Z Krstic, V.D. Krstic/Joumal of the European Ceramic Sociery 28(2008)1723-1730 1725 3 Mould =1: Plaster Substrate Relative 101214161820 Fig. 5. Change of Y 10 wt. Si3 N4 in the interface(SN-(BN+SN). Fig. 3. Schematic view of slip-casting process. thickness of the previous Si3 N4 layers determines the thick Ltd, Japan), with Cu Ko radiation and a scanning rate of ness of the next layer. As the layer thickness becomes larger the 2°/min. removal of the water through the wall of the green body becomes The Youngs modulus of the samples was measured using an more difficult and, for a given casting time, the layer thickness impulse-excitation of vibration technique( Grindo-Sonic MK5, becomes smaller and smaller. J.W. Lemmens, Inc. St Louis, MO, USA)according to ASTM The effect of the number of layers on sintered density(rela- andards C1259-94. This method covers a dynamic determina- tive density)andelastic modulus( Youngs modulus)is presented tion of the elastic properties of materials at ambient temperature. in Fig. 5 for laminates whose interface consists of 50 wt%BN and 50 wt %Al2O3(SN-(BN+Al2O3))and in Fig. 6 for lam 3. Results and discussion nates whose interface consists of 90 wt. BN and 10 wt %o Si3N4 (SN-(BN+SN)). Both Figs. 5 and 6 show a decrease in rela One of the objectives of this work was to design and fabri- tive density with the number of layers. This decrease in densi cate laminated structures possessing no direction of easy crack is caused by the presence of an increased level of porosity in the propagation, termed self-sealed laminates. Fig. 4 shows the interfacial layers and is also due to the addition of a lower den- cross-sections of the cast rectangular forms. The structure is sity BN component. The highest density is found with samples produced by slip-casting and pressureless sintering of alternate containing the smallest number of layers and the lowest density layers of Si3 N4 and BN. As shown in Fig 4, the structure consists was found with sample containing the largest number of lay of homogeneous Si3Na layers(grey phase) separated by the uni- ers. The effect of the number of layers and porosity on Youngs form bN (white phase) interface. No delamination is observed modulus is also shown in Figs. 5 and 6. As mentioned before during sintering and cooling to room temperature an increase in the number of layers decreases the relative den- In these samples the largest thickness of a Si3N4 layer was sity, which in turn, lowers the Youngs modulus of the structure approximately 1000 um and the smallest was 50 um. The The decrease in density (or increase in porosity) has a strong Fig 4. SEM micrographs of self-sealed Si3 N4/BN laminated structures with(a)6 and(b)13 Si3 N4 layers.Z. Krstic, V.D. Krstic / Journal of the European Ceramic Society 28 (2008) 1723–1730 1725 Fig. 3. Schematic view of slip-casting process. Ltd., Japan), with Cu K radiation and a scanning rate of 2◦/min. The Young’s modulus of the samples was measured using an impulse-excitation of vibration technique (Grindo-Sonic MK5, J.W. Lemmens, Inc. St. Louis, MO, USA) according to ASTM standards C 1259-94. This method covers a dynamic determina￾tion of the elastic properties of materials at ambient temperature. 3. Results and discussion One of the objectives of this work was to design and fabri￾cate laminated structures possessing no direction of easy crack propagation, termed self-sealed laminates. Fig. 4 shows the cross-sections of the cast rectangular forms. The structure is produced by slip-casting and pressureless sintering of alternate layers of Si3N4 and BN. As shown in Fig. 4, the structure consists of homogeneous Si3N4 layers (grey phase) separated by the uni￾form BN (white phase) interface. No delamination is observed during sintering and cooling to room temperature. In these samples the largest thickness of a Si3N4 layer was approximately 1000 m and the smallest was ∼50m. The Fig. 5. Change of Young’s modulus and relative density for samples containing 10 wt.% Si3N4 in the interface (SN − (BN + SN)). average thickness of the BN-based interface was approximately 12–15m. Due to the nature of the slip-casting process, the thickness of the previous Si3N4 layers determines the thick￾ness of the next layer. As the layer thickness becomes larger the removal of the water through the wall of the green body becomes more difficult and, for a given casting time, the layer thickness becomes smaller and smaller. The effect of the number of layers on sintered density (rela￾tive density) and elastic modulus (Young’s modulus) is presented in Fig. 5 for laminates whose interface consists of 50 wt.% BN and 50 wt.% Al2O3 (SN − (BN + Al2O3)) and in Fig. 6 for lami￾nates whose interface consists of 90 wt.% BN and 10 wt.% Si3N4 (SN − (BN + SN)). Both Figs. 5 and 6 show a decrease in rela￾tive density with the number of layers. This decrease in density is caused by the presence of an increased level of porosity in the interfacial layers and is also due to the addition of a lower den￾sity BN component. The highest density is found with samples containing the smallest number of layers and the lowest density was found with sample containing the largest number of lay￾ers. The effect of the number of layers and porosity on Young’s modulus is also shown in Figs. 5 and 6. As mentioned before, an increase in the number of layers decreases the relative den￾sity, which in turn, lowers the Young’s modulus of the structure. The decrease in density (or increase in porosity) has a strong Fig. 4. SEM micrographs of self-sealed Si3N4/BN laminated structures with (a) 6 and (b) 13 Si3N4 layers.