Availableonlineatwww.sciencedirect.com SCIENCE Acta materialia ELSEVIER Acta Materialia 53(2005)289-296 www.actamat-journals.com Apparent fracture toughness of Si3 N4-based laminates with residual compressive or tensile stresses in surface layers M. Lugovy a,* V. Slyunyayev a, N. Orlovskaya b, G. Blugan, J. Kuebler M. Lewis d Institute for Problems of Materials Science, 3 Kchizhnouski St, 03142 Kiev, Ukraine Drexel Unirersity, Philadelphia, US.A EMPA, Duebendorf, Switzerland nirersity of Warwick, Coventry, UK Received 18 December 2003: received in revised form 17 September 2004: accepted 20 September 2004 Available online 28 October 2004 Abstract The effect of macroscopic residual stresses on the fracture resistance and stable/unstable crack growth in Si3N4/Si3N4-30 wt% TiN layered ceramics has been investigated. The laminates were manufactured using rolling and hot pressing techniques. An appar- ent fracture toughness Kapp was calculated as a function of the crack length parameter d= Y(a)a-for the laminates with residual compressive or tensile stresses in the top layers. The toughness increases in the layers with a compressive stress with increasing crack length, and it decreases in the layers with a tensile stress as the crack continues to grow. An explanation for the experimentally meas- ured and calculated Kapp values is proposed. The existence of the threshold stress and the stable/unstable crack growth conditions is 004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: A. Layered structures; B. Fracture toughness; Modeling: C. Crack; Residual stress 1. Introduction cation in layers with a bulk residual compression [41 phase transformation of zirconia grains under loading Ceramic matrix composites have a broad range of [5], and compositional gradient [6-8 industrial applications. They have been extensively used The mismatch of thermal expansion coefficients be- as structural components in order to improve the tween different layers inevitably generates thermal resid mechanical, thermal and chemical performance of engi- ual stresses during subsequent cooling of layered neering devices. However, despite a high hardness, an ceramics with strong interfaces [9]. The relative thick excellent oxidation resistance, and high temperature sta- ness of different layers determines the relative magni- bility, ceramics are inherently brittle. One of the strate- tudes of compressive and tensile stress, while the strain gies to decrease brittleness and improve composite mismatch between the layers dictates the absolute values performance is through the design of ceramic laminates of the residual stresses 1]. In recent years, a number of papers have been pub- A residual compression of layers results in laminates lished on laminates with weak interfaces for crack toughening, which is a crack shielding phenomenon [10] deflection [2], surface compressive stress [3], crack bifur- It has been shown that a residual compression of 500 MPa in a surface layer of a three-layered alumina-zirco- Corresponding author. Tel :+38 44 457 4890: fax: +38 44 296 nia composite can increase the fracture toughness by a 1684/380444442131. factor of 7. 5(up to 30 MPa m")for crack lengths equal E-mail address: lugovy @viptelecom net (M. Lugovy to the surface layer thickness [3]. This mechanism is 1359-6454$30.00 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved doi: 10. 1016/j. actamat. 2004.09.0Apparent fracture toughness of Si3N4-based laminates with residual compressive or tensile stresses in surface layers M. Lugovy a,*, V. Slyunyayev a , N. Orlovskaya b , G. Blugan c , J. Kuebler c , M. Lewis d a Institute for Problems of Materials Science, 3 Kzhizhnovski St., 03142 Kiev, Ukraine b Drexel University, Philadelphia, USA c EMPA, Duebendorf, Switzerland d University of Warwick, Coventry, UK Received 18 December 2003; received in revised form 17 September 2004; accepted 20 September 2004 Available online 28 October 2004 Abstract The effect of macroscopic residual stresses on the fracture resistance and stable/unstable crack growth in Si3N4/Si3N4–30 wt% TiN layered ceramics has been investigated. The laminates were manufactured using rolling and hot pressing techniques. An appar￾ent fracture toughness Kapp was calculated as a function of the crack length parameter a˜ = Y(a)a1/2 for the laminates with residual compressive or tensile stresses in the top layers. The toughness increases in the layers with a compressive stress with increasing crack length, and it decreases in the layers with a tensile stress as the crack continues to grow. An explanation for the experimentally meas￾ured and calculated Kapp values is proposed. The existence of the threshold stress and the stable/unstable crack growth conditions is discussed.
2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: A. Layered structures; B. Fracture toughness; Modeling; C. Crack; Residual stress 1. Introduction Ceramic matrix composites have a broad range of industrial applications. They have been extensively used as structural components in order to improve the mechanical, thermal and chemical performance of engi￾neering devices. However, despite a high hardness, an excellent oxidation resistance, and high temperature sta￾bility, ceramics are inherently brittle. One of the strate￾gies to decrease brittleness and improve composite performance is through the design of ceramic laminates [1]. In recent years, a number of papers have been pub￾lished on laminates with weak interfaces for crack deflection [2], surface compressive stress [3], crack bifur￾cation in layers with a bulk residual compression [4], phase transformation of zirconia grains under loading [5], and compositional gradient [6–8]. The mismatch of thermal expansion coefficients be￾tween different layers inevitably generates thermal resid￾ual stresses during subsequent cooling of layered ceramics with strong interfaces [9]. The relative thick￾ness of different layers determines the relative magni￾tudes of compressive and tensile stress, while the strain mismatch between the layers dictates the absolute values of the residual stresses. A residual compression of layers results in laminates toughening, which is a crack shielding phenomenon [10]. It has been shown that a residual compression of
500 MPa in a surface layer of a three-layered alumina–zirco￾nia composite can increase the fracture toughness by a factor of 7.5 (up to 30 MPa m1/2) for crack lengths equal to the surface layer thickness [3]. This mechanism is 1359-6454/$30.00
2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2004.09.022 * Corresponding author. Tel.: +38 44 457 4890; fax: +38 44 296 1684/380 44 444 21 31. E-mail address: lugovy@viptelecom.net (M. Lugovy). Acta Materialia 53 (2005) 289–296 www.actamat-journals.com