Effect of alumina diluent on the fabrication of in-situ Al 2 O 3 -Ti/ZrB 2 composite by self propagating high temperatur
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INTRODUCTION
THE incorporation of high-temperature boride particulates improves the properties of ceramic matrix composites. Among the various borides, zirconium boride (ZrB2) and titanium boride (TiB2) are important materials due to their high melting point, hardness, elastic modulus, and electrical conductivity and their excellent chemical resistance to HCl, HF, other nonferrous metals, cryolite, and nonbasic slags. Both borides of titanium and zirconium have their advantages. ZrB2 has the advantage over TiB2 that it does not have many phases like TiB2, but the higher density (6 g cm3) is the limitation for the addition in lighter matrix. TiB2 is better than ZrB2 in terms of hardness, 3300 Hv as compared to 2800 Hv for ZrB2, as well as lesser density (4.5 g cm3), which makes it a suitable dispersion for hard composite. TiB2 has the limitation that it has many stable phases, which makes the composite multiphase and hence the control of the reaction essentially critical. The combination of both borides may be one compromising solution for the hard composites and can be a suitable dispersion for alumina matrix composites for different applications. The addition of borides to alumina is expected to improve mechanical strength significantly.[1,2] Titanium diboride dispersion in alumina matrix has shown higher strength, hardness, fracture toughness, and impact resistance of the composite compared to monolithic alumina. TiB2-Al2O3 composites have been prepared by mixing alumina and titanium diboride particulates and, thereby, sintering. The aluminothermic reduction of oxides in a furnace has also been used.[3,4] The alumina matrix dispersed with TiB2/ZrB2 becomes electrically conducting even if only 20 wt pct of boride phases are present in the matrix, which makes the composites machinable by electrodischarge machining.[5] The dispersion of borides is likely to be more homogenous when it is prepared in situ by the SHS process than if the two components are mixed and milled, as done conventionally since the reaction takes at each powder; also, S.K. MISHRA, Scientist, P.K.P. RUPA, Quick Hire Fellow, and S.K. DAS, Scientist, are with the National Metallurgical Laboratory, Jamshedpur, India 831007. Contact e-mail: [email protected] V. SHCHERBAKOV, Scientist, is with the Institute of Mackrokinetics and Materials Science, Russian Academy of Sciences (ISMAN), Chernogolovka, Russia. Manuscript submitted March 31, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
the material will be purer due to the reduction of the milling step and high temperature during SHS synthesis.[6] The SHS synthesis of the composite not only makes an insitu composite powder, but is also energy efficient over the conventional processes. At the same time, defects induced during the SHS process due to a high rate of heating and cooling should reduce the sintering temperature.[7] The powders have been made by the SHS process, most frequently to fabricate the composite, [8,9] and then sintered at higher temperature. The different aspects of SHS have been di
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