INVESTIGATIONS ON THE OXIDATION BEHAVIOR OF MAX-PHASE BASED Ti_2AlC COATINGS ON γ-TiAl

摘要

The oxidation resistance of the MAX-phase based material Ti_2AlC was investigated as coating on γ-TiAl. A 10 μm thick layer was deposited by low temperature DC magnetron sputtering on the high temperature alloy Ti-45Al-8Nb (at.%). The MAX-phase formation was enhanced after the deposition process by a heat treatment at 1000°C under high vacuum conditions (10~6 mbar). The oxidation resistance of the treated coating system was tested at 950cC in air and compared with the results of not annealed samples. In comparison to bare MAX phase material, the exposure tests show no continuous alumina scale formation on top of the annealed coating, similar to the not annealed specimens. The annealed and not annealed coatings failed during cyclic testing after 40 and 60 lh-cycles by spallation. SEM and EDS analysis after 2 lh-cycles showed the growth of a thick mixed oxide scale on top of the annealed MAX-phase coating. Moreover, α_2-Ti3Al and a-Nb_2Al formed at the substrate/coating interface, indicating Al as well as Ti depletion by enhanced diffusion processes within the Ti_2AlC layer, that presumably leads to fast oxidation.