Coatings for TiAl

Abstract

The intermetallic compound, TiAl, and materials based on it have been receiving considerable attention because of their high specific strengths at elevated temperatures. Their mechanical properties are being extensively studied, with a view to application in aircraft or automobile engines. Before using these materials at elevated temperatures, their interactions with the environment should be investigated and suitable measures taken to overcome any degradation. It has, however, been reported that protective Al2O3 scales do not form when TiAl is exposed to an oxidizing atmosphere in spite of its high aluminum content. The formation of an A12O3 scale, or at least a scale containing a continuous A12O3 layer, is a prerequisite for providing sufficient protection. Therefore, much effort has been expended in choosing suitable alloying additions. However, relatively less effort has been devoted to surface coatings on such materials. The purpose of the alloying addition is two-fold: to improve mechanical properties, particularly ductility, and to improve oxidation resistance. However, compatible alloying elements are rare. For instance, some elements which improve the ductility, e.g., vanadium, chromium, and manganese, lower the oxidation resistance. Accordingly, a reasonable concept for the designer is to add effective alloying elements to improve the mechanical properties and to apply a coating to provide protection against the environment. For improved reliability, elements that improve the oxidation resistance can also be added to increase the protection unless they reduce the required mechanical properties.