Availableonlineatwww.sciencedirect.cor ° Science Direct CERAMICS INTERNATIONAL ELSEVIER Ceramics International 34(2008)197-203 www.elsevier.com/locate/ceramint Potential of Sic multilayer ceramics for high temperature applications in oxidising environment Matteo pavese Paolo Fino a. Alberto ortona Claudio badini a Politecnico di Torino, Dipartimento di Scienza dei Materiali e Ingegneria Chimica, corso Duca degli Abruzzi 24, 10129 Torino, Italy University of Applied Sciences(SUPSI). The iCIMSI Research Institute, Galleria 2, CH 6928 Manno, Switzerlan Received 25 November 2005: received in revised form 6 January 2006: accepted 28 September 2006 Available online 2 November 2006 Abstract Multilayered ceramics seem very promising for applications at very high temperatures in an oxidising environment. Actually, lower cost and better oxidation resistance than many conventional ceramic composites The multilayered SiC oxidation and shock resistance has been investigated on tubular specimens processed by tape casting and pressureless sintering. Microstructure, oxidation and mechanical behaviour were investigated by micro-XRD, SEM, TGA-DTA-MS, indentation and radial compressive tests The mechanical characterization showed that weak interfacial bonds are present between the layers. Together with the residual stresses left after the preparation phase, they caused crack deflection and improved toughness with respect to traditional ceramics. These mechanisms persisted even after long-term oxidation at 1600C or repeated thermal shock tests. The strength was found to depend on the thickness of the single Sic layer, however it was only slightly affected by thermal treatments Keywords: A. Tape casting: D. SiC: Multilayers; Thermal shock resistance 1. Introduction Multilayered ceramics can be obtained by several methods: tape casting, slip casting, rolling, extrusion, followed by Monolithic ceramics show a catastrophic fracture behaviour sintering or hot pressing. The most used method is however tape under applied stress due to the lack of energy absorbing casting [3-9] that consists in casting a thick film of a slurry mechanisms in the failure process. Several tough composites containing the ceramic powders on a polymeric support. Then and multilayer ceramics were, however, developed in recent the tape is dried and the organic substances are removed by years. The key factor improving the toughness of these slow heating. Finally, the sintering can be realised without materials is the presence of weak interfaces between fibres and pressure or by hot pressing. In any case, these materials result he composite matrix or between the ceramic layers. These cheaper than fibre reinforced composites. interfaces allow for energy dissipation before fracture through Several multilayer ceramics have been investigated in the mechanisms of crack deflection, crack bridging, fibre pull out past [4-22]; the most studied materials have been alumina or and interface delamination alumina-zirconia [4-6, 10-11, silicon nitride [12, 13], silicon On fibre-based composites, debonding and pull out are carbide [14-19], even if other composites have been tested frequently achieved by putting a thin interphase layer on the [7, 20-22 fibre surface. For instance, interphases, generally less than In the case of multilayers, two methods have been used to I um thick, made of carbon or boron nitride, were successfully enhance toughness over conventional ceramics, namely the used in SiC/SiCr composites [1, 2] introduction of weak interfaces or the presence of residual stresses In the first case porous interlayers can be used Email adres: matteo, paese polito it (M. pavese,, [7, 11, 12, 14, 18, 19, 23], where the porosity is given by layers not wholly sintered, generally of a different material with 2-8842/$32.00 2006 Elsevier Ltd and Techna Group S.r.L. All rights reserved 10.1016 1-ceramint.2006.09008Potential of SiC multilayer ceramics for high temperature applications in oxidising environment Matteo Pavese a, *, Paolo Fino a , Alberto Ortona b , Claudio Badini a aPolitecnico di Torino, Dipartimento di Scienza dei Materiali e Ingegneria Chimica, corso Duca degli Abruzzi 24, 10129 Torino, Italy b University of Applied Sciences (SUPSI), The iCIMSI Research Institute, Galleria 2, CH 6928 Manno, Switzerland Received 25 November 2005; received in revised form 6 January 2006; accepted 28 September 2006 Available online 2 November 2006 Abstract Multilayered ceramics seem very promising for applications at very high temperatures in an oxidising environment. Actually, they present lower cost and better oxidation resistance than many conventional ceramic composites. The multilayered SiC oxidation and shock resistance has been investigated on tubular specimens processed by tape casting and pressureless sintering. Microstructure, oxidation and mechanical behaviour were investigated by micro-XRD, SEM, TGA–DTA-MS, indentation and radial compressive tests. The mechanical characterization showed that weak interfacial bonds are present between the layers. Together with the residual stresses left after the preparation phase, they caused crack deflection and improved toughness with respect to traditional ceramics. These mechanisms persisted even after long-term oxidation at 1600 8C or repeated thermal shock tests. The strength was found to depend on the thickness of the single SiC layer, however it was only slightly affected by thermal treatments. # 2006 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Tape casting; D. SiC; Multilayers; Thermal shock resistance 1. Introduction Monolithic ceramics show a catastrophic fracture behaviour under applied stress due to the lack of energy absorbing mechanisms in the failure process. Several tough composites and multilayer ceramics were, however, developed in recent years. The key factor improving the toughness of these materials is the presence of weak interfaces between fibres and the composite matrix or between the ceramic layers. These interfaces allow for energy dissipation before fracture through mechanisms of crack deflection, crack bridging, fibre pull out and interface delamination. On fibre-based composites, debonding and pull out are frequently achieved by putting a thin interphase layer on the fibre surface. For instance, interphases, generally less than 1 mm thick, made of carbon or boron nitride, were successfully used in SiC/SiCf composites [1,2]. Multilayered ceramics can be obtained by several methods: tape casting, slip casting, rolling, extrusion, followed by sintering or hot pressing. The most used method is however tape casting [3–9] that consists in casting a thick film of a slurry containing the ceramic powders on a polymeric support. Then the tape is dried and the organic substances are removed by slow heating. Finally, the sintering can be realised without pressure or by hot pressing. In any case, these materials result cheaper than fibre reinforced composites. Several multilayer ceramics have been investigated in the past [4–22]; the most studied materials have been alumina or alumina–zirconia [4–6,10–11], silicon nitride [12,13], silicon carbide [14–19], even if other composites have been tested [7,20–22]. In the case of multilayers, two methods have been used to enhance toughness over conventional ceramics, namely the introduction of weak interfaces or the presence of residual stresses. In the first case porous interlayers can be used [7,11,12,14,18,19,23], where the porosity is given by layers not wholly sintered, generally of a different material with www.elsevier.com/locate/ceramint Ceramics International 34 (2008) 197–203 * Corresponding author. Tel.: +39 011 564 4708; fax: +39 011 564 4699. E-mail address: matteo.pavese@polito.it (M. Pavese). 0272-8842/$32.00 # 2006 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2006.09.008