Journal of the American Ceramic Sociery-Wang et al. Vol. 85. No microstructure. For comparison. the same SENB method was used to measure the fracture toughness of monolithic and laminated ceramics in this paper. For the reasons mentioned above, we used apparent fracture toughness rather than fracture toughness for laminated ceramics. The dimension of test bars was 4 mm x 6 mm X 30 mm. The testing was conducted with a crosshead speed of 0.05 mm/min and a span of 24 mm. The load-displacement urve was also determined for notched samples using the same conditions as those used in the determination of fracture toughness To assess the bonding strength of BN interfacial layers, a so-called double-shearing method was designed as shown in Fig 1. The samples were made with three layers of thick Si, N, layers and two BN layers using the same compositions and processing of laminated Si, N,/BN composites The microstructure of the laminated Si, N /BN composites was observed by scanning electron microscopy (SEM). The surface of a Si, N, matrix layer was polished to a mirror surface and etched with melted NaOH Ill. Results and Discussion 250um (1) Microstructure and Load-Displacement Curve of Laminated Si,N,/BN Composites Figures 2(a) and (b) show SEM photographs of the surfaces perpendicular and parallel to the stacking sheets, respectively. In Fig. 2(a), the laminated structure can be observed clearly, in which SiNa layers(black) are about 80-100 um thick and BN(white) separating layers are only-20 um thick. Figure 2(b) shows that in the Si,N4 matrix layers SiC whiskers are distributed homoge neously and preferentially oriented due to rolling compaction an hot-pressing. In the composites, the thickness of the matrix Si,N thickness of the interfacial BN layers depends on the viscosity of the BN slurry and coating times. Therefore, the thickness ratio between matrix SiaNa layers and interfacial BN layers can be easily controlled and adjusted The above specific microstructure makes this kind of laminated Si,N,/BN composite with the specific mechanical performance Figure 3 shows two typical load-displacement curves of laminated Si N/BN compo site and conventional monolithic Si, N, material respectively. It can be seen that the laminated Si, N,/BN composite exhibits a nonbrittle failure manner while the conventional mono- lithic Si,N, material fractures catastrophically. For a laminated 9 H Si,N,/BN composite, after the first load drop, the load-bearing ability of the tested laminated ceramic bar still retains over 50% of the peak load. Until totally fractured. this laminated Si N /BN Fig. 2. SEM photographs of the surfaces of laminated Si,N /BN com- composite gives a prolonged deflection besides the elastic defor- posites:(a) side view perpendicular to the stacking sheets and (b) matrix mation. This shows that a laminated composite exhibits a different Si, N, layer, in which whiskers were aligned parallel to the stacking sheets. fracture behavior from that of a monolithic ceramic. Hence it may Load laminated composite Sin Fig. I. Schematic chart of double-shearing method for determinat of bonding strength of BN interfacial layer in laminated Si,N 图N Fig 3. Typical load-deflection curve of laminated Si, N, /BN composites2458 Journal of the American Ceramic Society—Wang et al. Vol. 85. No. 10 microstruclure. For comparison, the same SENB method was used to measure the fraciure toughness of monolithic and laminated Leramics in this paper. For the reasons mentioned above, we u.sed apparent fracture loughness rather than fracture toughness for laminated ceramics. The dimension of test hars was 4 mm X 6 nmi X 30 mm. The testing was conducted with a crosshead .speed of 0.05 mm/min and a span of 24 mm. The load-displacement curve was also determined for notched samples using the same conditions as those used in the detemiination of fracture toughness. To assess the bonding strength of BN interfacial layers, a so-called double-shearing method was designed as shown in Fig, I, The samples were made with three layers of thick Si ,N^ layers and two BN layers using the same compositions and processing of laminated Si^Nj/BN composites. The microstructure of the laminated Si^Nj/BN composites was observed by scanning electron microscopy (SEM). The surface of a Si,N.j matrix layer was polished to a mirror surface and etched with melled NaOH, in. Results and Discussion (/) Microstructure and iMad-Displacement Curve of laminated Si^NyBN Composites Figures 2(a) and (b) show SEM photographs of the surfaces perpendicular and parallel to the stacking sheets, respectively. In I-ig. 2{a). the laminated structure can be observed clearly, in which Si,Nj layers (black) are about 80-100 |im thick and BN (white) separating layers are only -20 p.m thick. Figure 2(b) shows that in the Si,Nj matrix layers SiC whiskers are distributed homoge￾neously and preferentially oriented due to rolling compaction and hot-pressing. In the composites, the thickness of the matrix Si,N4 layers depends on the thickness of green SiiN,j sheets, while the thickness of the interfacial BN layers depends on the viscosity of the BN slurry and coating times. Therefore, the thickness ratio between matrix Si^tNj layers and interfaeial BN layers can be easily controlled and adjusted. The above specific microstruclure makes this kind of laminated .Si,N4/BN composite with the specific mechanical performance. Figure 3 shows two typical load-displacement curves of laminated Si.N^/BN composite and conventional monolilhic Si^Nj material. respectively. It can be seen that the laminated SiiNj/BN composite exhibits a nonbrittle failure manner while the conventional mono￾lithic Si,N., material fractures catastrophically. For a laminated SijNj/BN composite, after the first load drop, the load-bearing ability of the tested laminated ceramic bar still retains over 50% of the peak load. Until totally fractured, this laminated Si^Nj/BN composite gives a prolonged deflection besides the elastic defor￾mation. This shows that a laminated composite exhibits a different fracture behavior from that of a monolithic ceramic. Hence it mav Load • 566666^ V V V V V V S BN Support Fig. 2. SEM photographs of the surfaces of laminated Si^Nj/BN com￾posites: (a) side view perpendicular to the stacking sheets and (h) matrix Si,Nj layer, in which whiskers were aligned parallel to ihc stacking sheets. 600 400 laminated composite 100 200 300 Displacement V\^. I. Schematic charl of double-shearing method for determination nf honding strength of BN interfacial layer in laminated Si3N4/BN composites. 400 Fig. 3. Typical load-defleetion curve of laminated SijNi/BN composites with notch