《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_Al2O3-SiC

CERAMICS INTERNATIONAL ELSEVIER Ceramics International 26(2000)801-805 Damage resistance and R-curve behavior of multilayer Al2O3 /SiC ceramics Jihong She*, Takahiro Inoue, Kazuo Ueno Department of Energy Conversion, Osaka National Research Institute, Midorigaoka 1-8-31, Ikeda, Osaka 563-8577, Japan Received I September 1999: received in revised form 14 September 1999; accepted 12 January 2000 Damage resistance and R-curve behavior of multilayer Al,O /Sic ceramics were evaluated in bending by the indentation-strength and the single-edge-notched-beam methods Due to the crack deflection at the Al2O3 /SiC interfaces, a plateau indentation strength observed to increase from 8.0 to 15.5 MPa mk al resistance to contact-induced damage. Moreover, fracture toughness was ith increasing notch depth from 0.5 to 2.0 mm, indicative of a strong R-curve behavior. C 2000 Elsevier Science Ltd and Techna S.r. l. All rights reserved Keywords: B. Interfaces; C. Fracture; C. Mechanical properties; D. Al203 1. Introduction 2. Experimental procedure In recent years, there has been considerable interest in The fabrication process of multilayer Al2O3/SiC cera- le mechanical behavior of a variety of multilayer cera- mics has been described elsewhere [13]. Briefly, sub mic composites. It has been shown that enhanced micrometer a-Al2O3 powders(0.22 um, TM-D, Taimei strength and toughness in combination with improved Chemicals Co, Ltd, Japan)were extrusion-molded into damage resistance can be achieved for three-layer cera- 0.35-mm-thick sheets, then coated with a 50 vol% SiC mic laminates consisting either of two strong outer lay- containing slurry, and finally hot-pressed in a graphite ers and a tough inner layer [1, 2] or of two compressive die under a 25-MPa pressure at 1500C for I h. The outer layers and a tensile inner layer [3-7. On the other resultant layered structure is shown in Fig. I. As can be hand, increased apparent toughness and fracture energy seen, the Al2O3 / SiC interfaces are smooth and well as well as a non-catastrophic fracture behavior have defined. The thicknesses of the Al]O3 and SiC layers been demonstrated for multilayer ceramic laminates by were determined to be 182.7+10.1 and 12.9+1.4 um introducing weak interfaces [8-ll] or by inserting por- respectively To evaluate the fracture behavior, the hot-pressed billets were machined into the rectangular bars of 3 mm and fracture resistance behavior of these laminates (thickness)x4 mm(width)x40 mm (length), and inden More recently, we have successfully fabricated a mul- ted at the center of one polished tensile surface with a tilayer Al2O3/ SiC ceramic [13]. Due to the deflection of Vickers diamond pyramid indenter under loads ranging through-thickness cracks at the weak Sic interfaces, such from 3 to 500 N. All indentations were made a laminate fails gracefully, and its apparent toughness the indenter contacted the specimen surface for 30 s and fracture energy are up to 15. 1 MPa m and 3335 J/ before it was withdrawn to complete the load cycle m, respectively. In this work, the damage resistance and Care was taken to orient one set of the indentation R-curve behavior are further investigated cracks to be parallel to the longitudinal axis of the rec- orresponding author at present address: Institute of Materials, angular bars. After indentation, the specimens were German Aerospace Center (D tested in three-point bending with a loading span of 2203-69648 mm and a crosshead speed of 0.5 mm /min. Three tests E-mailaddress:jhshe(@hotmail.com(She) were performed at each indentation load 0272-8842/00/S20.00 C 2000 Elsevier Science Ltd and Techna S r l. All rights reserved. PII:S0272-8842(00)00020-1

Damage resistance and R-curve behavior of multilayer Al2O3/SiC ceramics Jihong She *, Takahiro Inoue, Kazuo Ueno Department of Energy Conversion, Osaka National Research Institute, Midorigaoka 1-8-31, Ikeda, Osaka 563-8577, Japan Received 1 September 1999; received in revised form 14 September 1999; accepted 12 January 2000 Abstract Damage resistance and R-curve behavior of multilayer Al2O3/SiC ceramics were evaluated in bending by the indentation-strength and the single-edge-notched-beam methods. Due to the crack de¯ection at the Al2O3/SiC interfaces, a plateau indentation strength response was achieved, suggesting an exceptional resistance to contact-induced damage. Moreover, fracture toughness was observed to increase from 8.0 to 15.5 MPa m1/2 with increasing notch depth from 0.5 to 2.0 mm, indicative of a strong R-curve behavior. # 2000 Elsevier Science Ltd and Techna S.r.l. All rights reserved. Keywords: B. Interfaces; C. Fracture; C. Mechanical properties; D. Al2O3 1. Introduction In recent years, there has been considerable interest in the mechanical behavior of a variety of multilayer cera￾mic composites. It has been shown that enhanced strength and toughness in combination with improved damage resistance can be achieved for three-layer cera￾mic laminates consisting either of two strong outer lay￾ers and a tough inner layer [1,2] or of two compressive outer layers and a tensile inner layer [3±7]. On the other hand, increased apparent toughness and fracture energy as well as a non-catastrophic fracture behavior have been demonstrated for multilayer ceramic laminates by introducing weak interfaces [8±11] or by inserting por￾ous layers [12] between ceramic substrates. However, little work [12] has been performed on the ¯aw tolerance and fracture resistance behavior of these laminates. More recently, we have successfully fabricated a mul￾tilayer Al2O3/SiC ceramic [13]. Due to the de¯ection of through-thickness cracks at the weak SiC interfaces, such a laminate fails gracefully, and its apparent toughness and fracture energy are up to 15.1 MPa m1/2 and 3335 J/ m2 , respectively. In this work, the damage resistance and R-curve behavior are further investigated. 2. Experimental procedure The fabrication process of multilayer Al2O3/SiC cera￾mics has been described elsewhere [13]. Brie¯y, sub￾micrometer a-Al2O3 powders (0.22 mm, TM-D, Taimei Chemicals Co., Ltd., Japan) were extrusion-molded into 0.35-mm-thick sheets, then coated with a 50 vol% SiC￾containing slurry, and ®nally hot-pressed in a graphite die under a 25-MPa pressure at 1500C for 1 h. The resultant layered structure is shown in Fig. 1. As can be seen, the Al2O3/SiC interfaces are smooth and well￾de®ned. The thicknesses of the Al2O3 and SiC layers were determined to be 182.710.1 and 12.91.4 mm, respectively. To evaluate the fracture behavior, the hot-pressed billets were machined into the rectangular bars of 3 mm (thickness)4 mm (width)40 mm (length), and inden￾ted at the center of one polished tensile surface with a Vickers diamond pyramid indenter under loads ranging from 3 to 500 N. All indentations were made in air, and the indenter contacted the specimen surface for 30 s before it was withdrawn to complete the load cycle. Care was taken to orient one set of the indentation cracks to be parallel to the longitudinal axis of the rec￾tangular bars. After indentation, the specimens were tested in three-point bending with a loading span of 30 mm and a crosshead speed of 0.5 mm/min. Three tests were performed at each indentation load. 0272-8842/00/$20.00 # 2000 Elsevier Science Ltd and Techna S.r.l. All rights reserved. PII: S0272-8842(00)00020-1 Ceramics International 26 (2000) 801±805 * Corresponding author at present address: Institute of Materials, German Aerospace Center (DLR), 51147 KoÈln, Germany. Fax: +49- 2203-696480. E-mail address: jhshe@hotmail.com (J. She).

J. She et al. Ceramics International 26(2000)801-805 On the other hand a straight notch was introduced at a crack propagates through the AlO3 layer itself, but is the center part of the test bars of 3 mm in width, 4 mm arrested at the interface with the adjacent SiC layer. As in height, and 40 mm in length. The notch width was shown in Fig. 3, crack deflection occurs along or at about 0. 1 mm, and the notch depth was varied between close proximity to the Al,O3/ SiC interface. Further 0.5 and 2.5 mm. Four-point bending tests were con- loading causes the formation of some new cracks in the ducted on the notched specimens using an inner span of next Al2O3 layer. This process is repeated until all the 10 mm and an outer span of 30 mm at a loading rate of Al2O3 layers have cracked, resulting in a step-like load- 0. 1 mm/min. Three to four specimens were tested for displacement response each notch depth From the maximum load, Pi, in the load-displace- ment curve of the indented specimen, the indentatio strength, Oi, was calculated according to 3. Results and discussion 3P.L Fig. 2 shows the load-displacement curves of the indented and notched specimens under bending tests idently, the fracture behavior of the indented speci- where L is the supporting span of the three-point bending men is similar to that of the notched specimen. Upon fixture, b the specimen width, and t the specimen thick- initial loading, both specimens behave elastically until a ness. Fig. 4 shows the strengths of the specimens after crack initiates at the notch tip or from the indent. Such indentation at different loads. Clearly, the indentation 400pm 200Jm Fig. 1. Optical micrograph of the cross section of a representative Fig 3. Propagation of a major crack through the specimen. Note that specimen, showing a perfect layered structure crack deflection occurs along or at close proximity to the Al2O3/ SiC interfaces 500 1000 z 30 tched 100 00.10.2030.40.5 Crosshead Displacement (mm) 1000 Indentation Load (N /SiC ceramics with an indent(solid line)or a notch(dashed line)or Fig. 4. Indentation strength response of multilayer AlO SiC ceramics

On the other hand, a straight notch was introduced at the center part of the test bars of 3 mm in width, 4 mm in height, and 40 mm in length. The notch width was about 0.1 mm, and the notch depth was varied between 0.5 and 2.5 mm. Four-point bending tests were con￾ducted on the notched specimens using an inner span of 10 mm and an outer span of 30 mm at a loading rate of 0.1 mm/min. Three to four specimens were tested for each notch depth. 3. Results and discussion Fig. 2 shows the load±displacement curves of the indented and notched specimens under bending tests. Evidently, the fracture behavior of the indented speci￾men is similar to that of the notched specimen. Upon initial loading, both specimens behave elastically until a crack initiates at the notch tip or from the indent. Such a crack propagates through the Al2O3 layer itself, but is arrested at the interface with the adjacent SiC layer. As shown in Fig. 3, crack de¯ection occurs along or at close proximity to the Al2O3/SiC interface. Further loading causes the formation of some new cracks in the next Al2O3 layer. This process is repeated until all the Al2O3 layers have cracked, resulting in a step-like load± displacement response. From the maximum load, Pi, in the load±displace￾ment curve of the indented specimen, the indentation strength, i, was calculated according to i ˆ 3PiL 2bt2 …1† where L is the supporting span of the three-point bending ®xture, b the specimen width, and t the specimen thick￾ness. Fig. 4 shows the strengths of the specimens after indentation at di€erent loads. Clearly, the indentation Fig. 1. Optical micrograph of the cross section of a representative specimen, showing a perfect layered structure. Fig. 2. Load±displacement curves of multilayer Al2O3/SiC ceramics with an indent (solid line) or a notch (dashed line) on the tensile surface. Fig. 4. Indentation strength response of multilayer Al2O3/SiC ceramics. Fig. 3. Propagation of a major crack through the specimen. Note that crack de¯ection occurs along or at close proximity to the Al2O3/SiC interfaces. 802 J. She et al. / Ceramics International 26 (2000) 801±805

J. She et al. Ceramics International 26 (2000)801-805 strengths are insensitive to the increases in the indenta- MPa. Again, this predicted value from Eq.(2)is con- ion load, indicative of excellent damage resistance. In sistent with the measured strengths of 376.5, 417.3 and fact, the indentation strength, i, is related to the 376. 7 MPa for an indentation load of 200, 300 and 500 indentation crack depth, a, by N, respectively. These results have clearly shown that multilayer Al2O3/SiC ceramics can retain a significant =b(1 (2) fraction of their strength even under contact damage conditions. This should allow them to exhibit superior resistance to impact- or abrasion- induced damage in where oo is the fracture strength of the unindented sp service cimens Substituting the measured value of 497 MPa for To evaluate the R-curve behavior, the fracture resis- go and taking the indentation crack depth as the thick ance, Kr, was computed from the results of the inden ness of the individual Al,O3 layers, one can obtain a tation strengt calculated strength of 450 MPa. This suggests that as equation [16] long as the indentation crack does not penetrate specime the y rta is srlength tick vase th bnodtn4sd K, =0.S9(6)(o,- play MPa. Based on the experiments of Lawn and co-work- ers [14, 15], the indentation crack depth, a, can be esti mated fro where ai is the fracture strength of the indented spec mens at a load P. The computed fracture resistance is illustrated in Fig. 5 as a function of the estimated a=0016 (3) indentation-crack depth from Eq (3). It can be seen in Fig. 5 that the fracture resistance ing increasing indentation-crack depth, suggesting a strong where E is the Youngs modulus, H is the Vickers R-curve effect of multilayer Al,O3/SiC ceramics hardness, P is the indentation load, and Kic is the frac Also, the rising R-curve behavior was observed in the ture toughness. Using the experimental values of E= fracture toughness measurements using the SENB(sin 343.2 GPa, H=16.1 GPa, and KIC =5.32 MPa m gle-edge notched beam)method. Fig. 6 presents the the indentation crack depths at different loads were cal- measured Kic values at different notch depths, in which culated and are listed in Table 1. when the indentation the Kic was calculated from the load maximum in the load is below 200 N, the estimated indentation-crack load-displacement curve of a notched specimen. As can depth is smaller than the Al2O3-layer thickness of 182.7 be seen in Fig. 6, the measured fracture toughness Hm, indicating that the indentation crack is completely increases from 8.0 to 15.5 MPa m 2 as the notch depth contained within the outer Al,O3 layer. In this case, the increases from 0.5 to 2.0 mm indentation strengths should be kept at M450 MPa. A On the other hand, the load-displacement curves of can be seen in Fig. 4, this strength value is in good notched specimens were observed to be essentially the agreement with the experimental data. When the inden- same, except for the decreased peak load with increasing tation load is between 200 and 500 N, the indentation- notch depth. By assuming that the net section stress in induced crack may penetrate through the surface layer and into the second Al2O3 layer. Due to the deflection of such a through-thickness crack along the interface with the second SiC layer, however, it is possible for the indented specimens to retain a fracture strength of -405 Table l Estimated indentation-crack depths at different loads for multilayer Al2O3/SiC ceramics Indentation load (N) 200 30040 258.9 363.9 Fig. 5. Dependence of fracture resistance on crack depth for multi-

strengths are insensitive to the increases in the indenta￾tion load, indicative of excellent damage resistance. In fact, the indentation strength, i, is related to the indentation crack depth, a, by i ˆ 0 1 ÿ a t  2 …2† where 0 is the fracture strength of the unindented spe￾cimens. Substituting the measured value of 497 MPa for 0 and taking the indentation crack depth as the thick￾ness of the individual Al2O3 layers, one can obtain a calculated strength of 450 MPa. This suggests that as long as the indentation crack does not penetrate through the outer Al2O3-layer thickness, the indented specimen may retain its strength at a value of about 450 MPa. Based on the experiments of Lawn and co-work￾ers [14,15], the indentation crack depth, a, can be esti￾mated from a ˆ 0:016 E H  1=2
P KIC " #2=3 …3† where E is the Young's modulus, H is the Vickers hardness, P is the indentation load, and KIC is the frac￾ture toughness. Using the experimental values of E ˆ 343:2 GPa, H ˆ 16:1 GPa, and KIC ˆ 5:32 MPa m1/2, the indentation crack depths at di€erent loads were cal￾culated and are listed in Table 1. When the indentation load is below 200 N, the estimated indentation-crack depth is smaller than the Al2O3-layer thickness of 182.7 mm, indicating that the indentation crack is completely contained within the outer Al2O3 layer. In this case, the indentation strengths should be kept at 450 MPa. As can be seen in Fig. 4, this strength value is in good agreement with the experimental data. When the inden￾tation load is between 200 and 500 N, the indentation￾induced crack may penetrate through the surface layer and into the second Al2O3 layer. Due to the de¯ection of such a through-thickness crack along the interface with the second SiC layer, however, it is possible for the indented specimens to retain a fracture strength of 405 MPa. Again, this predicted value from Eq. (2) is con￾sistent with the measured strengths of 376.5, 417.3 and 376.7 MPa for an indentation load of 200, 300 and 500 N, respectively. These results have clearly shown that multilayer Al2O3/SiC ceramics can retain a signi®cant fraction of their strength even under contact damage conditions. This should allow them to exhibit superior resistance to impact- or abrasion-induced damage in service. To evaluate the R-curve behavior, the fracture resis￾tance, Kr, was computed from the results of the inden￾tation strength measurements using the following equation [16] Kr ˆ 0:59 E H  1=8 i
P1=3 ÿ
3=4 …4† where i is the fracture strength of the indented speci￾mens at a load P. The computed fracture resistance is illustrated in Fig. 5 as a function of the estimated indentation-crack depth from Eq. (3). It can be seen in Fig. 5 that the fracture resistance increases with increasing indentation-crack depth, suggesting a strong R-curve e€ect of multilayer Al2O3/SiC ceramics. Also, the rising R-curve behavior was observed in the fracture toughness measurements using the SENB (sin￾gle-edge notched beam) method. Fig. 6 presents the measured KIC values at di€erent notch depths, in which the KIC was calculated from the load maximum in the load±displacement curve of a notched specimen. As can be seen in Fig. 6, the measured fracture toughness increases from 8.0 to 15.5 MPa m1/2 as the notch depth increases from 0.5 to 2.0 mm. On the other hand, the load±displacement curves of notched specimens were observed to be essentially the same, except for the decreased peak load with increasing notch depth. By assuming that the net section stress in Table 1 Estimated indentation-crack depths at di€erent loads for multilayer Al2O3/SiC ceramics Indentation load (N) Crack depth (mm) 3 12.0 10 26.8 50 78.4 100 124.5 200 197.6 300 258.9 500 363.9 Fig. 5. Dependence of fracture resistance on crack depth for multi￾layer Al2O3/SiC ceramics. J. She et al. / Ceramics International 26 (2000) 801±805 803

al. Ceramics International 26(2000)801-805 the plane of the notch is constant, the nominal stress, 4. Conclusions On, can be calculated from [17 Multilayer ceramic composites may exhibit some (5) unique mechanical properties. In this work, the damage resistance and R-curve behavior of multilayer AlO3/ iC ceramics were investigated using the indentation- where ao is the notch depth, and h is the specimen strength and the single-edge-notched-beam methods height. Fig. 7 shows the variation of nominal stress with Due to the deflection of the transverse cracks along the normalized notch depth, ao/h. For comparison, the AlO3/Sic interfaces, the indentation strengths were measured nominal stresses at different notch depths are observed to be insensitive to the increases in the indenta also shown in Fig. 7. Obviously, the calculated curve tion load, indicative of excellent damage resistance. Fur coincides well with the experimental data. This result thermore, a rising R-curve behavior was demonstrated indicates that the only effect of the notch is to reduce the from the results of the indentation-strength and the frac- net cross section of the beam. Due to the crack defec ture-toughness measurements tion at the Al]O3/ SiC interfaces, the stress concentration associated with the notch may be effectively eliminated esulting in the observed notch-insensitive behavior Acknowledgements Jihong She would like to thank the Agency of Indus- trial Science and Technology(AIST), Ministry of Inter- national Trade and Industry(MITI)for granting him an AIST Research Fellowship at Osaka National Research Institute, Japa References e11 []C. Russo, M.P. Harmer, H M. Chan, G.A. Miller, Design of a laminated ceramic composite for improved strength and tough- J.Am. Ceram.Soc.75(1992)3396-3400 2B.J. Choi, K.H. Koh, H E. Kim, Mechanical properties of SiN 5 (1998)2725-2728 2.0 2.5 3 J.H. She, S. Scheppokat, R. Janssen, N. Claussen, Reaction- Notch Depth(mm) bonded three-layer alumina-based composites with improved damage resistance, J. Am. Ceram Soc. 81(1998)1374-1376 Fig. 6. Fracture toughness as a function of notch depth for multilayer 4 O. Sbaizero, E. Lucchini, Influence of residual stresses on the echanical properties of a layered ceramic composites, J. Eur. Ceram.Soc.l6(1996)813-818 5R. Sathyamoorthy, A.V. Virkar, R.A. Cutler, Damage-resistant Sic-AIN layered composites with surface compressive stresses, J Am. Ceran.Soc.75(1992)1136-114l 600 6R.A. Cutler, J D. Bright, A.V. Virkar, D K. Shetty, Strength mprovement in transformation toughened alumina by selective phase transformation, J. Am. Ceram Soc. 70(1987)714-718 [7 J.J. Hansen, R.A. Cutler, D K. Shetty, A.V. Virkar, Indentation fracture response and damage resistance of Al,O -, compo sites strengthened by transformation-induced residual stresses, J Am. Ceram Soc. 71(1988)C501-C505. [8W.J. Clegg, The fabrication and failure of laminar ceramic com- posites, Acta Metall. Mater. 40(1992)3085-3093. 9W.J. Clegg, K. Kendall, N M. Alford, T w. Button, J D. Birch all, A simple way to make tough ceramics, Nature 347(1990) 455-457 [0 H. Liu, S.M. Hsu, Fracture behavior of multilayer silicon nitride/ boron nitride ceramics, J. Am. Ceram Soc. 79(1996)2452-2457. 0 0.20.40.60.8 [1 D. Kovar, M. D. Thouless, J.W. Halloran, Crack deflection and propagation in layered silicon nitride/boron nitride ceramics, J Normalized Notch Depth Am. Ceram.Soc.81(1998)1004-1012. [2]T. Ohji, Y. Shigegaki, T. Miyajima, s. Kanzaki, Fracture resis- Fig. 7. Effect of the initial notch depth on the maximum nominal stress tance behavior of multilayered silicon nitride, J. Am. Ceram Soc under 4-point bending tests. The solid line is calculated from Eq. (5) 80(1997)991-994

the plane of the notch is constant, the nominal stress, n, can be calculated from [17] n ˆ o 1 ÿ ao h  2 …5† where o is the notch depth, and h is the specimen height. Fig. 7 shows the variation of nominal stress with normalized notch depth, ao=h. For comparison, the measured nominal stresses at di€erent notch depths are also shown in Fig. 7. Obviously, the calculated curve coincides well with the experimental data. This result indicates that the only e€ect of the notch is to reduce the net cross section of the beam. Due to the crack de¯ec￾tion at the Al2O3/SiC interfaces, the stress concentration associated with the notch may be e€ectively eliminated, resulting in the observed notch-insensitive behavior. 4. Conclusions Multilayer ceramic composites may exhibit some unique mechanical properties. In this work, the damage resistance and R-curve behavior of multilayer Al2O3/ SiC ceramics were investigated using the indentation￾strength and the single-edge-notched-beam methods. Due to the de¯ection of the transverse cracks along the Al2O3/SiC interfaces, the indentation strengths were observed to be insensitive to the increases in the indenta￾tion load, indicative of excellent damage resistance. Fur￾thermore, a rising R-curve behavior was demonstrated from the results of the indentation-strength and the frac￾ture-toughness measurements. Acknowledgements Jihong She would like to thank the Agency of Indus￾trial Science and Technology (AIST), Ministry of Inter￾national Trade and Industry (MITI) for granting him an AIST Research Fellowship at Osaka National Research Institute, Japan. References [1] C.J. Russo, M.P. Harmer, H.M. Chan, G.A. Miller, Design of a laminated ceramic composite for improved strength and tough￾ness, J. Am. Ceram. Soc. 75 (1992) 3396±3400. [2] B.J. Choi, K.H. Koh, H.E. Kim, Mechanical properties of Si3N4± SiC three-layer composite materials, J. Am. Ceram. Soc. 81 (1998) 2725±2728. [3] J.H. She, S. Scheppokat, R. Janssen, N. Claussen, Reaction￾bonded three-layer alumina-based composites with improved damage resistance, J. Am. Ceram. Soc. 81 (1998) 1374±1376. [4] O. Sbaizero, E. Lucchini, In¯uence of residual stresses on the mechanical properties of a layered ceramic composites, J. Eur. Ceram. Soc. 16 (1996) 813±818. [5] R. Sathyamoorthy, A.V. Virkar, R.A. Cutler, Damage-resistant SiC±AlN layered composites with surface compressive stresses, J. Am. Ceram. Soc. 75 (1992) 1136±1141. [6] R.A. Cutler, J.D. Bright, A.V. Virkar, D.K. Shetty, Strength improvement in transformation toughened alumina by selective phase transformation, J. Am. Ceram. Soc. 70 (1987) 714±718. [7] J.J. Hansen, R.A. Cutler, D.K. Shetty, A.V. Virkar, Indentation fracture response and damage resistance of Al2O3±ZrO2 compo￾sites strengthened by transformation-induced residual stresses, J. Am. Ceram. Soc. 71 (1988) C501±C505. [8] W.J. Clegg, The fabrication and failure of laminar ceramic com￾posites, Acta Metall. Mater. 40 (1992) 3085±3093. [9] W.J. Clegg, K. Kendall, N.M. Alford, T.W. Button, J.D. Birch￾all, A simple way to make tough ceramics, Nature 347 (1990) 455±457. [10] H. Liu, S.M. Hsu, Fracture behavior of multilayer silicon nitride/ boron nitride ceramics, J. Am. Ceram. Soc. 79 (1996) 2452±2457. [11] D. Kovar, M.D. Thouless, J.W. Halloran, Crack de¯ection and propagation in layered silicon nitride/boron nitride ceramics, J. Am. Ceram. Soc. 81 (1998) 1004±1012. [12] T. Ohji, Y. Shigegaki, T. Miyajima, S. Kanzaki, Fracture resis￾tance behavior of multilayered silicon nitride, J. Am. Ceram. Soc. 80 (1997) 991±994. Fig. 6. Fracture toughness as a function of notch depth for multilayer Al2O3/SiC ceramics. Fig. 7. E€ect of the initial notch depth on the maximum nominal stress under 4-point bending tests. The solid line is calculated from Eq. (5). 804 J. She et al. / Ceramics International 26 (2000) 801±805

J. She et al. Ceramics International 26(2000)801-805 [13J H. She, T. Inoue, K. Ueno, Multilayer Al2O3/SiC cera toughness: I. Direct crack measurements. J. Am. Ceram. Soc. 64 improved mechanical behavior, J. Eur. Ceram. Soc.. 20(2000) (1981)533-538 [16 P. Chantikul, G.R. Anstis, B.R. Lawn. D B. Marshall, A critical [14 B.R. Lawn, A.G. Evans, D B. Marshall, Elastic/plastic indenta valuation of indentation techniques for measuring fracture tough- tion damage in ceramics: the median/radial crack system, J. Am. ness: I Strength method. J Am Ceram Soc. 64( 1981)539-543. Ceram.Soc.63(1980)574581 [17 C.A. Folsom, F.W. Zok, F.F. Lange, D B. Marshall, Mechanical [15 G.R. Anstis, P. Chantikul. B.R. Lawn, D B. Marshall. A critical behavior of a laminar ceramic/fiber-reinforced epoxy composite, evaluation of indentation techniques for uring fracture J.Am. Ceran.Soc.75(1992)2969-2975

[13] J.H. She, T. Inoue, K. Ueno, Multilayer Al2O3/SiC ceramics with improved mechanical behavior, J. Eur. Ceram. Soc., 20 (2000) 1771±1775. [14] B.R. Lawn, A.G. Evans, D.B. Marshall, Elastic/plastic indenta￾tion damage in ceramics: the median/radial crack system, J. Am. Ceram. Soc. 63 (1980) 574±581. [15] G.R. Anstis, P. Chantikul, B.R. Lawn, D.B. Marshall, A critical evaluation of indentation techniques for measuring fracture toughness: I. Direct crack measurements, J. Am. Ceram. Soc. 64 (1981) 533±538. [16] P. Chantikul, G.R. Anstis, B.R. Lawn, D.B. Marshall, A critical evaluation of indentation techniques for measuring fracture tough￾ness: I. Strength method, J. Am. Ceram. Soc. 64 (1981) 539±543. [17] C.A. Folsom, F.W. Zok, F.F. Lange, D.B. Marshall, Mechanical behavior of a laminar ceramic/®ber-reinforced epoxy composite, J. Am. Ceram. Soc. 75 (1992) 2969±2975. J. She et al. / Ceramics International 26 (2000) 801±805 805