High temperature alloys and their oxidation

Structural materials for high temperature applications depend on the formation of a protective scale limiting the attack of the surrounding environment and sufficient mechanical properties, e.g., creep resistance. Usually, Fe- or Ni-based alloys contain a certain amount of Cr so that a chromia scale is formed. Chromia can protect the components in dry atmospheres and at temperatures below ca. 1000°C. If such protective elements are not present in a sufficient amount, they have to be enriched in the surface zone. One possibility to do so is the enrichment via a CVD process, so called powder pack cementation. Protective elements form a stable, protective oxide layer at elevated temperatures in oxidizing environments, e.g., Cr, Si, Ti or Al. TiO₂ gets deteriorated at elevated temperatures so that it cannot be used. Si- and Cr-oxides suffer from evaporation in water vapor containing environments so that alumina is a better choice. The CVD parameters are strongly influencing the coating quality which is a crucial factor for a later use of treated components. Results of different CVD treatments on several Ni-based alloys by varying the ratio of activator and diffusing element (Al) will be shown and discussed in view of a prolonged use of the treated components for more efficient flights, i.e., less inspection intervals or lower fuel consumption by using lighter materials.