Oxidation characteristics of gamma-TiAl-8Nb coated with a CrAlYN/CrN nanoscale multilayer coating

The full exploitation of gamma-TiAl for high performance applications is currently hindered by its poor high temperature oxidation behaviour above 750 degrees C. To overcome this there is a need to develop coatings that protect against high temperature oxidation, bill do not adversely affect the mechanical properties. This work reports on the oxidation protection offered by a CrAlYN/CrN based multilayer coating deposited on gamma-TiAl-8Nb. Coated samples were subjected to quasi-isothermal oxidation in dry air at 850 degrees C for times Lip to 1030hrs. Excellent oxidation resistance was achieved with the coated samples exhibiting a four fold improvement compared to the uncoated intermetallic. Microstructural investigations revealed a mixed Cr/Al oxide scale at the surface of the coating after an exposure of 168hrs. However cracks in the coating, a result of thermal mismatch between the coating and substrate. led to oxidation of the substrate with the formation of an alumina rich oxide directly below the crack and the extensive growth of titania (rutile) at the surface. In addition, significant modification of the surface region of the substrate was observed with the formation of a thick polycrystalline TiN layer and a segregated (Ti)Al3Nb interface region between the TiN and the unaffected substrate