REFRACTORY METALS HARD MATERIALS ELSEVIER International Journal of Refractory Metals Hard Materials 19(2001)425-435 www.elsevier.com/locate/ijrmhm Processing and microstructure of Sic laminar composites C. Reynaud F. Thevenot a,, T. Chartier b Dept Ceramiques Speciales, Ecole Nationale Superieure des mines de St-Etienne, 42023, Saint-Etienne Cedex 2, france S.P. C.T.S., U.M.R CNR. 6638,ENS. de Ceramiques Industrielles, 87065, Limoges, france Received 5 March 2001; accepted 14 August 2001 Abstract Porous laminar materials and alternate laminates of dense and porous layers in silicon carbide have been elaborated by tape casting and liquid phase sintering (YAG-alumina eutectic)processing. Porosity was introduced by the incorporation of pore forming agents(PFA)(5-50 vol% )in the slurry. Two types of PFA with a narrow size distribution have been used(corn starch and polyamide powders). The effects of size, content, type of PFA on tape casting processing, sintering characteristics, porosity control and microstructure are investigated. For each PFA, the porosity attains a maximum value dependent on the PFA nature(41 vol% with corn starch). Only for starch, the volumetric shrinkage was unaffected by the PFA content up to 45 vol%. Homogeneou distribution of porosity has been obtained for both monolithic and composite laminates. An equiaxed and homogeneous silicon carbide microstructure has been obtained and was unaffected by PFA. Layered structures without defects have been obtained with parallel layers and uniform thickness(dense layer: 70 um; porous layer: 80 um).@ 2001 Published by Elsevier Science Ltd. Keywords: Alternate laminate; Pore forming agent; SiC; Liquid phase sintering: Tape casting 1. ntroduction stresses introduced during cooling, and may be better resistant to oxidation in air compared to graphite(de- Fracture toughness of ceramic materials can be im- pending on the intrinsic ceramic properties). A study proved by designing laminar structures with weak in- performed on solid phase sintered silicon carbide/porous terlayers that promote crack-deflection mechanisms and silicon carbide laminar composites by Blanks et al. [3] hence increase fracture energy. Clegg et al. [l] have has shown the porosity level required to ensure reliably elaborated laminar materials in alpha silicon carbide crack deflection is 37 vol%. Porous interlayers appear with graphite interfaces with an apparent toughness and the most straightforward approach to elaborate weak a fracture energy, respectively, 5 and 200 times higher interfaces for reinforced laminar materials used in severe than the typical value measured for monolithic sintered conditions a-SiC. Many systems have been studied Sic/C [1, 21, SiC/ This paper describes the elaboration of laminar ma- SiC [3]. AlO3/AlO3 [4], Si3 N4/BN [5, 6], Si3 N4/Si3 N4 terials made of silicon carbide layers, obtained by tape whiskers [7], ZrO2 toughened AlO3 [8, 9], AlO3/Sic casting, and cosintered in liquid phase processing. A [10] and have exhibited higher fracture energy than their liquid phase sintering processing has been chosen due to respective monolithic form. Several processings can be lower sintering temperature and the possible design of sed to elaborate multilayered structures: tape casting duplex microstructure. The porosity is controlled by [3, 4, 8, 9], extrusion [10] or electrophoretic deposition [2]. adding pyrolysable particles (pore forming agents Weak interfaces can be constituted by graphite [1, 2], (PFA)) in tape casting slurries as reviewed in numerous boron nitride [5, or porous ceramic material [3, 4, 7, 10]. studies [3, 4, 11, 12]. If the PFa diameters are sufficiently Porous interlayers offer some advantages such as large, they form after burnout pores thermodynamically emical compatibility between layers, no thermal stable that shrink by the same amount as the ceramic material surrounding them 3]. However, two problems Corresponding author. Tel. +33-4-7742-0020: fax: +33-4-7742. can appear with the liquid phase sintering process.On the one hand, the pore mobility is higher than in solid E-inail address: thevenot@emse. fr(F. Thevenot) phase sintering and can lead to higher pore coalescence see front matter c 2001 Published by Elsevier Sc PI:S0263-4368(01)00055-5
                     
 

 

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