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:10.1016msec20080400
Micro/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-mortarlike 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
X Li et al. Materials Science and Engineering C 28(2008)1501-1508 Hokko Chemical Industry Co, Ltd, Tokyo, Japan), 2.5 wt% Al203 (99.9% purity, Beijing Chemical Plant, Beijing, China), and 1.5 wt% Mgo (99.9% purity, Beijing Hong Xing Chemical Plant, Beijing. China was milled in an ethanol medium for 24 h 20 wt% SiC whiskers(TwS 400, Hokko Chemical Industry, Japan) were first dispersed by an ultrasonic method in an ethanol medium. and then added into the milled powder mixture. The Sic whiskers have a length of about 10- 50 um and a diameter of 1 um while the diameter of the Si3 Na particles ranges from 0.2 to 5 um. After adding SiC whiskers, the milling process was repeated. The twice-milled mixture was filtered and dried, and then sieved through a 60-mesh screen. Such mixed powders were added with polyvinyl alcohol as binder, glycerine as plasticizing agent, Fig. 1. A schematic of a single-edge-notched SigNa/BN bend specimen. and paraffin as lubricant to produce uniform dough. A rolling compaction system was then used to produce green sheets with a thickness of 200 um. Such sheets were dipped into slurry containing inA ders of respective 5 wt%, 10 wt% and 50 wt% of Si3 N4 in BN to roduce interfacial layers with different interfacial toughness. The dried sheets were laminated and pressed under a pressure of O1 MP and the binder burn-out was carried out in air. Finally the sample was Soft Hard sintered, by hot pressing, at 1760C for 1.5 h, under a pressure of layer layer 22 MPa and an atmosphere of N The sintered billet was cut into bend specimens, and then ground and polished using the routine metallographic techniques with abrasive powders down to 0.025 Hm. Three kinds of specimens with Fig. 2. A representative AFM image of as-sintered SiaNa/BN composites the interfacial layers of 5 wt%, 10 wt. and 50 wt% of Si3 Na in BN are designated as BS-5, BS-10 and Bs-50, respectively. The bn interlay mechanisms are discussed in conjunction of the laminated architecture, thickness of samples BS-10 and BS-50 is about 10-30 uum. In order to hardness, elastic modulus, and energy dissipation during cracking. clearly observe crack propagation within the interlayers and across matrix layers, for sample BS-5, its Bn interlayer thickness was 2. Experimental details controlled to about 30-80 um. 2.1. Materials preparatio 2. 2. Micro/nanomechanical characterization A powder mixture of 88 wt% a-Si3N4(>99.9% purity, Fou Inder high AFM observations were made with a Veeco dimension 3100 AFM Technology Ceramic Co., Beijing, China), 8 wt%Y2O3(99.9% purity, system(Veeco Metrology Group, Santa Barbara, CA) Nanoindentation nm (b) nm(c) 50 0 200.5 10 1.5 0 1.5 0.5 1.5 0.5 0 0 ur 3500 (d) 250 B 2500 A Elastic modulus ◆2 n00000no Contact depth, nm Fig. 3. Representative AFM images: A representing a Sia N4 particle and B representing Sic whisker. (d) Nanoindentation load-displacement curves, and tic moduli and a function of indentation contact depth of the sic whiskers and SiN4 particles in Sia N4 matrix layers of specimen BS-5
mechanisms are discussed in conjunction of the laminated architecture, hardness, elastic modulus, and energy dissipation during cracking. 2. Experimental details 2.1. Materials preparation A powder mixture of 88 wt.% α-Si3N4 (N99.9% purity, Founder High Technology Ceramic Co., Beijing, China), 8 wt.% Y2O3 (N99.9% purity, Hokko Chemical Industry Co., Ltd., Tokyo, Japan), 2.5 wt.% Al2O3 (N99.9% purity, Beijing Chemical Plant, Beijing, China), and 1.5 wt.% MgO (N99.9% purity, Beijing Hong Xing Chemical Plant, Beijing, China) was milled in an ethanol medium for 24 h. 20 wt.% SiC whiskers (TWS- 400, Hokko Chemical Industry, Japan) were first dispersed by an ultrasonic method in an ethanol medium, and then added into the milled powder mixture. The SiC whiskers have a length of about 10– 50 μm and a diameter of 1 μm while the diameter of the Si3N4 particles ranges from 0.2 to 5 μm. After adding SiC whiskers, the milling process was repeated. The twice-milled mixture was filtered and dried, and then sieved through a 60-mesh screen. Such mixed powders were added with polyvinyl alcohol as binder, glycerine as plasticizing agent, and paraffin as lubricant to produce uniform dough. A rollingcompaction system was then used to produce green sheets with a thickness of 200 µm. Such sheets were dipped into slurry containing powders of respective 5 wt.%, 10 wt.% and 50 wt.% of Si3N4 in BN to produce interfacial layers with different interfacial toughness. The dried sheets were laminated and pressed under a pressure of 0.1 MPa and the binder burn-out was carried out in air. Finally the sample was sintered, by hot pressing, at 1760 °C for 1.5 h, under a pressure of 22 MPa and an atmosphere of N2. The sintered billet was cut into bend specimens, and then ground and polished using the routine metallographic techniques with abrasive powders down to 0.025 µm. Three kinds of specimens with the interfacial layers of 5 wt.%, 10 wt.% and 50 wt.% of Si3N4 in BN are designated as BS-5, BS-10 and BS-50, respectively. The BN interlayer thickness of samples BS-10 and BS-50 is about 10–30 μm. In order to clearly observe crack propagation within the interlayers and across matrix layers, for sample BS-5, its BN interlayer thickness was controlled to about 30–80 μm. 2.2. Micro/nanomechanical characterization AFM observations were made with a Veeco Dimension 3100 AFM system (Veeco Metrology Group, Santa Barbara, CA). Nanoindentation Fig. 1. A schematic of a single-edge-notched Si3N4/BN bend specimen. Fig. 2. A representative AFM image of as-sintered Si3N4/BN composites. Fig. 3. (a)-(c) Representative AFM images; A representing a Si3N4 particle and B representing SiC whisker. (d) Nanoindentation load–displacement curves, and (e) elastic moduli and hardnesses as a function of indentation contact depth of the SiC whiskers and Si3N4 particles in Si3N4 matrix layers of specimen BS-5. 1502 X. Li et al. / Materials Science and Engineering C 28 (2008) 1501–1508
(b) nm 50 50 0 0.5 2.0 0.5 1.0 15 1.0 1.0 1.5 0.5 1.5 0.5 0 3000 1500 60 Contact depth, nm Contact depth, nm Fig 4. (aHc)Representative AFM images: A representing a bn particle and B representing a Si3 N4 particle (d) Nanoindentation load-displacement curves (e) Elastic moduli and ardnesses as a function of indentation contact depth of the Bn and siN4 particles and in BN interfacial layers of specimen BS-5 tests were performed using a Troboscope nanomechanical test- tion using Oliver and Pharr method [38. The hardness is given ing system(Hysitron Inc, Minneapolis, Minnesota, USA)in con- by junction with the Veeco AFM system. The Hysitron nanoindente monitored and recorded the load and displacement of the in- H denter, a diamond Berkovich three-sided pyramid, with a force resolution of about 1 nN and displacement resolution of about where Pmax is the peak indentation load, Ac is the real contact area. 0.1 nm. A typical nanoindentation experiment consists of four The elastic modulus was calculated using the following equations subsequent steps: approaching the surface; loading to peak load holding the indenter at peak load for 5 s: finally unloading E completely. The hold step was included to avoid the influence of creep on the unloading characteristics since the unloading curve vas used to obtain the elastic modulus of a material under test [36, 37]. The indentation impressions were then imaged with Ei Es the same indenter tip. The hardness and elastic modulus were where Er is the reduced modulus, S is the contact stiffness determined determined from the load-penetration curve of the indenta- from the initial part of the unloading curve, Ei, Es v and vs are the 250 Fig 5 SEM images of three fractured Si N4/BN composite specimens: (a)BS-50, (b)BS-10, and (c)BS-5
tests were performed using a Troboscope nanomechanical testing system (Hysitron Inc., Minneapolis, Minnesota, USA) in conjunction with the Veeco AFM system. The Hysitron nanoindenter monitored and recorded the load and displacement of the indenter, a diamond Berkovich three-sided pyramid, with a force resolution of about 1 nN and displacement resolution of about 0.1 nm. A typical nanoindentation experiment consists of four subsequent steps: approaching the surface; loading to peak load; holding the indenter at peak load for 5 s; finally unloading completely. The hold step was included to avoid the influence of creep on the unloading characteristics since the unloading curve was used to obtain the elastic modulus of a material under test [36,37]. The indentation impressions were then imaged with the same indenter tip. The hardness and elastic modulus were determined from the load–penetration curve of the indentation using Oliver and Pharr method [38]. The hardness is given by H ¼ Pmax Ac ð1Þ where Pmax is the peak indentation load, Ac is the real contact area. The elastic modulus was calculated using the following equations: Er ¼ ffiffiffi p p 2 d S ffiffiffiffiffi Ac p ð2Þ 1 Er ¼ 1 m2 i Ei þ 1 m2 s Es ð3Þ where Er is the reduced modulus, S is the contact stiffness determined from the initial part of the unloading curve, Ei, Es, νi and νs are the Fig. 4. (a)-(c) Representative AFM images; A representing a BN particle and B representing a Si3N4 particle. (d) Nanoindentation load–displacement curves. (e) Elastic moduli and hardnesses as a function of indentation contact depth of the BN and Si3N4 particles and in BN interfacial layers of specimen BS-5. Fig. 5. SEM images of three fractured Si3N4/BN composite specimens: (a) BS-50, (b) BS-10, and (c) BS-5. X. Li et al. / Materials Science and Engineering C 28 (2008) 1501–1508 1503
1504 X Li et al/ Materials Science and Engineering C 28(2008)1501-1508 (f) Fig. 6. Optical images of in situ observation of fracture behavior of specimen BS-5. (a)The initial state without any cracks in front of the notch (b)A crack initiated and quick propagated through the first couple of SiNa matrix and BN interfacial layers. (c) and (d) crack deflection. (e)and (DT-shaped crack path within the bn interfacial layer. elastic moduli and Poissons ratios of the indenter and specimen, propagations of cracks were observed in situ at the microscale i where E =1040 GPa, 1=0.07 for diamond indenter [ 36-38, vs=0.27 the optical microscope and at the nanoscale using the veeco for Si3N4 and vs=0.32 for BN [23]. system. 3. Results and discussion Three-point bending tests were performed on single-edge- 3. 1. Micro/ nanoscale mechanical characterization notched bend specimens of 4 mm in width, 2. 4 mm in thickness, and 40 mm in length. The span for bending test, 21, is 34 mm, as Fig. 2 shows a representative AFM image of as-sintered Si3N4/BN hown in Fig. 1. The specimens were notched using a low speed saw composites. It can be seen that Si3 N4 matrix and bn interfacial layers with a 0.3 mm thick diamond blade. The notch length is 0.5 mm. The are laminated alternately. A slight surface height difference was found three-point bending tests were carried out with a custom designed between matrix and interfacial layers due to their hardness difference micro mechanical tester with bending function, which was The Hysitron nanoindenter was used to scan specimen surfaces integrated with both optical microscope and AFM where the notch and locate Si3 N4 matrix and BN interfacial layers In Si3N4 matrix was monitored simultaneously During bending test, deflection was layers, the addition of Sic whiskers was used to reinforce the Si3N4 gradually applied to the notched specimen and the corresponding matrix to increase its fracture toughness by crack deflection and force was recorded through the load sensor, which was calibrated bridging [27. As the hardness difference of Sic whiskers and using the indentation technique before each bending test. The particles is not very big, the polished sample surface is relatively Fig. 7. Optical image of in situ observation of a T-shaped crack path within the BN interfacial layer
elastic moduli and Poisson’s ratios of the indenter and specimen, where Ei=1040 GPa, νi= 0.07 for diamond indenter [36–38], νs= 0.27 for Si3N4 and νs= 0.32 for BN [23]. 2.3. In situ observation of cracking during bending loading Three-point bending tests were performed on single-edgenotched bend specimens of 4 mm in width, 2.4 mm in thickness, and 40 mm in length. The span for bending test, 2l, is 34 mm, as shown in Fig. 1. The specimens were notched using a low speed saw with a 0.3 mm thick diamond blade. The notch length is 0.5 mm. The three-point bending tests were carried out with a custom designed micro mechanical tester with bending function, which was integrated with both optical microscope and AFM where the notch was monitored simultaneously. During bending test, deflection was gradually applied to the notched specimen and the corresponding force was recorded through the load sensor, which was calibrated using the indentation technique before each bending test. The propagations of cracks were observed in situ at the microscale using the optical microscope and at the nanoscale using the Veeco AFM system. 3. Results and discussion 3.1. Micro/nanoscale mechanical characterization Fig. 2 shows a representative AFM image of as-sintered Si3N4/BN composites. It can be seen that Si3N4 matrix and BN interfacial layers are laminated alternately. A slight surface height difference was found between matrix and interfacial layers due to their hardness difference. The Hysitron nanoindenter was used to scan specimen surfaces and locate Si3N4 matrix and BN interfacial layers. In Si3N4 matrix layers, the addition of SiC whiskers was used to reinforce the Si3N4 matrix to increase its fracture toughness by crack deflection and bridging [27]. As the hardness difference of SiC whiskers and Si3N4 particles is not very big, the polished sample surface is relatively flat. Fig. 6. Optical images of in situ observation of fracture behavior of specimen BS-5. (a) The initial state without any cracks in front of the notch. (b) A crack initiated and quickly propagated through the first couple of Si3N4 matrix and BN interfacial layers. (c) and (d) Crack deflection. (e) and (f) T-shaped crack path within the BN interfacial layer. Fig. 7. Optical image of in situ observation of a T-shaped crack path within the BN interfacial layer. 1504 X. Li et al. / Materials Science and Engineering C 28 (2008) 1501–1508
X Li et al./ Materials Science and Engineering C 28(2008)1501-1508 microvoids and microcracks initiated / induced crack deflection into 隧 the Bn interfacial layers. However, why did not the crack deflection occur at the interface between the Si3N4 matrix and Bn interfacial ayers? The AFM observation(Fig 9)shows the existence of a strong bonding between the matrix and interfacial layers. This may be why the crack deflected and propagated within the interfacial layer, rathe than at the interface between the si3N4 matrix and Bn interfacial layers. Bridging ligaments were observed along the crack paths (Fig. 10). As the crack propagated within the interfacial layer, some of the bridging ligaments were broken(Fig. 11). As the crack propagated in the BN interlayer, the pre-existing microvoids and microcracks in Fig. 8. AFM images of pre-existing(a) microvoids and(b)microcracks in the BN front of the crack tip along the crack propagation direction coalesced (Fig. 12). Once the crack in the interfacial layer reached its limit at both sides, instead of kinking out of the interfacial layer, through-thickness We distinguish them from the size and aspect ratio of whiskers and cracking occurred in the following Si3 Na matrix layer( Fig. 5C). Then articles. Fig 3 shows the representative AFM images, nanoindenta- the crack propagated in an unsteady-state manner with crack tion load-displacement curves, and elastic moduli and hardnesses of deflection and propagation within BN interfacial layers, kinking ou the Si3N4 particles and Sic whiskers in Si3Na matrix layers. Post- of the BN interfacial layers, and through-thickness cracking(Fig. 13) nanoindentation AFM imaging shows that the Si3N4 particles exhibit To study the effect of loading span on the fracture behavior of a larger indent than the Sic whiskers, as shown in Fig. 3b and c. This laminated SiaNa/BN composites, three-point bending tests were also indicates that the hardness of the Si3N4 particles is lower than that of carried out on specimen BS-5 with a loading span of 16 mm. It was the Sic whiskers. The measured elastic moduli and hardnesses are found that shorter span leads to a fracture process different from the also plotted as a function of indentation contact depth, as shown in longer one( fig. 14). In the shorter span test, the crack deflected for a Fig 3d and e. Both elastic modulus and hardness of the Sic whiskers shorter distance and then kinked out of the interfacial layer with are higher than those of the Si3Na particles. The average elastic further bending loading modulus and hardness for the Si3N4 parti re 207.6 GPa and In order to understand the nature of the toughening mechanisms 21.9 GPa, and for the Sic whiskers are 239.4 GPa and 28.9 GPa, of laminated Si3N4/BN composites and the relationship between respectively cracking and the corresponding characteristics in the bending load In BN interfacial layers, the addition of Si3 Na particles was used to displacement curve, a three-point bending load-displacement curve increase the fracture toughness of the Bn interfacial layers and corresponding crack paths are sketched in Figs. 15 and 16. Based [23, 26, 27, 29, 30]. Nanoindentations were carried out on both BN and on this study and previous work [30]. four stages in the bending Si3N4 particles in BN interfacial layers, as shown in Fig. 4. The Bn fracture of laminated Si3N4/BN composites appear to exist: (1)linear particles show lower elastic modulus and hardness than the Si3N4 elastic response;(2) crack initiation; (3) crack deflection and particles. The average elastic modulus and hardness for the bn propagation in a stable manner within the interfacial layers: (4) articles are 146.2 GPa and 16.3 GPa, and for the Si3N4 particles are crack propagation in an unstable manner in both matrix and 206.8 GPa and 20.5 GPa, respectively. The Si3N4 particles in both interfacial layers In the first stage, the bending load-displacement matrix and interfacial layers agree well in elastic modulus and curve exhibits a linear elastic response. No obvious changes were observed in front of the notch In the second stage, there exists a load 3. 2. In situ observation of cracking during bending loading Fig 5 shows the SEM images of three fractured Si3N4/E specimens. Specimen BS-50 exhibits a straight through-thickness crack without any crack deflection( Fig 5a). Specimen BS-10 shows an very limited within BN interfacial layers. Unlike specimens BS-50 and BS-10, crack deflection was found in a steady-state manner in pecimen BS-5. The crack was first deflected and propagated within a BN interfacial layer for a quite long distance. Then crack deflection occurred instantaneously within several other BN interfacial layers in an unstable manner, associated with crack kinking and through thickness cracking, rather than layer by layer as described in the racture model for ceramic laminates in bending [31. 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. Below we limit our focus to specimen BS-5 Fig. 6 shows the results of in situ optical microscope observation of the fracture behavior of specimen BS-5. Fig. 6a shows the initial state of specimen BS-5 without any cracks in front of the notch. with increasing bending deflection, a crack initiated and quickly propa- gated through the first couple of Si3 Na matrix and BN interfacial layers. Then the crack was deflected with a T-shaped crack path within the BN interfacial layer(Fig. 7). The AFM image of BN interfacial layers shows that microvoids and microcracks had already pre-exist Fig 9 (a) Low and(b) high magnification AFM images of the interface between the before bending loading(Fig. 8). It is believed that these pre-existing Si N4 matrix and the BN interfacial layers in specimen BS-5
We distinguish them from the size and aspect ratio of whiskers and particles. Fig. 3 shows the representative AFM images, nanoindentation load–displacement curves, and elastic moduli and hardnesses of the Si3N4 particles and SiC whiskers in Si3N4 matrix layers. Postnanoindentation AFM imaging shows that the Si3N4 particles exhibit a larger indent than the SiC whiskers, as shown in Fig. 3b and c. This indicates that the hardness of the Si3N4 particles is lower than that of the SiC whiskers. The measured elastic moduli and hardnesses are also plotted as a function of indentation contact depth, as shown in Fig. 3d and e. Both elastic modulus and hardness of the SiC whiskers are higher than those of the Si3N4 particles. The average elastic modulus and hardness for the Si3N4 particles are 207.6 GPa and 21.9 GPa, and for the SiC whiskers are 239.4 GPa and 28.9 GPa, respectively. In BN interfacial layers, the addition of Si3N4 particles was used to increase the fracture toughness of the BN interfacial layers [23,26,27,29,30]. Nanoindentations were carried out on both BN and Si3N4 particles in BN interfacial layers, as shown in Fig. 4. The BN particles show lower elastic modulus and hardness than the Si3N4 particles. The average elastic modulus and hardness for the BN particles are 146.2 GPa and 16.3 GPa, and for the Si3N4 particles are 206.8 GPa and 20.5 GPa, respectively. The Si3N4 particles in both matrix and interfacial layers agree well in elastic modulus and hardness. 3.2. In situ observation of cracking during bending loading Fig. 5 shows the SEM images of three fractured Si3N4/BN composite specimens. Specimen BS-50 exhibits a straight through-thickness crack without any crack deflection (Fig. 5a). Specimen BS-10 shows an unstable crack deflection (Fig. 5b). The crack deflection, however, was very limited within BN interfacial layers. Unlike specimens BS-50 and BS-10, crack deflection was found in a steady-state manner in specimen BS-5. The crack was first deflected and propagated within a BN interfacial layer for a quite long distance. Then crack deflection occurred instantaneously within several other BN interfacial layers in an unstable manner, associated with crack kinking and throughthickness cracking, rather than layer by layer as described in the fracture model for ceramic laminates in bending [31]. 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. Below we limit our focus to specimen BS-5. Fig. 6 shows the results of in situ optical microscope observations of the fracture behavior of specimen BS-5. Fig. 6a shows the initial state of specimen BS-5 without any cracks in front of the notch. With increasing bending deflection, a crack initiated and quickly propagated through the first couple of Si3N4 matrix and BN interfacial layers. Then the crack was deflected with a T-shaped crack path within the BN interfacial layer (Fig. 7). The AFM image of BN interfacial layers shows that microvoids and microcracks had already pre-existed before bending loading (Fig. 8). It is believed that these pre-existing microvoids and microcracks initiated/induced crack deflection into the BN interfacial layers. However, why did not the crack deflection occur at the interface between the Si3N4 matrix and BN interfacial layers? The AFM observation (Fig. 9) shows the existence of a strong bonding between the matrix and interfacial layers. This may be why the crack deflected and propagated within the interfacial layer, rather than at the interface between the Si3N4 matrix and BN interfacial layers. Bridging ligaments were observed along the crack paths (Fig. 10). As the crack propagated within the interfacial layer, some of the bridging ligaments were broken (Fig. 11). As the crack propagated in the BN interlayer, the pre-existing microvoids and microcracks in front of the crack tip along the crack propagation direction coalesced (Fig. 12). Once the crack in the interfacial layer reached its limit at both sides, instead of kinking out of the interfacial layer, through-thickness cracking occurred in the following Si3N4 matrix layer (Fig. 5C). Then the crack propagated in an unsteady-state manner with crack deflection and propagation within BN interfacial layers, kinking out of the BN interfacial layers, and through-thickness cracking (Fig. 13). To study the effect of loading span on the fracture behavior of laminated Si3N4/BN composites, three-point bending tests were also carried out on specimen BS-5 with a loading span of 16 mm. It was found that shorter span leads to a fracture process different from the longer one (Fig. 14). In the shorter span test, the crack deflected for a shorter distance and then kinked out of the interfacial layer with further bending loading. In order to understand the nature of the toughening mechanisms of laminated Si3N4/BN composites and the relationship between cracking and the corresponding characteristics in the bending load– displacement curve, a three-point bending load–displacement curve and corresponding crack paths are sketched in Figs. 15 and 16. Based on this study and previous work [30], four stages in the bending fracture of laminated Si3N4/BN composites appear to exist: (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. In the first stage, the bending load–displacement curve exhibits a linear elastic response. No obvious changes were observed in front of the notch. In the second stage, there exists a load Fig. 8. AFM images of pre-existing (a) microvoids and (b) microcracks in the BN interfacial layers in specimen BS-5. Fig. 9. (a) Low and (b) high magnification AFM images of the interface between the Si3N4 matrix and the BN interfacial layers in specimen BS-5. X. Li et al. / Materials Science and Engineering C 28 (2008) 1501–1508 1505
X Li et al/ Materials Science and Engineering C 28(2008)1501-1508 o Crack propagation direction 10. Optical image of in situ observation of bridging and crack deflection within the BN interfacial layer in specimen BS-5. Fig. 11. Optical image of in situ observation of broken bridging ligaments in the BN interfacial layer in specimen BS-5 2.5 Fig 12 AFM image of the area in front of the crack tip along the crack propagation direction in the bn interfacial layer in specimen BS-5, showing coalescence of the pre-exiting t Delamination cracking Crack kinking and propagation Fig. 13. Opt ges of in sin observation of crack propagation in an unsteady-state manner in specimen BS-5 with crack deflection, kinking out of the BN interfacial layers, and
Fig. 13. Optical images of in situ observation of crack propagation in an unsteady-state manner in specimen BS-5 with crack deflection, kinking out of the BN interfacial layers, and through-thickness cracking. Fig. 10. Optical image of in situ observation of bridging and crack deflection within the BN interfacial layer in specimen BS-5. Fig. 11. Optical image of in situ observation of broken bridging ligaments in the BN interfacial layer in specimen BS-5. Fig. 12. AFM image of the area in front of the crack tip along the crack propagation direction in the BN interfacial layer in specimen BS-5, showing coalescence of the pre-exiting microvoids and microcracks. 1506 X. Li et al. / Materials Science and Engineering C 28 (2008) 1501–1508
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)67-69
drop 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 microcracks in BN interfacial layers arrest the major crack. The crack reorients and propagates within BN interfacial layers, forming a Tshaped crack. In the final stage, crack propagates unstably with crack deflection, crack kinking, and through-thickness cracking, consequently 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
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