Enhanced fracture toughness by ceramic laminate design L A Gee*,R S. Dobedoe R Vann M. H. Lewis, G. Blugan and J Kuebler A review of the potential toughening and failure mechanisms for ceramic laminate materials presented. An integrated approach to the design of ceramic laminates incorporating biaxial residual stresses for specific applications is outlined. Restrictions placed on the laminate architecture to avoid spontaneous transverse cracking of the tensile layer are discussed. The phenomena of edge cracking and crack bifurcation are considered with reference to elastic moduli, Poissons ratio, mismatch in thermal expansion coefficients, temperature gradient and laminate architecture. The use of compressive layers to produce a material that exhibits a threshold strength and criteria for increasing the critical applied stress below which failure will not occur are reported. A single edge V-notched beam (SEVNB)test geometry was used to measure crack growth resistance(R curve) behaviour of multilayer Si3N4/Si3N4-TiN composites Fracture mechanics weight function analysis was applied to predict the R curve behaviour of multilayer composites having a stepwise change in composition. A conservative, non-optimised laminate design exhibiting apparent fracture toughness in excess of 17 MPa m is reported, in excellent agreement with the weight function analysis Keywords: Ceramic laminates, Crack bifurcation, Fracture toughness, Single edge V-notched beam test, Weight function analysis ntroduction the strength of such ceramics is technologically and furthermore weak interfaces are known to Ceramic materials are known to exhibit numerous mise certain properties. For example, high attractive properties including high strength, elastic ture corrosion resistance may be worsened due to modulus, hardness and chemical and thermal stability. defect density. The use of residual stres However, monolithic ceramic materials have intrinsic associated with thermal expansion mismatch within limitations owing to their I eliability and brittleness, ceramic laminate materials therefore offers an attractive which make them susceptible to catastrophic failure. An alternative mechanism to improve fracture behaviour. improvement in the fracture toughness of such ceramic Considerable attention has been concentrated on materials would inevitably expand the range of indus- residual stress induced cracking in laminates. This trial application problem is frequently encountered in electronic packa The design of ceramic composites with layered macro- ging, adhesive joining and other technologies. On structures is receiving considerable research attention cooling from an elevated temperature Toto a tempera because they exhibit decreased sensitivity to surface ture T. two materials with coefficients of thermal defects, and have been shown to demonstrate non- expansion(CTEs)aI and a2 forming a layered structure mechanical properties in multilayer systems is the ability to deflect propagating cracks. Two different mechanisms of crack deflection have previously been employed. EM=(a2-a1)dT (1) interfaces with adjacent layers or into layers exhibiting Now consider a balanced laminate (which experiences residual biaxial compressive stress.- Essentially, the first no bending forces)with alternating layers of thickness !1 and t, formed from materials I and 2 respectively. Away nechanism depends on matrix/interface strength ratio and from the free surface the residual stress perpendicular to has had varying degrees of success. However, controlling the layers is zero, whereas in the plane of the laminate it is uniform and biaxial. In material l( the layer with the ng Research Institute, Sheffield lower CTE), the residual, biaxial compressive stress on is dvanced Materials, Department of Physics, University of given by Coventry CV4 7AL, UK feral Laboratories for Materials Testing and Research, EMPA, 01- EM2 CH-8600 Bubendorf. Switzerland Corresponding author, email i gee @ shu. ac uk 2005 Institute of Materials, Minerals and Mining pted 2o February2。05 Dol1o.179/174367605X166 Advances in Applied Ceramics 2005 VOL 104 No 3 103Enhanced fracture toughness by ceramic laminate design I. A. Gee*1 , R. S. Dobedoe2 , R. Vann2 , M. H. Lewis2 , G. Blugan3 and J. Kuebler3 A review of the potential toughening and failure mechanisms for ceramic laminate materials is presented. An integrated approach to the design of ceramic laminates incorporating biaxial residual stresses for specific applications is outlined. Restrictions placed on the laminate architecture to avoid spontaneous transverse cracking of the tensile layer are discussed. The phenomena of edge cracking and crack bifurcation are considered with reference to elastic moduli, Poisson’s ratio, mismatch in thermal expansion coefficients, temperature gradient and laminate architecture. The use of compressive layers to produce a material that exhibits a threshold strength and criteria for increasing the critical applied stress below which failure will not occur are reported. A single edge V-notched beam (SEVNB) test geometry was used to measure crack growth resistance (R curve) behaviour of multilayer Si3N4/Si3N4–TiN composites. Fracture mechanics weight function analysis was applied to predict the R curve behaviour of multilayer composites having a stepwise change in composition. A conservative, non-optimised laminate design exhibiting apparent fracture toughness in excess of 17 MPa m1/2 is reported, in excellent agreement with the weight function analysis. Keywords: Ceramic laminates, Crack bifurcation, Fracture toughness, Single edge V-notched beam test, Weight function analysis Introduction Ceramic materials are known to exhibit numerous attractive properties including high strength, elastic modulus, hardness and chemical and thermal stability. However, monolithic ceramic materials have intrinsic limitations owing to their poor reliability and brittleness, which make them susceptible to catastrophic failure. An improvement in the fracture toughness of such ceramic materials would inevitably expand the range of indus￾trial applications. The design of ceramic composites with layered macro￾structures is receiving considerable research attention because they exhibit decreased sensitivity to surface defects, and have been shown to demonstrate non￾catastrophic failure.1–6 A key feature that imparts good mechanical properties in multilayer systems is the ability to deflect propagating cracks. Two different mechanisms of crack deflection have previously been employed. Propagating cracks can be deflected either along weak interfaces with adjacent layers1–7 or into layers exhibiting residual biaxial compressive stress.8–11 Essentially, the first mechanism depends on matrix/interface strength ratio and has had varying degrees of success. However, controlling the strength of such ceramics is technologically difficult, and furthermore weak interfaces are known to compro￾mise certain properties. For example, high tempera￾ture corrosion resistance may be worsened due to the high defect density. The use of residual stress patterns associated with thermal expansion mismatch within ceramic laminate materials therefore offers an attractive alternative mechanism to improve fracture behaviour. Considerable attention has been concentrated on residual stress induced cracking in laminates. This problem is frequently encountered in electronic packa￾ging, adhesive joining and other technologies. On cooling from an elevated temperature T0 to a tempera￾ture T, two materials with coefficients of thermal expansion (CTEs) a1 and a2 forming a layered structure suffer a mismatch strain eM of: eM~ ðT0 T ð Þ a2{a1 dT (1) Now consider a balanced laminate (which experiences no bending forces) with alternating layers of thickness t1 and t2 formed from materials 1 and 2 respectively. Away from the free surface the residual stress perpendicular to the layers is zero, whereas in the plane of the laminate it is uniform and biaxial. In material 1 (the layer with the lower CTE), the residual, biaxial compressive stress s1 is given by: s1~{ eME1 0 1z t1E1 0 t2E2 0
(2) 1 Materials and Engineering Research Institute, Sheffield Hallam University, Sheffield S1 1WB, UK 2 Centre for Advanced Materials, Department of Physics, University of Warwick, Coventry CV4 7AL, UK 3 Swiss Federal Laboratories for Materials Testing and Research, EMPA, CH-8600 Du¨ bendorf, Switzerland *Corresponding author, email i.gee@shu.ac.uk
2005 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 18 August 2004; accepted 20 February 2005 DOI 10.1179/174367605X16671 Advances in Applied Ceramics 2005 VOL 104 NO 3 103