Low temperature sintering of nano-SiC using a novel Al 8 B 4 C 7 additive
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Byung-Nam Kim National Institute for Materials Science, Tsukuba, Ibaraki, 305-0047 Japan
Hidehiko Tanaka National Institute for Materials Science, Tsukuba, Ibaraki, 305-0044 Japan (Received 28 April 2009; accepted 8 October 2009)
Al8B4C7 was used as a sintering additive for the densification of nano-SiC powder. The average grain size was approximately 70 nm after sintering SiC-12.5wt% Al8B4C7 at 1550 C. The densification rate strongly depended on the sintering temperature and the applied pressure. The rearrangement of SiC particles occurred at the initial shrinkage, while viscous flow and liquid phase sintering became important at the middle and final stage of densification. I. INTRODUCTION
Due to the covalent nature of Si-C bond and low selfdiffusion coefficient, high temperature and prolonged sintering time have generally been required for the densification of SiC,1 where grain growth is inevitable. Accordingly, the preparation of bulk SiC having an ultra fine microstructure has been a challenge.2 A mixture of Al, B (or B4C), and C (hereafter termed Al-B-C) has been used as a sintering additive for the densification of submicrometer scale SiC powder to decrease the sintering temperature below 1700 C.3 The additive system, however, may not be suitable for the sintering of nano-SiC powder, because Al particles larger than 2–3 mm cause inhomogeneous dispersion of the additive, whereas finer ones react with air rather strongly.3 For attaining fine microstructures, uniform distribution of the additive is required. The addition of Al8B4C7 rather than a mixture of the elementary materials may solve the problems of inhomogeneous mixing and safety if the ternary compound can easily be crushed into fine powders and does not have significant reactivity with air. In this research, the densification of nano-SiC powder containing Al8B4C7 additive was investigated with the variation of the heating rate, applied pressure, and the additive content during spark plasma sintering (SPS). II. EXPERIMENTAL PROCEDURE
Al (Reagent grade, Koso Chemical Inc., Tokyo, Japan), B4C (Grade HD20, H. C. Starck, Goslar, Germany), and C (carbon black, MA-600B, Mitsubishi Chem., Tokyo, a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0057 J. Mater. Res., Vol. 25, No. 3, Mar 2010
Japan) powders were mixed with a molar ratio of 8Al: 4B: 7C in ethanol for 10 min using an ultrasonifier, and were dried with stirring. The powder mixture was then calcined at 1800 C for 2 h in Ar within a sealed carbon crucible to synthesize Al8B4C7.4 The synthesized powder was gently crushed using a mortar and the size distribution of the resultant powder was analyzed using a particle size analyzer (1064, CILAS, Orleans, France). Al8B4C7 powder (2.5–12.5 wt%) was mixed with ultrafine b-SiC (T-1, Sumitomo-Osaka Cement Co., Tokyo, Japan, d50: 30 nm, O: 0.45 wt%, C: 3.73 wt%) or submicrometer scale a-SiC (UF-15, 6H-SiC, H. C. Starck, Goslar, Germany, d50: 0.55 mm) using a planetary mill with SiC balls, SiC jar, and eth
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