OXIDATION BEHAVIOR AND STRENGTH OF B4C-30 WT PERCENT SIC COMPOSITE-MATERIALS

The oxidation behaviour and the effect of oxidation on the room-temper ature flexural strength of B4C-30 wt % SiC composite material were inv estigated. The weight changes of the samples exposed to air at tempera tures between 500 and 1000 degrees C were continuously monitored with a microbalance. At temperatures below 800 degrees C, the weight change of the specimen was negligible. As the temperature was increased to 8 00 degrees C, parabolic weight gain was observed; The rate of the weig ht gain increased with exposure temperature. The oxidation product for med on the surface was found to be a crystalline boric oxide (B2O3) by X-ray analyses. The oxide layer was severely cracked due to the therm al expansion mismatch between the oxide layer and the substrate. Howev er, the room-temperature flexural strength was increased when the samp les were exposed at temperature between 700 and 900 degrees C, apparen tly due to the blunting of strength-limiting defects at the surface. W hen the temperature was higher than 1000 degrees C, a severe reduction in strength was observed. The reliability of the composite material w as also improved significantly by such exposures.