Fundamental studies of the influence of boron on the graphite-oxygen reaction using in situ electron microscopy techniqu
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Controlled atmosphere electron microscopy coupled with in situ electron diffraction has been used to follow the manner by which the addition of boron oxide influences the graphite-oxygen reaction. Continuous observations of the process show that at about 450 GC the boron oxide undergoes a strong interaction with both the graphite edge and the basal plane regions, and this results in a spreading of the oxide to form a uniform thin adherent film over the entire substrate. The coated graphite specimens appear to be impervious to attack by oxygen at temperatures below 815 °C. Above this temperature, however, there is a weakening of the additive-graphite interaction with the "armchair" {1120} faces, and these regions then become vulnerable to attack by oxygen. At the same time very shallow pits are observed to develop in the basal plane, and this action coincides with the appearance of boron carbide species in the electron diffraction pattern. In a complementary series of experiments, it is found that boron carbide is an extremely active catalyst for the graphite-oxygen reaction even at temperatures as low as 100 °C. The impact of these low pressure studies on the behavior of carbon structures used in aerospace applications is discussed.
I. INTRODUCTION Carbon/carbon composites are being considered for use in a number of aerospace applications where materials are expected to survive and maintain their integrity in a variety of environmentally hostile conditions. In an inert environment, carbon/carbon composites have been found to retain their strength and mechanical properties to temperatures in excess of 1600 "C.1 Unfortunately, there is a major problem in using these materials in an oxidizing environment as they start to undergo gasification at temperatures as low as 450 °C, and the presence of catalytically active impurities can decrease this onset temperature and increase the rate of oxidation.2 Two major approaches have been used in an attempt to improve the oxidation-resistance of these materials. The first involves the addition of oxidation inhibitors to the substrate surface and the second, the coating of the composite with protective layers which function as oxygen diffusion barriers. McKee3 reviewed the different types of oxidation inhibitors for carbon and concluded that besides halogens the most common compounds were phosphorous and boron oxides.4"11 Allardice and Walker12 compared the reactivity of substitutionally boron-doped graphite with uncontaminated samples in both dry and wet oxygen environments over the range 550 to 650 °C. These workers found that boronation reduced the rate of carbon gasification in both reactants in spite of the fact that it lowered the activation energy of these processes. They argued 1886
http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 8, Aug 1993
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that boron, in the form of B 2 O 3 and possibly HBO 2 (in moist oxygen), effectively blocked the active sites on the graphite surface. This claim was supported by the finding of a progressive dec
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