JOURNAL OF MATERIALS SCIENCE LETTERS 21, 2 0 0 2, 883 – 886 Microstructure and fracture toughness of liquid-phase-sintered SiC-Ti(CN) composites HYUN-GU AN, YOUNG-WOOK KIM ∗ Department of Materials Science and Engineering, The University of Seoul, Seoul 130-743, Korea E-mail: ywkim@uoscc.uos.ac.kr SHINHOO KANG School of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea By in situ-forming of plate-shaped SiC grains, signi- ficant improvements in toughness and flaw tolerance have been achieved in SiC-TiC composites [1–5]. Eff- orts to introduce the plate-shaped SiC grains can be summarized with the following two techniques: (i) use of chemical vapor deposition in fabricating SiC-TiC nano-composites with needle-like microstructure [1, 2] and (ii) heat treatment at high temperatures (≥1900 ◦ C) to induce the β → α phase transformation of SiC [3– 5]. The latter leads to the in situ growth of plate-shaped α-SiC grains. It has been claimed that toughness can be improved effectively by bridging and deflecting the cracks around the plate-shaped α-SiC grains [1, 6]. The effect of initial α-SiC content in the starting pow- ders on microstructure of SiC-TiC composites has been investigated previously [5]. It shows that an equiaxed grain morphology is obtained with high α-SiC powders, whereas an plate-shaped grain morphology can be re- sulted from β -SiC or β -SiC containing α-SiC seeds via β → α phase transformation during sintering or annealing. A new approach taken in this study is through the in- corporation of Ti(CN) in SiC with oxide additives. It has been shown that the presence of nitrogen in the grain- boundary phase of SiC ceramic retards the β → α phase transformation of SiC during sintering and improves the fracture toughness of SiC [7]. Thus, the microstruc- ture and fracture toughness of SiC-Ti(CN) composites are expected to vary as a function of nitrogen contents, x , in Ti(C 1-x N x ) starting powders. In the present work, various Ti(C 1-x N x ) were added to SiC and microstructure and fracture toughness of composites were studied. An yttrium aluminum gar- net (Y 3 Al 5 O 12 , YAG) composition was selected as a sintering additive for liquid-phase sintering. The par- ticle sizes and manufacturers of the powders used in this study are listed in Table I. Commercially avail- able Al 2 O 3 (AKP-30, Sumitomo Chemicals, Tokyo, Japan) and Y 2 O 3 (99.9% pure, Shin-Etsu Chemical Co., Ltd, Tokyo, Japan) powders were used as sinter- ing additives. Five batches of powder were mixed, each containing 59.4 wt% β -SiC, 0.6 wt% α-SiC, 30 wt% Ti(C 1-x N x ), 4.3 wt% Al 2 O 3 , and 5.7 wt% Y 2 O 3 . All in- dividual batches were milled in ethanol for 24 h using ∗ Author to whom all correspondence should be addressed. polyethylene jar and SiC grinding balls. The milled slurry was dried, sieved, and hot-pressed at 1820 ◦ C for 1 h under a pressure of 25 MPa in an argon atmo- sphere for the composition with x = 0 in Ti(C 1-x N x ) and in nitrogen atmosphere for the compositions with 0.3 ≤ x ≤ 1 in Ti(C 1-x N x ). The hot-pressed composites were heated further at 1930 ◦ C for 4 h under an atmo- spheric pressure of argon or nitrogen, depending on the compositions, to enhance grain growth and the β → α phase transformation of SiC. The relative densities were determined on the pol- ished surfaces using image analysis (Image-Pro Plus, Media Cybernetics, Silver Spring, MD, U.S.A.). This method was useful to estimate the theoretical densi- ties of various specimens that contain large amounts of amorphous glass phases [3]. X-ray diffraction (XRD) was performed on the ground powders to determine the crystalline phases. The microstructure was observed via scanning electron microscopy (SEM) on polished and etched specimens. The fracture toughness was esti- mated by measuring the lengths of cracks generated by a Vickers indentor [8]. The variation in fracture tough- ness with indentation load (R-curve-like behavior) was measured over a load range of 49–294 N. The toughness values that were measured in the steady-state region were reported in this study. Relative densities of >98% were achieved by hot- pressing and subsequent annealing for all compositions. Fig. 1 shows the microstructures of the SiC-Ti(CN) composites after annealing. The bright phase is Ti(CN) and the gray phase is SiC. As shown, the T A B L E I Characteristics of initial powders used for preparation of composites Average particle Powder size (μm) Manufacturer TiC 1.40 H. C. Starck, Germany Ti(C 0.7 N 0.3 ) 1.14 Kennametal, U.S.A. Ti(C 0.5 N 0.5 ) 1.10 H. C. Starck, Germany Ti(C 0.3 N 0.7 ) 1.20 Kennametal, U.S.A. TiN 0.85 Kennametal, U.S.A. α-SiC 0.45 Showa Denko, Japan β-SiC 0.27 Ibiden Co., Japan 0261–8028 C  2002 Kluwer Academic Publishers 883