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 res istance of MoSi2/20 vol pct Nb composites was investigated. Niobium wa s incorporated into MoSi2 as particles, random short fibers, and conti nuous aligned fibers. After oxidation, it was found that all the compo sites had lost weight and essentially disintegrated. This was attribut ed to spalling of both the Nb2O5 scale and the MoSi2 matrix. The spall ing of the matrix was a result of cracks originating in the oxidized N b and propagating through the MoSi2 matrix. These cracks arose from tw o sources: (1) the volume expansion associated with the transformation of Nb to Nb2O5 and (2) the difference in thermal expansion between Nb 2O5 and MoSi2. However, it was found that the addition of smaller diam eter Nb reinforcements tended to retard the disintegration of the comp osites. This was attributed to the effect of reinforcement size on CTE mismatch cracking.