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The chemical stability of TiB 2 and TiN in a silicon nitride matrix under various conditions of temperature and gaseous environments was investigated. The addition of TiB2 and TiN on the microstructure and mechanical properties was also studied. No trace of interactions between TiN and Si3N4 was noticed. The addition of TiB 2 to Si3N4 enhanced conversion of the a to /3 phase of the Si 3 N 4 matrix. Observations of BN and TiN indicated a possible reaction between TiB 2 and S13N4. The fracture toughness of Si 3 N 4 was substantially enhanced with the addition of TiB 2 or TiN, while the strength was decreased. Crack deflection was the major toughening mechanism in a Si 3 N 4 /TiB 2 composite. Most of the microcracks passed through TiN particles and cleavaged along preferred orientations with large deflection angles.

I. INTRODUCTION Silicon nitride is one of the best ceramic materials for structural applications mainly because of its excellent mechanical, thermal, and chemical properties. Many potential applications are, however, still limited by its lack of toughness and reliability. Two approaches are used to resolve these problems.1 One is control of the grain size and aspect ratio of the p phase in the microstructure,2'3 and the other is to improve toughness by incorporating reinforcements into the Si3N4 matrix.4'5 The area of ceramic-toughened Si3N4 has been studied by a few researchers and some have been quite successful. These investigated toughening materials include SiC whiskers and fibers, and TiC, WC, and TiB 2 particulates.6"10 Among ceramics, TiB 2 possesses a relatively high melting point, Young's modulus, hardness, thermal expansion coefficient, and thermal conductivity. It has been applied toward toughening a ceramic matrix and developed as an armor material. Toughness enhanced 45-90% through incorporation of TiB 2 into SiC was reported by Janney24 and McMurtry.11 Liu and Ownby12 also reported that toughness of A12O3 improved 70% by incorporation of TiB 2 . Only a few reports on the TiB 2 /Si 3 N 4 system are published.1'10'25 Although a possible reaction between TiB2 and Si 3 N 4 was reported,10 it remains unclear how TiB 2 affects the microstructure, chemical stability, and mechanical properties. Because of its good thermal conductivity, chemical inertness, and wear resistance, TiN was used for cutting tool applications. Bellosi and Fiegna added TiN (30-40%) into Si3N4 and were able to increase fracJ. Mater. Res., Vol. 9, No. 9, Sep 1994 http://journals.cambridge.org

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ture toughness by 100% and enhanced strength up to 1000 MPa.13 An electroconductive composite may have been obtained with resistivity of only 10~ 7 ohm-m with TiN and TiB 2 as the second phases.13 This property lends itself to electrical discharge machining (EDM) to fabricate components of complex shape. The major objectives of our experiments were to investigate the chemical stability of TiB 2 and TiN in silicon nitride under various conditions and to scrutinize their effects on the microstructure and mechanica