Effect of inert filler addition on microstructure and strength of porous SiC ceramics

Abstract

Porous ceramics are of technological interest because of their unique characteristics such as low density, low thermal conductivity, high thermal shock resistance, high permeability, and high specific strength [1–5]. Specifically, porous silicon carbide (SiC) ceramics are used as molten metal filters, diesel particulate filters, and preforms for metal matrix composites on account of their excellent permeability, thermal shock resistance, specific strength, and corrosion resistance at high temperatures [6–9]. Different processing routes for porous SiC ceramics have been developed for specific applications to satisfy the associated requirements of porosity, pore size, and degree of interconnectivity. These manufacturing techniques include replica techniques [10, 11], sacrificial template techniques [12, 13], and reaction techniques [8, 14]. Recently, a new processing method for fabricating porous SiC ceramics was developed based on the following strategy [15, 16]: (1) pyrolysis of carbon-filled polysiloxane at 1000 C, which leads to the conversion of polysiloxane to silicon oxycarbide (SiOC); (2) carbothermal reduction of SiOC and C mixture at 1450 C, which converts the mixture to SiC ceramic; and (3) liquid-phase sintering of SiC using Al2O3–Y2O3 as a sintering additive at 1750–1950 C. This letter reports the effect of inert filler addition on the microstructural development and flexural strength of porous SiC ceramics processed by the above method using polymer microbeads as a pore former and submicron SiC powder as an inert filler. The raw materials used in this experiment included: polysiloxane (GE Toshiba Silicones Co., Ltd, Tokyo, Japan), carbon black (Korea Carbon Black Co., Ltd., Inchon, Korea), SiC (FCP15C, Norton AS, Lillesand, Norway), poly (methyl methacrylate-co-ethylene glycol dimethacrylate) microbeads (*8 lm, Sigma–Aldrich Inc, St. Louis, MO, designated as PMMA), Al2O3 (AKP30, Sumitomo Chemical Co., Tokyo, Japan), Y2O3 (H.C. Starck GmbH & KG, Goslar, Germany), and MgO (High Purity Chemicals, Osaka, Japan). To prepare a powder composition without a filler (designated as PSC), 68.79% polysiloxane, 10.78% carbon, 3.10% Al2O3, 0.88% Y2O3, and 0.44% MgO were mixed with 16.01% PMMA microbeads. A powder composition with 20 wt% fillers (designated as PCSF) was prepared by mixing 59.79% polysiloxane, 9.37% carbon, 3.36% Al2O3, 0.96% Y2O3, and 0.48% MgO with 8.65% SiC and 17.39% PMMA microbeads. The batches were milled in ethanol for 24 h using SiC grinding balls. The milled powder was then dried and pressed uniaxially into rectangular bars at 28 MPa. The compacts formed were cross-linked by heating to 200 C in air. The cross-linked samples were pyrolyzed at 1000 C for 1 h in argon at a heating rate of 1 C/min. The heat treatment allows the conversion of polysiloxane in the specimens to silicon oxycarbide [17]. The pyrolyzed specimens were further heat treated in argon at 1450 C for 0.5 h at a heating rate of 10 C/min and subsequently sintered at 1750–1950 C for the liquidphase sintering of SiC using Al2O3, Y2O3, and MgO. For the flexural strength measurements, bar-shaped samples were cut and polished to a size of 4 9 5 9 30 mm. Bend tests were performed on five specimens for each condition at a crosshead speed of 0.5 mm/min using a four-point method with inner and outer spans of 10 and 20 mm, respectively. S.-H. Chae Y.-W. Kim (&) Department of Materials Science and Engineering, The University of Seoul, 90 Jeonnong-dong, Dongdaemun-gu, Seoul 130-743, Korea e-mail: ywkim@uos.ac.kr