THE ROLE OF MICROSTRUCTURE ON PESTING DURING OXIDATION OF MOSI2 AND MO(SI,AL)(2) AT 773 K

The pesting behavior of MoSi2 and Mo(Si,Al)(2) has been examined in ai r at 773 K to clarify the origin and mechanism of pesting phenomena an d the effect of aluminum on pesting phenomena. The initial cracks play a much more important role than the grain boundaries and the initial oxide layer in pesting. Mo and Si oxidize to amorphous Mo-Si-O simulta neously with about a 200% volume expansion. Therefore, large stress ap pears at the cracktips and induce many new cracks. MoO3 vaporizes from the Mo-Si-O layer on the external surface and crack surfaces causing the oxides in the initial cracks to become porous. Oxygen has a short- circuit path to enter the sample in the cracks. Therefore, the partial pressure of oxygen is sufficiently high to allow oxidation of Mo ill the materials. The platelet-like MoO3 grows on the external surface an d also in the cracks. Finally, the sample distintegrates into powder. Pesting of Mo(Si,Al)(2) occurs in the same way, however, its rate is m uch lower than that of MoSi2. The role of Al is to decrease the initia l crack density of the samples from the melt. Other effects of Al migh t be to decrease the oxygen flux toward the oxide-intermetallic interf ace and to increase the plasticity of the amorphous oxide being formed in the cracks.