Materials Science and Engineering C 28(2008)1501-1508 Contents lists available at Science Direct MATERIALS Materials Science and Engineering C 除N ELSEVIER journalhomepagewww.elsevier.com/locate/msec Micro/ nanoscale mechanical characterization and in situ observation of cracking of laminated Si3Na/bn composites Xiaodong Lia, * Linhua Zou, Hai Nia, Anthony P. Reynolds, Chang-an Wang b, Yong Huang b b The State Key Laboratory of New Ceramics and Fine Processing Department of Materials Science and Engineering, Tsinghua University, Beifing 100084, PR China ARTICLE INFO ABSTRACT Micro/nanoscale mechanical characterization of laminated Si3N4/ BN composites was carried out by Received 13 July 2007 anoindentation techniques A custom-designed micro mechanical tester was integrated with an optical eceived in revised form 13 December 2007 2008 icroscope and an atomic force microscope to perform in situ three-point bending tests on notched Si,Na/BN 22 April 2008 omposite bend specimens where the crack initiation and propagation were imaged simultaneously with the optical microscope and atomic force microscope during bending loading. The whole fracture process was in situ captured. It was found that crack deflection was initiated induced by the pre-existing microvoids and Ceramic matrix composites(CMC) microcracks in BN interfacial layers. New fracture mechanisms were proposed to provide guidelines for the esign of biomimetic nacre-like composites. Atomic force microscopy(AFM) o 2008 Elsevier B V. All rights reserved. Nanoinden 1 Introduction crack propagation normal to the interfaces, is increased by more than four times(up to 15 MPa m ) and the Among all natural biomaterials, nacre, the mother-of-pearl, which work of fracture required to break the materials is increased is found in the shinny interior of many mollusk shells, is one of the substantially, by more than one hundred times(over 4 kJ/m). This most attractive materials with superior mechanical properties that has been considered as a breakthrough in the development of have fascinated scientists and engineers over the decades 1-9 Nacre matrix composites, although the design of laminated Sic/C composites is composed of 95% inorganic aragonite(a mineral form of CaCO3)and is still at the micrometer scale level, unlike that of nacre which has the only a small percent of organic biopolymer. It has a brick-and-mortar- nanoscale structures. Essentially, the biomimetic design introduced like structure with highly organized polygonal aragonite platelets of a here is to decrease, as much as possible, the dependence of the thickness ranging 200 to 500 nm and an edge length about 5 um mechanical properties of a ceramic material on its original natural sandwiched with a 5-20 nm thick organic biopolymer interlayer, crack population, by the energy dissipation mechanism, thereby which assembles the aragonite platelets together. Its laminated imparting flaw tolerance to an otherwise classically brittle material. tructure achieves a twice increase in strength and a thousand-fold Laminated SiaNa/BN composites with BN layers as weak interfacial increase in toughness(work of fracture)over its constituent ceramic layers are another example [22-25. Such composites also exhibit materials[10]. Such remarkable properties have inspired chemists and a high fracture toughness of 28 MPa m 2, and work of fracture over materials scientists to develop biomimetic composites to reproduce 4 kJ /m2[26, 27 The fracture in such laminated Si3 N/BN systems nacre's achievements [ 11-20 dominated by crack deflection, bridging, and through-thickness Nacre 's structure has been widely adopted as a biomimetic model cracking. in the design of ceramics to improve their toughness In the early Although much work has been focused on the structural and 1990s, based on nacre 's design concept, Clegg et al. proposed a simple mechanical characterization of laminated ceramic materials way to make tough ceramics. They introduced weak interfacial layers [ 22, 23, 26-35. their toughening mechanisms are still, to a large of graphite (C)between silicon carbide(Sic) layers [21]. Compared extent, unknown, in particular, at the micro/nanoscale. In this study, with monolithic ceramics, the fracture toughness of laminated Sic/c local elastic modulus and hardness of laminated Si3 N4/BN composites were measured using a nanoindenter. A custom-designed micro mechanical tester was integrated with an optical microscope and an atomic force microscope(AFM)to perform in situ three-point bending onding author. tel:+18037778011;fax:+18037770106. tests on notched Si3 Na/BN composite bend specimens where the crack initiation and propagation were imaged simultaneously with the optical microscope and AFM during bending I ading. The fracture 0928-4931/s-see front matter o 2008 Elsevier B.V. All rights reserved. doi:10.1016msec20080400Micro/nanoscale mechanical characterization and in situ observation of cracking of laminated Si3N4/BN composites Xiaodong Li a, ⁎, Linhua Zou a , Hai Ni a , Anthony P. Reynolds a , Chang-an Wang b , Yong Huang b a Department of Mechanical Engineering, University of South Carolina, 300 Main Street, Columbia, SC 29208, USA b The State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China ARTICLE INFO ABSTRACT Article history: Received 13 July 2007 Received in revised form 13 December 2007 Accepted 8 April 2008 Available online 22 April 2008 Keywords: Ceramic matrix composites (CMC) Fracture Atomic force microscopy (AFM) Nanoindentation Micro/nanoscale mechanical characterization of laminated Si3N4/BN composites was carried out by nanoindentation techniques. A custom-designed micro mechanical tester was integrated with an optical microscope and an atomic force microscope to perform in situ three-point bending tests on notched Si3N4/BN composite bend specimens where the crack initiation and propagation were imaged simultaneously with the optical microscope and atomic force microscope during bending loading. The whole fracture process was in situ captured. It was found that crack deflection was initiated/induced by the pre-existing microvoids and microcracks in BN interfacial layers. New fracture mechanisms were proposed to provide guidelines for the design of biomimetic nacre-like composites. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Among all natural biomaterials, nacre, the mother-of-pearl, which is found in the shinny interior of many mollusk shells, is one of the most attractive materials with superior mechanical properties that have fascinated scientists and engineers over the decades [1–9]. Nacre is composed of 95% inorganic aragonite (a mineral form of CaCO3) and only a small percent of organic biopolymer. It has a brick-and-mortar￾like structure with highly organized polygonal aragonite platelets of a thickness ranging 200 to 500 nm and an edge length about 5 µm sandwiched with a 5–20 nm thick organic biopolymer interlayer, which assembles the aragonite platelets together. Its laminated structure achieves a twice increase in strength and a thousand-fold increase in toughness (work of fracture) over its constituent ceramic materials [10]. Such remarkable properties have inspired chemists and materials scientists to develop biomimetic composites to reproduce nacre’s achievements [11–20]. Nacre's structure has been widely adopted as a biomimetic model in the design of ceramics to improve their toughness. In the early 1990s, based on nacre’s design concept, Clegg et al. proposed a simple way to make tough ceramics. They introduced weak interfacial layers of graphite (C) between silicon carbide (SiC) layers [21]. Compared with monolithic ceramics, the fracture toughness of laminated SiC/C composites, for crack propagation normal to the interfaces, is increased by more than four times (up to 15 MPa m1/2), and the work of fracture required to break the materials is increased substantially, by more than one hundred times (over 4 kJ/m2 ). This has been considered as a breakthrough in the development of ceramic matrix composites, although the design of laminated SiC/C composites is still at the micrometer scale level, unlike that of nacre which has the nanoscale structures. Essentially, the biomimetic design introduced here is to decrease, as much as possible, the dependence of the mechanical properties of a ceramic material on its original natural crack population, by the energy dissipation mechanism, thereby imparting flaw tolerance to an otherwise classically brittle material. Laminated Si3N4/BN composites with BN layers as weak interfacial layers are another example [22–25]. Such composites also exhibit a high fracture toughness of 28 MPa m1/2, and work of fracture over 4 kJ/m2 [26,27]. The fracture in such laminated Si3N4/BN systems is dominated by crack deflection, bridging, and through-thickness cracking. Although much work has been focused on the structural and mechanical characterization of laminated ceramic materials [22,23,26–35], their toughening mechanisms are still, to a large extent, unknown, in particular, at the micro/nanoscale. In this study, local elastic modulus and hardness of laminated Si3N4/BN composites were measured using a nanoindenter. A custom-designed micro mechanical tester was integrated with an optical microscope and an atomic force microscope (AFM) to perform in situ three-point bending tests on notched Si3N4/BN composite bend specimens where the crack initiation and propagation were imaged simultaneously with the optical microscope and AFM during bending loading. The fracture Materials Science and Engineering C 28 (2008) 1501–1508 ⁎ Corresponding author. Tel.: +1 803 777 8011; fax: +1 803 777 0106. E-mail address: lixiao@engr.sc.edu (X. Li). URL: http://www.me.sc.edu/research/nano/ (X. Li). 0928-4931/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2008.04.009 Contents lists available at ScienceDirect Materials Science and Engineering C j o u r n a l h om e p a g e : www. e l s ev i e r. c om / l o c a t e /m s e c