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-edge￾notched 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