The figure schematically illustrates an apparatus and process in which a continuous tow of silicon carbide fibers is coated and infiltrated with a polymer/silicon slurry. A compact of such coated, infiltrated fiber tows is subsequently heated to a temperature > 1,200 °C in nitrogen to convert it to one of a variety of materials, denoted generally as "SiC/RBSN," that are composites of silicon carbide fibers in reaction-bonded silicon nitride matrices. SiC/RBSN composites are candidates for use in structural components for some high-temperature applications.
A slurry of the type used in this process consists of silicon particles with a mean diameter of 0.3 µm suspended in an isopropyl solution of a polymeric binder. The use of submicron silicon particles and the choice of proportions of the other ingredients to achieve low viscosity of the slurry are necessary to ensure infiltration of the slurry into the interstices of the tow. A fiber tow of the type used in this process typically consists of nearly 500 filaments of ceramic-grade SiC, each with a diameter of about 15 µm. The fibers as manufactured are coated with a sizing of polyvinylacetate, which must be removed in the process to make room for infiltration by the silicon particles.
The tow is drawn from a supply spool, through the apparatus and process, to a takeup spool. After coming off the supply spool, the tow first passes through a few cylinders that contain solutions of equal volume proportions of isopropyl alcohol and water. The solutions are heated to a temperature between 35 and 55 °C and agitated ultrasonically. The sizing would ordinarily be insoluble in static isopropyl alcohol/water solutions, but with the ultrasonic agitation, the solutions gradually dissolve the sizing.
After emerging wet from the last sizing-removal bath, the tow is drawn through a drying furnace. The dried tow passes into a spreading device, wherein air is blown across the tow, creating vibrations and spreading the fibers out.
In addition, an electrostatic generator puts electrostatic charges on the passing fibers, so that mutual electrostatic repulsion will enhance spreading.
The spread tow is plunged into a dip-coating device, where it is coated and infiltrated with the slurry. The coated, infiltrated tow then passes through a second drying furnace. As the tow emerges from the furnace, it is consolidated by drawing it through a heated circular die. Typical drying and die temperatures range from 110 to 190 °C.
This work was done by Rickmond C. Chan of Lewis Research Center.For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Materials category, or circle no. 165 on the TSPOrder Card in this issue to receive a copy by mail ($5 charge).
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