Development of TiAlCr-x coatings for TiAl based alloys.

Gamma TiAl based alloys are considered as alternative materials to Ni based superalloys for applications in gas turbine engines. However, they exhibit low oxidation resistance above 700 °C. The need to develop oxidation resistant coatings for gamma-TiAl based alloys was the motivation for this project. Three TiAlCr-X coatings were studied: These were the Ti-50Al-10Cr (TiAlCr) and Ti-48Al-9Cr-4B (TiAlCrB) coatings and the Ti-48Al-9Cr-4B with Ag interlayers multilayer (TiAlCrB/Ag) coating. The coatings were deposited on Ti-48Al-2Nb-2Mn alloy and Si wafer substrates by RF sputtering using optimised deposition parameters. A pure Ag and two alloy (Ti-50Al-10Cr and Ti-48Al-9Cr-4B) targets were used for sputtering. The coatings were studied by SEM, EPMA, XRD, XPS, TEM, DSC, DTA and TG. The morphology and the microstructure of the as-deposited coatings on Si and Ti- 48Al-2Nb-2Mn substrates were the same. All the coatings exhibited dense columnar morphology with ~1 mum thick columns for the TiAlCr and TiAlCrB and coarser columns for the TiAlCrB/Ag. The TiAlCr and TiAlCrB coatings were amorphous with randomly dispersed alpha-Ti nanocrystals. In the TiAlCrB the nanocrystals were smaller ( 50nm), was noticed when the amorphous TiAlCr was heated at 550 °C, but not in the TiAlCrB after heating at 600 °C, which confirmed the enhanced stability of the TiAlCrB. The oxidation of the coatings, the substrate and target alloys was assessed by TG at 800 °C for 200 hours. The weight change per unit area and the parabolic rate constants were determined for each material. Mixed alumina and titania scales were formed on the specimens. Clusters of coarse titania crystals tended to form on the surface of the scales. Their formation was favoured by the surface roughness of the specimens. The TiAlCrB exhibited the best oxidation resistance of the three coatings, followed by the TiAlCrB/Ag and the TiAlCr