Availableonlineatwww.sciencedirect.com ScienceDirect E噩≈RS ELSEVIER Journal of the European Ceramic Society 27(2007)351-356 www.elsevier.com/locate/jeurceramsoc Effects of residual stresses on the fracture properties of non-oxide laminated composites S Guicciardi a, c M. Nagliatia, c C. Melandri a, c G. Pezzotti b, c.D. Scitia,C, CNR-/STEC, Institute of Science and Technology for Ceramics, Via Granarolo 64. 1-48018 Faenza, lte, yoto, Japan b Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Sakyo-ku, Matsugasaki, 606-8585 Research Institute for Nanoscience(RIN), Kyoto Institute of Technology, Sakyo-ku, Matsugasaki, 606-8585 Kyoto, Japan Received 16 November 2005; received in revised form 26 April 2006: accepted 29 April 2006 Available online 16 june 2006 A layered ceramic composite in the AIN-SiC-MoSi2 system was prepared with the outer layers under residual compressive stress. The mechanical properties of the constituent layers and of the laminated composite were measured. Due to the residual compressive stress, the fracture strength of the laminated composite was higher than the strength of the outer layer material. The fracture toughness of the laminar composite was evaluated by SEVNB. The resulting values were compared with a fracture mechanics model and a good agreement was found between the experimental measurements and the calculated apparent fracture toughness profile 2006 Elsevier Ltd. All rights reserved. Keyword: Composites: Mechanical properties: Laminates; AIN; SiC; MoSiz Introduction was obtained by alternating layers with two different com positions in the AIN-SiC-MoSiz system and with a stack The design of ceramic laminates has been proved to be a ing sequence that left the outer layers under compression viable strategy to obtain significant increases of the fracture AIN-SiC-MoSiz ceramic composites were previously shown to resistance of ceramic materials. 1-7 The basic idea is to cou- couple good mechanical properties to electro-conductivity. 2-14 ple material layers with different thermal expansion coefficients In similar compositions and architecture, these composites are CTE)so that residual stresses arise during cooling from the currently employed in industrial applications such as heaters sintering temperature. The relative thickness of adjacent layers, and igniters. It is therefore mandatory from the engineering their Youngs modulus and CTEs affect these residual stresses point of view to understand the effects of the residual stresses whose sign and magnitude can be adjusted through the compo- on the mechanical properties of the layered architecture, with sition, the stacking sequence and the layer thicknesses particular emphasis on the fracture properties. To this purpose, Compressive layers placed within a laminate have been in the present paper mechanical properties of the constituent proved to be able to arrest cracks and this phenomenon can materials and of the layered composite were measured and produce composites with a threshold strength in which failure analysed does not occur until a critical stress is applied. Also the fra ture toughness benefits from the presence of compressive lay- 2. Experimental ers across laminated composites. Moreover, residual surface compressive stresses have been shown to improve the tribolog- 2.1. Materials preparation ical properties. In this work, a non-oxide ceramic multilayer was pro Two different layers with the following compositions(in duced by the tape casting technique. The layered composite vol %)were prepared Corresponding author. Tel. +39 699 748: fax: +39 0546 4638 55aIN +15SiC+30MoSi, labelled as C: E-mailaddress:dile@@istec.cnr.it(D.Sciti) 80aIN+10Sic+10MoSiz labelled as i 0955-2219/S-see front matter o 2006 Elsevier Ltd. All rights reserved. doi: 10.1016/j-jeurceramsoc200604. 173Journal of the European Ceramic Society 27 (2007) 351–356 Effects of residual stresses on the fracture properties of non-oxide laminated composites S. Guicciardi a,c, M. Nagliati a,c, C. Melandri a,c, G. Pezzotti b,c, D. Sciti a,c,∗ a CNR-ISTEC, Institute of Science and Technology for Ceramics, Via Granarolo 64, I-48018 Faenza, Italy b Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Sakyo-ku, Matsugasaki, 606-8585 Kyoto, Japan c Research Institute for Nanoscience (RIN), Kyoto Institute of Technology, Sakyo-ku, Matsugasaki, 606-8585 Kyoto, Japan Received 16 November 2005; received in revised form 26 April 2006; accepted 29 April 2006 Available online 16 June 2006 Abstract A layered ceramic composite in the AlN–SiC–MoSi2 system was prepared with the outer layers under residual compressive stress. The mechanical properties of the constituent layers and of the laminated composite were measured. Due to the residual compressive stress, the fracture strength of the laminated composite was higher than the strength of the outer layer material. The fracture toughness of the laminar composite was evaluated by SEVNB. The resulting values were compared with a fracture mechanics model and a good agreement was found between the experimental measurements and the calculated apparent fracture toughness profile. © 2006 Elsevier Ltd. All rights reserved. Keyword: Composites; Mechanical properties; Laminates; AlN; SiC; MoSi2 1. Introduction The design of ceramic laminates has been proved to be a viable strategy to obtain significant increases of the fracture resistance of ceramic materials.1–7 The basic idea is to cou￾ple material layers with different thermal expansion coefficients (CTE) so that residual stresses arise during cooling from the sintering temperature. The relative thickness of adjacent layers, their Young’s modulus and CTEs affect these residual stresses whose sign and magnitude can be adjusted through the compo￾sition, the stacking sequence and the layer thicknesses. Compressive layers placed within a laminate have been proved to be able to arrest cracks and this phenomenon can produce composites with a threshold strength in which failure does not occur until a critical stress is applied.8 Also the frac￾ture toughness benefits from the presence of compressive lay￾ers across laminated composites.9,10 Moreover, residual surface compressive stresses have been shown to improve the tribolog￾ical properties.11 In this work, a non-oxide ceramic multilayer was pro￾duced by the tape casting technique. The layered composite ∗ Corresponding author. Tel.: +39 0546 699 748; fax: +39 0546 46381. E-mail address: dile@istec.cnr.it (D. Sciti). was obtained by alternating layers with two different com￾positions in the AlN–SiC–MoSi2 system and with a stack￾ing sequence that left the outer layers under compression. AlN–SiC–MoSi2 ceramic composites were previously shown to couple good mechanical properties to electro-conductivity.12–14 In similar compositions and architecture, these composites are currently employed in industrial applications such as heaters and igniters. It is therefore mandatory from the engineering point of view to understand the effects of the residual stresses on the mechanical properties of the layered architecture, with particular emphasis on the fracture properties. To this purpose, in the present paper mechanical properties of the constituent materials and of the layered composite were measured and analysed. 2. Experimental 2.1. Materials preparation Two different layers with the following compositions (in vol.%) were prepared: • 55AlN + 15SiC + 30MoSi2, labelled as C; • 80AlN + 10SiC + 10MoSi2, labelled as I. 0955-2219/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2006.04.173