EFFECTS OF BURN-OFF ON THERMAL-SHOCK RESISTANCES OF NUCLEAR CARBON MATERIALS

The purpose of this study is to evaluate the effects of burn-off on th ermal shock resistances and thermal shock fracture toughnesses of a ca rbon fiber reinforced carbon composite (C/C composite) and three fine- grained isotropic graphites. These thermal shack resistances and therm al shock fracture toughnesses were degraded slightly by air oxidation at 500 degrees C. The extents of degradations of the thermal shock par ameters were less than those of the mechanical and fracture mechanics properties, however, they were larger than that of the thermal diffusi vity. In observations of the microstructures of the fracture surfaces after oxidation of the graphites, the size and the number of pores wer e increased and the fracture surfaces were rough due to oxidation of b oundaries of graphite particles. After oxidation of the C/C composite, there were preferential removal of the boundary layer between carbon fiber and pyrolytic carbon matrix and pull out of carbon fiber. (C) 19 97 Elsevier Science Ltd.