OXIDATION PROTECTION MECHANISMS DURING THE SINTERING OF Mo/FeSi 2 MIXTURES *

A powder mixture of molybdenum and iron disilicide can be caused to react exothermically without excessive oxidation of the molybdenum owing to the formation of an unreactive and self-healing coating. The method depends upon a chemical means, the formation of a surface barrier of a molybdate that provides an environment in which molybdenum losses are suppressed by the exclusion of oxygen. A sodium compound added to the mixture of metal powders to be alloyed reacts with the molybdenum oxide to form a sodium molybdate. During the sintering process, the sodium molybdate migrates to the surface where it forms a protective glaze which prevents oxygen in the environment from reacting with the metallic molybdenum during alloying. Once the alloy is formed, molybdenum losses through oxidation and sublimation cannot occur since the molybdenum is in the form of MoSi₂. A discussion of the chain of chemical reactions believed to occur in the sintering process is based upon identification of evolved reaction products, as well as X-ray identification of solid reaction products.