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 °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 Nb₂O₃ 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. D.E. Alman, Formerly Graduate Student, Materials Engineering Department, Rensselaer Polytechnic Institute