S. Tariolle et al. Journal of Solid State Chemistry 177(2004)487-492 where Gp is the fracture energy of the porous layer, Gd liquid phase sintered [1o](sintering aid: YAG/alumina that of the dense layer eutectic 5 wt %). According to Eq (3), a minimum of porosity is Boron carbide(B C powder: Tetrabor 3000F, Wacker required to initiate crack deflection at the interface Ceramics, mean diameter 0.75 um) samples were solid between porous and dense layers, this porosity is phase sintered [11-14(sintering aid: phenolic resin 37 vol% and it was confirmed experimentally in Sic [4] 9 wt%(Novolac PN320, Solutia France) and alumina [5] specimens. To verify these energetic criteria, we have underta 2. 2. Sample processing the study of different multi-layered materials fabric Proportions of different components used for the Sic(alternate dense-porous) densified by liquid fabrication of B.C tapes are given in Table 1, the volume phase sintering, this mode of sintering has not been percentage of the main constituents in green tapes in used for this type of laminar composite up to now. Fig. I, respectively B4C(alternate dense-porous)densified by solid phase Notably, there is a high amount of organic ac sintering, that also is a new laminar system. especially in tapes containing PFA, which B4C with weak graphite interfaces has been produced induces burn-out difficulties, requiring to perform LOO burn-out with very slow heating rate Preparation of SiC and BC slurries was very similar The used processing steps are shown in the flow chart 2. Experimental The different constituents were milled and mixed at Different types of materials were prepare different rotation speeds. After de-airing at a very slow rotation speed and verifying the viscosity, slurries were Monolithic rnate dense-porous specimens tape cast. The tapes were cut, stacked in a predefined for Sic (the on process is fully described in sequence and thermo-compacted under a pressure of Ref.四9] 60 MPa at a temperature of 70C. Then, the samples Monolithic and alternate dense-porous specimens for were debindered and pyrolyzed (in air for SiC and in B4C. argon atmosphere for B. C)at very slow heating rate and finally they were sintered in VAS (Vide App Tape casting and thermocompression processing was System)graphite fur chosen to prepare multi-layered materials. This techni- arace under argon atmosphere(at 1950C for I h for SiC and at 2150C for I h for B.C) que allows to obtain thick and uniform layers. First the different constituents of the tape casting slurry will Table I be presented. Then, the preparation of the samples Composition of green tapes for B,C in wt% will be explained. Finally, techniques of characterization will be given. Dense layer Porous layer Porous layer (PFA) (under-sintered) 2.1. Ceramic powders and organic additives Ceramic 63 Dispersant Sintering agent 12.5 0 Slurries for tape-casting usually contain a mix of PFA different organic compounds. As organic solvent, the MEK-Ethanol azeotrope(60 vol% butanone-2 /40 vol% Plasticizer ethanol) was used. As dispersant, Beycostat C213 (CECA-France) was used to disperse the ceram 100% wder. An Organic ensured the cohesion and the flexibility of the tape, 口PFA respectively. Polyamide powders(mean diameter: 4.7, ■ Ceramic 10.5 and 20.7 um, PAl2, Orgasol, Atofina, France) together with corn starch (mean diameter: 14 Roquette, france) were used as pore forming agents 20% PFA). For SiC, a powder of graphite platelets(mean size:8×8×3μm3, Union Carbide) was used. For B4c, a graphite spray(Graphene. Orapi) was utilized to make Silicon carbide (Sic powder: Sika Tech FCP13 Norton-Norway, mean diameter I um) samples were Fig. 1. Volume percentage of main constituents in green tapes for B.C.where Gp is the fracture energy of the porous layer, Gd that of the dense layer. According to Eq. (3), a minimum of porosity is required to initiate crack deflection at the interface between porous and dense layers, this porosity is 37 vol% and it was confirmed experimentally in SiC [4] and alumina [5] specimens. To verify these energeticcriteria, we have undertaken the study of different multi-layered materials fabricated by tape-casting: * SiC (alternate dense–porous) densified by liquid phase sintering, this mode of sintering has not been used for this type of laminar composite up to now. * B4C (alternate dense–porous) densified by solid phase sintering, that also is a new laminar system. * B4C with weak graphite interfaces has been produced too. 2. Experimental Different types of materials were prepared: * Monolithicand alternate dense–porous specimens for SiC (the elaboration process is fully described in Ref. [9]). * Monolithicand alternate dense–porous specimens for B4C. Tape casting and thermocompression processing was chosen to prepare multi-layered materials. This techni￾que allows to obtain thick and uniform layers. First, the different constituents of the tape casting slurry will be presented. Then, the preparation of the samples will be explained. Finally, techniques of characterization will be given. 2.1. Ceramic powders and organic additives Slurries for tape-casting usually contain a mix of different organiccompounds. As organicsolvent, the MEK-Ethanol azeotrope (60 vol% butanone-2/40 vol% ethanol) was used. As dispersant, Beycostat C213 (CECA-France) was used to disperse the ceramic powder. An acrylic binder and a phthalate plasticizer ensured the cohesion and the flexibility of the tape, respectively. Polyamide powders (mean diameter: 4.7, 10.5 and 20.7 mm, PA12, Orgasol, Atofina, France) together with corn starch (mean diameter: 14 mm, Roquette, France) were used as pore forming agents (PFA). For SiC, a powder of graphite platelets (mean size: 8  8  3 mm3 , Union Carbide) was used. For B4C, a graphite spray (Graphe`ne, Orapi) was utilized to make weak interlayers. Silicon carbide (SiC powder: Sika Tech FCP13, Norton-Norway, mean diameter 1 mm) samples were liquid phase sintered [10] (sintering aid: YAG/alumina eutectic 5 wt%). Boron carbide (B4C powder: Tetrabor 3000F, Wacker Ceramics, mean diameter 0.75 mm) samples were solid phase sintered [11–14] (sintering aid: phenolicresin 9 wt% (NovolacPN320, Solutia France). 2.2. Sample processing Proportions of different components used for the fabrication of B4C tapes are given in Table 1, the volume percentage of the main constituents in green tapes in Fig. 1, respectively. Notably, there is a high amount of organicadditives, especially in tapes containing PFA, which further induces burn-out difficulties, requiring to perform burn-out with very slow heating rate. Preparation of SiC and B4C slurries was very similar. The used processing steps are shown in the flow chart (Fig. 2). The different constituents were milled and mixed at different rotation speeds. After de-airing at a very slow rotation speed and verifying the viscosity, slurries were tape cast. The tapes were cut, stacked in a predefined sequence and thermo-compacted under a pressure of 60 MPa at a temperature of 70C. Then, the samples were debindered and pyrolyzed (in air for SiC and in argon atmosphere for B4C) at very slow heating rate and finally they were sintered in VAS (Vide Appareillage System) graphite furnace under argon atmosphere (at 1950C for 1 h for SiC and at 2150C for 1 h for B4C). ARTICLE IN PRESS Table 1 Composition of green tapes for B4C in wt% Dense layer Porous layer (PFA) Porous layer (under-sintered) Ceramic63 28.8 72 Dispersant 2.3 1 2.6 Sintering agent 12.5 5.7 0 PFA 0 50 0 Binder 9.5 6.2 12.7 Plasticizer 12.7 8.3 12.7 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Organic PFA Dense layer Porous layer (PFA) Porous layer (under￾sintered) Ceramic Fig. 1. Volume percentage of main constituents in green tapes for B4C. 488 S. Tariolle et al. / Journal of Solid State Chemistry 177 (2004) 487–492