Availableonlineatwww.sciencedirect.com Science Direct E噩≈RS ELSEVIER Joumal of the European Ceramic Society 28(2008)1723-1730 www.elsevier.com/locate/jeurceramsoc Youngs modulus, density and phase composition of pressureless sintered self-sealed Si3N/bn laminated structures Z. Krstic*V D. Krstic Centre for Manufacturing of Advanced Ceramics and Nanomaterials, Nicol Hall, Queen's University, Kingston, Ontario, Canada, K7L 3N6 Received 2 July 2007; received in revised form 1 November 2007; accepted 10 November 2007 Available online 14 February 2008 Abstract New self-sealed Si3N/BN-based laminated structures have been produced by a modified slip-casting process in a form of square cross-sections varying the number of layers from 3 to 20 and their thickness from 70 to 1000 um. The composition of Si, N4 layers consists of 7 wt. Y203(yttria) and 3 wt% Al,O3(alumina). The BN-based interfaces consist of 90 wt % o Bn and 10wt% Si3 Na in Sn-(bn +sN) laminates and 50 wt BN and 50 wt% Al3O3 in SN-(BN+ Al,O3) laminates. e. Si3N4/BN-based laminates were densified by pressureless sintering at temperatures ranging from 1720 to 1820C for I h under static N2 gas Dsphere The highest density was achieved with samples having 3 Si3 N4 layers in SN-(BN+SN) laminates and 5 Si3 N4 layers in SN-(BN+Al2O3) with an average layer thickness of 260 and 320 um, respectively. Also, it was found that samples with the highest density exhibit the highest Youngs modulus of 315 GPa in SN-(BN +SN) laminates and 320 GPa in SN +(BN+ Al,O3)laminates. The microstructure of the(BN+Al2O3) interface consists mainly of YAG phase with BN and Si,N4 as minor phases, while the microstructure of the(Bn + SN) interface consists of BN and Si3 N4 as major phases. A much higher level of porosity was observed in(Bn+SN) interfaces than in(Bn +Al2O3)interfaces Crown Copyright o 2007 Published by Elsevier Ltd. All rights reserved. Keywords: Laminates; Si3 N4/BN; Self-sealed structure; Interface: Youngs modulus 1. Introduction The weak interface serves to deflect the propagating crack and lower its stress intensity. Traditionally, the crack deflec Silicon nitride(Si4N4) is one of the most promising ceramic tion is achieved by incorporating fibers into ceramic matrices.2,3 materials for structural application because of its good mechan- However, the fabrication of fiber-reinforced ceramics is expen- ical properties over a wide temperature range, good wear and sive and often unsuccessful in achieving required mechanical corrosive resistance, low density and good thermal shock resis- properties. Another way of preventing catastrophic failure is tance. However, in spite of good properties, mentioned above, the use of porous or intermittent interlayers, first studied by low reliability of this ceramic is considered to be the major Atkins, then followed by Clegg, Zhang and Krstic'and Tu et limiting factor for wider use as a structural material al.. One of the requirements for achieving high properties of the One way of preventing catastrophic failure of monolithic laminate is that the interface material must be chemically and ceramics component is to design a structure with dense and physically compatible with the lamina, so that they can be both strong layers(e.g. Si3N4, SiC, Al2O3, ZrOz, etc. separated co-fired and used at an elevated temperature without any unde by weak or porous layers of the same or different materials sired reactions and delamination. This delamination, caused by (Fig. 1) internal stresses during sintering, can be prevented by incor- porating porosity in the interlayers and by creating self-sealed structure. Lately, a pre-determined level of interlayer porosity to ensure crack deflection for the situation where residual stresses Corresponding author. were not present, has been investigated and reported by Blanks E-mail address: Krsticz@ post. queensu ca(Z. Krstic) et al. and Davis et al. The process of adding Si3N4 fibers to 0955-2219/S-see front matter. Crown Copyright o 2007 Published by Elsevier Ltd. All rights reserved doi: 10.1016/j-jeurceramsoc 2007.1 1.020
Available online at www.sciencedirect.com Journal of the European Ceramic Society 28 (2008) 1723–1730 Young’s modulus, density and phase composition of pressureless sintered self-sealed Si3N4/BN laminated structures Z. Krstic ∗, V.D. Krstic Centre for Manufacturing of Advanced Ceramics and Nanomaterials, Nicol Hall, Queen’s University, Kingston, Ontario, Canada, K7L 3N6 Received 2 July 2007; received in revised form 1 November 2007; accepted 10 November 2007 Available online 14 February 2008 Abstract New self-sealed Si3N4/BN-based laminated structures have been produced by a modified slip-casting process in a form of square cross-sections, varying the number of layers from 3 to 20 and their thickness from 70 to 1000 m. The composition of Si3N4 layers consists of 7 wt.% Y2O3 (yttria) and 3 wt.% Al2O3 (alumina). The BN-based interfaces consist of 90 wt.% BN and 10 wt.% Si3N4 in SN − (BN + SN) laminates and 50 wt.% BN and 50 wt.% Al3O3 in SN − (BN + Al2O3) laminates. Si3N4/BN-based laminates were densified by pressureless sintering at temperatures ranging from 1720 to 1820 ◦C for 1 h under static N2 gas atmosphere. The highest density was achieved with samples having 3 Si3N4 layers in SN − (BN + SN) laminates and 5 Si3N4 layers in SN − (BN + Al2O3) with an average layer thickness of 260 and 320 m, respectively. Also, it was found that samples with the highest density exhibit the highest Young’s modulus of 315 GPa in SN − (BN + SN) laminates and 320 GPa in SN + (BN + Al2O3) laminates. The microstructure of the (BN + Al2O3) interface consists mainly of YAG phase with BN and Si3N4 as minor phases, while the microstructure of the (BN + SN) interface consists of BN and Si3N4 as major phases. A much higher level of porosity was observed in (BN + SN) interfaces than in (BN + Al2O3) interfaces. Crown Copyright © 2007 Published by Elsevier Ltd. All rights reserved. Keywords: Laminates; Si3N4/BN; Self-sealed structure; Interface; Young’s modulus 1. Introduction Silicon nitride (Si4N4) is one of the most promising ceramic materials for structural application because of its good mechanical properties over a wide temperature range, good wear and corrosive resistance, low density and good thermal shock resistance. However, in spite of good properties, mentioned above, low reliability of this ceramic is considered to be the major limiting factor for wider use as a structural material. One way of preventing catastrophic failure of monolithic ceramic’s component is to design a structure with dense and strong layers (e.g. Si3N4, SiC, Al2O3, ZrO2, etc.) separated by weak or porous layers of the same or different materials (Fig. 1).1 ∗ Corresponding author. E-mail address: Krsticz@post.queensu.ca (Z. Krstic). The weak interface serves to deflect the propagating crack and lower its stress intensity. Traditionally, the crack deflection is achieved by incorporating fibers into ceramic matrices.2,3 However, the fabrication of fiber-reinforced ceramics is expensive and often unsuccessful in achieving required mechanical properties.4 Another way of preventing catastrophic failure is the use of porous or intermittent interlayers, first studied by Atkins,5 then followed by Clegg,6 Zhang and Krstic7 and Tu et al.8. One of the requirements for achieving high properties of the laminate is that the interface material must be chemically and physically compatible with the lamina, so that they can be both co-fired and used at an elevated temperature without any undesired reactions and delamination. This delamination, caused by internal stresses during sintering, can be prevented by incorporating porosity in the interlayers and by creating self-sealed structure. Lately, a pre-determined level of interlayer porosity to ensure crack deflection for the situation where residual stresses were not present, has been investigated and reported by Blanks et al.9 and Davis et al.10. The process of adding Si3N4 fibers to 0955-2219/$ – see front matter. Crown Copyright © 2007 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2007.11.020
Z Krstic, V.D. Krstic/Jounal of the European Ceramic Sociery 28 (2008)1723-1730 Monolithic Plate-form Self-sealed Ceramic laminated ceramic aminated ceramic laminated ceramic An easy direction of An easy direction of crack propagaton Direction of no easy An easy direction of crack propagatio Fig. 2. New design concept involving transformation from plate-form to the Fig.1. Plate-form laminated ceramic with two directions of easy crack propa- self-sealed laminated struc This paper reports the results of a novel concept of the self sealed Si3 N4/BN laminated structure produced by slip-casting Si3N4 powder to produce porous interlayers has been reported and pressureless sintering. The research is focused on the new by Ohji et al. However, the major problem associated with structural design of laminates by creating concentric tree trunk a porous interlayer laminate is the difficulty in controlling the structures of rectangular cross-section and to examine the effect level of porosity and shape of the pores in the path of the growing of the number of layers and their chemical composition on interfacial crack Young modulus, density and phase compositions of sintered In the last decade, efforts have been directed to developing a laminates plate-form laminated ceramic composite with a weak interface such as SiC/graphite, which exhibits fracture toughness 2. Experimental procedure as high as 14 MPam. Also, many investigators centered their attention on designing and processing plate-form lam Commercially available, high purity a-Si3N4 powder (UBe inated structures such as zirconia/zirconia (ZrO2/ZrO2), 2 E-10)was used as a starting material. Also, commercially avail- alumina/monazite (Al2O3/LaPO4), 3 silicon nitride/boron able sub-micron size Al2O3 powder(A-16, Alcoa Chemical itride(Si3 N,/BN ), 4B-SiALON/Si3 N4, Al2O3/SiC, Bauxite, USA)and high-purity sub-micron size Y2O3 powder Al,O3/ZrO2, 6 mulite/Al2O3 7and Al,O3/Al TiOs. 8 (99.99%)(Alpha Aesar)were used as sintering additives Hexi So far, the only fabrication techniques for laminates have onal bn powder produced by Carborundum Company grac been hot-pressing and hot-isostatic pressing. Recently, Wang et HPP-325 was used to create the weak interfaces. al.produced planar laminates with apparent fracture tough- Si3 N4 water-based slurry was prepared by mixing 90 wt% ness as high as 15. 1 MPam"by controlling composition Si3N4, 7 wt %o Y203 and 3 wt %o Al2O3 with a solid-to-liquid and structure in Si3N4/BN laminates. Even higher fracture ratio of 30/70 by volume. BN slurry was prepared by mixin toughness of 28.1 MPa m was obtained by adding secondary BN and Al2O3, and BN and Si3N4 in a liquid consisting of reinforcement such as SiC whiskers to the BN interface Impres- ethyl alcohol and water with an ethyl alcohol-to-water ratio of sive increases in fracture strength and work of fracture of 30/70. The bn interface consisted of 90 wt %o bn and 10 wt%0 >600 MPa and 8000 J/'m, respectively, have been achieved Si3 N4(SN-(Bn+SN) and 50 wt %o BN and 50 wt %o Al2O3 by hot-pressing of Si3 N4/BN laminates, as reported by Kovar et (SN-(BN+ Al2O3). Both Bn slurries had a solid-to-liquid ratio of 5/95 by volume The major disadvantage of these plate-form laminates is that Si3 N4/BN-based laminates were slip-cast alternately with they possess two delamination directions of easy crack propaga- Si3 N4 layers and BN-based interfaces in plaster of Paris moulds tion which limit their wide application as structural components with casting chambers of rectangular/square cross-sections of (Fig. 2). One possible solution to the intrinsic delamination 8 mm x 8 mm and a length of 60mm(Fig 3). All laminates problem associated with the plate-form ceramic laminates is to were fabricated in such a way that the outside layer and core are imitate natural materials, such as the ring structure of wood trunk Si3N4 in three-dimensions. In this paper, a new so called self-sealed Pressureless sintering was carried out using wo to zero when the structure of a laminate changes from a temperatures ranging from 1740 to 1800C for I h under static plate-form to a self-sealed rectangular structure(Fig. 2). 2122 N2 gas atmosphere Furthermore, self-sealed laminated structures can be effec The densities of sintered Si3N4/BN laminates are given tively produced by employing traditional ceramic processing terms of relative density, which is the ratio of the measured den techniques such as modified slip casting, which allows varying sity of a sample to its theoretical density. The measured density the casting time, changing the composition and controlling the was determined by the water displacement method viscosity of the slurries in order to get different numbers of layers Phase analysis was conducted on polished samples using with different thicknesse n X-ray diffractometer (Model IGC-5, Rigaku Denki C
1724 Z. Krstic, V.D. Krstic / Journal of the European Ceramic Society 28 (2008) 1723–1730 Fig. 1. Plate-form laminated ceramic with two directions of easy crack propagation. Si3N4 powder to produce porous interlayers has been reported by Ohji et al.11. However, the major problem associated with a porous interlayer laminate is the difficulty in controlling the level of porosity and shape of the pores in the path of the growing interfacial crack. In the last decade, efforts have been directed to developing a plate-form laminated ceramic composite with a weak interface such as SiC/graphite,6 which exhibits fracture toughness as high as 14 MPa m1/2. 7 Also, many investigators centered their attention on designing and processing plate-form laminated structures such as zirconia/zirconia (ZrO2/ZrO2),12 alumina/monazite (Al2O3/LaPO4),13 silicon nitride/boron nitride (Si3N4/BN),14 -SiALON/Si3N4, 15 Al2O3/SiC,4 Al2O3/ZrO2, 16 mulite/Al2O3 17 and Al2O3/Al2TiO5. 18 So far, the only fabrication techniques for laminates have been hot-pressing and hot-isostatic pressing. Recently, Wang et al.19 produced planar laminates with apparent fracture toughness as high as 15.1 MPa m1/2 by controlling composition and structure in Si3N4/BN laminates. Even higher fracture toughness of 28.1 MPa m1/2 was obtained by adding secondary reinforcement such as SiC whiskers to the BN interface. Impressive increases in fracture strength and work of fracture of >600 MPa and ∼8000 J/m2, respectively, have been achieved by hot-pressing of Si3N4/BN laminates, as reported by Kovar et al.20. The major disadvantage of these plate-form laminates is that they possess two delamination directions of easy crack propagation which limit their wide application as structural components (Fig. 2). One possible solution to the intrinsic delamination problem associated with the plate-form ceramic laminates is to imitate natural materials, such as the ring structure of wood trunk in three-dimensions. In this paper, a new so called self-sealed structure decreases the potential delamination directions from two to zero when the structure of a laminate changes from a plate-form to a self-sealed rectangular structure (Fig. 2).21,22 Furthermore, self-sealed laminated structures can be effectively produced by employing traditional ceramic processing techniques such as modified slip casting, which allows varying the casting time, changing the composition and controlling the viscosity of the slurries in order to get different numbers of layers with different thicknesses. Fig. 2. New design concept involving transformation from plate-form to the self-sealed laminated structure. This paper reports the results of a novel concept of the selfsealed Si3N4/BN laminated structure produced by slip-casting and pressureless sintering. The research is focused on the new structural design of laminates by creating concentric tree trunk structures of rectangular cross-section and to examine the effect of the number of layers and their chemical composition on Young’s modulus, density and phase compositions of sintered laminates. 2. Experimental procedure Commercially available, high purity -Si3N4 powder (UBE E-10) was used as a starting material. Also, commercially available sub-micron size Al2O3 powder (A-16, Alcoa Chemical, Bauxite, USA) and high-purity sub-micron size Y2O3 powder (99.99%) (Alpha Aesar) were used as sintering additives. Hexagonal BN powder produced by Carborundum Company grade HPP-325 was used to create the weak interfaces. Si3N4 water-based slurry was prepared by mixing 90 wt.% Si3N4, 7 wt.% Y2O3 and 3 wt.% Al2O3 with a solid-to-liquid ratio of 30/70 by volume. BN slurry was prepared by mixing BN and Al2O3, and BN and Si3N4 in a liquid consisting of ethyl alcohol and water with an ethyl alcohol-to-water ratio of 30/70. The BN interface consisted of 90 wt.% BN and 10 wt.% Si3N4 (SN − (BN + SN)) and 50 wt.% BN and 50 wt.% Al2O3 (SN − (BN + Al2O3)). Both BN slurries had a solid-to-liquid ratio of 5/95 by volume. Si3N4/BN-based laminates were slip-cast alternately with Si3N4 layers and BN-based interfaces in plaster of Paris moulds with casting chambers of rectangular/square cross-sections of 8 mm × 8 mm and a length of ∼60 mm (Fig. 3). All laminates were fabricated in such a way that the outside layer and core are Si3N4. Pressureless sintering was carried out using a graphite resistance furnace (Vacuum Industries, Somerville, MA, USA) at temperatures ranging from 1740 to 1800 ◦C for 1 h under static N2 gas atmosphere. The densities of sintered Si3N4/BN laminates are given in terms of relative density, which is the ratio of the measured density of a sample to its theoretical density. The measured density was determined by the water displacement method. Phase analysis was conducted on polished samples using an X-ray diffractometer (Model IGC-5, Rigaku Denki Co.
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 determination of the elastic properties of materials at ambient temperature. 3. Results and discussion One of the objectives of this work was to design and fabricate 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 uniform 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 thickness 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 (relative 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 laminates whose interface consists of 90 wt.% BN and 10 wt.% Si3N4 (SN − (BN + SN)). Both Figs. 5 and 6 show a decrease in relative 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 density 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 layers. 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 density, 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.
Z Krstic, V.D. Krstic/Jounal of the European Ceramic Sociery 28 (2008)1723-1730 33 审审1 293 当汤 :m-+26 E SN-BN 681012141618 Number of layers Number of layers Fig. 6. Change of Youngs modulus and relative density for samples whic Fig. 7. Change of Youngs modulus with the number of Si3 N4 lay containing 50 wt %o BN +50wt %o Al2O3 the interface(SN-(BN+Al O3)) lining 50 wt% BN +50wt% AlO3 designated as SN-(BN +Al2O3)and 10 wt. Si3 N4 in Bn designated as SN-(BN+SN) effect on Youngs modulus in that any small increase in porosity BN and the other consists of 10 wt. Si3 N4 and 90 wt. BN. It (equivalent to decrease in density) leads to a large decrease in can be seen from Fig. 7 that the laminated structure with inter- Youngs modulus faces containing 50 wt% BN and 50 wt %o Al2O3 has a higher Besides the porosity, BNis known to have low Youngs modu- overall Youngs modulus than the laminated structure with an lus (33.86 GPa, in the c direction and 85.95 GPain the a direction interfaces containing 10 wt Si3N4 and 90 wt. BN (at room temperature)and this is the main reason for the reduc- Much larger reduction in Youngs modulus tion of Youngs modulus of the whole system. Fig. 7 shows SN-(BN+SN)laminates(Fig. 7)is caused by the apparent the change of Youngs modulus with the number of layers for level of porosity in the interfacial layers. Careful examination two different interlayer compositions. In one set of samples, the of the interfaces in Fig. 8 revealed the presence of a much weak interfacial layer consists of 50 wt %o Al2O3 and 50 wt %o larger amount of porosity in layers containing BN and Si3n4 Fig. 8. Fracture surface of the interfaces (a)in SN-(BN+SN) and(b)in SN-(Bn +Al O3)laminated structures
1726 Z. Krstic, V.D. Krstic / Journal of the European Ceramic Society 28 (2008) 1723–1730 Fig. 6. Change of Young’s modulus and relative density for samples which containing 50 wt.% BN + 50 wt.% Al2O3 the interface (SN − (BN + Al2O3)). effect on Young’s modulus in that any small increase in porosity (equivalent to decrease in density) leads to a large decrease in Young’s modulus. Besides the porosity, BN is known to have low Young’s modulus (33.86 GPa, in the c direction and 85.95 GPa in the a direction (at room temperature)) and this is the main reason for the reduction of Young’s modulus of the whole system. Fig. 7 shows the change of Young’s modulus with the number of layers for two different interlayer compositions. In one set of samples, the weak interfacial layer consists of 50 wt.% Al2O3 and 50 wt.% Fig. 7. Change of Young’s modulus with the number of Si3N4 layers for interlayer containing 50 wt.% BN + 50 wt.% Al2O3 designated as SN − (BN + Al2O3) and 10 wt.% Si3N4 in BN designated as SN − (BN + SN). BN and the other consists of 10 wt.% Si3N4 and 90 wt.% BN. It can be seen from Fig. 7 that the laminated structure with interfaces containing 50 wt.% BN and 50 wt.% Al2O3 has a higher overall Young’s modulus than the laminated structure with an interfaces containing 10 wt.% Si3N4 and 90 wt.% BN. Much larger reduction in Young’s modulus in SN − (BN + SN) laminates (Fig. 7) is caused by the apparent level of porosity in the interfacial layers. Careful examination of the interfaces in Fig. 8 revealed the presence of a much larger amount of porosity in layers containing BN and Si3N4. Fig. 8. Fracture surface of the interfaces (a) in SN − (BN + SN) and (b) in SN − (BN + Al2O3) laminated structures
Z Krstic, V.D. Krstic/Joumal of the European Ceramic Sociery 28(2008)1723-1730 1727 3 SiN, layers 0 0000 10 Si, N, layers 人 00000000000 9 Si,N, layers 7 SiN, layer 7 Si,N, layers B-SA N,B-s, 400队5 0000 S-S,N, S,N, 2 Theta(degree) 2 Theta(degree) Fig.9.X-ray diffraction pattems of SN-(BN+Al203)laminates with different Fig. 10. X-ray diffraction patterns of SN-(BN+SN) laminates with different number of layers sintered at 1760C. number of layers sintered at 1760( Laminates with interfacial layers containing 10 wt %o Si3N4 in with an increase in the number of Si3 N4 layers. It is found that a Bn appear to be less effective in promoting densification and certain amount of Al2O3 present in the interface diffuses into the results in a lower relative density and consequently a lower Si3N4 layers and some Y2O3(after decomposition of B-phase) diffuses into BN-based interfacial layers. Finally, both Y203 and Figs. 9 and 10 show the X-ray patterns of cross-section Al2 O3 react with each other to form the YAG phase. An increase of the Si3N4/BN-based laminates having different numbers in the number of Si3 N4 layers decreases the thickness of the lay of Si3N4 layers for compositions SN-(BN+Al2O3) and ers and increases the number of the BN-based interfaces. The SN-(BN+SN), respectively. According to the X-ray data, consequence of this is that the diffusion path for mass transport the microstructure of the Si3N4 layers consists of a major p- within the layers at the sintering temperature becomes shorter Si3N4 phase and two minor phases, Y2SiAIOSN(B-phase) and allowing easier diffusion and partitioning of Y203 and Al2O3 Y2035Al2O3(YAG). It can also be inferred from Figs. 9 and 10 within the layers that the amount of YAG phase increases continuously with the The X-ray diffraction patterns(Fig. 11)of the BN-based increase in the number of Si3N4 layers from 5 to 13 and 4 to interfaces, when layers were broken/separated along the Bn 16. It can also be noticed that the increase in the amount of interface, show that, in addition to the presence of B-Si3N4 YAG phase takes place at the expense of B-phase, indicating phase, three other phases are present: BN, B-phase and YAO that the Y2SiAIOSN(known as B-Phase)decomposes to YAG Again, the content of the YAG phase increases with an increase and Sin of the number of Si3N4 layers for up to 9 layers. When the It is also noticed that the entire amount of a-Si3N4 phase is number of layers is more than 9, the YAG phase becomes transformed to B-Si3N4 phase in both compositions. It appears the major phase in the interfaces. This behaviour is consis that the addition 10 wt %o of oxides(7 wt %o of Y203 and 3 wt tent with the ability of the Y203 component to diffuse fron of Al2O3) provides sufficient amount of liquid phase for com- the Si3N4 layer to the interfacial layer and react with Al2O plete a to B-Si3 N4 phase transformation. The slow cooling rate As pointed out earlier, as the number of layers increases, of 10C/min used in the present experiment was another favor- their thickness decreases creating shorter diffusion paths for ble factor which contributed to complete decomposition of Y2O3 and faster reaction with Al2 O3. The result of this is B-phase at temperatures between 1000 and 1100C. Another the larger amount of YAG phase formed in the interfacial interesting finding of this work is the increase of YAG phase layer. The decomposition of B-phase can be expressed by the
Z. Krstic, V.D. Krstic / Journal of the European Ceramic Society 28 (2008) 1723–1730 1727 Fig. 9. X-ray diffraction patterns of SN − (BN + Al2O3) laminates with different number of layers sintered at 1760 ◦C. Laminates with interfacial layers containing 10 wt.% Si3N4 in BN appear to be less effective in promoting densification and results in a lower relative density and consequently a lower Young’s modulus. Figs. 9 and 10 show the X-ray patterns of cross-section of the Si3N4/BN-based laminates having different numbers of Si3N4 layers for compositions SN − (BN + Al2O3) and SN − (BN + SN), respectively. According to the X-ray data, the microstructure of the Si3N4 layers consists of a major - Si3N4 phase and two minor phases, Y2SiAlO5N (B-phase) and 3Y2O35Al2O3 (YAG). It can also be inferred from Figs. 9 and 10 that the amount of YAG phase increases continuously with the increase in the number of Si3N4 layers from 5 to 13 and 4 to 16. It can also be noticed that the increase in the amount of YAG phase takes place at the expense of B-phase, indicating that the Y2SiAlO5N (known as B-phase) decomposes to YAG and Si3N4. It is also noticed that the entire amount of -Si3N4 phase is transformed to -Si3N4 phase in both compositions. It appears that the addition 10 wt.% of oxides (7 wt.% of Y2O3 and 3 wt.% of Al2O3) provides sufficient amount of liquid phase for complete to -Si3N4 phase transformation. The slow cooling rate of 10 ◦C/min. used in the present experiment was another favorable factor which contributed to complete decomposition of B-phase at temperatures between 1000 and 1100 ◦C. Another interesting finding of this work is the increase of YAG phase Fig. 10. X-ray diffraction patterns of SN − (BN + SN) laminates with different number of layers sintered at 1760 ◦C. with an increase in the number of Si3N4 layers. It is found that a certain amount of Al2O3 present in the interface diffuses into the Si3N4 layers and some Y2O3 (after decomposition of B-phase) diffuses into BN-based interfacial layers. Finally, both Y2O3 and Al2O3 react with each other to form the YAG phase. An increase in the number of Si3N4 layers decreases the thickness of the layers and increases the number of the BN-based interfaces. The consequence of this is that the diffusion path for mass transport within the layers at the sintering temperature becomes shorter allowing easier diffusion and partitioning of Y2O3 and Al2O3 within the layers. The X-ray diffraction patterns (Fig. 11) of the BN-based interfaces, when layers were broken/separated along the BN interface, show that, in addition to the presence of -Si3N4 phase, three other phases are present: BN, B-phase and YAG. Again, the content of the YAG phase increases with an increase of the number of Si3N4 layers for up to 9 layers. When the number of layers is more than 9, the YAG phase becomes the major phase in the interfaces. This behaviour is consistent with the ability of the Y2O3 component to diffuse from the Si3N4 layer to the interfacial layer and react with Al2O3. As pointed out earlier, as the number of layers increases, their thickness decreases creating shorter diffusion paths for Y2O3 and faster reaction with Al2O3. The result of this is the larger amount of YAG phase formed in the interfacial layer. The decomposition of B-phase can be expressed by the
Z Krstic, V.D. Krstic/Jounal of the European Ceramic Sociery 28 (2008)1723-1730 13 Si N, laye wvw B-4 BsiN sIn, layers 6 SiN, layers B-Phase-Y. SiAJON YAG3Yo5A°3 5001 BNB-SiN, ss,N B-SiN, B-SI B-Si 50 2 Theta(degree) 2 Theta(degree) Fig. 11. X-ray diffraction patterns of BN interface with a different number of Fig. 12. X-ray diffraction patterns of BN-based interface with different number Si3N4 layers for SN-(BN+Al2O3) laminated structures sintered at 1760C. for Si3 N4 layers of SN-(BN+SN)laminated structures sintered at 1760C eacti small undetected amount of YAG may be present in the inter 10Y2 SiAIOSN→6Y2O3+3￥2O35Al2O3+Y2SigO6 face. The absence of YAG phase makes BN-based interfaces in SN-(BN+SN) laminated structures more porous than in +Si2N2O2↑+4N2↑ () SN-(BN+ Al203)laminates. Considering that the bN phase In turn, Y2O3 formed by Reaction(1)diffuses into the BN- has a low diffusivity, its sintering will be limited and it is xpected that a much larger level of porosity will remain in based interface layer and reacts with Al2 O3 to form YAG phase these interfacial layers than in the BN+ Al2O3 interfacial lay- according to the reaction: ers.Support for this statement is found from the results obtained 3Y203+5Al2O3=3Y203. 5Al20 (2) from on Youngs modulus measurement presented in Figs 5-7 which show a much larger drop in Youngs modulus for sam- The support for this reaction mechanism is found in Fig. 11 ples with BN+ Si3 N4 interfacial layers than for samples with which shows a continuous decrease of B-phase peak with the BN+AlO3 interfacial layers. The results are in line with pre- increase in the number of Si3N4 layers along with the increase diction that a sharp reduction in Youngs modulus is associated of the intensity of X-ray peaks generated by the YAG phase with porosity Quite different X-ray diffraction patterns were produced in Initial mechanical properties measurements revealed a sig- the laminates containing BN+ Si3N4 interfacial layers(Fig. 12). nificant increase in apparent fracture toughness compared to Besides Si3N4 phase, only bn phase is detected and its amount the monolithic terpart. The measurements also showed increases with an increase of the number of Si3N4 layers. the effect of the Si3N4 layers number on the apparent frac When the number of Si3N4 layers exceeds 10, the bn phase ture toughness, fracture strength and work of fracture(Table 1) becomes the major phase in the system. Expectedly, no YAG The highest apparent fracture toughness of 22 MPa mand phase was observed in any of the laminates. Since the inter- fracture strength of 470 MPa, respectively, were found for 7 face in the SN-(Bn+ SN)laminates consists of 90wt. Bn Si3 N4 layers in SN-(BN+Al2O3) laminates. Somewhat lower and 10 wt% Si N, there is no Y2O3 or Al2O3 present to form apparent fracture toughness of 19.5 MPa/2 but higher frac YAG phase. Although the X-ray diffraction did not detect the ture strength of 515 MPa were found for 4 Si3N4 layers in the presence of YAG phase it should be emphasized that some SN-(BN+SN) laminates
1728 Z. Krstic, V.D. Krstic / Journal of the European Ceramic Society 28 (2008) 1723–1730 Fig. 11. X-ray diffraction patterns of BN interface with a different number of Si3N4 layers for SN − (BN + Al2O3) laminated structures sintered at 1760 ◦C. reaction: 10Y2SiAlO5N → 6Y2O3 + 3Y2O3·5Al2O3 + Y2Si8O6 + Si2N2O2↑ + 4N2↑ . (1) In turn, Y2O3 formed by Reaction (1) diffuses into the BNbased interface layer and reacts with Al2O3 to form YAG phase according to the reaction: 3Y2O3 + 5Al2O3 = 3Y2O3·5Al2O3. (2) The support for this reaction mechanism is found in Fig. 11 which shows a continuous decrease of B-phase peak with the increase in the number of Si3N4 layers along with the increase of the intensity of X-ray peaks generated by the YAG phase. Quite different X-ray diffraction patterns were produced in the laminates containing BN + Si3N4 interfacial layers (Fig. 12). Besides Si3N4 phase, only BN phase is detected and its amount increases with an increase of the number of Si3N4 layers. When the number of Si3N4 layers exceeds ∼10, the BN phase becomes the major phase in the system. Expectedly, no YAG phase was observed in any of the laminates. Since the interface in the SN − (BN + SN) laminates consists of 90 wt.% BN and 10 wt.% Si3N4 there is no Y2O3 or Al2O3 present to form YAG phase. Although the X-ray diffraction did not detect the presence of YAG phase it should be emphasized that some Fig. 12. X-ray diffraction patterns of BN-based interface with different number for Si3N4 layers of SN − (BN + SN) laminated structures sintered at 1760 ◦C. small undetected amount of YAG may be present in the interface. The absence of YAG phase makes BN-based interfaces in SN − (BN + SN) laminated structures more porous than in SN − (BN + Al2O3) laminates. Considering that the BN phase has a low diffusivity, its sintering will be limited and it is expected that a much larger level of porosity will remain in these interfacial layers than in the BN + Al2O3 interfacial layers. Support for this statement is found from the results obtained from on Young’s modulus measurement presented in Figs. 5–7 which show a much larger drop in Young’s modulus for samples with BN + Si3N4 interfacial layers than for samples with BN + Al2O3 interfacial layers. The results are in line with prediction that a sharp reduction in Young’s modulus is associated with porosity. Initial mechanical properties measurements revealed a significant increase in apparent fracture toughness compared to the monolithic counterpart. The measurements also showed the effect of the Si3N4 layers number on the apparent fracture toughness, fracture strength and work of fracture (Table 1). The highest apparent fracture toughness of ∼22 MPa m1/2 and fracture strength of 470 MPa, respectively, were found for 7 Si3N4 layers in SN − (BN + Al2O3) laminates. Somewhat lower apparent fracture toughness of 19.5 MPa m1/2 but higher fracture strength of 515 MPa were found for 4 Si3N4 layers in the SN − (BN + SN) laminates
Z Krstic, V.D. Krstic/Joumal of the European Ceramic Sociery 28(2008)1723-1730 1729 The effect of the number of Si3 Na layers on mechanical properties of the self-sealed Si3N4/BN laminated structures Number of Si3 N4 layers Materials S Work of Apparent fracture Strength toughness(MPam )(MPa) fracture(kJ/m) toughness(MPam2) (MPa) fracture(kJ/m) 10 310 Monolithic si N4~92 92 In the both laminates, there is an increase in the apparent fracture toughness and strength with number of Si3N4 layers followed by a decrease in the both by further increase in the ayers numbers. It is believed that this increase of apparent fracture toughness and strength is attributed to the crack interac tion(crack deflections)with the BN interphase. After deflection along the weak bn interphase(Fig. 13), crack propagation occurred in both radial and axial directions( Fig. 14). This led to an increase in apparent fracture toughness and fracture strength Also, Table 1 reveals the effects of the number of Si3N4 layers on the work of fracture. the highest work of fracture of 230 and 320kJ/m, respectively, are observed in SN-(BN-Al203)and in SN-(Bn+SN) laminates 4. Conclusion Fig. 13. A crack propagation through the Si3 N4 layer and deflection at BN Self-sealed Si3 N4/BN-based laminated structures with uare cross-section having different number of with different thickness have been fabricated for the fir using modified slip-casting method and densified by pressure- less sintering process either delamination nor peeling during sintering or cool ing from sintering temperature were observed in either SN-(BN+ Al2O3)or SN-(BN +SN) laminated structures. The highest apparent density of over 3.22 g/em' was achieved in SN-(BN+ Al2O3) with 5 Si3N4 layers, and the highest apparent density of 3.20 g/cm was achieved in SN-(BN+SN) both I SN-(BN+AlO3)and sn-(Bn+SN), it was found that the samples with highest apparent density were also samples with highest Youngs modulus with values of over 310 and 315 GPa. The microstructure of Si3 N4 layers in both laminates con- sists of B-Si3N4 phase, YAG phase and smaller amount of B-phase. The microstructure of the BN-based interface in ly of YAG phase with BN and Si3N4 as minor phases. Low level of porosity w observed in this interfaces The microstructure of the bn-based interfaceinSN-(BN+SN)laminates consists of BN and Si3N4 Fig. 14. Radial and axial direction of a crack propagatio as major phases without the presence of YAG phase. A much
Z. Krstic, V.D. Krstic / Journal of the European Ceramic Society 28 (2008) 1723–1730 1729 Table 1 The effect of the number of Si3N4 layers on mechanical properties of the self-sealed Si3N4/BN laminated structures Number of Si3N4 layers Materials SN − (BN + Al2O3) SN − (BN + SN) Apparent fracture toughness (MPa m1/2) Strength (MPa) Work of fracture (kJ/m3) Apparent fracture toughness (MPa m1/2) Strength (MPa) Work of fracture (kJ/m3) 3 – – – 17.2 337 150 4 10.4 100 100 19.5 515 225 5 11.2 290 125 16 412 279 7 22.3 470 200 16.3 375 300 9 16 367 230 13.2 300 320 11 12.8 322 150 13 220 246 13 10 310 140 – – – Monolithic Si3N4 ∼9.2 ∼790 80 ∼9.2 ∼790 80 Fig. 13. A crack propagation through the Si3N4 layer and deflection at BN interface. Fig. 14. Radial and axial direction of a crack propagation22. In the both laminates, there is an increase in the apparent fracture toughness and strength with number of Si3N4 layers followed by a decrease in the both by further increase in the layers numbers. It is believed that this increase of apparent fracture toughness and strength is attributed to the crack interaction (crack deflections) with the BN interphase. After deflection along the weak BN interphase (Fig. 13), crack propagation occurred in both radial and axial directions (Fig. 14). This led to an increase in apparent fracture toughness and fracture strength. Also, Table 1 reveals the effects of the number of Si3N4 layers on the work of fracture. The highest work of fracture of 230 and 320 kJ/m3, respectively, are observed in SN − (BN-Al2O3) and in SN − (BN + SN) laminates. 4. Conclusion Self-sealed Si3N4/BN-based laminated structures with square cross-section having different number of Si3N4 layers with different thickness have been fabricated for the first time using modified slip-casting method and densified by pressureless sintering process. Neither delamination nor peeling during sintering or cooling from sintering temperature were observed in either SN − (BN + Al2O3) or SN − (BN + SN) laminated structures. The highest apparent density of over 3.22 g/cm3 was achieved in SN − (BN + Al2O3) with 5 Si3N4 layers, and the highest apparent density of 3.20 g/cm3 was achieved in SN − (BN + SN) laminates in samples having 3 Si3N4 layers. In both laminates, SN − (BN + Al2O3) and SN − (BN + SN), it was found that the samples with highest apparent density were also samples with highest Young’s modulus with values of over 310 and 315 GPa, respectively. The microstructure of Si3N4 layers in both laminates consists of -Si3N4 phase, YAG phase and smaller amount of B-phase. The microstructure of the BN-based interface in SN − (BN + Al2O3) laminates consists mainly of YAG phase with BN and Si3N4 as minor phases. Low level of porosity was observed in this interfaces. The microstructure of the BN-based interface in SN − (BN + SN) laminates consists of BN and Si3N4 as major phases without the presence of YAG phase. A much
Z Krstic, V.D. Krstic/Jounal of the European Ceramic Sociery 28 (2008)1723-1730 higher level of porosity was found in these interfaces than in 10. Davis,J.B,Kristofferson,A,Carlstorm, Eand Clegg,WJ.Fabrication BN+Al2O3 interface. The level of porosity in both BN-based and crack deflection in ceramic laminates with porous interlayers. J. Am. interfaces is controlled by the amount of YAG phase present in Ceran.Soc.,2000,83,2369-2374 the interface. The interface with higher content of YAG phase 1. OhjL, T, Shigegaki, Y, Miyajima, T. and Kanzaki, S. Fracture resistance contains lower level porosity, as well as higher Youngs modu- 991-994 12. Sanchez-Herencia, A.J., Pascual. C, He, J and Lange, F. F. ZrO/ZrO Due to the crack interaction with bn interface in radial and layered composites for crack bifurcation. J. Am. Ceram. Soc., 1999,8 axial direction, very high apparent fracture toughness, strength 13. Mawdsley, I. Kover, D. and Halloran, I. w. Fracture behavior of alu. mina/monazite multilayer laminates. J Am. Ceram. Soc., 2000, 83, 802-808 14. Liu, H and Hsu, S M., Fracture behavior of multilayer silicon nitride/boron References nitride ceramics. J. Am. Ceram. Soc.. 1996.79.2452-2457 5. Shigegaki, Y, Brito, M. E, Hirao, K, Toriyama, M. and Kanzaki. S,B- 1. Clegg, w.J., Kandall, K Alford, N M, Birchall, D and Button, T w,A SiALON-silicon nitride multilayered composites. J Am Ceram Soc. 1997 mple way to make tough ceramics. Nature, 1990, 347, 455-457 80.2624-2628. 2. Kovar,D. King. B.H. Trice.R. W and Halloran,J W. fibrous monolithic 16. Plucknett, K P, Caceres, C H, Hughers, C and wilkinson, D S, Process- m.SoC,1997,80,2471-248 ing of tape cast laminates prepared from fine alumina/zirconia powders 3. Koh, Y. H, Kim, H. W. and Kim, H. E, Mechanical properties of Am Ceram.Soc.,1994,77,2145-2153 three-layered monolithic silicon nitride-fibrous silicon nitride/boron nitride 17. Katsuki, H and Hirata, Y, Coat of alumina sheet with needle-like mulite. J. Ceram Soc. Jpn., 1990, 98, 1114-1119 4.She,J,Inoe,T and Ueno,K, Multilayer Al203/SiC ceramics with improved 18. Russo, C J, Harmer, M P, Chan, M. H and Miller, G. A Design of mechanical behavior. J. Eur Ceram Soc. 2000. 20. 1771-1775 laminated ceramic composites for improved strength and toughness. J Am. 5. Atkins, A. G, Imparting h and toughness to brittle composites. Ceran.Soc.,1992,75,3396-3400 Nature,1974,252,116-11 19. Wang, C, Huang, Y, Zan, Q, Zou, L and Cai, S, Control of composition 6. Clegg, w.J., The fracture and failure of laminar ceramic composites. Acta and structure in laminated silicon nitride/boron nitride composites. J. Am. Metall. Mater,1992,40,3085-3093 Ceram.Soc,2002,85,2457-2461 7.Zhang, L and Krstic, VD, High toughness silicon carbide/graphite laminar 20. Kovar, D Thouless, M. D and Halloran, J.W., Crack deflection and propa. composite by slip casting. Theor Appl. Fract. Mech., 1995, 24, 13-19 gation in layered silicon nitride/boron nitride ceramics. J. Am. Ceram Soc. 8. Tu, w.C., Lange, F. F and Evans, A. G, Concept for a damage tolerant 998,81,1004-1012. omposite with"strong" interfaces. J. Am. Ceram Soc., 1996, 79(2), 41 21. Yu. Z.B. and Krstic. V.D. Fabrication and characterization of laminated SiC ceramics with self-sealed ring structure. J Mater. Sci., 2003. 38.4735-4738 9. Blanks.K. S. Kristofferson. A. Carlstorm. E and Clegg. W. J.J. Enr 22. Yu, Z, Krstic, Z and Krstic, V. D, Laminated Si3 N4/SiC composites with Cerm.Soc.,1998,18,1945-1951 self-sealed structure. Key Eng. Mater, 2005, 280-283, 1873-1876
1730 Z. Krstic, V.D. Krstic / Journal of the European Ceramic Society 28 (2008) 1723–1730 higher level of porosity was found in these interfaces than in BN + Al2O3 interface. The level of porosity in both BN-based interfaces is controlled by the amount of YAG phase present in the interface. The interface with higher content of YAG phase contains lower level porosity, as well as higher Young’s modulus. Due to the crack interaction with BN interface in radial and axial direction, very high apparent fracture toughness, strength and work of fracture are found in these laminates. References 1. Clegg, W. J., Kandall, K., Alford, N. M., Birchall, D. and Button, T. W., A simple way to make tough ceramics. Nature, 1990, 347, 455–457. 2. Kovar, D., King, B. H., Trice, R. W. and Halloran, J. W., Fibrous monolithic ceramics. J. Am. Ceram. Soc., 1997, 80, 2471–2487. 3. Koh, Y. H., Kim, H. W. and Kim, H. E., Mechanical properties of three-layered monolithic silicon nitride-fibrous silicon nitride/boron nitride monolith. J. Am. Ceram. Soc., 2002, 85, 2840–2842. 4. She, J., Inoe, T. and Ueno, K., Multilayer Al2O3/SiC ceramics with improved mechanical behavior. J. Eur. Ceram. Soc., 2000, 20, 1771–1775. 5. Atkins, A. G., Imparting strength and toughness to brittle composites. Nature, 1974, 252, 116–118. 6. Clegg, W. J., The fracture and failure of laminar ceramic composites. Acta Metall. Mater., 1992, 40, 3085–3093. 7. Zhang, L. and Krstic, V. D., High toughness silicon carbide/graphite laminar composite by slip casting. Theor. Appl. Fract. Mech., 1995, 24, 13–19. 8. Tu, W. C., Lange, F. F. and Evans, A. G., Concept for a damage tolerant composite with “strong” interfaces. J. Am. Ceram. Soc., 1996, 79(2), 417– 424. 9. Blanks, K. S., Kristofferson, A., Carlstorm, E. and Clegg, W. J., J. Eur. Ceram. Soc., 1998, 18, 1945–1951. 10. Davis, J. B., Kristofferson, A., Carlstorm, E. and Clegg, W. J., Fabrication and crack deflection in ceramic laminates with porous interlayers. J. Am. Ceram. Soc., 2000, 83, 2369–2374. 11. Ohji, T., Shigegaki, Y., Miyajima, T. and Kanzaki, S., Fracture resistance behavior of multilayered silicon nitride. J. Am. Ceram. Soc., 1997, 80, 991–994. 12. Sanchez-Herencia, A. J., Pascual, C., He, J. and Lange, F. F., ZrO2/ZrO2 layered composites for crack bifurcation. J. Am. Ceram. Soc., 1999, 82, 1512–1518. 13. Mawdsley, J., Kover, D. and Halloran, J. W., Fracture behavior of alumina/monazite multilayer laminates. J. Am. Ceram. Soc., 2000, 83, 802–808. 14. Liu, H. and Hsu, S. M., Fracture behavior of multilayer silicon nitride/boron nitride ceramics. J. Am. Ceram. Soc., 1996, 79, 2452–2457. 15. Shigegaki, Y., Brito, M. E., Hirao, K., Toriyama, M. and Kanzaki, S., - SiALON-silicon nitride multilayered composites. J. Am. Ceram. Soc., 1997, 80, 2624–2628. 16. Plucknett, K. P., Caceres, C. H., Hughers, C. and Wilkinson, D. S., Processing of tape cast laminates prepared from fine alumina/zirconia powders. J. Am. Ceram. Soc., 1994, 77, 2145–2153. 17. Katsuki, H. and Hirata, Y., Coat of alumina sheet with needle-like mulite. J. Ceram. Soc. Jpn., 1990, 98, 1114–1119. 18. Russo, C. J., Harmer, M. P., Chan, M. H. and Miller, G. A., Design of laminated ceramic composites for improved strength and toughness. J. Am. Ceram. Soc., 1992, 75, 3396–3400. 19. Wang, C., Huang, Y., Zan, Q., Zou, L. and Cai, S., Control of composition and structure in laminated silicon nitride/boron nitride composites. J. Am. Ceram. Soc., 2002, 85, 2457–2461. 20. Kovar, D., Thouless, M. D. and Halloran, J. W., Crack deflection and propagation in layered silicon nitride/boron nitride ceramics. J. Am. Ceram. Soc., 1998, 81, 1004–1012. 21. Yu, Z. B. and Krstic, V. D., Fabrication and characterization of laminated SiC ceramics with self-sealed ring structure. J. Mater. Sci., 2003, 38, 4735–4738. 22. Yu, Z., Krstic, Z. and Krstic, V. D., Laminated Si3N4/SiC composites with self-sealed structure. Key Eng. Mater., 2005, 280–283, 1873–1876.