently coated with a slurry of BN adjusted with AUO, or SiiNj. and then dried and packed into a graphite die. After the binders were burned out at 430"C (bi- 2 h in air, the green body was hot-pressed in a graphite resistance furnace in flowing N, al I82O''C for \.5 h. For comparison, SiiNj monolithic samples were fabricated using the same composition and processing of Si,N_,/BN laminated compos￾ites, but without the BN coating on ihe Si,Nj green sheets. The mechanical properties of the laminated Si^N/BN compos￾ites were determined on an A-2000 Shimad/u universal materials testing machine. Six samples were tested for each property. The loads were applied perpendicular to the plane of laminates. Bending strength was determined by three-point bend testing (test bars 4 mm x 3 mm X Mt mm) with a span of 30 mm and a crosshead speed of 0.5 mm/min. The tensile surface of the samples was polished with diamond paste down to I |xm. and the corners were chamfered with a l^i mm diamond grinding wheel. As for the fracture toughness of laminated ceramics, even though there are only a very few references mentioned, in which the fracture toughness was measured by SENB.'' SEVNB,'" and the indentation method.'* up to the present, there has not been a suitable method to measure the fracture toughness of laminated ceramics because these methods are suitable only for homoge￾neous ceramics while laminated ceramics are not homogeneous in 2457