Protection of TiAl-components against high temperature oxidation with fluorine

Titanium aluminides are very promising light weight materials for several high temperature applications e.g. in aero or automotive engines but due to their insufficient oxidation resistance at temperatures above roughly 800°C they can not be used for longer times. The oxidation behaviour of TiAl-alloys can be improved significantly by microalloying of small amounts of fluorine into the subsurface zone. The use of TiAl-components after fluorine treatment is possible up to temperatures above 1000°C in oxidizing atmospheres due to the formation of a thin protective alumina layer. One possibility to apply fluorine to the surface of complex TiAl-components is the PI³-technique (plasma immersion ion implantation). The use of an Ar/CH2F2-plasma for the F-PI³ into small coupons leads to a positive effect which is as good as the beamline implantation of elemental fluorine gas into samples of the same size. Other treatments are using liquid or gaseous fluorine containing compounds which react with the surface of the specimens. The oxidation kinetics of F-treated TiAl-alloys will be discussed in the paper. Several more complex TiAl-samples and turbine blades as examples for real TiAl-components have been treated. Their oxidation behaviour will be shown. Post exposure investigations like SEM reveal a thin protective alumina scale on the surface of the implanted samples in contrast to a thick mixed scale (TiN/TiO2/Al2O3) growing on untreated samples during high temperature exposure in air.