THE EFFECT OF NB MORPHOLOGY ON THE CYCLIC OXIDATION RESISTANCE OF MOSI2/20 VOL PCT NB COMPOSITES

The effect of Nb morphology on the 1200-degrees-C cyclic oxidation resistance of MoSi2/20 vol pct Nb composites was investigated. Niobium was incorporated into MoSi2 as particles, random short fibers, and continuous aligned fibers. After oxidation, it was found that all the composites had lost weight and essentially disintegrated. This was attributed to spalling of both the Nb2O5 scale and the MoSi2 matrix. The spalling of the matrix was a result of cracks originating in the oxidized Nb and propagating through the MoSi2 matrix. These cracks arose from two sources: (1) the volume expansion associated with the transformation of Nb to Nb2O5 and (2) the difference in thermal expansion between Nb2O5 and MoSi2. However, it was found that the addition of smaller diameter Nb reinforcements tended to retard the disintegration of the composites. This was attributed to the effect of reinforcement size on CTE mismatch cracking.