Microstructure and mechanical properties of Ir-Ta coatings on nickel-base single-crystal superalloy TMS-75

In the present study, a methodology for the controlled addition of tantalum in iridium-alloy coatings prepared by dc magnetron sputtering from a composite target has been proposed. Ir-Ta coatings with 16.2, 23.9, 40.7, and 65.1 at. % Ta were deposited at 573 K on the 100 oriented nickel-base single-crystal superalloy, TMS-75, by suitably selecting the ratio of the surface area of the iridium target to the surface area of the tantalum target on the basis of reported values of the sputtering yield of the respective elements. The micro structural analysis indicates that pure coatings of iridium and tantalum form polycrystalline films with rms roughness of similar to5 nm, while the Ir-Ta coatings form a nanocrystalline structure whose crystallite size and rms roughness decrease with the increase in tantalum content. Measurement of mechanical properties using the nanoindentation technique shows that the Young's modulus and hardness of the coatings generally decrease with the increase in the tantalum content. However, there is a peaking of hardness in the composition range 16.2-23.9 at. % tantalum. This could be due to the formation of the nanocrystalline Ir3Ta phase. The data on the mechanical properties are presented to explore the possibility of using Ir-Ta as an oxidation-resistant material on nickel-base single-crystal superalloy. (C) 2004 American Vacuum Society.