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-5We distinguish them from the size and aspect ratio of whiskers and particles. Fig. 3 shows the representative AFM images, nanoindenta￾tion load–displacement curves, and elastic moduli and hardnesses of the Si3N4 particles and SiC whiskers in Si3N4 matrix layers. Post￾nanoindentation 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 through￾thickness 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 propa￾gated 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