《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_8Mechanical properties of fibrous monolithic Si3N4

Availableonlineatwww.sciencedirect.com SCIENCE E噩≈S ournal of the European Ceramic Society 24(2004)699-703 www.elsevier.com/locate/jeurceramsoc Mechanical properties of fibrous monolithic Si3 N4/BN ceramics with different cell boundary thicknesses Young- Hag Koh*, Hae-Won Kim, Hyoun-Ee Kim chool of Materials Science and Engineering, Seoul National University, Seoul, 151-742, South Korea Received 3 February 2003: received in revised form 12 March 2003; accepted 15 March 2003 Fibrous Si3 N/BN monoliths with various cell boundary thickness were fabricated by hot-pressing. The extruded Si3 N4 polymer was dip-coated with the BN-containing slurry with a different BN concentration. a cell boundary thickness increase up to 42 um by adjusting the bn concentration from 0 to 25 wt. in the slurry, and this cell boundary thickness was found to play an important ole in determining the fracture behavior of the fibrous monoliths On increasing cell boundary thickness, the flexural strength decreased, but the apparent work-of-fracture and toughness increased remarkably due to the extensive crack interactions with weak C 2003 Elsevier Ltd. All rights reserved Keywords: BN; Composites: Si3N4; Strength; Toughness; Work-of-fracture 1. Introduction Research on fibrous Si3 N4/BN monoliths has been focused on finding the mechanism of crack deflection Silicon nitride(Si N4) is regarded as one of the most and delamination+ 12,22-24 and on the optimization of promising materials for high-temperature structural their mechanical properties through processing applications due to its excellent thermomechanical improvements. 2,25 Crack interactions are strongly properties, such as high strength, hardness, and resis- dependent on the characteristics of both the cell and the tance to creep and oxidation at elevated temperatures. cell boundaries, especially their relative elastic moduli, however, its wider utilization has been limited mainly strengths, toughnesses, and thermal expansion coeffi- ecause of its catastrophic fracture behavior. Many cients. Also, the cell boundary thickness is expected to efforts have been made to prevent the catastrophic play a key role in the fracture behavior of fibrous fracture pattern of ceramics, including fiber reinfor- monoliths cement, 3 lamination, 4-10 and the production of fibrous Therefore, in the present research, we investigated the ceramics 1-2I Among those composites, fibrous mono- effect of cell boundary thickness on mechanical proper lithic ceramics consist of a primary pl l1) ties, including strength, apparent work-of-fracture, and and a tailored phase(BN cell boundary). 2 Non-catas- fracture toughness of fibrous monolithic Si3 N4/BN trophic failures are frequently observed in fibrous composites. The thickness of the cell boundaries was monolithic materials because of crack interactions with controlled by changing the bn concentration in the e weak cell boundaries 12-21 In other words. cracks slurry used for the dip-coating process propagate through the cell boundaries, which results in a high fracture energy requirement, as is the case of crack deflection or crack delamination 2. Experimental procedures A high purity a-Si3 N4 powder (E-10, Ube Industries, 4 Corresponding author. Now with University of Michigan. Tokyo, Japan)was mixed with 5 wt. yttria(Grade F, H. C. Starck GmbH Co., Berlin, Germany) and 2 0955-2219/03/S. see front matter C 2003 Elsevier Ltd. All rights reserved. doi:10.1016S0955-221903)00266-8

Mechanical properties of fibrous monolithic Si3N4/BN ceramics with different cell boundary thicknesses Young-Hag Koh*, Hae-Won Kim, Hyoun-Ee Kim School of Materials Science and Engineering, Seoul National University, Seoul, 151-742, South Korea Received 3 February 2003; received in revised form 12 March 2003; accepted 15 March 2003 Abstract Fibrous Si3N4/BN monoliths with various cell boundary thickness were fabricated by hot-pressing. The extruded Si3N4 polymer was dip-coated with the BN-containing slurry with a different BN concentration. A cell boundary thickness increase up to 42 mm by adjusting the BN concentration from 0 to 25 wt.% in the slurry, and this cell boundary thickness was found to play an important role in determining the fracture behavior of the fibrous monoliths. On increasing cell boundary thickness, the flexural strength decreased, but the apparent work-of-fracture and toughness increased remarkably due to the extensive crack interactions with weak cell boundaries. This is related to the change in stored energy before fracture initiation and in interfacial fracture resistance. # 2003 Elsevier Ltd. All rights reserved. Keywords: BN; Composites; Si3N4; Strength; Toughness; Work-of-fracture 1. Introduction Silicon nitride (Si3N4) is regarded as one of the most promising materials for high-temperature structural applications due to its excellent thermomechanical properties, such as high strength, hardness, and resis￾tance to creep and oxidation at elevated temperatures. However, its wider utilization has been limited mainly because of its catastrophic fracture behavior. Many efforts have been made to prevent the catastrophic fracture pattern of ceramics,1,2 including fiber reinfor￾cement,3 lamination,4
10 and the production of fibrous ceramics.11
21 Among those composites, fibrous mono￾lithic ceramics consist of a primary phase (Si3N4 cell) and a tailored phase (BN cell boundary).12 Non-catas￾trophic failures are frequently observed in fibrous monolithic materials because of crack interactions with the weak cell boundaries.12
21 In other words, cracks propagate through the cell boundaries, which results in a high fracture energy requirement, as is the case of crack deflection or crack delamination. Research on fibrous Si3N4/BN monoliths has been focused on finding the mechanism of crack deflection and delamination4,12,22
24 and on the optimization of their mechanical properties through processing improvements.12,25 Crack interactions are strongly dependent on the characteristics of both the cell and the cell boundaries, especially their relative elastic moduli, strengths, toughnesses, and thermal expansion coeffi- cients.12 Also, the cell boundary thickness is expected to play a key role in the fracture behavior of fibrous monoliths. Therefore, in the present research, we investigated the effect of cell boundary thickness on mechanical proper￾ties, including strength, apparent work-of-fracture, and fracture toughness of fibrous monolithic Si3N4/BN composites. The thickness of the cell boundaries was controlled by changing the BN concentration in the slurry used for the dip-coating process. 2. Experimental procedures A high purity a-Si3N4 powder (E-10, Ube Industries, Tokyo, Japan) was mixed with 5 wt.% yttria (Grade F, H. C. Starck GmbH & Co., Berlin, Germany) and 2 0955-2219/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0955-2219(03)00266-8 Journal of the European Ceramic Society 24 (2004) 699–703 www.elsevier.com/locate/jeurceramsoc * Corresponding author. Now with University of Michigan. E-mail address: younghag@engin.umich.edu (Y.-H. Koh)

Y-H. Koh et al. Journal of the European Ceramic Society 24(2004)699-703 wt% alumina(HP-DBM, Reynolds, Bauxite, AK, 3. Results and discussion USA)as sintering additives. It was then ball-milled in ethanol with silicon nitride balls as media for 24 h. After The typical microstructure of the Si3 N4/BN fibrous milling, the mixture was dried in a rotary vacuum eva monolith is shown in Fig. 1. Flat hexagonal Si3 Na cells orator and subsequently passed through a 60-mesh were surrounded by continuous BN cell boundaries screen. The powder was mixed again with a polymer According to the XRD analyses, the Si3 N4 was com binder(methylcellulose), plasticizer(glycerol), and sol- pletely transformed into the B-phase and no reaction vent(distilled water)at room-temperature for extrusion. occurred between the Si3N4 and the BN. However, EDS The cell boundary material was prepared by mixing analyses showed that a glass phase(Si-Y-Al-N-O)was the bn powder (Grade MBN, Boride Ceramics present at the cell boundaries as well as in the cells Composites Ltd, UK) with 20 wt Al2O3 as a sintering implying that the glass phase migrated into the cell aid. A polymer binder(PVB), dispersant Menhaden boundaries during the hot-pressing fish oil), and solvent(trichloroethyleneethanol) were The thickness of the cell boundary layer was con- also added, and mixed by ball milling for 6 h. The trolled by adjusting the bn concentration in the dip amount of BN-containing material increased up to 25 coating slurry, and was measured by image analyses of wt% for different cell boundary thicknesses. The as- SEM micrographs(see Fig. 1). The boundary layer received bn powder showed platelets with a dimension thickness was found to be linearly dependent on the bn of 1-10 um and a thickness of 0.1-0.5 um. The total concentration as shown in Fig. 2. Therefore, fibrous oxygen content, including B2O3, was less than 5 wt % monoliths with desired cell boundary thicknesses were The cell was made by extruding the Si3 N4-polymer fabricated simply by changing the bn concentration in compound into 300 um diameter fibers and was the slurry. The spec coated with bn by passing through the BN-containing thickness represents the monolithic Si3N slurry, and subsequently arrayed by an automatic The cell boundary thickness was found to have a winding machine. The layered green billets were cut into strong influence on the fracture behavior of the fibrous the desired dimension, and then dried in the oven at materials. Representative stress versus deflection curves 80C for 12 h to improve the shape and strength of cell of the specimens with different cell boundary thick by hardening. The layered green billets were inserted nesses are shown in Fig. 3. Monolithic Si3 N4 exhibited into a mold of 40x40 mm and pressed at 0.5 MPa typical catastrophic failure [Fig. 3(A)] as is frequently Binder burnout of billets occurred in 900oC with heat- observed in other ceramics. However, the Si3 N4 fibrous ing rate of 2-3C/min and maintained for 3 h in flowing monolith with 18-um thick bn cell boundary was not nitrogen. Billets were hot-pressed at 1800C for I h completely fractured even after reaching critical stress, under an applied load of 30 MPa in a flowing nitrogen as shown in Fig. 3(B). The gradual fractures became atmosphere. more profound with increased cell boundary thickness Specimens for mechanical testing were machined into When the cell boundary thickness reached 37 um, the a bar shape with dimensions of 3x4x25 mm and material was fractured in a non-brittle manner [Fig. 3 ground with a 800-grit diamond wheel. The tensile side ( C)], which implied active crack interactions at the cell specimens was p with diamond slurries boundaries down to I um, and then chamfered to minimize the machining flaws. The flexural strength was measured using a four-point flexural configuration with a cross- head speed of 0.05 mm/min, and inner-and outer-spans of 10 and 20 mm, respectively. From the load versus crosshead deflection response, the apparent work-of- fracture was calculated by estimating the area under the load-deflection curve and dividing by twice the cross- sectional area of the sample. Apparent fracture toughness was measured using the single-edge-notched beam (SENB)method. A straight notch with depth and width of 1. 2 and 0.3 mm, respectively, was made at the center of the tensile surface using a thin diamond blade. The notched specimen was fractured using the four-point flexural configuration mentioned earlier. The density of the specimens was measured using the Archimedes 400m method. The specimens were characterized by scanning electron microscopy(SEM), energy dispersive spectro- Fig. 1. A representative SEM micrograph of cross-section of a fibrous metry(EDS), and X-ray diffraction(XRD) Si3N4/BN monolith

wt.% alumina (HP-DBM, Reynolds, Bauxite, AK, USA) as sintering additives. It was then ball-milled in ethanol with silicon nitride balls as media for 24 h. After milling, the mixture was dried in a rotary vacuum eva￾porator and subsequently passed through a 60-mesh screen. The powder was mixed again with a polymer binder (methylcellulose), plasticizer (glycerol), and sol￾vent (distilled water) at room-temperature for extrusion. The cell boundary material was prepared by mixing the BN powder (Grade MBN, Boride Ceramics & Composites Ltd, UK) with 20 wt.% Al2O3 as a sintering aid. A polymer binder (PVB), dispersant (Menhaden fish oil), and solvent (trichloroethylene/ethanol) were also added, and mixed by ball milling for 6 h. The amount of BN-containing material increased up to 25 wt.% for different cell boundary thicknesses. The as￾received BN powder showed platelets with a dimension of 1–10 mm and a thickness of 0.1–0.5 mm. The total oxygen content, including B2O3, was less than 5 wt.%. The cell was made by extruding the Si3N4-polymer compound into 300 mm diameter fibers and was coated with BN by passing through the BN-containing slurry, and subsequently arrayed by an automatic winding machine. The layered green billets were cut into the desired dimension, and then dried in the oven at 80 C for 12 h to improve the shape and strength of cell by hardening. The layered green billets were inserted into a mold of 4040 mm and pressed at 0.5 MPa. Binder burnout of billets occurred in 900 C with heat￾ing rate of 2–3 C/min and maintained for 3 h in flowing nitrogen. Billets were hot-pressed at 1800 C for 1 h under an applied load of 30 MPa in a flowing nitrogen atmosphere. Specimens for mechanical testing were machined into a bar shape with dimensions of 3425 mm and ground with a 800-grit diamond wheel. The tensile side of the specimens was polished with diamond slurries down to 1 mm, and then chamfered to minimize the machining flaws. The flexural strength was measured using a four-point flexural configuration with a cross￾head speed of 0.05 mm/min, and inner-and outer-spans of 10 and 20 mm, respectively. From the load versus crosshead deflection response, the apparent work-of￾fracture was calculated by estimating the area under the load-deflection curve and dividing by twice the cross￾sectional area of the sample. Apparent fracture toughness was measured using the single-edge-notched beam (SENB) method. A straight notch with depth and width of 1.2 and 0.3 mm, respectively, was made at the center of the tensile surface using a thin diamond blade. The notched specimen was fractured using the four-point flexural configuration mentioned earlier. The density of the specimens was measured using the Archimedes method. The specimens were characterized by scanning electron microscopy (SEM), energy dispersive spectro￾metry (EDS), and X-ray diffraction (XRD). 3. Results and discussion The typical microstructure of the Si3N4/BN fibrous monolith is shown in Fig. 1. Flat hexagonal Si3N4 cells were surrounded by continuous BN cell boundaries. According to the XRD analyses, the Si3N4 was com￾pletely transformed into the b-phase and no reaction occurred between the Si3N4 and the BN. However, EDS analyses showed that a glass phase (Si–Y–Al–N–O) was present at the cell boundaries as well as in the cells, implying that the glass phase migrated into the cell boundaries during the hot-pressing. The thickness of the cell boundary layer was con￾trolled by adjusting the BN concentration in the dip￾coating slurry, and was measured by image analyses of SEM micrographs (see Fig. 1). The boundary layer thickness was found to be linearly dependent on the BN concentration as shown in Fig. 2. Therefore, fibrous monoliths with desired cell boundary thicknesses were fabricated simply by changing the BN concentration in the slurry. The specimen with 0 mm-cell boundary thickness represents the monolithic Si3N4. The cell boundary thickness was found to have a strong influence on the fracture behavior of the fibrous materials. Representative stress versus deflection curves of the specimens with different cell boundary thick￾nesses are shown in Fig. 3. Monolithic Si3N4 exhibited typical catastrophic failure [Fig. 3(A)] as is frequently observed in other ceramics. However, the Si3N4 fibrous monolith with 18-mm thick BN cell boundary was not completely fractured even after reaching critical stress, as shown in Fig. 3(B). The gradual fractures became more profound with increased cell boundary thickness. When the cell boundary thickness reached 37 mm, the material was fractured in a non-brittle manner [Fig. 3 (C)], which implied active crack interactions at the cell boundaries. Fig. 1. A representative SEM micrograph of cross-section of a fibrous Si3N4/BN monolith. 700 Y.-H. Koh et al. / Journal of the European Ceramic Society 24 (2004) 699–703

Y-H. Koh et al. Journal of the European Ceramic Society 24(2004)699-703 且 三苏 BN Concentration in Slurry Iwt %l Cell Boundary Thickness lumI Fig. 2. Cell boundary thickness of the specimens as a function of BN Fig. 4. Flexural strength of the specimens as a function of cell concentration in the slurry. boundary thickness For structural ceramic applications, energy dissipa- tion capacity during fracture is one of the most impor 700(B)Thickness=18 um tant criteria. The energy dissipated by the sample during (C)Thickness=37 the non-catastrophic failure is estimated by the appar ent work-of-fracture, which corresponds to the area under the load-deflection curve after the first load drop (inelastic region). Despite the decrease in strength, the ∽ work-of-fracture increased markedly for fibrous mono- liths, as shown in Fig. 4. When the cell boundary was very thin(10 um), the specimen fractured in a brittle manner. However, with increasing the cell boundary thickness. the work-of-fracture also increased. max imum work-of-fracture in the present research was observed for the specimen with a boundary thickness of 37 um. When the cell boundary was thicker than this, the work-of-fracture decreased sharply, as shown in Fig. 5. These results suggest that the work-of-fracture is Fig 3. Flexural response of specimens with cell boundary thicknesses influenced by the strength of the material as well as by of(A)0(B)18. and (C)37 um. the crack propagation pattern. When the strength is too high, i.e. when too much strain energy is stored in the material, the crack propagates through the cells or cell The maximum stress recorded in Fig 3 corresponds boundaries without any significant crack interactions to the flexural strength of the material. The strength of On the other hand when the strength is too low, the fibrous monoliths with respect to the cell boundary work-of-fracture is low because the material can with thickness is shown in Fig 4. As expected, the strength of stand only limited stress fibrous monoliths was lower than that of monolithic The influence of cell boundary thickness on the crack Si3N4, and decreased steadily with increasing cell pattern is demonstrated in the sEM micrographs, in boundary thickness. This decrease in flexural strength Fig. 6. The fracture pattern of the fibrous monolithic was due to the increase in flaw size which might be pre- specimen with boundary thickness of 18 um is shown in sent in cell boundaries. However, the strengths were not Fig. 6(A). Unlike the monolithic materials, extensive very sensitive to the boundary thickness except for o-um crack deflections occurred at the cell boundaries. When cell boundary thickness (i.e. monolithic Si3 N4), pre- the cell boundary thickness was increased to 37 um, not sumably because the size of the critical flaws responsible only the crack deflections but also extensive crack dela for fracture was not strongly dependent on the cell minations occu rred, as illustrated in Fig. 6(B). The boundary thickness. improved work-of-fracture with increasing the cell

The maximum stress recorded in Fig. 3 corresponds to the flexural strength of the material. The strength of fibrous monoliths with respect to the cell boundary thickness is shown in Fig. 4. As expected, the strength of fibrous monoliths was lower than that of monolithic Si3N4, and decreased steadily with increasing cell boundary thickness. This decrease in flexural strength was due to the increase in flaw size which might be pre￾sent in cell boundaries. However, the strengths were not very sensitive to the boundary thickness except for 0-mm cell boundary thickness (i.e. monolithic Si3N4), pre￾sumably because the size of the critical flaws responsible for fracture was not strongly dependent on the cell boundary thickness. For structural ceramic applications, energy dissipa￾tion capacity during fracture is one of the most impor￾tant criteria. The energy dissipated by the sample during the non-catastrophic failure is estimated by the appar￾ent work-of-fracture, which corresponds to the area under the load–deflection curve after the first load drop (inelastic region). Despite the decrease in strength, the work-of-fracture increased markedly for fibrous mono￾liths, as shown in Fig. 4. When the cell boundary was very thin (10 mm), the specimen fractured in a brittle manner. However, with increasing the cell boundary thickness, the work-of-fracture also increased. Max￾imum work-of-fracture in the present research was observed for the specimen with a boundary thickness of 37 mm. When the cell boundary was thicker than this, the work-of-fracture decreased sharply, as shown in Fig. 5. These results suggest that the work-of-fracture is influenced by the strength of the material as well as by the crack propagation pattern. When the strength is too high, i.e. when too much strain energy is stored in the material, the crack propagates through the cells or cell boundaries without any significant crack interactions. On the other hand, when the strength is too low, the work-of-fracture is low because the material can with￾stand only limited stress. The influence of cell boundary thickness on the crack pattern is demonstrated in the SEM micrographs, in Fig. 6. The fracture pattern of the fibrous monolithic specimen with boundary thickness of 18 mm is shown in Fig. 6(A). Unlike the monolithic materials, extensive crack deflections occurred at the cell boundaries. When the cell boundary thickness was increased to 37 mm, not only the crack deflections but also extensive crack dela￾minations occurred, as illustrated in Fig. 6(B). The improved work-of-fracture with increasing the cell Fig. 3. Flexural response of specimens with cell boundary thicknesses of (A) 0, (B) 18, and (C) 37 mm. Fig. 2. Cell boundary thickness of the specimens as a function of BN concentration in the slurry. Fig. 4. Flexural strength of the specimens as a function of cell boundary thickness. Y.-H. Koh et al. / Journal of the European Ceramic Society 24 (2004) 699–703 701

Y-H. Koh et al. Journal of the European Ceramic Society 24(2004)699-703 10 Cell Boundary Thickness uml Cell Boundary Thickness um Fig.5. Apparent work-of-fracture of the specimens as a function of Fig. 7. Apparent fracture toughness of the specimens as a function of cell boundary thickness. cell boundary thicknes toughness"which the original intact specimen would have experienced at that load with a given notch depth ) which represents the material,s resistance to the crack propagation, of the fibrous materials is shown in sured by the senB method using a 1. 2 mm crack depth Monolithic Si3 N4 is known to have a high fracture toughness (7.5 MPa m/2)due to the interactions between the cracks and elongated grains. Even though such elongated grains were not formed, the fracture toughness of Si3 N4 was enhanced by incorpor 1 mm ating BN interlay Like the work-of-fracture. the apparent fracture toughness increased with cell bound- y ary thickness until 37 um(10 MPa m /)and decreased thereafter. This improvement was again attributed to the crack interactions (i.e. crack deflections and delani nations) with the weak cell boundaries The advantage of the fibrous monolith is most clearly illustrated by the stress-deflection curve of the notched specimens. When the specimens had a notch on the sur- face, which corresponded to a surface crack, the fibrous monolith had a similar value of critical stress at failure as its monolithic counterpart (i.e. exhibits notch-insensitiv- ty). Moreover, the fracture did not occur in a catastrophic 1 mm fashion but in a stable manner, i.e. the load bearing capacity kept increasing after the first drop of stress Fig. 6. SEM micrographs of crack propagations of the specimens with The fracture behaviors of the fibrous monolith are cell boundary thickness of (A)18 and (B)37 um strongly dependent on the properties of cell and cell boundary, such as elastic modulus, interfacial strength, boundary thickness is attributed to these combined surface flaw, and coefficient of thermal expansion effects of crack deflections and delaminations (CTE). 2 Fibrous monoliths can show two possible Fracture toughness(Fracture toughness measurement fracture behaviors depending on the magnitude of crack by SEnB method assumes elastic behaviour up to the interactions, that is, brittle and non-catastrophic failure load of failure. Therefore, the apparent fracture tough- Considering the stored energy before fracture initiation, ness for fibrous monolithus is reported as the"normal when the strength is too high, i. e when too much strain

boundary thickness is attributed to these combined effects of crack deflections and delaminations. Fracture toughness (Fracture toughness measurement by SENB method assumes elastic behaviour up to the load of failure. Therefore, the apparent fracture tough￾ness for fibrous monolithus is reported as the ‘‘normal toughness’’ which the original intact specimen would have experienced at that load with a given notch depth.), which represents the material’s resistance to the crack propagation, of the fibrous materials is shown in Fig. 7 as a function of cell boundary thickness, mea￾sured by the SENB method using a 1.2 mm crack depth. Monolithic Si3N4 is known to have a high fracture toughness (7.5 MPa m1/2) due to the interactions between the cracks and elongated grains. Even though such elongated grains were not formed, the apparent fracture toughness of Si3N4 was enhanced by incorpor￾ating BN interlayers. Like the work-of-fracture, the apparent fracture toughness increased with cell bound￾ary thickness until 37 mm (10 MPa m1/2) and decreased thereafter. This improvement was again attributed to the crack interactions (i.e. crack deflections and delami￾nations) with the weak cell boundaries. The advantage of the fibrous monolith is most clearly illustrated by the stress–deflection curve of the notched specimens. When the specimens had a notch on the sur￾face, which corresponded to a surface crack, the fibrous monolith had a similar value of critical stress at failure as its monolithic counterpart (i.e. exhibits notch-insensitiv￾ity). Moreover, the fracture did not occur in a catastrophic fashion but in a stable manner, i.e. the load bearing capacity kept increasing after the first drop of stress. The fracture behaviors of the fibrous monolith are strongly dependent on the properties of cell and cell boundary, such as elastic modulus, interfacial strength, surface flaw, and coefficient of thermal expansion (CTE).12 Fibrous monoliths can show two possible fracture behaviors depending on the magnitude of crack interactions, that is, brittle and non-catastrophic failure. Considering the stored energy before fracture initiation, when the strength is too high, i.e. when too much strain Fig. 5. Apparent work-of-fracture of the specimens as a function of cell boundary thickness. Fig. 6. SEM micrographs of crack propagations of the specimens with cell boundary thickness of (A) 18 and (B) 37 mm. Fig. 7. Apparent fracture toughness of the specimens as a function of cell boundary thickness. 702 Y.-H. Koh et al. / Journal of the European Ceramic Society 24 (2004) 699–703

Y.H. Ko et al. Journal of the European Ceramic Society 24(2004)699-703 Table I 4. Kovar. D, Thouless, M. D. and Halloran, J. H. Crack deflection Summarized mechanical properties of monolithic Si3N4 and fibrous and propagation in layered silicon nitride-boron nitride ceramics. monoliths with cell boundary thickness of 37 um J.Am. Ceran.Soc.,l998,81,10041012 5. She J, Inoue. T and Ueno, K. Damage resistance and R-curve Samples MOR(MPa) WOF (J/m2) KIc(MPa m/2) behavior of multilayer AlO/SiC ceramics. Ceramic Inter- Monolithic Sin4697±2 1 Negligible7.5±0.16 ationa,2000,26.801-805. Fibrous monolith 227-1l 1216±23310±0.19 6. Russo, C. J. Harmer. M. P. Chan. H. M. and Miller. G.A. Design of a laminated ceramic composite for improved strength and toughness. Am. Ceram. Soc. 1992.. 3396-3400 energy is stored in the material, the crack propagates 7. She, J, Inoue, T and Uneo, K, Multilayer Al,O3/SiC ceramics J. Eur. Ceram. Soc. 2000 through the cells and the cell boundaries without any sig- 20.1771-1775 nificant crack interactions. On the other hand when the 8. Liu, H and Hsu, S. M, Fracture behavior of multilayer silicon strength is too low, the work-of-fracture is low because nitride/boron nitride ceramics. J. Anm. Ceram. Soc., 1996, 79 the material can withstand only limited stress(see Fig. 5). 2-2457 fracture behavior is dependent on the stored energy 9. Ohji, T, Shigegaki, Y, Miyajima, T. and Kanzaki, S resistance behavior of multilayered silicon nitride. J. Am c before fracture initiation, as well as the change in inter Soc.1997,80,991-994. facial fracture resistance by different cell boundary thick- 10.Clegg, WJ,Kendall,X,Alford, KMcN.Button,TWand ness. Therefore, cell boundary thickness is one of the most critical factors for obtaining high wOF. The mechanical London),1990,357,455-457 properties of monolithic Si3 N4 and fibrous monolith with I1. Coblenz, w.S., Fibrous Monolithic Ceramic and Method for cell boundary thickness of 37 um are listed in Table 1. The 12. Kovar, D. King, B. H, Trice, R.w. and Halloran, J.H WOF and facture toughness increased remarkably for Fibrous monolithic ceramics. J. Am. Ceram. Soc.. 1997.80 2471- fibrous monolith with non-catastrophic failure 13. Baskaran, S. Nunn, S. D, Popovic, D. and Halloran, J Fibrous monolithic ceramics: 1. fabrication. microstructure and indentation behavior. Am. Ceram. Soc. 1993.76.2209-2216. 4. Conclusion 14. Baskaran, S and Halloran, J. H. Fibrous monolithic ceramic Il, flexural strength and fracture behavior of the silicon carbide/ brous monolithic ceramics consisting of strong graphite system. J. Am. Ceram Soc., 1993, 76, 2217-222 Si3N4 cells surrounded by weak BN cell boundaries with 15. Baskaran S and Halloran. J H. Fibrous monolithic various thicknesses were fabricated by hot-pressing Si3N4-polymer was extruded, and then coated with a 16. Baskaran. S. Nunn. S D and Halloran. J H. Fibrous mono- BN-containing slurry by dip-coating. Cell boundary thickness was controlled by adjusting the concentration of the alumina/nickel system. J. Am. Ceram. Soc., 1994, 77, 1256- of BN in the slurry. On increasing the cell boundary thickness, the density decreased and the fracture beha 17. Trice.R. W. and Halloran.J I temperature on the interfacial fracture energy of silicon nitride/ vior changed from a brittle pattern to one of the non- boron nitride fibrous monolithic ceramics.. dm. ceram. soc catastrophic failure type. Mechanical properties of these fibrous monoliths were significantly affected by these 18. Trice, R. w and Halloran, J.H. Elevated-temperature mechan- distinctive fracture behaviors. When the cell boundary ical properties of silicon nitride/ boron nitride fibrous monolithic thickness was increased, the flexural strength decreased ceramics. J. m. Ceram. Soc. 2000.83. 311-316. 19. Trice, R. W.and Halloran, J. H, Effect of sintering aid compo- due to the reduction in the volume fraction of the si3 N4 tion on the processing of Si3 N4/BN fibrous monolithic ceramics However, the apparent work-of-fracture and the frac J. Am. Ceram. Soc. 82.2943-2947 ture toughness increased significantly due to the exten 20. Hai, G, Yong. H and An, w. C, Preparation and ve crack interactions with the weak bn cell fibrous monolithic nics by in-situ synthesizing. J. Mater boundaries. these different fracture behaviors were Sci,1999,34,2455-2459 related to the energy stored before fracture initiation 21. She. J, Inoue. T, Suzuki, M. Sodeoka. S. and Ueno, K, ties and fracture behavior of fibrous AlO3/ and the change in the interfacial fracture resistance. Sic ceram ok. F. w. and Lange. F.F. Flexur of brittle multilayer materials: I, modeling. J. Am. Ceram. Soc References 1. Harmer, M. P. Chan. H. M. and Miller. G. A. Un ce between dissimilar elastic materials. Int. Solids. Struct tunities for microstructural engineering with duplex 1989,125,1053-1067 ceramic composites. J. Am. Ceram Soc., 1992, 75, I 24. Camus. G. Modeling of the Mechanical behavior and damage Evans, A G, Perspective on the development of high-toughness process of fibrous ceramic matrix composites: application to a 2- ceramics. J. Anm. Ceram. Soc. 1990. 73. 187-206. 3. Kerans, R.J. and Parthasarathy, T.A., Crack deflection in cera- 25. King, B. H, Influcence of Architecture on the Mechnaical Prop- mic composites and fiber coating design criteria. Composites. erties of Fibrous Monolithic Ceramics. PhD Thesis, University 1999,A30,521-524. Michigan. Ann Arbor. MI. 1997

energy is stored in the material, the crack propagates through the cells and the cell boundaries without any sig￾nificant crack interactions. On the other hand, when the strength is too low, the work-of-fracture is low because the material can withstand only limited stress (see Fig. 5). Fracture behavior is dependent on the stored energy before fracture initiation, as well as the change in inter￾facial fracture resistance by different cell boundary thick￾ness. Therefore, cell boundary thickness is one of the most critical factors for obtaining high WOF. The mechanical properties of monolithic Si3N4 and fibrous monolith with cell boundary thickness of 37 mm are listed in Table 1. The WOF and facture toughness increased remarkably for fibrous monolith with non-catastrophic failure. 4. Conclusion Fibrous monolithic ceramics consisting of strong Si3N4 cells surrounded by weak BN cell boundaries with various thicknesses were fabricated by hot-pressing. Si3N4-polymer was extruded, and then coated with a BN-containing slurry by dip-coating. Cell boundary thickness was controlled by adjusting the concentration of BN in the slurry. On increasing the cell boundary thickness, the density decreased and the fracture beha￾vior changed from a brittle pattern to one of the non￾catastrophic failure type. Mechanical properties of these fibrous monoliths were significantly affected by these distinctive fracture behaviors. When the cell boundary thickness was increased, the flexural strength decreased due to the reduction in the volume fraction of the Si3N4. However, the apparent work-of-fracture and the frac￾ture toughness increased significantly due to the exten￾sive crack interactions with the weak BN cell boundaries. These different fracture behaviors were related to the energy stored before fracture initiation and the change in the interfacial fracture resistance. References 1. Harmer, M. P., Chan, H. M. and Miller, G. A., Unique oppor￾tunities for microstructural engineering with duplex and laminar ceramic composites. J. Am. Ceram. Soc., 1992, 75, 1715–1728. 2. Evans, A. G., Perspective on the development of high-toughness ceramics. J. Am. Ceram. Soc., 1990, 73, 187–206. 3. Kerans, R. J. and Parthasarathy, T. A., Crack deflection in cera￾mic composites and fiber coating design criteria. Composites, 1999, A30, 521–524. 4. Kovar, D., Thouless, M. D. and Halloran, J. H., Crack deflection and propagation in layered silicon nitride–boron nitride ceramics. J. Am. Ceram. Soc., 1998, 81, 1004–1012. 5. She, J., Inoue, T. and Ueno, K., Damage resistance and R-curve behavior of multilayer Al2O3/SiC ceramics. Ceramic Inter￾national, 2000, 26, 801–805. 6. Russo, C. J., Harmer, M. P., Chan, H. M. and Miller, G. A., Design of a laminated ceramic composite for improved strength and toughness. J. Am. Ceram. Soc., 1992, 75, 3396–3400. 7. She, J., Inoue, T. and Uneo, K., Multilayer Al2O3/SiC ceramics with improved mechanical behavior. J. Eur. Ceram. Soc., 2000, 20, 1771–1775. 8. Liu, H. and Hsu, S. M., Fracture behavior of multilayer silicon nitride/boron nitride ceramics. J. Am. Ceram. Soc., 1996, 79, 2452–2457. 9. Ohji, T., Shigegaki, Y., Miyajima, T. and Kanzaki, S., Fracture resistance behavior of multilayered silicon nitride. J. Am. Ceram. Soc., 1997, 80, 991–994. 10. Clegg, W. J., Kendall, X., Alford, K.McN., Button, T. W. and Birchall, J. D., A simple way to make tough ceramics. Nature (London), 1990, 357, 455–457. 11. Coblenz, W. S., Fibrous Monolithic Ceramic and Method for Production. US Patent, No. 4, 772524, 1998. 12. Kovar, D., King, B. H., Trice, R. W. and Halloran, J. H., Fibrous monolithic ceramics. J. Am. Ceram. Soc., 1997, 80, 2471– 2487. 13. Baskaran, S., Nunn, S. D., Popovic, D. and Halloran, J. H., Fibrous monolithic ceramics: I, fabrication, microstructure, and indentation behavior. J. Am. Ceram. Soc., 1993, 76, 2209–2216. 14. Baskaran, S. and Halloran, J. H., Fibrous monolithic ceramics: II, flexural strength and fracture behavior of the silicon carbide/ graphite system. J. Am. Ceram. Soc., 1993, 76, 2217–2224. 15. Baskaran, S. and Halloran, J. H., Fibrous monolithic ceramics: III, mechanical properties and oxidation behavior of the silicon carbide/ boron nitride system. J. Am. Ceram. Soc., 1994, 77, 1249–1255. 16. Baskaran, S., Nunn, S. D. and Halloran, J. H., Fibrous mono￾lithic ceramics: IV, mechanical properties and oxidation behavior of the alumina/nickel system. J. Am. Ceram. Soc., 1994, 77, 1256– 1262. 17. Trice, R. W. and Halloran, J. H., Influence of microstructure and temperature on the interfacial fracture energy of silicon nitride/ boron nitride fibrous monolithic ceramics. J. Am. Ceram. Soc., 1999, 82, 2502–2508. 18. Trice, R. W. and Halloran, J. H., Elevated-temperature mechan￾ical properties of silicon nitride/boron nitride fibrous monolithic ceramics. J. Am. Ceram. Soc., 2000, 83, 311–316. 19. Trice, R. W. and Halloran, J. H., Effect of sintering aid compo￾sition on the processing of Si3N4/BN fibrous monolithic ceramics. J. Am. Ceram. Soc., 1999, 82, 2943–2947. 20. Hai, G., Yong, H. and An, W. C., Preparation and properties of fibrous monolithic ceramics by in-situ synthesizing. J. Mater. Sci., 1999, 34, 2455–2459. 21. She, J., Inoue, T., Suzuki, M., Sodeoka, S. and Ueno, K., Mechanical properties and fracture behavior of fibrous Al2O3/ SiC ceramics. J. Eur. Ceram. Soc., 2000, 20, 1877–1881. 22. Folsom, C. A., Zok, F. W. and Lange, F. F., Flexural properties of brittle multilayer materials: I, modeling. J. Am. Ceram. Soc., 1994, 77, 689–696. 23. He, M.-Y. and Hutchinson, J. W., Crack deflection at an inter￾face between dissimilar elastic materials. Int. J. Solids. Struct., 1989, l25, 1053–1067. 24. Camus, G., Modeling of the Mechanical behavior and damage process of fibrous ceramic matrix composites: application to a 2- D SiC/SiC. Int. J. Solids. Struct, 2000, 37, 919–942. 25. King, B. H., Influcence of Architecture on the Mechnaical Prop￾erties of Fibrous Monolithic Ceramics. PhD Thesis, University of Michigan, Ann Arbor, MI, 1997. Table 1 Summarized mechanical properties of monolithic Si3N4 and fibrous monoliths with cell boundary thickness of 37 mm Samples MOR (MPa) WOF (J/m2 ) KIC (MPa m1/2) Monolithic Si3N4 69721 Negligible 7.50.16 Fibrous monolith 22711 1216233 100.19 Y.-H. Koh et al. / Journal of the European Ceramic Society 24 (2004) 699–703 703