Declassfied 30 Aug 1973. An investigation was conducted to determine the manufacturability of hot-pressed 75--25 wt% NbC--graphite composites. Commercially available and custom ground superfine carbide and graphite powder combinations were evaluated; the hot pressing operation was studied; and machining techniques were developed. The relationship of the physical properties of the starting powders to thermal and mechanical properties of the composites was studied. Room temperature and 2400 deg C tensile, compressive, and flexural strengths were determined; tensile creep at 24OO deg C and compressive deformation (creep) at 2200 deg C were studied; and thermal expansion and specific heat to 5000 deg F were determined. In addition, the effect of a post hot-pressing heat treatment was studied. Evaluation of results shows that equally dense, high strength composites can be obtained by hot-pressing large or small billets. For the composition studied, the resulting structure of the superfine carbide graphite powders was a weak carbide continuous phase in a dominant continuous graphite phase. The resulting structure from commercially available carbide/graphite powders was a dominant carbide network. Optimum mechanical properties were obtained both at room temperature and at high temperature, using normal commercially available high purity powders. Thermal properties do not appear to be dependent on the physical properties of the powders. A post hot-pressing hightemperature heat treatment is important in improving the dimensional stability of the billets. Ultrasonic material removal techniques developed for rough machining and diamond tooling developed for final machining provided an economical high production method for manufacturing composite hardware. (auth)
Declassified 30 Aug 1973. Recent analysis of the NERVA hot end support system revealed that the high with-grain (WG) thermal expansion and the high creep deformation of NRX-A6 NERVA-type 75 wt% niobium carbide -- 25 wt% graphite hot pressed composite material may not be entirely satisfactory in future designs. Design requirements have shown the need for a better match between the normally high average coefficient of thermal expansion ( alpha ) of the composite material and the fueled graphite. A manufacturing engineering program was initiated to investigate methods for reducing the difference in the coefficient of thermal expansion (CTE) between the fuel and the carbide composite and, in addition, methods for reducing the creep rate. These objectives were targeted to be accomplished on 75--25 wt% composite material. It was demonstrated that the coefficient of thermal expansion can be r by about 50 percent of the difference between the coefficient for fueled graphite and that for the NRX-A6 type of composite material without compositional changes. In addition, by a slight modification of the carbide-- graphite ratio, the process should permit the matching of the CTE of fueled graphite and composite material. Improvements can be accomplished by a compressive heat treatment process in which the billet is subjected to a laterally unrestrained axial compression at an elevated temperature. The process can be accelerated by the substitution of calcined- petroleum needle coke in place of the graphite flour. A reduction of approximately 50% of the difference between the NRX-A6 and fueled graphite CTEs was accomplished with a compressive heat treatment without a die at 2700 deg C and 2000 psi for 60 minutes. A further reduction of 10% was accomplished when these pararmeters were changed to 2500 deg C and 3000 psi for 120 minutes. In addition to the reduction in the CTE ...