A MULTILAYER SILICON CARBIDE FIBER COATING FOR TOUGHENED, NEUTRON RADIATION-RESISTANT SILICON CARBIDE/SILICON CARBIDE COMPOSITES

SILICON CARBIDE (SIC) HAS BEEN IDENTIFIED AS A PROMISING MATERIAL FOR FUSION EN4ERGY APPLICATIONS BECAUSE OF ITS LOW ACTIVATION, LOW DECAY HEAT, AND OUTSTANDING HIGH-TEMPERATURE MECHANICAL PROPERTIES. CONTINUOUS SIC FIBER-REINFORCED SIC-MATRIX COMPOSITES (SIC/SIC) ARE OF PARTICULAR INTEREST BECAUSE OF THEIR RELATIVELY HIGH STRENGTH AND TOUGHNESS COMPARED TO THAT OF MONOLITHIC SIC. CURRENTLY AVAILBLE FIBER COATINGS SUCH AS CARBON AND BORON NITRIDE HAVE DEMONSTRATED THE DESIRED MECHANICAL CHARACTERISTICS NECESSARY TO PROMOTE HIGH COMPOSITE STRENGTH AND TOUGHNESS. HOWEVER, THE UTILITY OF THESE MATERIALS IS SEVERELY LIMITED BY THEIR SUSCEPTIBILITY TO MECHANICAL DEGRADATION WHEN EXPOSED TO NEUTRON IRRADIATION. THIS LIMITATION IS DUE TO THE DIMENSIONAL AND MICROSTRUCTURAL INSTABILITY OF THE FIBE2R COATING AS A RESULT OF RADIATION EXPOSURE. SIC/SIC COMPOSITES EXHIBITING HIGH TENSILE STRENGTH AND FRACTURE TOUGHNESS WERE DEVELOPED INCORPORATING AN ENGINEERED SIC FIBER COATING SYSTEM. BY METHODS OF CHEMICAL VAPOR INFILTRATION (CVI), SIC/SIC COMPOSITES, TOUGHENED BY AN UNBONDED MULTILAYER SIC FIBER COATING, WERE PRODUCED AND EVALUATED. COMPARISON OF THE TENSILE TEST RESULTS WAS MADE WITH THAT OF A BASELINE STATE-OF-THE-ART SIC/SIC COMPSOITE SYSTEM EMPLOYING AN OPTIMIZED PYROLYTIC CARBON (PYC) FIBER COATING. RESULTS INDICATE THAT THE MULTILAYER SIC FIBER COATING EMULATES THE DESIRABLE MECHANICAL ATTRIBUTES OF PYC NECESSARY TO IMPART HIGH COMPOSITE STRENGTH AND TOUGHNESS, AND MAY POSSESS THE DESIRED PROPERTIES FOR INCREASED NEUTRON RADIATION RESISTANCE. THE OBJECTIVE OF THIS PHASE I PROJECT IS TO INVESTIGATE THE FEASIBILITY OF OPTIMIZING THE MULTILAYER SIC FIBER COATING SYSTEM SO AS TO MAXIMIZE THE STRENGTH AND TOUGHNESS OF SIC/SIC COMPOSITES FOR POTENTIAL FUSION ENERGY APPLICATIONS. OPTIMIZATION WILL CONSIST OF EVALUATING THE EFFECT OF THE NUMBE2R OF UNBONDED, UNIFORM THICKNESS SIC LAYERS IN THE MULTILAYER FIBER COATING SYSTEMS. FIVE CVI-DENSIFIED SIC/SIC COMPOSITE PLATES INCORPORATING ONE, TWO, FOUR, EIGHT, AND SIXTEEN UNBONDED SIC LAYERS IN THEIR RESPECTIVE MULTILAYER FIBER COATING SYSTEMS WILL BE PRODUCED AND EVALUATED TO DETERMINE THEIR CORRESPONDING TENSILE STRENGTH AND FRACTURE TOUGHNESS CHARACTERISTICS. AN SIC/SIC COMPOSITE UTILIZING