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)991-994the 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