The structure of carbon in chemically vapor deposited SiC monofilaments
- PDF / 7,994,660 Bytes
- 12 Pages / 593.28 x 841.68 pts Page_size
- 42 Downloads / 238 Views
J. DiCarlo NASA Lewis Research Center, Cleveland, Ohio 44135 (Received 29 May 1990; accepted 23 August 1990) The microstructures of carbon-rich regions in chemically vapor deposited SCS-6 SiC fibers have been studied by electron diffraction and high resolution transmission electron microscopy. The microstructures of the carbon in the substrate filament and the outer coating layers of the fiber are consistent with different distributions of Oberlin's model of Basic Structural Units (BSUs)1 while that of the inner substrate coating is consistent with turbostratic carbon (TC).
I. INTRODUCTION
SiC fibers are some of the most important reinforcing agents used in high performance metal-matrix or ceramic-matrix composites. The SCS family of fibers is manufactured by Textron Co. (Textron Speciality Materials, Lowell, MA) using a chemical vapor deposition (CVD) technique to deposit SiC on a continuous carbon filament. Typically, they have high tensile strength (3950 MPa), high tensile modulus (400 GPa), and low density (3045 kg/m3).2 The microstructure of these fibers, which determines their mechanical and physical properties, has been previously studied by transmission electron microscopy (TEM).3'4 The SCS fibers consist of many different layers produced by changing the deposition conditions during the CVD process. The doublecoated SCS-6, which is the latest member of this family, has two additional outermost layers which were studied in Ref. 4. About half the volume of the SCS-6 fiber, including the outermost coating, consists of carbon while the other half is SiC. Since carbon is such an important constituent of the fiber, and plays a very significant role in the mechanical properties of the composites which contain the fibers,5 it is worthwhile to study the microstructure of the carbonaceous parts of the fiber in more detail. In this paper, the results of a TEM study of the different regions of carbonaceous material in the SCS-6 fiber are presented. These are interpreted in terms of a unified model due to Oberlin.1 II. STRUCTURE OF CARBONACEOUS MATERIALS
In addition to its two well-defined allotropic forms, diamond and graphite, carbon can take on any number of partially crystalline forms ranging continuously from a near-amorphous to a highly crystalline state.6 In this J. Mater. Res., Vol. 5, No. 12, Dec 1990
http://journals.cambridge.org
Downloaded: 13 Mar 2015
paper we shall not discuss diamond, where the bonding between the carbon atoms is of the spz type, and is in a class by itself. Franklin7 classified all carbonaceous material (except diamond) into two distinct groups: graphitizing (or soft carbon) and nongraphitizing (or hard carbon). Soft carbons, such as petroleum coke and pyrolitic carbon, are readily converted to graphite by heating up to 3000 K. On the other hand, hard carbons, such as glassy or cellulose carbon, are not completely converted to graphite even at temperatures in excess of 3300 K.s Graphite consists of a stack of "basal" planes of carbon atoms. Within each basal plane, the C atoms are a
Data Loading...
