Oxidation Behavior of Silicides

The oxidation behavior of Mo, Nb, and Ti-silicides has received significant attention in past few decades for their potential to be used as high temperature structural materials. These Si-bearing intermetallic alloys have the ability to form an oxide scale containing SiO2, which is protective if formed as a continuous and impervious layer, so that the ingress of oxygen from atmosphere to the underneath alloy is restricted. To form a continuous and stable SiO2 scale, it is important to have sufficient activity of Si along with thermodynamic and kinetic conditions favoring its growth in comparison to that of oxides of other alloying elements. MoSi2 has superior oxidation resistance compared to that of Mo3Si or Mo5Si3, because of its higher Si content. Furthermore, a continuous film of SiO2 is able to form at temperatures in the range of 800-1700 oC on MoSi2 due to vaporization of MoO3, but not on NbSi2 or TiSi2 due to competitive growth of Nb2O5 or TiO2, respectively. During past two decades, a significant effort has been devoted to development of Mo-Si-B alloys containing Mo-rich solid solution, Mo3Si and Mo5SiB2 as constituent phases, due to their ability to form a protective borosilicate scale. The presence of B2O3 contributes to fluidity of borosilicate scale, thereby contributing to closure of porosities. Efforts have been also made to develop multicomponent Nb-silicide based alloys with optimum combination of mechanical properties and high temperature oxidation resistance with limited success. There have been efforts to develop silicide based coatings for protection oxidation for Mo-rich Mo-Si-B alloys and Nb-Si based ternary or multicomponent alloys with inadequate oxidation resistance. Oxidation behavior of selected silicides with potential for structural application, along with mechanisms for protection against oxidation has been reviewed and discussed.