CLARIFICATION AND SUPPRESSION OF ACCELERATED OXIDATION IN MoSi_2 AS A ULTRA-HIGH TEMPERATURE MATERIAL.

点击翻译按钮获取中文摘要

MoSi_2 has been known for a promising material for ultra-high temperature applications. Although MoSi2 has outstanding oxidation resistance, it also shows accelerated oxidation behavior, resulting in pesting, especially at low temperatures. The accelerated oxidation and posting are based on simultaneous oxidation of Mo and Si. However, the mechanism of the simultaneous oxidation has not been clear. Aims of the present research are to clarify the mechanism and the method of the suppression. The main results are as follows.(1) The accelerated oxidation is most severe at temperatures ranging from 673 to 773 K and became much milder as temperature increases.(2) The accelerated oxidation and pesting are liable to occur under a combination of defects on surface of MoSi2 and H2O vapor in atmospheres.(3) As the atomic ratio of Mo/Si increases, the oxidation resistance decreases. However, the effect is appreciably small.(4) Defects such as pores and cracks are responsible for the accelerated oxidation and pesting. Conversely, the accelerated oxidation and pesting are completely suppressed by using fully dense MoSi2.(5) The initiation sites of the simultaneous oxidation are deffects.The accelerated oxidation and pesting, after all, are most strongly affected by preexisting defects located on the surface of MoSi2. Thermodynamical consideration also shows that the simultaneous oxidation is not an intrinsic property of MoSi2.