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

一些金属二硅酸盐作为一种高温材料，由于能形成保护性的硅垢而具有极高的抗氧化性，因此受到了广泛的关注。然而，二硅化剂氧化的机理尚未明确。换句话说，我们不知道保护二氧化硅的水垢是在什么样的二硅化剂中形成的。因此，本研究的重点是二硅酸盐氧化方式的分类。在773 ~ 1773 K的空气温度范围内对烧结好的二硅酸盐进行了高温氧化试验。氧化垢可分为三大类：(1)由金属氧化物和SiO_2组成的混合氧化垢（TaSi_2, NbSi_2, CrSi_2）；(2)由硅选择性氧化形成的SiO_2垢（FeSi_2, CoSi_2,）；(3)由金属氧化物蒸发形成的SiO_2垢（MoSi_2, WSi_2, VSi_2, ressi_ <1.75>），尽管氧化垢的结构与温度有关。结果表明，FeSi_2、CoSi_2和CrSi_2在1273 K以下具有优异的抗氧化性能，而MoSi_2、WSi_2和VSi_2在更高的温度下具有优异的抗氧化性能。氧和硅通过氧化垢的扩散率、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二硅化物表面氧化垢结构的因素。在此基础上，提出了金属二硅化物氧化模型（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.