Manufacturing of aluminide coatings modified via electroless platinum plating on different Ni-based superalloys for high temperature applications

Diffusion aluminide bond coats are widely used to provide oxidation resistance on Ni-base superalloy components in advanced gas turbine engines. Modification of these coatings by deposition of platinum onto the superalloy, prior to aluminizing, considerably improves their resistance in harsh operating environments, preventing degradation and increasing service life [1]. Nevertheless, the only method for Pt deposition currently employed at industrial scale is electroplating, that produces non-uniform coatings on the sharp geometries typical of turbine blades [2,3]. This limit, intrinsically related to the effect of the electric field, dramatically restricts process flexibility and applicability. A new process capable of depositing uniform coatings on Ni-based superalloys is therefore desirable Electroless plating process can be a valid alternative since it allows reduction of metal ions by incorporation of a reducing agent in the plating solution [4]; yet, its applicability to deposition of pure platinum films is still not very well-established [5]. To this purpose, this work takes the challenge of developing a simple and stable electroless solution for platinum plating and produce electroless Pt-modified aluminide on Renè N4 and GTD111 superalloy substrates. The proposed plating solution is based on hydrazine monohydrate as reducing agent in acidic environment. Deposition properties were assessed investigating the effect of several parameters (i.e. temperature, pH and bath composition) on plating rate and morphology by means of SEM imaging. The growth mechanism was evaluated by SEM imaging at different stages of deposition and it was found that nucleation of platinum nanoparticles starts in correspondence of catalytic sites with high surface energy and growth occurs with a high degree of internal stresses that lead to cracking. Effect of substrate composition in the plating rate was also evaluated and it was found that a higher Cr content slows down formation of the first Pt monolayer. The best parameters were selected according to plating rate and morphologic features, in order to produce coating that comply with the practical necessities of industry. Pt-modified aluminide were therefore produced by slurry aluminizing on the two different Ni-based superalloys. Microstructure and phase composition were investigated by SEM, EDS and XRD analysis. The new produced coatings were compared to standard Pt-modified aluminides, showing similar microstructural features. These results confirm that electroless plating process can be successfully employed as a strategy for Pt-modification of aluminide coatings, enhancing its flexibility and applicability on complex-shaped geometries.