Reactive melt infiltration of silicon-niobium alloys in microporous carbons
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Studies of the reactive melt infiltration of silicon-niobium alloys in microporous carbon preforms prepared by the pyrolysis of a polymer precursor have been carried out using modeling, DTA, and melt infiltration. Mercury porosimetry results indicate a very narrow pore size distribution with virtually all the porosity within the carbon preforms open to infiltrants. The morphology and amount of the residual phases (niobium disilicide and silicon) in the infiltrated material can be tailored according to requirements by careful control of the properties (pore size and pore volume) of the porous carbon preforms and alloy composition. The average room temperature four-point flexural strength of a reaction-formed silicon carbide material (made by the infiltration of medium pore size carbon preform with S i - 5 at. % Nb alloy) is 290 ± 40 MPa (42 ± 6 ksi) and the fracture toughness is 3.7 ± 0.3 MPaVm. The flexural strength decreases at high temperatures due to relaxation of residual thermal stresses and the presence of free silicon in the material.
I. INTRODUCTION In recent years, there has been an increasing demand for high performance ceramics and composite materials for high temperature aerospace applications. A number of these applications require materials that have good strength and toughness, oxidation resistance, and high thermal conductivity at temperatures approaching 1400 °C. A variety of silicon-based ceramics and composites have attracted a great deal of attention for the above purposes.1"6 Commercially available reactionbonded silicon carbide is fabricated by the melt infiltration of silicon into preforms containing silicon carbide and carbon. The residual silicon in commercially available reaction-bonded silicon carbide ceramics is detrimental to its high temperature strength, especially above 1350 °C (mp of Si = 1410 °C). In addition, there are other critical issues in the fabrication of silicon carbide based ceramics which are mainly related to fabrication of complex shapes, processing time, and temperature. A combination of these factors leads to the high manufacturing cost of the final components. Owing to the above considerations, there is a strong need to develop a processing approach for silicon carbide based advanced ceramics which yields low residual free silicon content, complex shape capabilities, high strength and toughness, high thermal conductivity, good oxidation resistance, and is cost effective. We have used a reaction-forming process for the processing of silicon carbide ceramics.5"8 This process has near-net-shape capabilities, shorter processing times, and lower processing temperatures in comparison to conventional processes such as sintering and hot isostatic J. Mater. Res., Vol. 9, No. 7, Jul 1994
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pressing. In this process, a microporous carbon preform is infiltrated with molten silicon or a silicon-refractory metal (molybdenum or niobium) alloy. In the case of silicon infiltration, the final products are silicon carbide and res
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