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In the combustion synthesis of TiC-Al 2 O 3 composites from TiO2, Al, and C, different types of precursors, i.e., anatase and rutile TiO2, and graphite and carbon black were used. The measurements of combustion wave velocity showed that the reactivity was higher with graphite as a carbon source and anatase as a TiO2 source. This was also confirmed by analyzing the degree of conversion of reactants to products. For the precursors investigated, the wave propagation of the 3TiO 2 -4Al-3C system diluted by A12O3 showed that the limit of wave propagation was more dependent on carbon source than on TiO 2 . The wave front of all the samples proceeded in unstable combustion mode. Temperature profiles in the reaction zone suggested that the heat-affected zone was broader in the case of graphite carbon source than carbon black. The microstructures and chemical composition of the products were also studied.

I. INTRODUCTION TiC-Al 2 O 3 composites are used in applications such as cutting tools and tape heads in the electronic industry because of their high hardness (20-22 GPa), good strength (500-600 MPa), moderate fracture toughness (4-4.5 MPa • m 1/2 ), and fine grain size ( 1 - 5 /im). At the present time, commercial TiC-Al 2 O 3 ceramics have been primarily manufactured by pressureless sintering or hot-pressing of TiC and A12O3 powders whose composition is usually 30 wt. % TiC-Al 2 O 3 . 1 3 The method that utilizes self-sustained exothermic reactions between reactants to prepare materials such as refractory ceramic and intermetallic compounds is called self-propagating high-temperature synthesis (SHS), or combustion synthesis.4 Recently, the synthesis of TiC-Al 2 O 3 composites by the aluminothermic reduction of TiO2 in the presence of carbon has drawn much attention as an alternative to conventional processes such as hot-pressing of TiC and A12O3 powder mixture. It is because the combustion synthesis of the composite is not only simple and energy-efficient, but also uses low-cost precursors. 58 The combustion synthesis of the TiC-Al 2 O 3 composite can be described as follows9: 3TiO 2 + 4A1 + 3C —* 3TiC + 2Al 2 O 3 (a), A#/°,298.i5K

= -1088.7 kJ/mole,

AG/,298.i5K

= -1057.3 kJ/mole (in the case of anatase TiO 2 ).

A//y 2 9 8 1 5 K = —1076.6 kJ/mole , AG/,298.i5K

= -1038.9 kJ/mole (in the case of rutile TiO 2 ). J. Mater. Res., Vol. 9, No. 7, Jul 1994

http://journals.cambridge.org

Downloaded: 27 Nov 2014

It has been reported that TiC-Al 2 O 3 composites combustion-synthesized could be sintered pressurelessly during the reaction and have microstructures consisting of layered patterns of TiC in A12O3.5 The thermal explosion method6 under high pressure and the dynamic consolidation7 of the hot products using explosives were carried out to fabricate the dense TiC-Al 2 O 3 composites in one step. The thermally exploded samples have unique microstructures consisting of round TiC grain with a size of a few micrometers and A12O3 occupying the spaces between TiC grains. In this case, the fracture toughness was measured to