Influence of Ta content on microstructure and mechanical properties of Ir-Ta as diffusion barrier coatings on nickel base single crystal superalloy TMS-75

A methodology for the controlled addition of tantalum in iridium alloy coatings prepared by DC magnetron sputtering from a composite target has been proposed in the present investigation. Ir-Ta coatings with 16.2, 23.9, 40.7 and 65.1 at% Ta were deposited at 573 K on 〈100〉 oriented nickel base single crystal superalloy, TMS-75 by suitably selecting the ratio of surface area of tantalum to surface area iridium on the basis of reported values of the sputtering yield of the respective elements. The microstructural analysis indicates that pure coatings of iridium and tantalum form polycrystalline films with RMS roughness of ∼5 nm, while the Ir-Ta coatings form nanocrystalline structure whose crystallite size and RMS roughness decrease with increase in tantalum content. Measurement of mechanical properties using nanoindentation technique shows that the Young's modulus and hardness of the coatings generally decrease with increase in the tantalum content. However, there is a peaking of hardness in the composition range 16.2-23.9at % tantalum because of the formation of If₃Ta phase with Ll₂ structure. The data on the mechanical properties are presented to explore the possibility to use Ir-Ta as a post-coat diffusion barrier material on nickel base single crystal superalloy.