Availableonlineatwww.sciencedirect.com Science Direct E噩≈RS ELSEVIER Joumal of the European Ceramic Society 28(2008)1961-1971 www.elsevier.comlocate/jeurceramsoc fracture behaviour of microcrack -free alumina-aluminium titanate ceramics with second phase nanoparticles at alumina grain boundaries S Bueno a m.H. Berger b.R. Moreno a c.Baudin a a Instituto de Ceramica y Vidrio(CSIC). C. Kelsen 5, 28049 Madrid, Spain Ecole des Mines de Paris, Centre des Materiaur, 91003 Evry Cedex, france Received 12 October 2007: received in revised form 19 December 2007; accepted 4 January 2008 Available online 4 March 2008 Alumina+10 vol %o aluminium titanate composites were obtained by colloidal filtration and reaction sintering of alumina and titania. The materials were dense with aluminium titanate grains of average sizes 2.2-2. 4 um located mainly at alumina triple points. The reaction sintering schedule promoted the formation of additional nanometric grains, identified as aluminium titanate using STEM-EDX analysis between the alumina grains. This special microstructure led to a change of the toughening mechanism from the typical crack bridging reported for microcrack-free composites fabricated from alumina and aluminium titanate powders to microcracking The identification of microcracking as the main toughening mechanism was done from the analysis of stable fracture tests of SENVB samples in three points bending and fractographic observations. Monophase alumina materials with similar grain sizes were used as referer Different fracture toughness parameters were derived from the load-displacement curves: the critical stress intensity factor, KiC, the critical energy release rate, Gic, the J-Integral and the work of fracture, ywoF, and the R curves were also built. The comparison between the linear elastic fracture arameters and the non-linear ones revealed significant toughening and faw tolerance o 2008 Elsevier Ltd. all rights reserved. Keywords: D. Al2O3; D. Al2 TiOs; C Mechanical properties: C. Toughening: B. Nanocomposites Introduction 10-6°C-1,a25-100c=-2:7×10-6°C-)13and mina shows limited anisotropy(aa25-10000C=8.4 x 10-6 6oc-I The use of ceramic materials in structural applications is ac25-10000C=9.2 x 10-6oC-), 4 thus, high tensile or com- limited by the"ilaw sensitive"fracture, occurring sponta- pressive stresses, depending on the particular crystallographic neously from natural flaws, inherent to the brittle behaviour. orientation of the grains, would develop during cooling from The"flaw tolerance"approach deals with the development of the sintering temperature at the grain-matrix interfaces due to microstructures that originate toughening mechanisms to reduce thermal expansion mismatch. Depending on grain size and the the sensitivity of the strength to the size of any processing or characteristics of the grain boundaries, microcracking might induced flaw, thus improving the reliability of the materials. occur during cooling from sintering and/or during fracture. Such mechanisms originate an increasing resistance with con- In the early 90s alumina-aluminium titanate composites tinued crack extension, rising R-curve behaviour, and most of with aluminium titanate contents 20-30 vol% obtained from them are caused by localized internal residual stresses in the alumina and aluminium titanate mixtures, were studied by materials other authors. Crack bridging by second phase agglomerates Alumina(Al2O3-aluminium titanate(Al2TiO5) materials nd by lar was e n offer improved flaw tolerance and toughness 4-12 Ther- ing mechanism leading to R-curve behaviour, assessed by hal expansion of aluminium titanate is highly anisotr- the indentation-strength method; no toughness values were opic(a25-100c010.9×10-60C-1,ab25-100c20.5× Corresponding author. Tel: +3491 7355840: fax: +3491 7355843 In this work. B-Al2TiOs orthorhombic crystal is described by a b-face cen- E-mail address: cbaudin @icv csices(C. Baudin) tered unit cell, space group Bbmm, a=9.439A, b=9.647A c=3.593A 0955-2219/S-see front matter o 2008 Elsevier Ltd. All rights reserved. doi: 10.1016/j-jeurceramsoc 2008.01.01Available online at www.sciencedirect.com Journal of the European Ceramic Society 28 (2008) 1961–1971 Fracture behaviour of microcrack-free alumina–aluminium titanate ceramics with second phase nanoparticles at alumina grain boundaries S. Bueno a, M.H. Berger b, R. Moreno a, C. Baud´ın a,∗ a Instituto de Cer ´amica y Vidrio (CSIC). C. Kelsen 5, 28049 Madrid, Spain b Ecole des Mines de Paris, Centre des Mat´eriaux, 91003 Evry Cedex, France Received 12 October 2007; received in revised form 19 December 2007; accepted 4 January 2008 Available online 4 March 2008 Abstract Alumina + 10 vol.% aluminium titanate composites were obtained by colloidal filtration and reaction sintering of alumina and titania. The materials were dense with aluminium titanate grains of average sizes 2.2–2.4 m located mainly at alumina triple points. The reaction sintering schedule promoted the formation of additional nanometric grains, identified as aluminium titanate using STEM–EDX analysis between the alumina grains. This special microstructure led to a change of the toughening mechanism from the typical crack bridging reported for microcrack-free composites fabricated from alumina and aluminium titanate powders to microcracking. The identification of microcracking as the main toughening mechanism was done from the analysis of stable fracture tests of SENVB samples in three points bending and fractographic observations. Monophase alumina materials with similar grain sizes were used as reference. Different fracture toughness parameters were derived from the load–displacement curves: the critical stress intensity factor, KIC, the critical energy release rate, GIC, the J-Integral and the work of fracture, γWOF, and the R curves were also built. The comparison between the linear elastic fracture parameters and the non-linear ones revealed significant toughening and flaw tolerance. © 2008 Elsevier Ltd. All rights reserved. Keywords: D. Al2O3; D. Al2TiO5; C. Mechanical properties; C. Toughening; B. Nanocomposites 1. Introduction The use of ceramic materials in structural applications is limited by the “flaw sensitive” fracture, occurring sponta￾neously from natural flaws, inherent to the brittle behaviour. The “flaw tolerance” approach deals with the development of microstructures that originate toughening mechanisms to reduce the sensitivity of the strength to the size of any processing or induced flaw, thus improving the reliability of the materials.1–3 Such mechanisms originate an increasing resistance with con￾tinued crack extension, rising R-curve behaviour, and most of them are caused by localized internal residual stresses in the materials. Alumina (Al2O3)–aluminium titanate (Al2TiO5) materials can offer improved flaw tolerance and toughness.4–12 Ther￾mal expansion of aluminium titanate is highly anisotr￾opic (αa25–1000 ◦C = 10.9 × 10−6 ◦C−1, αb25–1000 ◦C = 20.5 × ∗ Corresponding author. Tel.: +34 91 7355840; fax: +34 91 7355843. E-mail address: cbaudin@icv.csic.es (C. Baud´ın). 10−6 ◦C−1, αc25–1000 ◦C = −2.7 × 10−6 ◦C−1) 1 13 and alu￾mina shows limited anisotropy (αa25–1000 ◦C = 8.4 × 10−6 ◦C−1, αc25–1000 ◦C = 9.2 × 10−6 ◦C−1),14 thus, high tensile or com￾pressive stresses, depending on the particular crystallographic orientation of the grains, would develop during cooling from the sintering temperature at the grain–matrix interfaces due to thermal expansion mismatch. Depending on grain size and the characteristics of the grain boundaries, microcracking might occur during cooling from sintering and/or during fracture. In the early 90s alumina–aluminium titanate composites with aluminium titanate contents 20–30 vol.% obtained from alumina and aluminium titanate mixtures, were studied by other authors.4–7 Crack bridging by second phase agglomerates and by large alumina grains was identified as the toughen￾ing mechanism leading to R-curve behaviour, assessed by the indentation–strength method; no toughness values were 1 In this work, -Al2TiO5 orthorhombic crystal is described by a b-face cen￾tered unit cell, space group Bbmm, a = 9.439 A, ˚ b = 9.647 A, ˚ c = 3.593 A. ˚ 0955-2219/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2008.01.017