Low temperature oxidation of fully dense and porous MoSi2

The effects of pore and H2O vapor on accelerated oxidation and pesting were observed from fully dense and porous molybdenum disilicide (MoSi2) at 500 degrees C in air and air-H2O vapor atmospheres. Occurrence of simultaneous oxidation of Mo and Si, resulting in pesting, was hindered by using fully dense MoSi2. In addition, H2O vapor has no influence on the oxidation behavior of fully dense MoSi2. On the other hand, porous MoSi2 showed the accelerated oxidation behavior after an induction period. In the induction period, simultaneous oxidation occurred preferentially at pores and gradually spreaded over the surface. After this period, the mass gain increased linearly with a high rate due to generation and development of cracks, resulting in pesting. In porous MoSi2, H2O vapor in air accelerated the occurrence of pesting. This is probably due to the formations of metastable hydrates, H2Si3O7 and H4Si8O18, at pores. (C) 1999 Elsevier Science S.A. All rights reserved.