Field-activated pressure-assisted combustion synthesis of polycrystalline Ti 3 SiC 2
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Field-activated pressure-assisted combustion synthesis of polycrystalline Ti3 SiC2 Aiguo Feng,a) Tim Orling, and Z. A. Munirb) Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616 (Received 2 July 1998; accepted 14 October 1998)
The simultaneous synthesis and densification of the ternary Ti3 SiC2 was investigated by the field-activated, pressure-assisted combustion method. Depending on temperature and time at temperature, relatively pure and nearly fully dense materials can be synthesized by this approach. The optimum conditions to produce this phase were a reaction temperature of 1525 ±C and a time at temperature of 2 h. The product contained TiC as a second phase at a level of # 2 mol%. The resulting ternary phase has typically elongated grains which were about 25 mm in size. Within a range of applied force of 1–4 N, the microhardness of the product was relatively constant, ranging from 6 to 7 GPa. Investigations on the thermal and chemical stabilities of the ternary were also conducted. Vacuum annealing at temperatures of 1600 and 2000 ±C resulted in the formation of a surface layer of TiC, while the air annealing at 1000 ±C resulted in the formation of TiO2 . Oxidation studies on the prepared Ti3 SiC2 were made at temperatures ranging from 800 to 1100 ±C. The results suggest a two-mechanism process, one dominating in the approximate range of 800–950 ±C and the other in the range 950–1100 ±C with corresponding activation energies of 137.7 and 312.5 kJ ? mol21 . The results are explained in terms of two proposed reactions on the basis of microprobe analyses.
I. INTRODUCTION
Although the ternary Ti3 SiC2 was first synthesized and crystallographically characterized in the late 1960s,1 only recently has it attracted attention as a potentially useful material. It belongs to a class of materials, referred to as the 312, which has a hexagonal structure with silicon layers separated by two layers of edgesharing Ti octahedra, each containing a central carbon atom.1 Renewed interest in this material stems from its reported attractive properties, including good electrical and thermal conductivities, relatively high ductility and fracture toughness, and good oxidation resistance.2–9 The material is reported to have a hardness in the range of 2 to 13 GPa, with the measured values showing a dependence on the load.2,10 Young and shear moduli for this material are reported to be 325 and 135 GPa, respectively.2,11 The relatively low ratio of the hardness to Young’s modulus has led some authors to use the term “ductile ceramic” when referring to Ti3 SiC2 , although this is subject to debate.5 Regardless of whether the term is appropriate, the fact that this ternary has a considerably higher fracture toughness relative to refractory carbide phases (a room-temperature value of ø 7 MPa ? m1/2 )4 a)
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b)
J. Mater. Res., Vol. 14
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