Fluorine for protection of aero engine parts against high temperature oxidation

Titanium based alloys or intermetallics form a TiO₂ layer which is protective at rather low temperatures, passive layer, but loses its protective properties at temperatures above approximately 650°C. All common alloys based on the γ-TiAl phase do not form a protective alumina layer during high temperature exposure in oxidizing environments despite their high aluminum content above 40 at.%. These materials form a mixed scale of TiO₂ and Al₂O₃. Such a scale does not show long term protective behavior at service temperature. In addition, also nitrides are formed in air which deteriorate the oxidation behavior even more. The oxidation behavior can be improved to a certain point, e.g. by alloying with additional elements such as Nb. On the other hand, alloying influences the mechanical properties and does not prevent mixed scale formation or even has a detrimental effect which can be seen by the embrittlement of Ti₂AlNb where the Nb content is too high. In contrast to the alloying attempt the desired formation of a protective alumina layer can be achieved by fluorination, i.e. so called “fluorine effect”. Distinct amounts of fluorine in the surface zone of Ti based alloys change the oxidation mechanism towards alumina formation if the Al content is sufficient, > 40 at.%. This can be reached via an Al-enrichment before exposure, e.g. by powder pack aluminizing. This fluorination works for different alloys, independent of their chemical composition, their production route or their microstructure. Additionally, the fluorine effect is effective at temperatures above 1000°C, in wet environments and under thermocyclic load. Results of high temperature exposure tests of untreated and treated Ti based alloys will be presented and discussed.