Availableonlineatwww.sciencedirect.com DIRECT E噩≈3S SEVIER Journal of the European Ceramic Society 25(2005)589-597 www.elsevier.com/locate/jeurceramsoc Mechanical properties and mechanical behaviour of Sic dense-porous laminates C. Reynaud a F. Thevenot a T. Chartier b, *.J.-L. Besson b Dept. Ceramiques Speciales, E.N.S. des Mines de Saint-Etienne, 42023 Saint-Etienne, france b SPCTS, U.M.R. C.N.R.S. 6638, E.N.S. de Ceramiques Industrielles, 87065 Limoges, france Received 3 December 2003; received in revised form 23 February 2004; accepted 28 February 2004 Available online 15 June 2004 Abstract Porous laminar materials and alternate laminates of silicon carbide dense and porous layers have been elaborated by tape casting and liquid phase sintering processing. Porosity was introduced by incorporation of pore forming agents(corn starch or graphite platelets) in the slurry. Homogeneous distributions of porosity have been obtained for both monolithic and composite laminates. The microstructure of the SiC matrix was equiaxed and was not affected by the porosity. The porosity(P) dependence of Young s modulus(e), modulus of rupture(or), toughness (Kic)and fracture energy(Gic)was found to be well described on the entire range of porosity by relations of the form Xo(1-P)x proposed by Wagh et al. from a model that takes into account the tortuosity of the porosity In the case of our materials, mE= 2.7, mog Kic= mE +0.5 and mGIc =mE+ 1. All the ex-corn starch composites behaved in a brittle manner, even those having weak interlayers with a porosity content higher than the critical value of about 0. 4 predicted by the model developed by Blanks et al. A non-purely brittle behaviour started to be obtained with ex-graphite laminar composites in which the pores are elongated and oriented parallel to the interfaces Keywords: Composites; Porosity; Mechanical properties; SiC; Laminates 1. Introduction As a necessary condition for the interfacial material that it must be chemically compatible with the material of Among the strategies developed to improve the flaw tol- the strong layers, Clegg and co-workers studied ceramic erance of non-transforming ceramics, the designing of lami- laminates with alternating dense and porous layers made nar structures with weak interfaces or interphases promoting of silicon carbide or alumina. This approach has also the crack deflection mechanisms have proved to be a successful advantage of avoiding the building up of any internal stresses way to increase fracture energy. The weak interfaces most due to difference in thermal expansion coefficients which mmonly used are either graphite or boron nitride. 2,3 For can be the source of delamination during cooling. 9 instance, in the case of graphite interfaces, Clegg et al. have According to theoretical analysis by He and Hutchinson obtained an apparent toughness and a fracture energy re- of the kinking of a crack out of an interface, 0 crack deflec- spectively 5 and 200 times higher than the typical values of tion is to occur when the ratio between the fracture energy monolithic a-SiC. However, since graphite and boron ni- of the homogeneous weak interphase, Gi, and the fracture tride have a low oxidation resistance and cannot be used at energy of the strong layer, Gs, is lower than 0.57. However high temperature in oxidising atmosphere without a protec- in the case of dense-porous laminates, the porous layer is tive barrier, other systems have been investigated, specially inhomogeneous, and the fracture energy to be considered is with oxidation resistant weak interphases such as monazite, 4 that of the ligament of matter between the crack and the pore fluorophlogopite or MoSi2+ Mo2 B5.6 ahead of it, Glig?. Than the criterion for crack deflection in the weak interphase becomes Glig/Gs <0.57. Though these ligaments of matter are made of the same material as Corresponding author the dense layers, and might be expected to have the same E-mail address: t chartier(aensci fr (T. Chartier). fracture energy, theoretical analyses show that if the tip of a 0955-2219/s-see front matter o 2004 Published by Elsevier Ltd doi: 10.1016/j jeurceramsoc 2004.02.009Journal of the European Ceramic Society 25 (2005) 589–597 Mechanical properties and mechanical behaviour of SiC dense-porous laminates C. Reynaud a, F. Thévenot a, T. Chartier b,∗, J.-L. Besson b a Dept. Céramiques Spéciales, E.N.S. des Mines de Saint-Etienne, 42023 Saint-Etienne, France b S.P.C.T.S., U.M.R. C.N.R.S. 6638, E.N.S. de Céramiques Industrielles, 87065 Limoges, France Received 3 December 2003; received in revised form 23 February 2004; accepted 28 February 2004 Available online 15 June 2004 Abstract Porous laminar materials and alternate laminates of silicon carbide dense and porous layers have been elaborated by tape casting and liquid phase sintering processing. Porosity was introduced by incorporation of pore forming agents (corn starch or graphite platelets) in the slurry. Homogeneous distributions of porosity have been obtained for both monolithic and composite laminates. The microstructure of the SiC matrix was equiaxed and was not affected by the porosity. The porosity (P) dependence of Young’s modulus (E), modulus of rupture (σR), toughness (K1C) and fracture energy (G1C) was found to be well described on the entire range of porosity by relations of the form X0(1 − P)mX proposed by Wagh et al. from a model that takes into account the tortuosity of the porosity. In the case of our materials, mE = 2.7, mσR = mK1C = mE + 0.5 and mG1C = mE + 1. All the ex-corn starch composites behaved in a brittle manner, even those having weak interlayers with a porosity content higher than the critical value of about 0.4 predicted by the model developed by Blanks et al. A non-purely brittle behaviour started to be obtained with ex-graphite laminar composites in which the pores are elongated and oriented parallel to the interfaces. © 2004 Published by Elsevier Ltd. Keywords: Composites; Porosity; Mechanical properties; SiC; Laminates 1. Introduction Among the strategies developed to improve the flaw tol￾erance of non-transforming ceramics, the designing of lami￾nar structures with weak interfaces or interphases promoting crack deflection mechanisms have proved to be a successful way to increase fracture energy. The weak interfaces most commonly used are either graphite1 or boron nitride.2,3 For instance, in the case of graphite interfaces, Clegg et al. have obtained an apparent toughness and a fracture energy re￾spectively 5 and 200 times higher than the typical values of monolithic
-SiC.1 However, since graphite and boron ni￾tride have a low oxidation resistance and cannot be used at high temperature in oxidising atmosphere without a protec￾tive barrier, other systems have been investigated, specially with oxidation resistant weak interphases such as monazite,4 fluorophlogopite5 or MoSi2 + Mo2B5. 6 ∗ Corresponding author. E-mail address: t.chartier@ensci.fr (T. Chartier). As a necessary condition for the interfacial material is that it must be chemically compatible with the material of the strong layers, Clegg and co-workers studied ceramic laminates with alternating dense and porous layers made of silicon carbide7 or alumina.8 This approach has also the advantage of avoiding the building up of any internal stresses due to difference in thermal expansion coefficients which can be the source of delamination during cooling.9 According to theoretical analysis by He and Hutchinson of the kinking of a crack out of an interface,10 crack deflec￾tion is to occur when the ratio between the fracture energy of the homogeneous weak interphase, Gi, and the fracture energy of the strong layer, GS, is lower than 0.57. However, in the case of dense-porous laminates, the porous layer is inhomogeneous, and the fracture energy to be considered is that of the ligament of matter between the crack and the pore ahead of it, Glig7. Than the criterion for crack deflection in the weak interphase becomes Glig/GS < 0.57. Though these ligaments of matter are made of the same material as the dense layers, and might be expected to have the same fracture energy, theoretical analyses show that if the tip of a 0955-2219/$ – see front matter © 2004 Published by Elsevier Ltd. doi:10.1016/j.jeurceramsoc.2004.02.009