X et al./Materials Science and Engineering C 28(2008)1501-1508 c Crack initiation Crack deflection and propagation 100um Fig 14. Optical images of in situ observation of fracture behavior of specimen BS-with a loading span of 16 mm (a)The initial state without any cracks in front of the notch (b)A crack vitiated in the SiaNa matrix layer. (c)Crack deflection and kinking in the Bn interfacial layer. (d)A T-shaped crack path within the BN interfacial layer.(e)Final fracture. drop in the bending load-displacement curve, indicating the difficult such that through-thickness cracking is dominant, leading occurrence of through-thickness cracking in front of the notch In to brittle failure. The composite with 5 wt% Si3 N4 in BN interfacial the third stage, crack deflects and propagates in a stable manner layers(specimen BS-5) exhibits fully developed crack deflection. It within the interfacial layers. The pre-existing microvoids and micro- was found that microvoids and microcracks had already pre-existed cracks in BN interfacial layers arrest the major crack. The crack in BN interfacial layers before bending loading. These pre-existing reorients and propagates within BN interfacial layers, forming a T- microvoids and microcracks arrested the major crack, leading to shaped crack In the final stage, crack propagates unstably with crack crack deflection and propagation in BN interfacial layers. The deflection, crack kinking, and through-thickness cracking, conse- fracture process was found to progress in four stages:(1)linear quently leading to fracture failure of the whole specimen. Note that elastic response; (2)crack initiation; (3) crack deflection and through-thickness cracking does not always occur in the middle of the propagation in a stable manner within the interfacial layers;(4) span. The crack deflection does not always happen symmetrically crack propagation in an unstable manner in both matrix and from the centre toward two sides within the interfacial layers; instead, interfacial layers. some cracks go across the matrix and interfacial layers directly in the through-thickness manner. Such observations were not included in Acknowledgements the previous model on the facture behavior of ceramic laminates in This work was supported by the National Science Foundation (Grant No. EPS-0296165 and CMMI-0653651), the ACS Petroleum 4. Conclusions Research Fund(ACS PRF# 40450-AC10), the National Aeronautical and Space Administration(NASA), South Carolina EPSCoR office, and the Relatively l layers are needed to realize crack University of South Carolina Nano Center Seed Grant. The content of deflection in BN composites. A higher Si3N4 content this information does not necessary reflect the position or policy of in bn interfa tance, specimens BC-50 and Bs-10) the government and no official endorsement should be referred. makes the it stronger and crack deflection more References [1]S. Kamat, X Su, R Ballarini, A.H. Heuer, Nature 405(2000)1036-1040. in stable state I Song, X.H. Zhang YL Bai, Journal of Materials Research 17(2002)1567-1570 5V. Laraia, A.H. Heuer, Journal of the American Ceramic Society 72(1989) 77-2179 [6].Z Wang. H.B. Wen, FZ. Cui, H.B. Zhagn, H D Li, Journal of Materials Science 995)2299-2304 Crack propagation!. [7 QL Feng, F Z Cui, G Pu, R.Z. Wang, H.D. Li, Materials Science Engineering [8 RZ Wang Z Suo, A.G. Evans, N Yao, LA Aksay, Journal of Materials Research 16 (2001)2485-2493. Crack initiation [9]AG. Evans, Z Suo, R.Z. Wang, LA. Aksay, M.Y. He, .W. Hutchinson, Journal of P.M. Weiss, A Nguyen, Y F Lu, RA. Assink, W L Gong, C Brinker, Nature Fig15.A schematic of a three-point bending load-displacement curve of a laminated /11/ L.A. Aksay, M.Trau. L Honma, N. Yao, L Zhou, P Fenter, P. M. Eisenberger M. Gruner, Science 273(1996)892-898 [12]G. Falini, S. Albeck, S. Weiner, L Addadi, Science 271(1996)67-69drop in the bending load–displacement curve, indicating the occurrence of through-thickness cracking in front of the notch. In the third stage, crack deflects and propagates in a stable manner within the interfacial layers. The pre-existing microvoids and micro￾cracks in BN interfacial layers arrest the major crack. The crack reorients and propagates within BN interfacial layers, forming a T￾shaped crack. In the final stage, crack propagates unstably with crack deflection, crack kinking, and through-thickness cracking, conse￾quently leading to fracture failure of the whole specimen. Note that through-thickness cracking does not always occur in the middle of the span. The crack deflection does not always happen symmetrically from the centre toward two sides within the interfacial layers; instead, some cracks go across the matrix and interfacial layers directly in the through-thickness manner. Such observations were not included in the previous model on the facture behavior of ceramic laminates in bending [31]. 4. Conclusions Relatively weak interfacial layers are needed to realize crack deflection in laminated Si3N4/BN composites. A higher Si3N4 content in BN interfacial layers (for instance, specimens BC-50 and BS-10) makes the interfacial layers stronger and crack deflection more difficult such that through-thickness cracking is dominant, leading to brittle failure. The composite with 5 wt.% Si3N4 in BN interfacial layers (specimen BS-5) exhibits fully developed crack deflection. It was found that microvoids and microcracks had already pre-existed in BN interfacial layers before bending loading. These pre-existing microvoids and microcracks arrested the major crack, leading to crack deflection and propagation in BN interfacial layers. The fracture process was found to progress in four stages: (1) linear elastic response; (2) crack initiation; (3) crack deflection and propagation in a stable manner within the interfacial layers; (4) crack propagation in an unstable manner in both matrix and interfacial layers. Acknowledgements This work was supported by the National Science Foundation (Grant No. EPS-0296165 and CMMI-0653651), the ACS Petroleum Research Fund (ACS PRF# 40450-AC10), the National Aeronautical and Space Administration (NASA), South Carolina EPSCoR office, and the University of South Carolina NanoCenter Seed Grant. The content of this information does not necessary reflect the position or policy of the Government and no official endorsement should be referred. References [1] S. Kamat, X. Su, R. Ballarini, A.H. Heuer, Nature 405 (2000) 1036–1040. [2] L. Addadi, S. Weiner, Nature 389 (1997) 912–915. [3] F. Song, X.H. Zhang, Y.L. Bai, Journal of Materials Research 17 (2002) 1567–1570. [4] J.D. Currey, A.J. Kohn, Journal of Materials Science 11 (1976) 1615–1623. [5] V.J. Laraia, A.H. Heuer, Journal of the American Ceramic Society 72 (1989) 2177–2179. [6] R.Z. Wang, H.B. Wen, F.Z. Cui, H.B. Zhagn, H.D. Li, Journal of Materials Science 30 (1995) 2299–2304. [7] Q.L. Feng, F.Z. Cui, G. Pu, R.Z. Wang, H.D. Li, Materials Science & Engineering C-Biomimetic and Supramolecular Systems 11 (2000) 19–25. [8] R.Z. Wang, Z. Suo, A.G. Evans, N. Yao, I.A. Aksay, Journal of Materials Research 16 (2001) 2485–2493. [9] A.G. Evans, Z. Suo, R.Z. Wang, I.A. Aksay, M.Y. He, J.W. Hutchinson, Journal of Materials Research 16 (2001) 2475–2484. [10] A. Sellinger, P.M. Weiss, A. Nguyen, Y.F. Lu, R.A. Assink, W.L. Gong, C.J. Brinker, Nature 394 (1998) 256–260. [11] I.A. Aksay, M. Trau, S. Manne, I. Honma, N. Yao, L. Zhou, P. Fenter, P.M. Eisenberger, S.M. Gruner, Science 273 (1996) 892–898. [12] G. Falini, S. Albeck, S. Weiner, L. Addadi, Science 271 (1996) 67–69. Fig. 14. Optical images of in situ observation of fracture behavior of specimen BS-with a loading span of 16 mm. (a) The initial state without any cracks in front of the notch. (b) A crack initiated in the Si3N4 matrix layer. (c) Crack deflection and kinking in the BN interfacial layer. (d) A T-shaped crack path within the BN interfacial layer. (e) Final fracture. Fig. 15. A schematic of a three-point bending load–displacement curve of a laminated Si3N4/BN composite. X. Li et al. / Materials Science and Engineering C 28 (2008) 1501–1508 1507