JOURNAL OF MATERIALS SCIENCE 39 (2 0 0 4 ) 3613 – 3617 Microstructure stability of fine-grained silicon carbide ceramics during annealing YOUNG-IL LEE, YOUNG-WOOK KIM ∗ Department of Materials Science and Engineering, The University of Seoul, Seoul 130-743, Korea E-mail: ywkim@uos.ac.kr M. MITOMO National Institute for Materials Science, Ibaraki 305-0044, Japan Fine-grained silicon carbide ceramics with an average grain size of ∼140 nm or smaller were prepared by low-temperature hot-pressing of very fine β -SiC powders using Al 2 O 3 -Y 2 O 3 -CaO (AYC) or Y-Mg-Si-Al-O-N glass (ON) as sintering additives. The microstructure stability of the resulting fine-grained SiC ceramics was investigated by annealing at 1850 ◦ C and by evaluating quantitatively the grain growth behavior using image analysis. The β → α phase transformation of SiC in AYC-SiC was responsible for the accelerated abnormal grain growth of platelet-shaped grains. In contrast, the β → α phase transformation in ON-SiC was suppressed, which resulted in a very stable microstructure. C  2004 Kluwer Academic Publishers 1. Introduction Silicon carbide is difficult to densify without sintering additives because of its low self-diffusion coefficients and the covalent nature of the Si C bond. Therefore, sintering additives are commonly used to attain full den- sification. Densification is achieved in solid-state sin- tering by using B and C additives, and in liquid-phase sintering by using metal oxide additives. The interest in liquid-phase sintered SiC has grown continually during recent years because such materials are easier to pro- cess and seem to have superior mechanical properties than solid-state sintered SiC [1–4]. The development of platelet grains into fine matrix grains during sin- tering or annealing is advantageous in the toughening of ceramics. These platelet grains can act as reinforc- ing agents that promote crack bridging and deflection, which result in improved toughness [1, 5]. Thus, many attempts have been made to develop the composite- type microstructures during sintering and/or annealing [6–8]. The evolution of the bimodal microstructure has been attributed to the β → α phase transformation of SiC and/or the accelerated solution-reprecipitation by seeding during liquid-phase sintering [1, 6, 8]. Opti- mization of mechanical properties has been conducted through microstructure control [7, 8]. It has also been shown that the SiC ceramics with fine grains (<300 nm) and homogeneous microstruc- tures exhibit superplasticity. The superplasticity of SiC was reported in both the solid-state-sintered β -SiC with a grain size of 200 nm (∼140% elongation) [9] and the liquid-phase-sintered β -SiC with a grain size of 230 nm (∼150% elongation) [10]. A fine initial grain ∗ Author to whom all correspondence should be addressed. size and a low grain-growth rate are especially im- portant for superplastic deformation. It has been sug- gested that both grain size and grain size distribution are important parameters for obtaining superplasticity because dynamic grain growth is responsible for strain- hardening in ceramics during superplastic deformation [10, 11]. Thus, it is important to evaluate microstruc- tural stability by investigating grain growth behavior during annealing. Although the effects of processing parameters on microstructural development have been studied extensively [6, 12, 13], the evaluation of mi- crostructure stability with respect to sintering addi- tive composition has received less attention. Nader et al. [6] reported that pure α-SiC and β -SiC do not transform with an Y 2 O 3 -AlN additive system, and thus result in a stable microstructure. They also showed that the β → α phase transformation rate decreases with an increasing β -content in the starting powder as well as in the presence of nitrogen. Ortiz et al. [14] found that the presence of nitrogen stabilizes the β - phase, which also correspondingly generates stable microstructures. In this work, the fabrication of fine-grained SiC ce- ramics have been attempted by starting from very fine (∼30 nm) β -SiC powder with an Al 2 O 3 -Y 2 O 3 -CaO (AYC) or a Y-Mg-Si-Al-O-N glass (ON) as sinter- ing additives. The microstructure stability of the fine- grained SiC ceramics was investigated by annealing the ceramics for up to 12 h at 1850 ◦ C, followed by an obser- vation of the resulting microstructures using scanning electron microscopy (SEM). The SEM images were then characterized using image analysis. 0022–2461 C  2004 Kluwer Academic Publishers 3613