Dual–Layer PVD Coating System With Integrated Diffusion Barrier for Oxidation Protection of γ–Tial Based Alloys

Abstract

Titanium aluminides are used as structural materials for turbine blades in jet engines due to their low density and the resulting weight reduction. However‚ their application is limited to service temperatures below 800 °C due to insufficient oxidation resistance. A feasible way to increase the service temperature is the application of Al-rich oxidation protective coatings to ensure the formation of a thermally grown alumina layer. Nevertheless‚ interdiffusion processes‚ especially of Al‚ between the intermetallic coatings and the TiAl-based substrate materials have detrimental effects on the coating as well as on the substrate alloy. Therefore‚ the interdiffusion processes should be suppressed or at least reduced. In the present work‚ the properties of a 1 µm thick layer of the Ti5Si3 phase as a potential diffusion barrier for an afterwards applied oxidation resistant Al–Ti coating was investigated. For this purpose‚ DC magnetron sputtering was used to deposit a coating of partially crystalline Ti5Si3 phase prior to the deposition of an Al2O3 forming Al-30Ti (in at.%) top layer. The deposition was performed as a one-batch process in an industrial scale multisource sputter coater. This allowed depositing the Ti5Si3 interlayer from pure Ti and Si targets. Afterwards‚ the Al-30Ti top layer was deposited in the same sputter process by changing the substrate position and using Ti and Al targets. Cyclic oxidation tests at 900 °C for 1000 cycles (1 h each) combined with thermogravimetric analysis were performed in laboratory air in order to study the oxidation behavior of the coating systems. The morphologies of the coating systems and the microstructural development during the oxidation tests were investigated by scanning electron microscopy and high-resolution transmission electron microscopy. The intermetallic phases and the phase evolution were monitored by in–situ high temperature x-ray diffraction as well as standard x–ray diffraction. The results show that the Ti5Si3 phase could effectively slow down the Al depletion in the Al-30Ti at.% coating due to minor Al inwards diffusion into the γ–TiAl substrates. As a result‚ the dual–layer coating system provided a high oxidation resistance due to the long-term stable formation of thermally grown alumina on the surface. The stability of the interlayer of the Ti5Si3 phase was excellent and it was still present even after 1000 1-h-cycles of oxidation at 900 °C without any changes in thickness. In contrast‚ the single layer coating of Al-30Ti suffered from breakaway oxidation after already 800 cycles of exposure to 900°C in air.