JOURNAL OF MATERIALS SCIENCE 33 (1998) 211 – 214 In situ enhancement of toughness of SiC–TiB 2 composites KYEONG-SIK CHO, HEON-JIN CHOI, JUNE-GUNN LEE Division of Ceramics, Korea Institute of Science and Technology, PO Box 131, Cheongryang, Seoul 130-650, Korea YOUNG-WOOK KIM Department of Materials Science and Engineering, The University of Seoul, 90 Jeonnong-Dong, Seoul 130-743, Korea A process based on liquid phase sintering and subsequent annealing for grain growth is presented to obtain the in situ enhancement of toughness of SiC—30 wt %, 50 wt %, and 70 wt % TiB 2 composites. Its microstructures consist of uniformly distributed elongated -SiC grains, relatively equiaxed TiB 2 grains, and yttrium aluminium garnet (YAG) as a grain boundary phase. The composites were fabricated from -SiC and TiB 2 powders with the liquid forming additives of Al 2 O 3 and Y 2 O 3 by hot-pressing at 1850 °C and subsequent annealing at 1950 °C. The annealing led to the in situ growth of elongated -SiC grains, due to the P phase transformation of SiC, and the coarsening of TiB 2 grains. The fracture toughness of the SiC—50 wt % TiB 2 composites after 6 h annealing was 7.3 MPa m 1/2 , approximately 60% higher than that of as-hot-pressed composites (4.5 MPa m 1/2 ). Bridging and crack deflection by the elongated -SiC grains and coarse TiB 2 grains appear to account for the increased toughness of the composites. 1. Introduction Composites of SiC—TiB  can be fabricated by hot pressing with the aid of C and Al or B [1, 2] or pressureless sintering with in situ synthesis of TiB  through a reaction between TiC and boron [3] to a near full density at temperatures in excess of 2000 °C. Several investigations have shown that the dispersion of TiB  particles results in the improved fracture toughness of SiC ceramics [3—5]. It is claimed that the residual stresses due to the thermal expansion mis- match between TiB  (8.610 °C) and SiC (4.210 °C) improve toughness by deflecting the cracks around the TiB  particles [2, 3]. Recently, several reports have been published on in situ toughened SiC [6—10], akin to Si  N  . The improvement of fracture toughness was achieved through the development of elongated -SiC grains [7—11]. In this study, we present an alternative microstruc- tural design for enhancing the fracture toughness of SiC—TiB  composites. We have used liquid phase sintering to fabricate SiC—30 wt %, 50 wt %, and 70 wt % TiB  composites at relatively low temper- ature (1850 °C) [12], and subsequent annealing at 1950 °C to develop the in situ growth of elongated -SiC grains as well as coarse TiB  grains. The frac- ture toughness and strength of the composites were presented as a function of TiB  content and annealing time. 2. Experimental procedure Commercially available -SiC (Ibiden Co., Ltd, Nagoya, Japan, grade Ultrafine), TiB  (H.C. Starck, Berlin, Germany, grade F), Al  O  (Sumitomo Chem- icals, Tokyo, Japan, grade AKP-30), and Y  O  pow- ders (H.C. Starck, Berlin, Germany, grade Fine) were used as the starting powders. The powder mixtures of 90 wt % -SiC with 7 wt % Al  O  and 3 wt % Y  O  for monolithic SiC, and the powder mixtures of SiC—TiB  containing 30—70 wt % TiB  with 7 wt % Al  O  and 3 wt % Y  O  for SiC—TiB  composites were ball milled in ethanol with SiC grinding balls for 24 h. The milled slurry was dried and sieved through a 60 mesh screen. The granulated powders were hot pressed in a graphite resistance furnace at 1850 °C for 1 h at 25 MPa to final dimensions of 30 mm in dia- meter by 20 mm high. The hot-pressed SiC—TiB  com- posites were subsequently heat treated at 1950 °C for 6 and 12 h under flowing argon in the same furnace without pressure to enhance the grain growth of TiB  and the P phase transformation of SiC. Densities of composites prepared by grinding off the surface layers were measured using the Archimedes method and the relative densities of the specimens were calculated based on the densities of SiC (3.215 g cm), TiB  (4.495 g cm), Al  O  (3.987 g cm), and Y  O  (5.031 g cm) assuming a rule of mixtures. Crystalline phases in the sintered specimens were determined by X-ray diffractometry 0022—2461  1998 Chapman & Hall 211