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/BN10 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 struc￾ture 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 plas￾tic 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 fi￾brous 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