Establishment of oxidation model of metal disilicides for ultra-high temperature application

一些金属二硅化物由于在高温下形成保护性二氧化硅膜而具有极高的抗氧化性，因而备受关注。然而，二硅化物的氧化机理尚不清楚。换句话说，目前还不知道在哪种二硅化物中形成了保护性的二氧化硅氧化膜。因此，本研究重点对硅化物的氧化方式进行了分类。在773~1773K的温度范围内，对烧结硅化物进行了高温氧化试验。氧化膜可以方便地分为三类：(1)金属氧化物和SiO_2(TaSi_2，NbSi_2，CrSi_2)形成的混合氧化膜；(2)Si(FeSi_2，CoSi_2)选择性氧化形成SiO_2膜；(3)金属氧化物蒸发形成SiO_2膜(MoSi_2，WSi_2，VSi_2，Resi_lt；1.75&gT；)，尽管膜的结构与温度有关。这些结果表明，FeSi2、CoSi2和CrSi2在1273K以下具有良好的高温抗氧化性，而MoSi2、WSi2和VSi2在更高的温度下具有良好的抗氧化性能。氧和硅通过氧化膜的扩散系数、金属氧化物的蒸发速率以及金属和硅对氧的亲和力是决定金属硅化物上形成的氧化膜结构的因素。在此基础上，提出了金属硅化物的氧化模型(机理)。

Some of metal disilicides are receiving much attention as high-temperature materials having extremely-high oxidation resistance based on the formation of a protective silica scale. However, the mechanism of oxidation of the disilicides has not been clarified. In other words, it is not known that a protective silica scale is formed in what kind of disilicides. Therefore, this study focuses on the classification of oxidation mode in the disilicides.High temperature oxidation tests of the sintered disilicides were carried out at the temperature range, from 773 to 1773 K in air. The oxide scales were conveniently classified into three groups : (1) formation of a mixed oxide scale consisting of metal oxide and SiO_2 (TaSi_2, NbSi_2, CrSi_2) (2) formation of a SiO_2 scale by selective oxidation of Si (FeSi_2, CoSi_2,), and (3) formation of a SiO_2 scale by evaporation of metal oxide (MoSi_2, WSi_2, VSi_2, ReSi_<1.75>), though the scale structure depends on temperature. These results demonstrated that FeSi_2, CoSi_2, and CrSi_2 had high potential as high temperature materials having outstanding oxidation resistance at the temperatures below 1273 K, and MoSi_2, WSi_2, and VSi_2 at higher temperatures. The diffusivity of oxygen and Si through oxide scale, evaporation rate of metal oxide, and affinity of metal and Si for oxygen were deciding factors in structures of the oxide scales formed on the metal disilides Based on the results, the oxidation model (mechanism) of metal disilicides was proposed.