Availableonlineatwww.sciencedirect.com Part B: engineering ELSEVIER Composites: Part B 37(2006)481-489 Design and production of ceramic laminates with high mechanical reliability Vincenzo M. Sglavo*Massimo bertoldi I Dipartimento di Ingegneria dei Materiali e Tecnologie Industriali, Universita di Trento, Via Mesiano, 77, 38050 Trento, Italy Available online 20 March 2006 Abstract a procedure for designing innovative ceramic laminates characterized by high mechanical reliability is proposed in this work. A fracture mechanics approach has been considered to define the stacking sequence, thickness and composition of the different laminae on the basis of the requested strength and of the defect size distribution. Once the different laminae are stacked together a residual stress profile is generated upon cooling after sintering because of the differential thermal expansion coefficient. Such residual stress profile is conceived in order to allow stable growth of surface defects upon bending and guarantee limited strength scatter. As an example, the proposed approach is used to design and produce ceramic laminates in the alumina-zirconia and alumina-mullite system. Mechanical performances of the produced materials are discussed in terms of the generated residual stress profile and compared to parent monolithic ceramics. C 2006 Elsevier ltd. all rights reserved Keywords: A. Ceramic-matrix composites; B. Residual/internal stress; B. Strength; B. Fracture toughness; Ceramic laminates 1. Introduction been improved by introducing low-energy paths for growin crack in laminated structures [2-6] or by introducing Ceramics are commonly considered as brittle materials. In compressive residual stresses [7, 8]. Laminates presenting spite of this, their very attractive physical and chemical threshold strength have been also successfully produced by properties make such materials suitable for different appli- alternating thin compressive layers and thicker tensile layers cations. The limited fracture toughness associated to the [9]. Unfortunately, the most important limitations of such presence of flaws generated either upon processing and in laminates is that they can be used only with specific service is responsible for their limited mechanical reliability. orientations with respect to the applied load and, for example, The resulting strength scatter is usually too large to allow safe they are not easily suitable to produce real components such design,unless statistical approaches identifying acceptable plates, shells or tubes as usually required in typical minimum failure risk are used. In addition, fracture usually applications occurs in a catastrophic manner in absence of any warning of The idea that surface stresses can hinder the growth of the incipient rupture [1] surface cracks has been extensively exploited in the past Many efforts have been made in the past to increase the especially on glasses [10-11]. Sglavo and Green have recently mechanical reliability of glasses and ceramics. Higher fracture proposed that the creation of a residual stress profile in glass toughness have been attained through the exploitation of the with a maximum compression at a certa pth from the reinforcing action of grain anisotropy or second phases or the surface can arrest surface cracks and result in higher failure promotion of crack shielding effects associated to phase- stress and limited strength variability [12-14 One can point transformation or micro-cracking [1]. In such cases, a precise out that surface flaws represent the most typical defects in microstructure control is always required and this is achievable ceramic and glasses: in fact, once the processing procedures are conditions. As an alternativefracture behavior of ceramics has generate volume defects [15, 16], surface flaws are normally only surface defects become critical when a body is subjected rresponding author Tel. +39 461 882468: fax: +39 40 to bending and not to tension, as it is usually the case in ceramic E-mail address: vincenzo. sglavo@ unitn. it(V M. Sglavo) components Now at Eurocoating Spa, Via Al Dos de la Roda 60, 38057 Pergine Residual stresses in ceramic materials can arise either from differences in the thermal expansion coefficient of the 1359-8368/S- see front matter 2006 Elsevier Ltd. All rights reserved. constituting grains or phases, uneven sintering rates or doi: 10. 1016/ martensitic phase transformations. As described below, if theDesign and production of ceramic laminates with high mechanical reliability Vincenzo M. Sglavo *, Massimo Bertoldi 1 Dipartimento di Ingegneria dei Materiali e Tecnologie Industriali, Universita` di Trento, Via Mesiano, 77, 38050 Trento, Italy Available online 20 March 2006 Abstract A procedure for designing innovative ceramic laminates characterized by high mechanical reliability is proposed in this work. A fracture mechanics approach has been considered to define the stacking sequence, thickness and composition of the different laminae on the basis of the requested strength and of the defect size distribution. Once the different laminae are stacked together a residual stress profile is generated upon cooling after sintering because of the differential thermal expansion coefficient. Such residual stress profile is conceived in order to allow stable growth of surface defects upon bending and guarantee limited strength scatter. As an example, the proposed approach is used to design and produce ceramic laminates in the alumina–zirconia and alumina–mullite system. Mechanical performances of the produced materials are discussed in terms of the generated residual stress profile and compared to parent monolithic ceramics. q 2006 Elsevier Ltd. All rights reserved. Keywords: A. Ceramic-matrix composites; B. Residual/internal stress; B. Strength; B. Fracture toughness; Ceramic laminates 1. Introduction Ceramics are commonly considered as brittle materials. In spite of this, their very attractive physical and chemical properties make such materials suitable for different appli￾cations. The limited fracture toughness associated to the presence of flaws generated either upon processing and in service is responsible for their limited mechanical reliability. The resulting strength scatter is usually too large to allow safe design, unless statistical approaches identifying acceptable minimum failure risk are used. In addition, fracture usually occurs in a catastrophic manner in absence of any warning of the incipient rupture [1]. Many efforts have been made in the past to increase the mechanical reliability of glasses and ceramics. Higher fracture toughness have been attained through the exploitation of the reinforcing action of grain anisotropy or second phases or the promotion of crack shielding effects associated to phase￾transformation or micro-cracking [1]. In such cases, a precise microstructure control is always required and this is achievable only with a careful control of starting material and process conditions. As an alternative, fracture behavior of ceramics has been improved by introducing low-energy paths for growing crack in laminated structures [2–6] or by introducing compressive residual stresses [7,8]. Laminates presenting threshold strength have been also successfully produced by alternating thin compressive layers and thicker tensile layers [9]. Unfortunately, the most important limitations of such laminates is that they can be used only with specific orientations with respect to the applied load and, for example, they are not easily suitable to produce real components such as plates, 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]. Sglavo and Green have recently proposed that the creation of a residual stress profile in glass with a maximum compression at a certain depth from the surface can arrest surface cracks and result in higher failure stress and limited strength variability [12–14]. One can point out that surface flaws represent the most typical defects in ceramic and glasses: in fact, once the processing procedures are optimized to reduce or remove heterogeneities that can generate volume defects [15,16], surface flaws are normally created during surface finishing or upon service. In addition, only surface defects become critical when a body is subjected to bending and not to tension, as it is usually the case in ceramic components. Residual stresses in ceramic materials can arise either from differences in the thermal expansion coefficient of the constituting grains or phases, uneven sintering rates or martensitic phase transformations. As described below, if the Composites: Part B 37 (2006) 481–489 www.elsevier.com/locate/compositesb 1359-8368/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2006.02.001 * Corresponding author Tel.: C39 461 882468; fax: C39 461 881945. E-mail address: vincenzo.sglavo@unitn.it (V.M. Sglavo). 1 Now at Eurocoating Spa, Via Al Dos de la Roda 60, 38057 Pergine Valsugana, Italy