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

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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.