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

MoSi_2 因其在超高温应用中很有前景的材料而闻名。尽管MoSi2具有出色的抗氧化性，但它也表现出加速氧化行为，导致虫害，尤其是在低温下。加速氧化和后处理基于Mo和Si的同时氧化。然而，同时氧化的机理尚不清楚。本研究的目的是阐明抑制的机制和方法。主要结果如下：(1)加速氧化在673~773 K温度范围内最为严重，随着温度的升高而变得更加温和。(2)MoSi2表面缺陷和大气中的H2O蒸气共同作用下容易发生加速氧化和虫害。(3)随着Mo/Si原子比的增加，抗氧化性降低。但影响较小。(4)气孔、裂纹等缺陷是加速氧化和虫害的原因。相反，使用完全致密的MoSi2可以完全抑制加速氧化和有害现象。(5)同时氧化的起始位点是缺陷。毕竟，加速氧化和有害现象最强烈地受到位于MoSi2表面上预先存在的缺陷的影响。热力学考虑还表明，同时氧化并不是 MoSi2 的固有特性。

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.