A Novel Route to Silocon Based Ceramic Coatings on Carbon Substrates
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A NOVEL ROUTE TO SILOCON BASED CERAMIC COATINGS ON CARBON SUBSTRATES
ANDREW N. MACINNES and ANDREW R. BARRON Department of Chemistry and Materials Research Laboratory Harvard University, Cambridge, Massachusetts 02138. ABSTRACT Carbon fiber tows have been impregnated by ethanolic solutions of organo-silicon chlorides, and fired at temperatures up to 900 0 C to form silicon based coatings. Fired tows were subsequently examined by X-ray photoelectron spectroscopy and scanning electron microscopy to characterize the coated material. A uniform silicon oxycarbide is formed at temperatures upwards of 400 0C, which provides an oxidation barrier in carbon fiber reinforced metals. INTRODUCTION The continuing drive to produce hybrid composites materials that are stronger, stiffer, yet more lightweight has led to increasing efforts to incorporate high performance carbon fibers into metallic matrices. 1.2 However, a major obstacle in the realization of carbon fiber reinforcement of metals is that to fully exploit the composite properties the metal matrix must wet the carbon reinforcement yet at the same time not undergo adverse reactions impairing mechanical properties. Unfortunately, no pure metal satisfies this criterion. For the particular case of aluminum, it appears to be inert and non wetting toward carbon fibers at low temperatures but wetting and extremely reactive at higher temperatures, 3 forming aluminium carbide, A1 4 C3 , by the reaction: 4 Al
+
3C ---------------- > A14 C3
The formation of the friable carbide and its subsequent oxidation during processing, or service at elevated temperatures, ultimately leads to disbondment of the fibers and severe reduction of shear load transfer between matrix and reinforcement. It becomes clear, therefore, that some form of coating is needed to alleviate the fiber/matrix interfacial reaction (i.e. SiC, TiB 2 , BN via CVD 4). In this regard, we have investigated the formation of silicon carbide based coatings on carbon fibers by a reactive solution process. This follows our recently reported Author to whom all correspondence should be addressed. Mat. Res. Soc. Symp. Proc. Vol. 249. 01992 Materials Research Society
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work on the application of a refractory metal carbide coating on to carbon fiber tows by a reactive solution process,5 aimed to produce a graded carbide/carbon interface. Carbon fiber tows were infiltrated by an ethanolic solution of transition metal chlorides, and resistively fired to form a coating of the appropriate metal carbide. The presence of chloride in the precursor was proposed to aid the reaction process by activating the carbon surface, when released as either chlorine or phosgene gas.6 We have attempted to extend this work to the formation of a graded silicon carbide/carbon interface, however, the inability to utilize SiC14 in the above process (due to its low boiling point, 57 OC) has led us to investigate other possible precursors. Working within the framework of utilizing silicon containing compounds that are non-volatile, and decompose to l
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