L. Zhang V.D. Krstic/Theoretical and Applied fracture Mechanics 24(1995)13-19 value for apparent fracture toughness is signifi- much larger strain to failure and that after the cantly smaller than the measured values. How- peak load is reached failure is still not catas- ever, when the strength of the individual layer is trophic used(i.e. 800 MPa. ) the predicted fracture tough- ness is 14.4 MPam/2, which is very close to experimentally measured Kic. It should be stressed however, that one of the problems asso- References ciated with Eq. (3)is that it does not include the effect of graphite layer thickness. As already dis- .E. Gordon, Mechanisam for the control gation in all-brittle system, Proc. R Soc. cussed, the strength and fracture toughness of 32,508520(1964) partially converted graphite layer depends criti [2]RA.. Sambell, D H. Bowen and D. C. Phillips, Carbon cally upon the thickness of the SiC layers. These fibre composites with ceramic and glass matrices, J. in turn, controls the fracture response of the 3] K.M. Prewo and J.J. Brennan, High-strength carbide entire structure fibre-reinforced glass-matrix composites. J. Mater. Sci. 15,463468(1980) [4]WJ. Clegg, K. Kendall, N M. Alford, D. Birchall and 4. Conclusion T.W. Button, A simple way to make tough ceramics Nature347,455-457(1990) Using a simple slip casting technique it has [5]wJ. Clegg and J D. Birchall, in: Proc. 4th int. Conf. on Fibre Reinforced Composites, edited by G. Gibson, Inst proven possible to produce laminated Sic ceram- Mechan Eng. Liv ics with apparent fracture toughness one order of [6] wJ. Clegg, The fracture and failure of laminar ceramic magnitude higher than that of a monolithic mate- composites, Acta Metall. Mater. 40, 3085-3093(1992). rial. Due to the presence of weak interfaces and [7] w.J. Clegg, K Kendall, N MCN. Alford, T w. Button and repeated crack initiation across each SiC layer, J D. Birchall, in: Brit. Ceram. ics, Vol 9, Institute of Ceramics. Stoke-on-Trent, UK, p. the structure exhibits no notch width sensitivity 263(1980) and is capable of supporting the same load as the [8] V D. Ktrstic, Prodaction of fine, high-purity beta silicon sample without the notch but of the same height bide powders, J. Am. Ceram. Soc. 75(1)170-174 It is shown that the toughening is controlled by (1992) both the Sic and graphite layer thickness. A [9] D.J. Munz, J. L. Shannon and R.t. Fracture calculation from Maximum four poir maximum in apparent toughness is achieved at of chevron notch specimens, fract. 16 Sic to graphite layer thickness of 30. This ratio provides the condition under which complete [10] L Zhang and V.D. Krstic, High Toughness Carbide and Method Thereof, U.s occurs leading to neration of an optimum interfacial strength Patent No.5443770(1995) [11]M.A. Mulla and V D. Krstic, Pressureless sintering of Another important characteristic of this new class B-SiC with Al2O, additions, J. Mater. Sci. 29, 934-938 of sic ceramics is that the material fails with (1994L. Zhang, V.D. Krstic /Theoretical and Applied Fracture Mechanics 24 (1995) 13-19 19 value for apparent fracture toughness is signifi￾cantly smaller than the measured values. How￾ever, when the strength of the individual layer is used (i.e. 800 MPa.) the predicted fracture tough￾ness is 14.4 MPa. m 1/2, which is very close to experimentally measured K1c. It should be stressed, however, that one of the problems asso￾ciated with Eq. (3) is that it does not include the effect of graphite layer thickness. As already dis￾cussed, the strength and fracture toughness of partially converted graphite layer depends criti￾cally upon the thickness of the SiC layers. These in turn, controls the fracture response of the entire structure. 4. Conclusions Using a simple slip casting technique it has proven possible to produce laminated SiC ceram￾ics with apparent fracture toughness one order of magnitude higher than that of a monolithic mate￾rial. Due to the presence of weak interfaces and repeated crack initiation across each SiC layer, the structure exhibits no notch width sensitivity and is capable of supporting the same load as the sample without the notch but of the same height. It is shown that the toughening is controlled by both the SiC and graphite layer thickness. A maximum in apparent toughness is achieved at SiC to graphite layer thickness of 30. This ratio provides the condition under which complete conversion of graphite to SiC occurs leading to generation of an optimum interracial strength. Another important characteristic of this new class of SiC ceramics is that the material fails with much larger strain to failure and that after the peak load is reached failure is still not catas￾trophic. References [1l J. Cook and J.E. Gordon, Mechanisam for the control of crack propagation in all-brittle system, Proc. K Soc. London A282, 508-520 (1964). [2l R.A.J. Sambell, D.H. Bowen and D.C. Phillips, Carbon fibre composites with ceramic and glass matrices, J. Mater. Sci. 7, 663-675 (1972). [3] K.M. Prewo and J.J. Brennan, High-strength carbide fibre-reinforced glass-matrix composites. J. Mater. Sci. 15, 463-468 (1980). [4] W.J. Clegg, K. Kendall, N.M. Alford, D. Birchall and T.W. Button, A simple way to make tough ceramics, Nature 347, 455-457 (1990). [5l W.J. Clegg and J.D. Birchall, in: Proc. 4th Int. Conf. on Fibre Reinforced Composites, edited by G. Gibson, Inst. of Mechan. Eng., Liverpool, pp. 179-184 (1990). [6] W.J. Clegg, The fracture and failure of laminar ceramic composites, Acta Metall. Mater. 40, 3085-3093 (1992). [7] W.J. Clegg, K. Kendall, N.McN. Alford, T.W. Button and J.D. Birchall, in: Brit. Ceram. Soc. Conf. Special Ceram￾ics, Vol. 9, Institute of Ceramics. Stoke-on-Trent, UK, p. 263 (1980). [8] V.D. Ktrstic, Prodaction of fine, high-purity beta silicon carbide powders, J. Am. Ceram. Soc. 75(1) 170-174 (1992). [9] D.J. Munz, J.L Shannon and R.T. Bubsey, Fracture toughness calculation from Maximum load in four point bend tests of chevron notch specimens, Int. J. Fract. 16, 137 (1980). [10] L. Zhang and V.D. Krstic, High Toughness Carbide Ceramics by Slip Casting and Method Thereof, U.S. Patent No. 5,443,770 (1995). [11] M.A. Mulla and V.D. Krstic, Pressureless sintering of B-SiC with AI203 additions, J. Mater. ScL 29, 934-938 (1994)