Analysis of SCS-6 silicon carbide fibers by Fourier transform infrared spectroscopy
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Analysis of SCS-6 silicon carbide fibers by Fourier transform infrared spectroscopy S. Krishnamurthy UES, Inc., 4401 Dayton-Xenia Rd., Dayton, Ohio 45432-1894 (Received 20 February 1996; accepted 19 August 1996)
Fourier transform infrared (FTIR) spectroscopy analysis of SCS-6 silicon carbide fibers was performed to detect the presence of oxygen in the form of SiO2 . The results showed infrared peaks corresponding to SiC fundamental lattice absorption as well as reflection. No absorption peak due to SiO2 was observed. These results are in agreement with previously reported data indicating low levels of oxygen in this fiber.
Silicon carbide fiber reinforced titanium matrix composites are of considerable interest due to their high specific mechanical properties and are candidate materials for advanced aerospace applications. Much of the past work on these composites involving a variety of titanium and titanium aluminide alloy matrices has utilized the SCS-6 SiC fiber, which is commercially produced by Textron Specialty Materials using chemical vapor deposition (CVD). This fiber measures 142 mm in diameter and consists of CVD SiC deposited on a 33 mm diameter carbon monofilament substrate. The outer surface of this fiber is coated with two carbon-rich C 1 SiC layers with a total coating thickness of ,3 mm. A thorough work on the microstructural characterization of this fiber has been previously published.1 One of the earlier studies dealing with the compositional analysis of the SCS-6 fiber using scanning Auger spectroscopy indicated the presence of high levels of oxygen (.10 at. %) in the bulk of this fiber, presumably in the form of SiO2 .2 Based on this data, it was suggested that the outward diffusion of oxygen from the fiber into the surrounding titanium may contribute to increased hardness and embrittlement of the titanium alloy matrix and can have deleterious effects on composite properties.2 A subsequent evaluation of this fiber using Auger spectroscopy and neutron activation analysis (NAA) disputed these results, and the NAA data showed that the bulk oxygen content of the SCS-6 fiber is quite low (,0.12 at. %).3 This latter study concluded that the previously observed high oxygen levels were artifacts probably due to inadequate vacuum and specimen contamination during scanning Auger analysis. The work reported here presents a re-evaluation of the SCS-6 fiber using Fourier transform infrared (FTIR) spectroscopy, which is a useful technique for measuring the oxidation of silicon carbide.4,5 The SiO2 absorption at 1090 cm 21 and the SiC absorption at 790 cm21 are sufficiently far apart6 and, therefore, each band area can be measured in the presence of the other. The work of Falk and Karunanithy has demonstrated that the J. Mater. Res., Vol. 12, No. 2, Feb 1997
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ratio of the two band areas can provide a quantitative measure of the extent of oxidation of SiC to SiO2 .5 The data of these authors are shown in Fig. 1
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