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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426 C. Reynaud et al. International Journal of Refractory Metals Hard Materials 19(2001)425-435 On the other hand, pores left after burnout may be filled Table 2, and are compared with compositions reviewed by the liquid phase. The effects of size, content, type of in the literature; the organics contents are significantly 'A on tape casting processing, sintering characteris- lower in our case. The volume fraction of PFA incor tics, porosity control and microstructure are presented porated in the slurry is referred to the theoretical volume of the dry gree e ceramic volume tape casting organic volume PFA volume) 2. Processing and experimental procedures Two different types of PFAs: polyamide powders ( PAl2, Orgasol Elf-Atochem-France) and corn starch A fine alpha silicon carbide powder has been chosen (Roquette-France), with a spheroidal shape and a nar- (Sika Tech FCP13, Norton-Norway). Alpha polytype row particle size distribution have been used. PFA was preferred to beta polytype to avoid excessive grain characteristics are listed in Table I growth during beta-alpha transformation. An amount The incorporation of polymer particles in the suspen- of 5 wt%(related to all ceramic powders) of the YAG- sion without precautions has led after casting to the alumina eutectic(60 wt% Al2O3-40 wt%Y203) com- elaboration of brittle and less flexible tapes. Three strat position was added. Characteristics of raw materials are egies have been envisaged to take into account the Pfa listed in Table 1 () Volume compensation: a part of ceramic volume Tape casting slurries have been elaborated in four was replaced by the PFA. Consequently the volume tages. In the first stage, ceramic powders were mixed, fraction (ceramic PFA)versus organic additives by using ball milling alumina media, during 4 h in a volume was constant [4]. This method led to a high MEK-ethanol azeotrope solvent(60 vol% butanone-2/ nts: the tapes we 40 vol% ethanol) containing 0.6 wt% of a phosphate ened. Therefore, the pyrolysis of organic components ester as dispersant (Beycostat C213, CECA-France) was too problematic(cracking and swelling) accord Then(second stage), an acrylic binder and a phthalate ing to Corbin and Apte [12] plasticiser were added to the suspension and mixed (i) Weight compensation: the weight ratio between during 14-16 h. The third stage corresponds to the in- (ceramic+ PFA) and organic additives was kept con- A s oration of the PFA into the slurry mixing time of stant [12]. A cracking behaviour was obtained during h leads to an uniform distribution of the Pfa. fi the drying stage nally slurry was de-aired at a slow rotation speed during (ini) No compensation: the organic content was inde- 24h. Typical compositions of suspensions are given in pendent of the PFA. This method was retained. The Table I Characteristic parameters of the ceramic powders and of the different PFAs incorporated into the slurry Raw ceramic materi P-SiC FCP13 Alumina Cr15 Yttrium oxide dso(um)a 0.3 BET S(m/g) PFA PA-A PA-B PA-C Corn starch Chemical nature olyamide-I Polyamide-12 Polyamide-I Starch .5 122 Aspect rati 1.37 1.29 a Laser grain sizer analysis. Data obtained by image analysis. Table 2 Concentration of components in the dry tape for our tapes compared to compositions reviewed in the literature Refs Our study Corbin and Apte [12] Corbin and Apte [12 Davis et al. [4] Volume fraction of PFA referred 0.55 0 to ceramic volume Ceramic volume fraction"(vol % 68.4 37 38.7 31.5 fraction(vol % PFA volume fraction"(vol % 0 The PFA volume fraction incorporated is referred to the total volume composed of ceramic material+ pore forming agent(PFA). The volume fractions are referred to the theoretical volume of the dry green tape (i.e. ceramic volume tape casting organic volume PFA volume)
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