Method of Forming a Porous Carbonaceous Preform From Water-Based Slurry

A carbonaceous material was formed by blending about 24.4 grams of carbon fiber, about 34.7 grams molybdenum carbide powder, 17.7 grams carbon powder, and 23.41 grams of silicon carbide powder. A solution comprised of 90 cubic centimeters water, 30 cubic centimeters furfuryl alcohol, 5 cubic centimeters isopropyl alcohol, and 1 cubic centimeter glycerol was mixed for several minutes, and adjusted to a pH of 12 by adding an aqueous solution of 25 weight percent tetramethylammonium hydroxide. A mixture of the carbonaceous material and about 6 grams of polyethylene oxide, polyox WSR-205, Union Carbide, was ball milled for about 10 minutes. A slurry of the mixture and about 90 cubic centimeters of the solution was vibration milled for 5 minutes in a plastic container with 350 grams of zirconia balls having an average diameter of 0.635 centimeter. The slurry was deaired in a vacuum desiccator.

The slurry was cast by the doctor blade method on a glass plate covered with a sheet of TEFLON FEP to form tapes about 0.5 milliliters in thickness. The tapes were dried in air for several hours to a thickness of about 0.3 millimeters. The tapes were formed having a bottom surface facing the TEFLON coated glass plate, and an oppositely facing top surface. The tapes were cut into sections about 2.73 by 2.73 centimeters. Laminated tapes were formed by stacking about 12 of the tape sections on a sheet of TEFLON. The sections were sprayed with a thin film of furfuryl alcohol on the top surface of each tape section, and stacked one on top of the other so that bottom surfaces faced top surfaces.

The laminated tapes were pressed at about 20 pounds between sheets of TEFLON to flatten the laminated tapes. The laminated tapes were dried in an oven at about 100.degree. C. to increase strength and remove liquid. After several hours the laminated tapes were removed and heated to 250.degree. C. for 1 hour to decompose the polyethylene oxide and furfuryl alcohol, and form tape preforms. The porosity of one of the preforms was measured at about 39 percent. A cross section of another of the preforms was examined metallographically, and was found to have a uniform distribution of carbon powder, carbon fibers, silicon carbide, molybdenum carbide, porosity, and no delamination of the tape sections.

One tape preform and a deposit of 6.5 grams of a silicon alloy comprised of 8 weight percent molybdenum were placed on a carbon fiber cloth, Union Carbide, to form an assembly. A mixture of molybdenum disilicide and isopropyl alcohol was brushed onto the carbon fiber cloth. The assembly was placed in a covered carbon crucible, and heated in the bell-jar furnace to about 1430.degree. C. for 15 minutes in a vacuum. The silicon alloy became molten and infiltrated the preform, reacting silicon with carbon to form a silicon carbide body having a minor phase of molybdenum silicides and silicon filling porosity that remained in the reacted preform. The furnace was cooled to room temperature and the silicon carbide body was recovered. Metallographic examination of the silicon carbide body showed no detectable porosity, no delamination of the original tape layers, and uniform dispersion of the molybdenum silicides and silicon so there was no cracking in the silicon carbide body. The difference in thermal expansion between the molybdenum silicides, silicon carbide, and silicon in the reaction formed body would have caused cracks to form if they had been distributed non-uniformly.