Applications of in situ reactions to MoSi2-based materials

The influence of silica in MoSi2-based materials is briefly reviewed and it is shown that these materials would benefit from the elimination of this oxide. In situ displacement reactions in which aluminum is used to convert silica to alumina are shown to be a viable processing route for that purpose. It is shown how, with the adequate selection of precursors, this technique can be extended to generate in situ coatings on refractory metal reinforcements, preventing their interaction with MoSi2 as well as in situ alumina fibrous reinforcements using silica-rich fiber precursors. Furthermore, in situ displacement reactions are used to generate boride reinforcements in MoSi2 and aluminum additions can also be used to eliminate silica in these composites. Since the critical aspects for the design of in situ processes are the knowledge of the thermodynamics and kinetics of the possible reactions, the thermodynamics of the most relevant in situ processes described are analyzed using the data currently available. This analysis is limited by the extent of the data available but is helpful in guiding new process development.