CVD Coating of Ceramic Monofilaments
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CVD COATING OF CERAMIC MONOFILAMENTS
JASON R. GUTH Applied Sciences,
Inc.,
PO Box 579,
Cedarville,
OH 45314
ABSTRACT In many composite systems it has become apparent that coatings on the reinforcements are necessary to achieve high toughness materials. In order to examine materials which may be used as coatings on ceramic monofilaments and remain stable in high temperature, oxidizing environments, the deposition of a number of refractory metals has been attempted. The results of coating experiments using silicon carbide fibers as substrates as well as general observations concerning the prospects of continuously coating long lengths of fibers will be discussed. The materials studied include carbon, cobalt, zirconium, molybdenum, tantalum, tungsten, and iridium. Carbon has been deposited from methane and propylene onto both SiC and sapphire fibers. Deposition of the metals has been achieved by direct chlorination of the metals followed by hydrogen reduction at the fiber. Iridium(III)2,4-pentanedionate has been used to deposit iridium metal. All metals were deposited at low pressure in a hot wall reactor with fibers continuously spooled through the reactor. INTRODUCTION Ceramic materials have found uses in many high temperature structural applications. However, due to the inherent brittleness of most ceramic materials the interest in monolithic ceramics is being replaced by composites incorporating fibers or filaments as reinforcements in ceramic matrices[l]. In recent years it has become evident that weak interfacial strengths are a necessary condition to obtain a tough ceramic composite material[2,3]. To date, the materials found to best generate such weak interfaces are carbon and boron nitride, usually applied as a coating to the reinforcement fibers[3-5]. Both carbon and boron nitride are easily oxidized at fairly low temperature. Therefore, it is desirable to find other materials or compounds which are stable in 0 oxidizing environments to above 1000 C, easily applied to fibers or filaments, and lead to a sufficiently weak interface for composite toughening. The purpose of this work is to investigate these first two issues and feed experiments aimed at defining the third. The systems chosen to investigate the issues mentioned above consist of SiC (Textron SCSO) and single crystal sapphire (Saphikon) ceramic monofilaments and glass or ceramic matrices. In general, the composites consist of unidirectionally aligned filaments in a matrix densified by hot pressing. To apply coatings to the fibers before consolidation into composites, chemical vapor deposition (CVD) was chosen. The advantages of the CVD method include the wide range of elements and compounds that can be deposited, uniform surface coating that is not line-ofsight, and the high purity of deposited materials[5]. The fibers Mat. Res. Soc. Symp. Proc. Vol. 250. Q1992 Materials Research Society
264
of interest are received and coated as single, long filaments which can be wound on spools and laid-up into tapes for composite fabrication. EXPERIMEN
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