Effect of additive composition on microstructure and strength of porous silicon carbide ceramics

Abstract

Porous silicon carbide (SiC) ceramics have many potential applications due to their unique properties, which include high temperature stability, chemical stability, excellent abrasion resistance, high thermal shock resistance, high specific strength, and controlled permeability [1–9]. For example, SiC ceramics can be used as catalytic supports, molten metal filters, membrane supports, gas-burner media, and light-weight structural materials for high-temperature applications. It has frequently been observed that the composition of the sintering additives affects the microstructural development of sintered SiC ceramics. Al2O3–Y2O3 [10], Al2O3– Y2O3–CaO [11], and AlN–Y2O3 [12] additives generally lead to the growth of SiC platelet grains when the SiC is sintered or annealed at a temperature above 1950 C. In contrast, Y–Mg–Si–Al oxynitrides [13], B4C–C [14], and AlN [15] additives lead to equiaxed microstructures, regardless of the sintering temperature. Thus, the mechanical properties of porous SiC ceramics may be affected by the sintering additives. Chi et al. [16] investigated the effect of incorporating various amounts of Al2O3 on the strength of porous SiC ceramics. They observed a maximum strength of *17 MPa at a porosity of 61% when 5 wt% Al2O3 was added. Lee and Kim [17] fabricated porous SiC ceramics by powder processing using polymer microbeads as a template. The ceramics showed a strength of *30 MPa at a porosity of 50% when 8 wt% Al2O3–Y2O3 was added in a 7:3 weight ratio. Ma et al. [18] fabricated porous SiC ceramics by adding silicone resin as a binder. The ceramics typically showed a strength of *21 MPa at a porosity of 45%. However, there has been no systematic research on the effect of sintering additives in the processing of porous SiC ceramics. In this study, porous SiC ceramics were fabricated by carbothermal reduction of a polysiloxane-derived SiOC with hollow microspheres, followed by sintering. The effects of the additive composition on the porosity, microstructure, and strength of the resulting porous SiC ceramics were investigated. The potential advantages of using polysiloxane for fabricating porous SiC ceramics are the utilization of low-cost polymer processing such as extrusion and the easiness of porosity control [19]. The following raw materials were used: a polysiloxane (YR3370, GE Toshiba Silicones Co., Ltd, Tokyo, Japan), carbon black (Corax MAF, Korea Carbon Black Co., Ltd., Inchon, Korea), b-SiC (Ultrafine grade, Betarundum, Ibiden Co. Ltd., Ogaki, Japan), hollow microspheres (461DE20, Expancel, Sundsvall, Sweden), Al2O3 (AKP30, Sumitomo Chemical Co., Tokyo, Japan), Y2O3 (H C. Starck GmbH & KG, Goslar, Germany), Y3Al5O12 (YAG, High Purity Chemicals, Osaka, Japan), MgO (High Purity Chemicals, Osaka, Japan), SiO2 (High Purity Chemicals, Osaka, Japan), CaO (High Purity Chemicals, Osaka, Japan), and AlN (grade F, Tokuyama Soda Co., Tokyo, Japan). The SiC was used as an inert filler, while the oxides and AlN were used as sintering additives. The inert filler was added to minimize shrinkage during sintering and to increase the strength of the resulting porous SiC ceramics [20]. The microspheres were hollow poly(methyl methacrylate) spheres with diameters ranging from 15 to 25 lm. Eight batches of powder were prepared (Table 1). The microsphere content was fixed at 5 wt%. An example of the sample notation is as follows: 3A2Y denotes a specimen containing 3 wt% Al2O3 and 2 wt% Y2O3 as sintering J.-H. Eom Y.-W. Kim (&) Department of Materials Science and Engineering, The University of Seoul, 90 Jeonnong-dong, Dongdaemun-gu, Seoul 130-743, Republic of Korea e-mail: ywkim@uos.ac.kr