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 rolling￾compaction 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