Effect of additive composition on microstructure and mechanical properties of SiC ceramics sintered with small amount of RE2O3 (RE: Sc, Lu, Y) and AlN

Silicon carbide is a potential material for various structural applications because of its unique combination of properties such as high hardness, high temperature strength, and excellent resistance to wear and corrosion [1–4]. In order to improve the fracture toughness, several attempts have been made to fabricate SiC ceramics consisting of heterogeneous microstructure with weak grain boundary phases [5–7]. The liquid-phase sintering using metal oxides, Al–B–C, AlN– metal oxides as sintering additives is recommended for obtaining SiC ceramics with tailored microstructure [8, 9]. Among various additive systems, AlN and RE2O3 (RE: rareearth elements) systems are preferred due to relatively less weight loss [10], easy control of oxynitride decomposition, and absence of gas-phase reactions below 2000 C [6]. The use of RE2O3 renders SiC ceramics with excellent high temperature strength and toughness properties [6, 11]. The platelet-reinforced microstructures exhibit enhanced toughness due to combination of intergranular crack mode, introduced by the glassy grain boundary phase, and energy dissipating processes in the crack wake [12]. Though most of the studies reported amorphous grain boundary or ternary junction phase, it has been recently identified that sintering additive composition plays a dominant role in affecting the crystallinity of grain boundary phase. Amorphous intergranular phase was obtained using AlN–Y2O3 [13, 14] and AlN–Er2O3 [9, 11] additives systems, whereas AlN– Sc2O3 render clean grain boundary [15, 16] and AlN–Lu2O3 systems render clean [17] or crystallized grain boundary [18]. It is to note that most of the published research has been conducted toward characterization and property evaluation of SiC ceramics prepared with relatively large amounts of additives (C5 wt%), while its performance with small additive contents is poorly understood. Therefore, an attempt has been made in this study, for the first time, to sinter SiC ceramics using small amount (3 wt%) of AlN–RE2O3 (RE: Sc, Lu and Y) additives. The major purpose is to investigate the influence of additive composition on microstructural and mechanical characteristics of SiC ceramics sintered with small additive amounts. Commercially available b-SiC (Ultrafine grade, Betarundum, Ibiden Co. Ltd., Ogaki, Japan), AlN (Grade F, Tokuyama Soda Co., Tokyo, Japan), and metal oxides (Sc2O3, Lu2O3, and Y2O3, 99.9% pure, Shin-Etsu Chemical Co., Tokyo, Japan) were used as the starting powders. The mean particle size and the specific surface area of the b-SiC powders were 0.27 lm and 17.5 m/g, respectively. A powder mixture of SiC and 3 wt% additives AlN and RE2O3 (RE: Sc, Lu, Y) was prepared. The relative amount of RE2O3 in the total content of additives was 20 mol.%. The respective powder batches were milled in ethanol for 24 h using SiC grinding balls. The milled slurry was dried, sieved, and hot-pressed at 2050 C for 6 h under a pressure of 25 MPa in N2 atmosphere. The details of specimen designation, batch composition, and additive systems are given in Table 1. The designation SCRE represents silicon carbide specimen prepared using 3 wt% RE–AlN (RE: Sc, Lu, Y) additive. The bulk density of the sintered specimen was measured using the Archimedes method in water. The theoretical B. V. Manoj Kumar M.-H. Roh Y.-W. Kim (&) W. Kim Department of Materials Science and Engineering, The University of Seoul, Seoul 130-743, Korea e-mail: ywkim@uos.ac.kr