The initiation of spontaneous infiltration of alloys into carbon preforms in air
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The initiation of spontaneous infiltration of alloys into carbon preforms in air X. F. Yang,a) X. M. Xi, and L. M. Xiao Materials Research and Education Center, 201 Ross Hall, Auburn University, Auburn, Alabama 36849 (Received 16 January 1996; accepted 14 July 1997)
It was found in a previous work that the Al–Si alloy could spontaneously infiltrate into carbon preforms in air. In this study, the initiation stage of the infiltration process was investigated in detail through two different infiltration experiments. In one experiment, carbon preforms were fully dipped into an alloy bath that was exposed to air, and in the other experiment a carbon preform was only partially dipped into an alloy bath that was protected with a flowing Ar or N2 gas. Experimental results have suggested that the initiation of infiltration is controlled by the pressure of oxidizing gases such as O2 or CO at the infiltration front and is not affected by the presence or absence of N2 gas. The critical pressure of oxidizing gases is estimated to be on the order of 10–4 atm for systems investigated in our experiments. An effective way to reduce the O2 or CO pressure is to flush a preform with nonoxidizing gases during or before infiltration, or to use an active metal to reduce the O2 pressure and thus the corresponding CO pressure.
I. INTRODUCTION
An inherent difficulty encountered in fabricating ceramic-reinforced metal matrix composites is that the molten alloys normally do not wet the ceramic reinforcements well and it is thus difficult to infiltrate ceramic preforms with molten alloys. As a result, application of these materials has been limited by the high fabrication cost. One way to reduce the cost is to produce nearnet-shape composites by rapid pressureless infiltration. It has been recognized that pressureless infiltration can be achieved by utilizing certain chemical reactions that lead to enhanced wetting between the ceramic fillers and the molten alloys. Various reactions have been proposed in the past,1 and the reaction between Si and C in particular has been studied by many researchers.2–6 It is well known that reaction between Si and C can lead to pressureless infiltration of molten Si into C preforms in a nonoxidizing atmosphere. According to Ness and Page,5 the solid carbon first dissolves into the molten Si and subsequently precipitates out as SiC. Most of the molten metals used for infiltration, such as Al and Si, are very active and can be oxidized rapidly in air. Formation of even a thin layer of oxide can significantly increase the contact angle between the molten metals and the preforms. Infiltration with these metals is thus usually performed in a vacuum or nonoxidizing environment. However, recently we have discovered that Al–Si alloys can spontaneously infiltrate into C-containing ceramic preforms in an openair environment, and we have been able to fabricate nearnet-shape composites of SiC–Al–Si by simply dipping a)
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