Availableonlineatwww.sciencedirect.com ScienceDirect E噩≈RS ELSEVIER Joumal of the European Ceramic Society 27(2007)1449-1453 www.elsevier.comlocate/jeurceramsoc Effective fracture toughness in Al,O3-Al2O3/zrO laminates Tanja Lube a, * Javier Pascuala, Francis Chalvet b, Goffredo de portu b a Institut fiir Struktur- und Funkionskeramik, Montanuniversitat Leoben, Peter Tinner StraBe 5, Leoben A-8700, Austria b Istituto di Scienza e Technologia dei Materiali Ceramici-CNR Via Granarolo 64. 48018 Faenza, Italy Available online 30 May 2006 During the processing of laminar ceramics, biaxial residual stresses can arise due to the thermal mismatch between different layers. For ceramic multilayers, the beneficial consequences of compressive stresses at the surface are well known: increase in strength, apparent toughness and reliability. Nevertheless, the resulting tensile stresses may induce a negative influence in the effective fracture toughness if the tensile stresses are hi The weight function technique is used to assess the stress intensity factor corresponding to the residual stresses field. The infuence of geometrical parameters such as thickness, number of layers and tension/compression thickness ratio is analyzed. For different multilaye (AlO3-xAl2O3 /(1-x)ZrO2), effective R-curves are presented. The existence of an optimal architecture that maximizes the toughening is exposed as well as two tendencies on the apparent R-curve that define different fracture patterns: brittle failure or layer-by-layer fracture 2006 Elsevier Ltd. All rights reserved. Keywords: Composites; Toughening: Al2 O3: ZrO2: Laminate 1. Introductio This paper examines laminates with strong interfaces, in par ticular multilayers made of alumina(A)and an alumina-zirconia Ceramic composites have a broad range of industrial appli- composite(AZ). Those multilayers with an A-outer layer are cations. They have been extensively developed for structural shielded due to the minor thermal expansion of A compared to components in order to improve the mechanical, chemical and the composite AZ. thermal performance of engineering devices. However, despite Although fracture toughness of a layered composite can be a high hardness, an excellent oxidation resistance, and high tem- experimentally measured, it is only an apparent or effective value perature stability, ceramics are inherently brittle. One of the because of the influence of the residual stress. Besides, different laminates with residual stresses. 1,2gt strategies to decrease brittleness is through the design of ceramic shielding effects or intrinsic properties of the structure, such as bridging associated to grain size, rend difficult the interpretation Laminates can improve mechanical performance since sur- of toughness measurements. face compression introduces a closure stress that protects against The apparent R-curve of a laminate can be calculated con- faws. Two strategies of laminate design have been previ- sidering the equilibrium condition at the crack tip, i.e. crack ously presented: first, laminates with a weak interface that propagation is possible if the stress intensity at the crack tip deflects cracks, thus preventing catastrophic failure 4 and sec- Ktip, equals or exceeds the intrinsic material toughness Ko ond, laminates with strong interfaces. Since strong interfaces will transmit residual stresses during cooling from sintering Ktip(a)>Ke.o being Ktip(@)=Kappl(a)+Kres(a),(1) temperature, one can benefit of a phase transformation nermal mismatch to induce compressive stresses at the where Kapp(a)is the applied stress intensity and Kres(a)is the stress intensity contribution from the residual stress. Solving for Kappl(a) holds Kappl(a)>ke.0-Kres(a)=KR.effective Corresponding author. Tel. +43 3842 402 4111: fax: +43 3842 402 4102 E-mailaddresses:tanjalube@mu-leobenat(TLube),deport@istec.cnr.twhereKapp(a)equalsthedesiredeffectiveR-curve (G. de Portu). KReffective(a) 0955-2219/S-see front matter o 2006 Elsevier Ltd. All rights reserved. doi: 10. 1016/j-jeurceramsoc. 2006.04.063Journal of the European Ceramic Society 27 (2007) 1449–1453 Effective fracture toughness in Al2O3–Al2O3/ZrO2 laminates Tanja Lube a,∗, Javier Pascual a, Francis Chalvet b, Goffredo de Portu b a Institut f ¨ur Struktur- und Funktionskeramik, Montanuniversit ¨at Leoben, Peter Tunner Straße 5, Leoben A-8700, Austria b Istituto di Scienza e Technologia dei Materiali Ceramici-CNR, Via Granarolo 64, 48018 Faenza, Italy Available online 30 May 2006 Abstract During the processing of laminar ceramics, biaxial residual stresses can arise due to the thermal mismatch between different layers. For ceramic multilayers, the beneficial consequences of compressive stresses at the surface are well known: increase in strength, apparent toughness and reliability. Nevertheless, the resulting tensile stresses may induce a negative influence in the effective fracture toughness if the tensile stresses are high. The weight function technique is used to assess the stress intensity factor corresponding to the residual stresses field. The influence of geometrical parameters such as thickness, number of layers and tension/compression thickness ratio is analyzed. For different multilayers (Al2O3 − xAl2O3/(1 − x)ZrO2), effective R-curves are presented. The existence of an optimal architecture that maximizes the toughening is exposed as well as two tendencies on the apparent R-curve that define different fracture patterns: brittle failure or layer-by-layer fracture. © 2006 Elsevier Ltd. All rights reserved. Keywords: Composites; Toughening; Al2O3; ZrO2; Laminate 1. Introduction Ceramic composites have a broad range of industrial appli￾cations. They have been extensively developed for structural components in order to improve the mechanical, chemical and thermal performance of engineering devices. However, despite a high hardness, an excellent oxidation resistance, and high tem￾perature stability, ceramics are inherently brittle. One of the strategies to decrease brittleness is through the design of ceramic laminates with residual stresses.1,2 Laminates can improve mechanical performance since sur￾face compression introduces a closure stress that protects against flaws. Two strategies of laminate design have been previ￾ously presented: first, laminates with a weak interface that deflects cracks, thus preventing catastrophic failure3,4 and sec￾ond, laminates with strong interfaces. Since strong interfaces will transmit residual stresses during cooling from sintering temperature, one can benefit of a phase transformation4 or a thermal mismatch5 to induce compressive stresses at the surface. ∗ Corresponding author. Tel.: +43 3842 402 4111; fax: +43 3842 402 4102. E-mail addresses: tanja.lube@mu-leoben.at (T. Lube), deportu@istec.cnr.it (G. de Portu). This paper examines laminates with strong interfaces, in par￾ticular multilayers made of alumina (A) and an alumina–zirconia composite (AZ). Those multilayers with an A-outer layer are shielded due to the minor thermal expansion of A compared to the composite AZ. Although fracture toughness of a layered composite can be experimentally measured, it is only an apparent or effective value because of the influence of the residual stress. Besides, different shielding effects or intrinsic properties of the structure, such as bridging associated to grain size, rend difficult the interpretation of toughness measurements. The apparent R-curve of a laminate can be calculated con￾sidering the equilibrium condition at the crack tip, i.e. crack propagation is possible if the stress intensity at the crack tip, Ktip, equals or exceeds the intrinsic material toughness K0: Ktip(a) ≥ Kc,0 being Ktip(a) = Kappl(a) + Kres(a), (1) where Kappl(a) is the applied stress intensity and Kres(a) is the stress intensity contribution from the residual stress. Solving for Kappl(a) holds Kappl(a) ≥ Kc,0 − Kres(a) = KR,effective, (2) where Kappl(a) equals the desired effective R-curve. KR,effective(a). 0955-2219/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2006.04.063