Availableonlineatwww.sciencedirect.com SCIENCE E噩≈S Journal of the European Ceramic Society 24(2004)825-831 www.elsevier.com/locatejeurceramsoc Effect of porous interlayers on crack deflection in ceramic laminates J. Ma Hongzhi Wang, Luqian Weng, G.E. B. Tan a school of Materials Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore bDSO National Laboratories, 20 Science Park Drive, Singapore 118230, Singape Received 13 March 2003: received in revised form 10 April 2003; accepted 27 April 2003 Abstract Ceramic layered systems with interlayers of various porosities were fabricated using tape casting technique. Submicron size alu nina powders were used to make the tapes for both the strong dense laminae and the weak porous interlayers. Porosity was ntroduced into the interlayers by the addition of PMMa powders. The pores generated were found to be spherical and uniformly distributed. The crack deflection capability of the layered systems with interlayers of different porosity were then investigated. To acilitate the study, the fracture energies of the different porosity monolithic porous layers, and also the dense alumina layer, were quantified using four point bending tests. It was observed that there exists an optimum porosity that the crack deflection, hence the fracture energy of the system, can be maximized. Theories proposed in the literature on crack deflection in layered systems were also discussed and compared with the present experimental findings C 2003 Elsevier Ltd. All rights reserved Keywords: Al2O3; Crack deflection: Fracture: Laminates; Porosity 1. Introduction presence of residual stresses, they proposed that the cri- tical interface to bulk fracture energy ratio for crack Ceramic layered systems have attracted wide attention deflection is 0. 25. In their subsequent work, they in recent years as such configurations have shown to be incorporated the effect of in-plane and residual stresses effective in improving the toughness of the ceramic in their model to illustrate the influences of these stres components. ,2 It is noted that such enhancement in ses in crack deflection of layered systems. The results of fracture property is mainly attributed to the crack their studies were also found to be consistent with that deflection capability in the interlayers of such systems. computed numerically by other researchers. 10 Both experimental and theoretical works reported in the The above studies have shown that to build a tough literature have indicated that the ability to deflect crack ness enhanced layered system, not only a weak interface lepends on the fracture energy ratio of the interlayer is required to promote crack deflection, a chemically and the laminae in the layered systems. 3-7 Cook and compatible interface is also required to avoid the build Gordon have analyzed the problem based on the stres- ing up of internal stresses. An easy way to construct ses at a crack tip and suggested that a crack will be such a system has been proposed by Clegg et al deflected at an interface if the strength of the interface is where a porous interlayer of the same material as that of about 1/5 of that of the matrix. Based on an energy the bulk is to be employed as the weak interface. In their approach, Kendall, on the other hand, has proposed work, natural starches, such as rice and potato starch that crack deflection will occur if the fracture energy of were used to generate the pores in the porous interlayer the interface is less than 10 to 20% of the matrix, where via burnt-off during sintering. In the present work, the exact value depends on the thickness ratio of the however, PMMa powders are used to generate the interface and the matrix layer In a theoretical study by desired porosity in the porous interlayers. It is found He and Hutchinson% for layered systems without the that the PMMa powder particles produce uniformly distributed spherical pores after they burnt out during Corresponding author. Tel +65-67906214; fax: +65-67900920. sintering. Both the dense and porous layers were E-mail address: asima(@ ntu.ed u sg (J. Ma). fabricated using tape casting technique 0955-2219/03/S. see front matter C 2003 Elsevier Ltd. All rights reserved. doi:10.1016S0955-221903)00338-8Effect of porous interlayers on crack deflection in ceramic laminates J. Maa,*, Hongzhi Wanga , Luqian Wenga , G.E.B. Tanb a School of Materials Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore bDSONational Laboratories, 20 Science Park Drive, Singapore 118230, Singapore Received 13 March 2003; received in revised form 10 April 2003; accepted 27 April 2003 Abstract Ceramic layered systems with interlayers of various porosities were fabricated using tape casting technique. Submicron size alu￾mina powders were used to make the tapes for both the strong dense laminae and the weak porous interlayers. Porosity was introduced into the interlayers by the addition of PMMA powders. The pores generated were found to be spherical and uniformly distributed. The crack deflection capability of the layered systems with interlayers of different porosity were then investigated. To facilitate the study, the fracture energies of the different porosity monolithic porous layers, and also the dense alumina layer, were quantified using four point bending tests. It was observed that there exists an optimum porosity that the crack deflection, hence the fracture energy of the system, can be maximized. Theories proposed in the literature on crack deflection in layered systems were also discussed and compared with the present experimental findings. # 2003 Elsevier Ltd. All rights reserved. Keywords: Al2O3; Crack deflection; Fracture; Laminates; Porosity 1. Introduction Ceramic layered systems have attracted wide attention in recent years as such configurations have shown to be effective in improving the toughness of the ceramic components.1,2 It is noted that such enhancement in fracture property is mainly attributed to the crack deflection capability in the interlayers of such systems. Both experimental and theoretical works reported in the literature have indicated that the ability to deflect crack depends on the fracture energy ratio of the interlayer and the laminae in the layered systems.3
7 Cook and Gordon8 have analyzed the problem based on the stres￾ses at a crack tip and suggested that a crack will be deflected at an interface if the strength of the interface is about 1/5 of that of the matrix. Based on an energy approach, Kendall,3 on the other hand, has proposed that crack deflection will occur if the fracture energy of the interface is less than 10 to 20% of the matrix, where the exact value depends on the thickness ratio of the interface and the matrix layer. In a theoretical study by He and Hutchinson4 for layered systems without the presence of residual stresses, they proposed that the cri￾tical interface to bulk fracture energy ratio for crack deflection is 0.25. In their subsequent work,9 they incorporated the effect of in-plane and residual stresses in their model to illustrate the influences of these stres￾ses in crack deflection of layered systems. The results of their studies were also found to be consistent with that computed numerically by other researchers.10 The above studies have shown that to build a tough￾ness enhanced layered system, not only a weak interface is required to promote crack deflection, a chemically compatible interface is also required to avoid the build￾ing up of internal stresses. An easy way to construct such a system has been proposed by Clegg et al.,1,11 where a porous interlayer of the same material as that of the bulk is to be employed as the weak interface. In their work, natural starches, such as rice and potato starch, were used to generate the pores in the porous interlayer via burnt-off during sintering. In the present work, however, PMMA powders are used to generate the desired porosity in the porous interlayers. It is found that the PMMA powder particles produce uniformly distributed spherical pores after they burnt out during sintering. Both the dense and porous layers were fabricated using tape casting technique. 0955-2219/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0955-2219(03)00338-8 Journal of the European Ceramic Society 24 (2004) 825–831 www.elsevier.com/locate/jeurceramsoc * Corresponding author. Tel.: +65-67906214; fax: +65-67900920. E-mail address: asjma@ntu.ed.u.sg (J. Ma)