J.Am. Ceran.Soe,88102826-2832(2005) Dol:l0.l11551-2916.2005.00479x journal C) 2005 The American Ceramic Society Tailored Residual Stresses in High Reliability Alumina-Mullite Ceramic Laminates Vincenzo m. sglavo, * I Massimo Paternoster, and Massimo bertoldi Dipartimento di Ingegneria dei Materiali e Tecnologie Industriali, Universita di Trento, 38050 Trento, Italy a design and processing approach to fabricate ceramic lami- As an alternative fracture behavior of ceramics has been im- nates with high mechanical reliability, i. e, high failure resist- proved by introducing low-energy paths for growing cracks in e, limited strength scatter, and increased damage tolerance is laminated structures. This has been achieved using either sented in this paper. Different ceramic layers are stacked rous or weak interlayer. 4to promote delamination and crack together to develop a specific residual stress profile after sinte- deflection. In this way the strength is usually not increased, but ing. By changing the composition of the laminae and the com- the deformation and the energy absorbed before failure are am- architecture it is possible to produce a material with plified many times. In other cases, sandwiched structures de redefined failure stress which can be evaluated from the frac- signed to improve the mechanical performance were proposed ture toughness curve correlated to the residual stresses. In ad- based on other microstructural mechanisms. 5, 6 urface defects can be forced to grow in a stable way before reaching the critical tures in which the strength is controlled by the presence of com- condition, thus obtaining a unique-value strength ceramic ma- pressive residual stresses. Laminates presenting threshold terial Laminates composed of alumina mullite composite layers strength, i.e a minimum stress value below which rupture are designed and created in this work by the implementation of does not occur, have been successfully produced by lange and he proposed approach. The material obtained shows a"con- o-workers by alternating thin compressive layers and thicker stant"strength of 456 MPa(standard deviation <7%) even tensile layers. The most significant limitations of such laminates when large surface damage is produced by vickers indentation. is that they can be used only with specific orientations to the applied load and, for example, they require complex manufac- turing to produce shells or tubes as usually required in typical L. Introduction ROBABly the fundamental reason for the scarce employ of The idea that surface stresses can hinder the growth of surface ceramic materials in structural applications is their limited cracks has been extensively exploited in the past especially on mechanical reliability. Although ceramics possess many glasses. 0, II It is important to point out that surface flaws rep- tive properties suitable for different applications, such materials resent the most typical defects in ceramic and glasses: in fact, have low fracture toughness. In addition, processing and dam- once the processing procedures are optimized to reduce or elim- age and degradation in service, results in flaws of varying sizes hate heterogeneities that can produce volume defects, sur- The resulting strength scatter is usually too large to allow safe ace flaws are normally generated during surface finishing or design, unless statistical approaches embodying acceptable min- when a body is subjected to bending and not to tension, as is imum failure risk are used. In addition, fracture usually occurs in a catastrophic manner in absence of any warning of the usually, the case in ceramic components. Recently, Sglavo and Green have proposed that the creation of a residual stress profile Much effort has been made in the past to overcome such prob- with a maximum compression at a certain depth from the sur lems. Solutions proposed to improve the mechanical behavior of and limited strength variability. 4 This approach has been ap- defects, or to increase fracture toughness by microstructural con- plied to silicate glasses by producing the residual stress field via a trol. Higher fracture toughness has been achieved through the double ion-e exploitation of the reinforcing action of grain anisotropy or sec- Residual stresses in ceramic materials can arise either from ond phases or the promotion of crack shielding effects associated differences in the thermal expansion coefficient of the constitut- to phase-transformation or micro-cracking. Unfortunately, all ing grains or phases, from uneven sintering rates or from mar- these solutions overcome the problem of the wide strength scatter tensitic phase transformations associated to specific volume to a limited extent only. Moreover, a precise microstructure con- lange. As described in the present paper, if the development trol is always required and this is achievable only with a careful of the residual stresses in ceramic multilayer is opportunely con- control of starting material and process conditions. The same trolled, materials characterized by high fracture resistance and limited strength scatter can be designed and produced. By var- strict requirements are needed when the reduction of flaw severity ying the nature, the thickness and the stacking order of the lam- which allows cutting the low-stress tail of critical flaw popula- inae, the residual stress profile developed after sintering can be tion.Nevertheless, costs associated with the preloading of all the tailored to promote the growth of surface cracks in a stable bodies are usually too high for most applications ibrated and varied as required is obtained by changing of the multilayer"structure". This approach, as described in this pa- per, has been implemented for the production of alur Manuscript No. 20268 Received July 31, 2004: approved March 7, 2005. Il. Theory and design Procedure n order to analyze the effect of residual stresses on crack Author to whom correspondence should be addressed. e-mail: sglavo a ingunitn.it agation and resistance failure in brittle material, theTailored Residual Stresses in High Reliability Alumina-Mullite Ceramic Laminates Vincenzo M. Sglavo,* ,w Massimo Paternoster, and Massimo Bertoldi Dipartimento di Ingegneria dei Materiali e Tecnologie Industriali, Universita` di Trento, 38050 Trento, Italy A design and processing approach to fabricate ceramic lami￾nates with high mechanical reliability, i.e., high failure resist￾ance, limited strength scatter, and increased damage tolerance is presented in this paper. Different ceramic layers are stacked together to develop a specific residual stress profile after sinte￾ring. By changing the composition of the laminae and the com￾posite architecture it is possible to produce a material with predefined failure stress which can be evaluated from the frac￾ture toughness curve correlated to the residual stresses. In ad￾dition, by tailoring the fracture toughness curve, surface defects can be forced to grow in a stable way before reaching the critical condition, thus obtaining a unique-value strength ceramic ma￾terial. Laminates composed of alumina/mullite composite layers are designed and created in this work by the implementation of the proposed approach. The material obtained shows a ‘‘con￾stant’’ strength of 456 MPa (standard deviation o7%) even when large surface damage is produced by Vickers indentation. I. Introduction PROBABLY the fundamental reason for the scarce employ of ceramic materials in structural applications is their limited mechanical reliability. Although ceramics possess many attrac￾tive properties suitable for different applications, such materials have low fracture toughness. In addition, processing and dam￾age and degradation in service, results in flaws of varying sizes. The resulting strength scatter is usually too large to allow safe design, unless statistical approaches embodying acceptable min￾imum failure risk are used. In addition, fracture usually occurs in a catastrophic manner in absence of any warning of the incipient rupture.1 Much effort has been made in the past to overcome such prob￾lems. Solutions proposed to improve the mechanical behavior of ceramics aimed either to reduce the presence or the severity of defects, or to increase fracture toughness by microstructural con￾trol. Higher fracture toughness has been achieved through the exploitation of the reinforcing action of grain anisotropy or sec￾ond phases or the promotion of crack shielding effects associated to phase-transformation or micro-cracking.1 Unfortunately, all these solutions overcome the problem of the wide strength scatter to a limited extent only. Moreover, a precise microstructure con￾trol is always required and this is achievable only with a careful control of starting material and process conditions. The same strict requirements are needed when the reduction of flaw severity is pursued. In some cases, the sole solution is the ‘‘proof testing’’ which allows cutting the low-stress tail of critical flaw popula￾tion.1 Nevertheless, costs associated with the preloading of all the bodies are usually too high for most applications. As an alternative, fracture behavior of ceramics has been im￾proved by introducing low-energy paths for growing cracks in laminated structures. This has been achieved using either po￾rous2 or weak interlayer3,4 to promote delamination and crack deflection. In this way the strength is usually not increased, but the deformation and the energy absorbed before failure are am￾plified many times. In other cases, sandwiched structures de￾signed to improve the mechanical performance were proposed based on other microstructural mechanisms.5,6 A different approach has been proposed for laminated struc￾tures in which the strength is controlled by the presence of com￾pressive residual stresses.7–9 Laminates presenting threshold strength, i.e., a minimum stress value below which rupture does not occur, have been successfully produced by Lange and co-workers9 by alternating thin compressive layers and thicker tensile layers. The most significant limitations of such laminates is that they can be used only with specific orientations to the applied load and, for example, they require complex manufac￾turing to produce shells or tubes as usually required in typical applications. The idea that surface stresses can hinder the growth of surface cracks has been extensively exploited in the past especially on glasses.10,11 It is important to point out that surface flaws rep￾resent the most typical defects in ceramic and glasses: in fact, once the processing procedures are optimized to reduce or elim￾inate heterogeneities that can produce volume defects,12,13 sur￾face flaws are normally generated during surface finishing or upon service. In addition, surface defects only become critical when a body is subjected to bending and not to tension, as is usually the case in ceramic components. Recently, Sglavo and Green have proposed that the creation of a residual stress profile with a maximum compression at a certain depth from the sur￾face can arrest surface cracks and result in higher failure stress and limited strength variability.14 This approach has been ap￾plied to silicate glasses by producing the residual stress field via a double ion-exchange process.15,16 Residual stresses in ceramic materials can arise either from differences in the thermal expansion coefficient of the constitut￾ing grains or phases, from uneven sintering rates or from mar￾tensitic phase transformations associated to specific volume change. As described in the present paper, if the development of the residual stresses in ceramic multilayer is opportunely con￾trolled, materials characterized by high fracture resistance and limited strength scatter can be designed and produced. By var￾ying the nature, the thickness and the stacking order of the lam￾inae, the residual stress profile developed after sintering can be tailored to promote the growth of surface cracks in a stable manner before final failure. Thus, the strength that can be cal￾ibrated and varied as required is obtained by changing of the multilayer ‘‘structure’’. This approach, as described in this pa￾per, has been implemented for the production of alumina/mul￾lite composite laminates. II. Theory and Design Procedure In order to analyze the effect of residual stresses on crack prop￾agation and resistance failure in brittle material, the simple Journal J. Am. Ceram. Soc., 88 [10] 2826–2832 (2005) DOI: 10.1111/j.1551-2916.2005.00479.x r 2005 The American Ceramic Society 2826 S. J. Glass—contributing editor Supported by University of Trento (Italy). Originally presented at the 2004 Annual Meeting in the symposium in honor of Ed Fuller. *Member, American Ceramic Society. w Author to whom correspondence should be addressed. e-mail: sglavo@ing.unitn.it Manuscript No. 20268. Received July 31, 2004; approved March 7, 2005