E≈S Journal of the European Ceramic Society 20(2000)1877-1881 Mechanical properties and fracture behavior of fibrous Al2O3/SiC ceramIcs Jihong She *, Takahiro Inoue, Masato Suzuki, Satoshi Sodeoka, Kazuo Ueno Department of Energy Conversion, Osaka National Research Institute, Midorigaoka 1-8-31, Ikeda, Osaka 563-8577, Japan Received 18 August 1999: received in revised form 8 February 2000; accepted 12 February 2000 Fibrous Al,O3 ceramics with a mixture of Sic and Al,O3 as the cell boundaries were fabricated by extrusion-molding and hot pressing techniques. The effects of the cell boundary composition on the mechanical properties and fracture behavior are investi- gated. It is shown that a 65: 35 mixture of SiC: Al2O3 can act as a suitable cell boundary for Al2O3 cells. In bending tests, such a ceramic displays a non-catastrophic fracture behavior with reasonable load-carrying capability, and its fracture energy and appar ent toughness are up to 1349 J/m2 and 6.0 MPa m/, respectively. C 2000 Elsevier Science Ltd. All rights reserved Keywords: Al2Or-SiC; Composites: Fibres; Fracture; Mechanical properties 1. Introduction SiC/BN and Si3N4/BN. Due to the preferred crack propagation along the weak C or Bn boundaries, these The incorporation of high-strength ceramic fiber fibrous ceramics fail non-catastrophically in a similar ceramic matrices has led to a new generation of manner to fiber-reinforced ceramics. However, this th high tough d gracefu ceful failure has not been demonstrated for fibro failure characteristics. However, the fabrication of fiber- Al,O3 ceramics, although several systems reinforced ceramic composites is time-consuming, Al2O3 ZrO2, Al2O3/AlTiOs and Al2O3/Ni have been complex, and expensive. Therefore, some simple and investigated. inexpensive processes are expected. Coblenz! have suc The objective of the present work is to show that a cessfully developed a novel and powerful method to non-catastrophic fracture behavior can be achieved for make ceramic composites with a distinct fibrous texture, fibrous Al2O3 ceramics with a mixture of SiC and Al2O consisting of high-aspect-ratio polycrystalline cells and as the cell boundary. The fracture behavior in combi thin cell boundaries. This structure can be obtained nation with the mechanical properties such as strength, from ordinary ceramic powders via conventional cera- toughness and fracture energy are evaluated in bending mic- and polymer-processing ogy. Specifically thegreen"polymer filaments containing with a sec- 2. Experimental procedure ceramIc powder as the polycrystalline cell are coated ondary phase as the cell boundary, then compacted and sintered. resulting in a wood-like fibrous microstructure Green Al2O3 fibers were prepared using an extrusion The resultant ceramics are called " fibrous ceramics process. The starting material was a-Al2O3 powder(0.22 Following Coblenz's method, Baskaran et al. have um, TM-D, Taimei Chemicals Co., Ltd. Japan).17.5 fabricated a variety of fibrous ceramics such as SiC/C, vol% ZrO2(0.30 m, TZ-8Y, Toyo Soda Manufacturing Co, Ltd, Japan) was added to develop a fine-grained microstructure. To form ers by ext must be used In this work, 4 wt% methyl cellulose and address:jhshe(@hotmail.com(J.She) 3.5 wt% dynamite glycerol were employed as binder 0955-2219/00/S. see front matter C 2000 Elsevier Science Ltd. All rights reserved PII:S0955-2219(00)00067-4
Mechanical properties and fracture behavior of ®brous Al2O3/SiC ceramics Jihong She *, Takahiro Inoue, Masato Suzuki, Satoshi Sodeoka, Kazuo Ueno Department of Energy Conversion, Osaka National Research Institute, Midorigaoka 1-8-31, Ikeda, Osaka 563-8577, Japan Received 18 August 1999; received in revised form 8 February 2000; accepted 12 February 2000 Abstract Fibrous Al2O3 ceramics with a mixture of SiC and Al2O3 as the cell boundaries were fabricated by extrusion-molding and hotpressing techniques. The eects of the cell boundary composition on the mechanical properties and fracture behavior are investigated. It is shown that a 65:35 mixture of SiC:Al2O3 can act as a suitable cell boundary for Al2O3 cells. In bending tests, such a ceramic displays a non-catastrophic fracture behavior with reasonable load-carrying capability, and its fracture energy and apparent toughness are up to 1349 J/m2 and 6.0 MPa m1/2, respectively. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Al2O3±SiC; Composites; Fibres; Fracture; Mechanical properties 1. Introduction The incorporation of high-strength ceramic ®bers into ceramic matrices has led to a new generation of advanced composites with high toughness and graceful failure characteristics. However, the fabrication of ®berreinforced ceramic composites is time-consuming, complex, and expensive. Therefore, some simple and inexpensive processes are expected. Coblenz1 have successfully developed a novel and powerful method to make ceramic composites with a distinct ®brous texture, consisting of high-aspect-ratio polycrystalline cells and thin cell boundaries. This structure can be obtained from ordinary ceramic powders via conventional ceramic- and polymer-processing technology. Speci®cally, the ``green'' polymer ®laments containing the ceramic powder as the polycrystalline cell are coated with a secondary phase as the cell boundary, then compacted and sintered, resulting in a wood-like ®brous microstructure. The resultant ceramics are called ``®brous ceramics''. Following Coblenz's method, Baskaran et al.2ÿ4 have fabricated a variety of ®brous ceramics such as SiC/C, SiC/BN and Si3N4/BN. Due to the preferred crack propagation along the weak C or BN boundaries, these ®brous ceramics fail non-catastrophically in a similar manner to ®ber-reinforced ceramics. However, this graceful failure has not been demonstrated for ®brous Al2O3 ceramics, although several systems such as Al2O3/ZrO2, Al2O3/Al2TiO5 and Al2O3/Ni have been investigated. The objective of the present work is to show that a non-catastrophic fracture behavior can be achieved for ®brous Al2O3 ceramics with a mixture of SiC and Al2O3 as the cell boundary. The fracture behavior in combination with the mechanical properties such as strength, toughness and fracture energy are evaluated in bending. 2. Experimental procedure Green Al2O3 ®bers were prepared using an extrusion process. The starting material was a-Al2O3 powder (0.22 mm, TM-D, Taimei Chemicals Co., Ltd., Japan). 17.5 vol% ZrO2 (0.30 m, TZ-8Y, Toyo Soda Manufacturing Co., Ltd., Japan) was added to develop a ®ne-grained microstructure. To form green ®bers by extrusion, some organic agents such as binder, softener and dispersant must be used. In this work, 4 wt% methyl cellulose and 3.5 wt% dynamite glycerol were employed as binder 0955-2219/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(00)00067-4 Journal of the European Ceramic Society 20 (2000) 1877±1881 * Corresponding author present address: Institute of Materials Research, German Aerospace Centre (DLR), 51147 KoÈln, Germany. Fax: +49-2203-696480. E-mail address: jhshe@hotmail.com (J. She)
1878 J. She et al. Journal of the European Ceramic Society 20(2000)1877-1881 oftener, while 2 wt% glycol together with 1.5 wt% 3. Results and discussion polyglycol were used as dispersants AlO3 and Zro powders were mixed with methyl Fig. I shows the polished cross-sectional surface of a cellulose in acetone by ball milling using alumina representative specimen, in which the cell and cell grinding media. After drying at room temperature and boundary structures are clearly visible. The polycrystal sieving through a 32-mesh screen, the mixture was fur- line Al2O3 cells(gray) are uniformly separated by the ther mixed with a"liquid"mixture, which was prepared continuous Sic cell boundaries (dark). Due to the by mixing dynamite glycerol, glycol and polyglycol in deformation of the green fibers along the compression water using magnetic stirrer. The relative amount of axis during warm-pressing and hot-pressing, the Al2O water to(Al_O3+ ZrO2)powders was taken as 15 wt% cells appear as the flattened hexagons with an aspect of to ensure a relatively homogeneous distribution of 2. The width of the cells are 540+8 um. The volume ceramic particles and organic components. Extrusion fraction of the cell boundaries, VBoundary, was deter was performed on a laboratory-scale extruder. The dia- mined to be about 10% from 300-cell encompassed area meter of green fibers was fixed at about 0.5 mm. on the cross section using optical microscope with an Sheets of unidirectionally aligned green fibers were image processor-analyzer. On the other hand, SEM produced by winding the fibers around a mandrel and observations revealed that the Al2O3 cells were almost fixing them into place with an adhesive. After trimming free of pores, but a few of pores were presented in the to 39.5 mm long and 20.5 mm wide with green fibers SiC cell boundaries due to the poor sinterability of SiC. aligned in the long direction, these sheets were coated by During grinding and polishing, the Sic-containing spraying with SiC AlO3 slurries, which were prepared boundaries are easy to be removed, and thus appears as by milling SiC and Al2O3 powders in ethanol. The con- grooves on the polished surfaces, as shown in Fig. 1 tent of powder solids in the slurries was x7.5 wt%. The Furthermore, the density of the SiC cell boundaries was volume ratio of SiC to Al2O3 was taken as 50: 50. 65: 35 estimated from and 80: 20 to determine the effects of the cell-boundary composition. Fibrous specimens were assembled from Ceramic=Peel (1-Vboundary)+BoundaryBoundary (1) the coated fiber sheets. Typically, 18 sheets were stacked graphite die of 21 x 40 mm, and uniaxially pressed where Ceramic, Pell and Boundary are the bu at 250 C using a 30-MPa pressure. This collapsed and the fibrous ceramics, the Al,O3 cells and the Sic cell consolidated the fibers into a solid billet. Hot-pressing boundaries, respectively. In this work, the densities of was carried out in vacuum under 25 MPa at 1500C for the fibrous specimens were measured to be 4.1504, I h, and a low heating rate of 5C/min was used below 4.0459 and 3.9927 g/cm'when the cell boundaries con- 600C to remove the organic polymers. After hot-press- tained 50, 65 and 80 vol% SiC. Table I presents the ing, the billets were cut and ground into rectangular estimated densities of the Sic cell boundaries for speci- bars of 3 mm(width)x 4 mm(height)x 40 mm mens with different SiC contents in the boundaries (length), with the tensile surface normal to the hot- Also, the relative densities and the total porosities for pressing direction and with the grinding direction the sic cell boundaries are given in Table 1. In calcula allel to the length of the specimen tions, the density of the Al,O3 cells was taken as the Density was measured by water immersion method Flexural strength was determined by a three-point bending test with a support distance of 30 mm and a cross-head speed of 0.5 mm/min. The tensile surfaces were polished and the edges were chamfered. Fracture toughness was measured by a four-point SENB techni que with a 10-mm inner span and a 30-mm outer span at a cross-head speed of 0. 1 mm/min. The notch depth and width were about 1. 5 and 0. I mm, respectively Strength and toughness measurements were conducted in a universal testing machine with the tensile surface perpendicular to the loading direction. Load and dis- placement data were recorded with a computerized data- acquisition system. At least three bars were tested for each cell-boundary composition. Fracture energy was calculated from the area under the load-displace- 400um ment curve of the notched bar. Cross sections and frac ture surfaces were observed by optical and scanning Fig. 1. Optical micrograph of the cross-section of a representative electron microscopy, respectively
and softener, while 2 wt% glycol together with 1.5 wt% polyglycol were used as dispersants. Al2O3 and ZrO2 powders were mixed with methyl cellulose in acetone by ball milling using alumina grinding media. After drying at room temperature and sieving through a 32-mesh screen, the mixture was further mixed with a ``liquid'' mixture, which was prepared by mixing dynamite glycerol, glycol and polyglycol in water using magnetic stirrer. The relative amount of water to (Al2O3 + ZrO2) powders was taken as 15 wt% to ensure a relatively homogeneous distribution of ceramic particles and organic components. Extrusion was performed on a laboratory-scale extruder. The diameter of green ®bers was ®xed at about 0.5 mm. Sheets of unidirectionally aligned green ®bers were produced by winding the ®bers around a mandrel and ®xing them into place with an adhesive. After trimming to 39.5 mm long and 20.5 mm wide with green ®bers aligned in the long direction, these sheets were coated by spraying with SiC+Al2O3 slurries, which were prepared by milling SiC and Al2O3 powders in ethanol. The content of powder solids in the slurries was 7.5 wt%. The volume ratio of SiC to Al2O3 was taken as 50:50, 65:35 and 80:20 to determine the eects of the cell-boundary composition. Fibrous specimens were assembled from the coated ®ber sheets. Typically, 18 sheets were stacked in a graphite die of 21 40 mm, and uniaxially pressed at 250C using a 30-MPa pressure. This collapsed and consolidated the ®bers into a solid billet. Hot-pressing was carried out in vacuum under 25 MPa at 1500C for 1 h, and a low heating rate of 5C/min was used below 600C to remove the organic polymers. After hot-pressing, the billets were cut and ground into rectangular bars of 3 mm (width) 4 mm (height) 40 mm (length), with the tensile surface normal to the hotpressing direction and with the grinding direction parallel to the length of the specimen. Density was measured by water immersion method. Flexural strength was determined by a three-point bending test with a support distance of 30 mm and a cross-head speed of 0.5 mm/min. The tensile surfaces were polished and the edges were chamfered. Fracture toughness was measured by a four-point SENB technique with a 10-mm inner span and a 30-mm outer span at a cross-head speed of 0.1 mm/min. The notch depth and width were about 1.5 and 0.1 mm, respectively. Strength and toughness measurements were conducted in a universal testing machine with the tensile surface perpendicular to the loading direction. Load and displacement data were recorded with a computerized data- acquisition system. At least three bars were tested for each cell-boundary composition. Fracture energy was calculated from the area under the load±displacement curve of the notched bar. Cross sections and fracture surfaces were observed by optical and scanning electron microscopy, respectively. 3. Results and discussion Fig. 1 shows the polished cross-sectional surface of a representative specimen, in which the cell and cell boundary structures are clearly visible. The polycrystalline Al2O3 cells (gray) are uniformly separated by the continuous SiC cell boundaries (dark). Due to the deformation of the green ®bers along the compression axis during warm-pressing and hot-pressing, the Al2O3 cells appear as the ¯attened hexagons with an aspect of 2. The width of the cells are 540 8 mm. The volume fraction of the cell boundaries, VBoundary, was determined to be about 10% from 300-cell encompassed area on the cross section using optical microscope with an image processor-analyzer. On the other hand, SEM observations revealed that the Al2O3 cells were almost free of pores, but a few of pores were presented in the SiC cell boundaries due to the poor sinterability of SiC. During grinding and polishing, the SiC-containing boundaries are easy to be removed, and thus appears as grooves on the polished surfaces, as shown in Fig. 1. Furthermore, the density of the SiC cell boundaries was estimated from ceramic cell 1 ÿ Vboundary ÿ boundary Vboundary 1 where ceramic, cell and boundary are the bulk densities of the ®brous ceramics, the Al2O3 cells and the SiC cell boundaries, respectively. In this work, the densities of the ®brous specimens were measured to be 4.1504, 4.0459 and 3.9927 g/cm3 when the cell boundaries contained 50, 65 and 80 vol% SiC. Table 1 presents the estimated densities of the SiC cell boundaries for specimens with dierent SiC contents in the boundaries. Also, the relative densities and the total porosities for the SiC cell boundaries are given in Table 1. In calculations, the density of the Al2O3 cells was taken as the Fig. 1. Optical micrograph of the cross-section of a representative specimen. 1878 J. She et al. / Journal of the European Ceramic Society 20 (2000) 1877±1881
J. She et al. Journal of the European Ceramic Society 20(2000)1877-1881 Table I after testing showed that fracture originated at the Density and porosity for the SiC-containing boundaries in fibrous ensile surface between two supporting points, followed by shear delamination almost parallel to the surface Boundary Bulk density Relative Total along the cell boundaries. This should be attributed to a density(%) porosity(%) relatively low shear strength of the 65 vol% SiC- containing boundaries. The terraced topography in Fig 滋 21.3 489 3(b) indicates the preferential crack growth along the 37.0 63.0 cell boundaries, which appear bright in the SEM micrograph Specimens with the cell boundaries containing 80 theoretical value of 4. 298 g/cm, and the theoretical vol% Sic display a graceful failure, with some load densities of the Sic-containing boundaries were esti- retention at specimen deflections as large as 1.0 mm mated using a rule of mixtures with the values of 3.965 The mean flexural strength was 106 MPa, with strengths g/cm' for Al2O3 and 3.21 g/cm for SiC. It can be seen ranging from 90 to 131 MPa. The low flexural strength in Table I that the total porosities in the 65 and 80 suggests the decrease in the shear strength of the cell vol% SiC-containing boundaries are up to 48.9 and boundaries 63.0%. This may greatly weaken the cell boundaries ig. 4 shows the load displacement curves of notched Fig. 2 shows the flexural strength behavior of repre- specimens under four-point bending tests. As shown, sentative specimens. As shown, all the specimens behave the fracture behavior of all the notched specimens is like monolithic ceramics until a maximum flexural stress non-catastrophic, with the evidence of a gradual load is reached on the tensile surface. Beyond the stress decrease after the peak load. During tests, the shear maximum, the load-bearing ability depends strongly delaminations were observed to occur along the cell upon the composition of the cell boundaries. When the Sic content in the cell boundaries was 50 vol%. failure occurred in a brittle manner. The average flexural strength of three specimens was 263 MPa. Two speci mens with higher strengths broke into two parts; but the third specimen with the lowest strength remained intact with some load-carrying ability(as shown in Fig. 2) The fracture surface of a broken specimen in Fig. 3(a) shows the stepped fracture at only several boundaries With 65 vol% Sic in the cell boundaries, the speci mens exhibit a non-catastrophic fracture behavior, with significant load-retaining capability beyond the peak stress. The average flexural strength is 283 MPa, slightly higher than that of the specimens with 50 vol% SiC- containing boundaries. Examination of the specimens 100Nm NRI52-25KU X535如m (b) 50 vol% SiC 65 vol% Sic 80 vol% SiC 150 0.20.4 TOaN Crosshead Displacement ONRI57 25KV X50 35m m Fig. 2. Nominal stress as a function of crosshead displacement for Fig 3. Fracture surfaces of fibrous Al2O3 ceramics with (a)50 vol% unnotched specimens with 50, 65 and 80 vol% SiC in the boundaries. and(b)65 vol% SiC in the cell boundarie
theoretical value of 4.298 g/cm3 , and the theoretical densities of the SiC-containing boundaries were estimated using a rule of mixtures with the values of 3.965 g/cm3 for Al2O3 and 3.21 g/cm3 for SiC. It can be seen in Table 1 that the total porosities in the 65 and 80 vol% SiC-containing boundaries are up to 48.9 and 63.0%. This may greatly weaken the cell boundaries. Fig. 2 shows the ¯exural strength behavior of representative specimens. As shown, all the specimens behave like monolithic ceramics until a maximum ¯exural stress is reached on the tensile surface. Beyond the stress maximum, the load-bearing ability depends strongly upon the composition of the cell boundaries. When the SiC content in the cell boundaries was 50 vol%, failure occurred in a brittle manner. The average ¯exural strength of three specimens was 263 MPa. Two specimens with higher strengths broke into two parts; but the third specimen with the lowest strength remained intact, with some load-carrying ability (as shown in Fig. 2). The fracture surface of a broken specimen in Fig. 3(a) shows the stepped fracture at only several boundaries. With 65 vol% SiC in the cell boundaries, the specimens exhibit a non-catastrophic fracture behavior, with signi®cant load-retaining capability beyond the peak stress. The average ¯exural strength is 283 MPa, slightly higher than that of the specimens with 50 vol% SiCcontaining boundaries. Examination of the specimens after testing showed that fracture originated at the tensile surface between two supporting points, followed by shear delamination almost parallel to the surface along the cell boundaries. This should be attributed to a relatively low shear strength of the 65 vol% SiCcontaining boundaries. The terraced topography in Fig. 3(b) indicates the preferential crack growth along the cell boundaries, which appear bright in the SEM micrograph. Specimens with the cell boundaries containing 80 vol% SiC display a graceful failure, with some load retention at specimen de¯ections as large as 1.0 mm. The mean ¯exural strength was 106 MPa, with strengths ranging from 90 to 131 MPa. The low ¯exural strength suggests the decrease in the shear strength of the cell boundaries. Fig. 4 shows the load displacement curves of notched specimens under four-point bending tests. As shown, the fracture behavior of all the notched specimens is non-catastrophic, with the evidence of a gradual load decrease after the peak load. During tests, the shear delaminations were observed to occur along the cell Table 1 Density and porosity for the SiC-containing boundaries in ®brous Al2O3/SiC ceramics Boundary composition Bulk density (g/cm3 ) Relative density (%) Total porosity (%) Al2O3±50 vol% SiC 2.822 78.7 21.3 Al2O3±65 vol% SiC 1.777 51.1 48.9 Al2O3±80 vol% SiC 1.245 37.0 63.0 Fig. 2. Nominal stress as a function of crosshead displacement for unnotched specimens with 50, 65 and 80 vol% SiC in the boundaries. Fig. 3. Fracture surfaces of ®brous Al2O3 ceramics with (a) 50 vol% and (b) 65 vol% SiC in the cell boundaries. J. She et al. / Journal of the European Ceramic Society 20 (2000) 1877±1881 1879
1880 J. She et al. Journal of the European Ceramic Society 20(2000)1877-1881 50 vol% SiC 65 vol% SiC 乙 0 vol% SiC 0 0.20.40.60 Crosshead Displacement(mm) Fig 4. Load-displacement curves of notched specimens with 50, 65 and 80 vol% SiC in the boundaries boundaries, as indicated in Fig. 5(a) by arrows. After tests, all the specimens do not fall apart Table 2 summarizes the mechanical properties of fibrous Al,O3 ceramics with different volume fractions of Sic in the cell boundaries when the cell boundaries contain 80 vol% SiC, the measured apparent toughness from the maximum load is only about 2.4 MPa m",but the calculated fracture energy from the area under the load displacement curve is up to 1616 J/m. In contrast, the specimens with 50 vol% SiC-containing boundaries have a high apparent toughness of 5.1 MPa m/ but a ow fracture energy of 521 J/m. These results are con- 50 um sidered to be mainly associated with the shear strength of the cell boundaries. Due to a very low shear strength Fig. 5. High-magnification optical micrograph of the side surface of a fibrous Al2O3 ceramic with(a)65 vol% and(b)50 vol% SiC in the cell of 80 vol% SiC-containing boundaries, delaminations may occur along almost each successive cell boundary giving rise to a high energy absorption during the crack extension. When the cell boundaries contain 50 vol% of non-oxide systems such as SiC/C and Sic/BN, 4but SiC, the shear strength is not low enough for effective to those of oxide systems such as Al2O3/ delaminations. In this case. the delamination cracks will Al TiOs and AlO3 /Ni 2.5 kink out of the cell boundaries after propagating only a short distance, as shown in Fig. 5(b). This may greatly decrease the energy dissipation capability during frac- 4. Conclusions ture. Only in 65 vol% SiC-containing boundaries, the shear strength is at a moderate level. As a result. the Fibrous AlO3/SiC ceramics, in which high-aspect specimens exhibit sufficient flexural strength(283 MPa), ratio polycrystalline Al_O3 cells were separated by thin apparent toughness(6.0 MPa m/ 2)and fracture energy SiC-containing boundaries, were fabricated by extru (1349 J/m2). These properties are comparable to those sion-molding and hot-pressing. The mechanical proper ties and fracture behavior were evaluated in bending with a ground or notched tensile surface. When the cell Mechanical properties of fibrous Al,,/SiC ceramics with different boundaries contained 65 vol% SiC, pronounced shear boundary compositions delamination occurred along the cell boundaries. Boundary resulting in a graceful failure. The flexural strength MPa m/- energy (m?) fracture energy and apparent toughness of such com- posites were measured to be 283 MPa, 1349 J/m2 and AlOr50 vol% SiC 63±23 5.1±0.2 52l±125 Al2Ox65vol%SiC283±4l6.0±0.5 6.0 MPa m /, respectively. These properties have rarely AlOr-80 vol% SiC 106+16 2.4±0.2 16l6±141 been observed in oxide systems, and are comparable to those of non-oxide systems
boundaries, as indicated in Fig. 5(a) by arrows. After tests, all the specimens do not fall apart. Table 2 summarizes the mechanical properties of ®brous Al2O3 ceramics with dierent volume fractions of SiC in the cell boundaries. When the cell boundaries contain 80 vol% SiC, the measured apparent toughness from the maximum load is only about 2.4 MPa m1/2, but the calculated fracture energy from the area under the load displacement curve is up to 1616 J/m2 . In contrast, the specimens with 50 vol% SiC-containing boundaries have a high apparent toughness of 5.1 MPa m1/2 but a low fracture energy of 521 J/m2 . These results are considered to be mainly associated with the shear strength of the cell boundaries. Due to a very low shear strength of 80 vol% SiC-containing boundaries, delaminations may occur along almost each successive cell boundary, giving rise to a high energy absorption during the crack extension. When the cell boundaries contain 50 vol% SiC, the shear strength is not low enough for eective delaminations. In this case, the delamination cracks will kink out of the cell boundaries after propagating only a short distance, as shown in Fig. 5(b). This may greatly decrease the energy dissipation capability during fracture. Only in 65 vol% SiC-containing boundaries, the shear strength is at a moderate level. As a result, the specimens exhibit sucient ¯exural strength (283 MPa), apparent toughness (6.0 MPa m1/2) and fracture energy (1349 J/m2 ). These properties are comparable to those of non-oxide systems such as SiC/C and SiC/BN,3,4 but far superior to those of oxide systems such as Al2O3/ Al2TiO5 and Al2O3/Ni.2,5 4. Conclusions Fibrous Al2O3/SiC ceramics, in which high-aspectratio polycrystalline Al2O3 cells were separated by thin SiC-containing boundaries, were fabricated by extrusion-molding and hot-pressing. The mechanical properties and fracture behavior were evaluated in bending with a ground or notched tensile surface. When the cell boundaries contained 65 vol% SiC, pronounced shear delamination occurred along the cell boundaries, resulting in a graceful failure. The ¯exural strength, fracture energy and apparent toughness of such composites were measured to be 283 MPa, 1349 J/m2 and 6.0 MPa m1/2, respectively. These properties have rarely been observed in oxide systems, and are comparable to those of non-oxide systems. Fig. 4. Load±displacement curves of notched specimens with 50, 65 and 80 vol% SiC in the boundaries. Fig. 5. High-magni®cation optical micrograph of the side surface of a ®brous Al2O3 ceramic with (a) 65 vol% and (b) 50 vol% SiC in the cell boundaries. Table 2 Mechanical properties of ®brous Al2O3/SiC ceramics with dierent boundary compositions Boundary composition Strength (MPa) Toughness (MPa m1/2) Fracture energy (J/m2 ) Al2O3±50 vol% SiC 26323 5.10.2 521125 Al2O3±65 vol% SiC 28341 6.00.5 1349109 Al2O3±80 vol% SiC 10616 2.40.2 1616141 1880 J. She et al. / Journal of the European Ceramic Society 20 (2000) 1877±1881
Acknowledgement 2. Baskaran, S, Nunn. S. D. Popovic, D. and Halloran, J. W. Fibrous monolithic ceramics: I. Fabrication microstructure and Jihong She would like to express his gratitude towards indentation behavior. J. Am. Ceram Soc.1993. 76. 2209-2216 e Agency of Industrial Science and Technology 3. Baskaran. S. and Halloran. J. W.. Fibrous monolithic ceramics II. Flexural strength and fracture behavior of the silicon carbide/ (AIST), Ministry of International Trade and Industry graphite system. J. Am. Ceram Soc., I (MITI) for an AIST Research Fellowship 4. Baskaran S and Halloran. J. W. Fibrous monolithic ceramics: Ill. Mechanical properties and oxidation behavior of the silicon arbide/boron nitride system. J. Am. Ceram. Soc., 1994. 77 1249-1255 References 5. Baskaran. S. Nunn, S. D. and Halloran. J. w. Fibrous mono- lithic ceramics: IV. Mechanical properties and oxidation behavior 1. Coblenz, W. S. Fibrous monolithic ceramic and method for of the alumina/ nickel system. J. Am. Ceram. Soc. 1994. 77, 1256- roduction. US Patent No. 4772524 1988
Acknowledgement Jihong She would like to express his gratitude towards the Agency of Industrial Science and Technology (AIST), Ministry of International Trade and Industry (MITI) for an AIST Research Fellowship. References 1. Coblenz, W. S., Fibrous monolithic ceramic and method for production. US Patent No. 4772524, 1988. 2. Baskaran, S., Nunn, S. D., Popovic, D. and Halloran, J. W., Fibrous monolithic ceramics: I. Fabrication, microstructure, and indentation behavior. J. Am. Ceram. Soc., 1993, 76, 2209±2216. 3. Baskaran, S. and Halloran, J. W., Fibrous monolithic ceramics: II. Flexural strength and fracture behavior of the silicon carbide/ graphite system. J. Am. Ceram. Soc., 1993, 76, 2217±2224. 4. Baskaran, S. and Halloran, J. W., Fibrous monolithic ceramics: III. Mechanical properties and oxidation behavior of the silicon carbide/boron nitride system. J. Am. Ceram. Soc., 1994, 77, 1249±1255. 5. Baskaran, S., Nunn, S. D. and Halloran, J. W., Fibrous monolithic ceramics: IV. Mechanical properties and oxidation behavior of the alumina/nickel system. J. Am. Ceram. Soc., 1994, 77, 1256± 1262. J. She et al. / Journal of the European Ceramic Society 20 (2000) 1877±1881 1881