PROTECTION OF GRAPHITE BY IMPREGNATION. Quarterly Progress Report No. 3 for September 1-November 30, 1960. Vitro Job 2195

The hypothesis that sample resistivity is the controlling factor in electrokinetic impregnation was substantiated. Experiments show that dense clay- bonded silicon carbide can be impregnated beneath the surface pores whereas only surface pore impregnation is achieved with both dense and porous graphite. The resistivity of the materials differ by a factor of about 1O/sup 10/. Although deep penetration of the substrate by the particles of the coating system is not achieved during deposition, there is evidence of considerable diffusion of the binder-carbide solid solution into the pores of the graphite after sintering. A number of sintered, crack-free, and adherent TaC coatings on ATJ graphite panels were prepared which showed evidence of surface pore penetration. The attainment of satisfactory reproducibility was difficult because of preferential deposition of the binder metal and contamination from erosion of the chrome-manganese steel ball mill. Efforts to prevent contamination by lining the mill with rubber were unsatisfactory. A silicon carbide liner for the mill was ordered and will be received in December. To prevent preferential deposition, precoating of the tantalum carbide particles with nickel by chemical precipitation and electroless nickel plating was investigated. Nickel coatings were obtained by both techniques; however, the chemically precipitated nickel was not adherent when the coated particles were placed in the electrophoretic bath. Good adherence was obtained by electroless plating; but this material, when sintered, cracked during the cooling cycle. Further investigation established that both iron and nickel are required as binders to obtain well-sintered Tar. Work was begun on the preparation of coated rocket nozzle liners for testing at NOL. Graphite nozzles were machined to the WM-D-800-2 configuration, and a cell was designed for obtaiining a uniform electrophoretic deposit. Test coatings were within the required thickness tolerance of plus or minus 0.002 in. The coatings, thus far, have cracked at the edge of the nozzle throat during the heat-up cycle. This is believed to result from a temperature differential in the furnace and from improper control of the binder composition. The furnace is being modified to minimize the temperature gradient, and variability in the binder composition will be eliminated when the carbide liner is installed in the ball mill. Thermal shock tests were performed on coated graphite panels. The specimens were plunged into a 2500 deg C zone, held at temperature for 2 min, and then removed from the hot zone. Microscopic examination of the coating after testing indicated no ill effects. (auth