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

MoSi_2是一种很有前途的超高温材料。虽然MoSi 2具有出色的抗氧化性，但它也显示出加速的氧化行为，导致虫害，特别是在低温下。加速氧化和张贴是基于Mo和Si的同时氧化。然而，同时氧化的机制还不清楚。本研究的目的是阐明抑制的机制和方法。主要结果如下。(1)加速氧化是最严重的温度范围从673至773 K，并成为温和得多，随着温度的升高。(2)MoSi_2表面缺陷与水蒸气在大气中的共同作用下，易发生加速氧化和穿孔。(3)随着Mo/Si原子比的增加，抗氧化性降低。然而，影响是相当小的。(4)气孔和裂纹等缺陷是加速氧化和穿孔的原因。相反，通过使用完全致密的MoSi 2，完全抑制了加速的氧化和穿孔。(5)同时氧化的起始点是缺陷，而加速氧化和穿孔的影响最大的是位于MoSi 2表面的缺陷。热力学的考虑也表明，同时氧化是不是一个内在的属性的MoSi 2。

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.