Phd in ELECTRICAL, MATERIALS, RAW MATERIALS AND NANOTECHNOLOGY
ENGINEERING

Thesis title: Aluminide coatings modified via electroless platinum plating for high temperature applications

The performance and durability of gas turbine engine components operating in high-temperature environments critically depend on the effectiveness of protective coatings. Among these, platinum-modified aluminide coatings are widely employed as bond coats in thermal barrier coating (TBC) systems applied to Ni-based superalloys. Pt-aluminide coatings protect the underlying layers from oxidation through the formation of a stable and adherent α-Al₂O₃ scale during high-temperature exposure. Platinum modification is achieved by depositing a Pt layer on the base superalloy prior to aluminizing, and electroplating is currently the only industrial method used for this purpose. However, this technique has several limitations, primarily related to the edge effect of the electric field: the formation of coating with non-uniform thickness on complex geometries, the complexity of the required apparatus and the need for custom-designed anodes for each component to be coated. Difficulties in controlling the Pt deposition process often lead to microstructural non-uniformities in the resulting aluminide, with formation of brittle phases where excess platinum accumulates, and hindered control over degradation phenomena related to thermodynamic imbalance and diffusion processes. This thesis addresses the above issues by developing and optimizing a Pt deposition method based on electroless plating, and by exploring the potential of innovative microstructural modifications aimed at improving the performance of Pt-aluminide coatings. The work is divided into five main research phases: (i) the design of a new electroless Pt deposition process and (ii) its validation for the production of platinum-modified aluminides, (iii) the optimization of an aluminizing procedure via slurry aluminizing, (iv) the microstructural tailoring via the introduction of an electroless Ni interlayer, and (v) the preliminary development of nanocomposite Pt-aluminide coatings reinforced with α-Al₂O₃ nanoparticles. Extensive coating characterization was carried out throughout the study, using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), quantitative image analysis, grain size measurement, and oxidation kinetics modeling. Oxidation behavior was assessed through mass gain measurements and interpreted using the Monceau parabolic rate model, which provided insights into oxide growth kinetics. Structural parameters such as grain size and nanoparticle distribution were correlated with performance to establish structure–property relationships. The first part of the thesis focuses on the formulation and validation of a new electroless Pt plating solution, specifically developed for the production of aluminide coatings for high-temperature applications. A completely novel bath formulation was designed and optimized for stability, deposition rate, and Pt film quality. The process showed excellent flexibility for deposition on Ni-based superalloys with varying chemical compositions, ensuring uniform Pt layers even on complex geometries, and overcoming typical current-density-related issues found in electrolytic processes. The resulting coatings exhibited high purity, controlled thickness, and a nanocrystalline structure suitable for subsequent diffusion treatments. Moreover, the bath formulation ensured high deposition repeatability, platinum consumption comparable to commercial electrolytic solutions, supported bath regeneration via metal turnover, and, most importantly, eliminated the need for an anode, thereby simplifying equipment and increasing industrial flexibility. In the second phase, coatings obtained via electroless deposition were compared to those produced through conventional electroplating, both in the as-deposited state and after aluminizing. The Pt layers obtained by electroless and electrolytic techniques exhibited comparable morphology, composition, and microstructure; however, the electroless coatings showed significantly better thickness uniformity. After aluminizing, the coatings showed similar phase composition, primarily ζ-PtAl₂ and β-(Ni,Pt)Al, but electroless coatings demonstrated greater microstructural uniformity, lower porosity, and slightly superior oxidation resistance. These results confirmed electroless deposition as a valid industrial alternative to electroplating, offering both performance benefits and process simplification. The third phase involved systematic optimization of the aluminizing step, aimed at controlling aluminum distribution along coating thickness and improving microstructural control. By varying slurry thickness and concentration, aluminum availability during diffusion was modulated, resulting in coatings composed entirely of the β-NiAl phase throughout the thickness. The optimized process was also validated for the production of Pt-aluminides, confirming its versatility for both standard and modified coatings. The effect of the initial Pt layer thickness, ranging from 4.5 to 12 μm, was also studied. Thicker Pt layers favored the formation of an outer ζ-PtAl₂ phase and led to increased total coating thickness. Although thicker Pt layers provided improved oxidation resistance, a 7 μm Pt thickness was identified as the best compromise between performance and Pt consumption, supporting the design of more cost-effective and efficient coatings. In the fourth part, an electroless Ni interlayer was introduced after the Pt layer, aiming to modify diffusion dynamics. The microstructural changes and oxidation resistance were studied for Ni layers of 5, 10, 20, and 40 μm thickness. As Ni thickness increased, a progressive suppression of brittle Pt-rich phases in the outer region was observed, along with stabilization of the β-NiAl phase, reduction of the IDZ thickness, and decreased precipitation of TCP phases (topologically close-packed), which are detrimental to mechanical resistance. However, for Ni layers exceeding 10 μm, the reduced availability of Pt near the external interface led to lower oxidation resistance. As such, the 10 μm Ni interlayer was selected as the optimal configuration, offering the best compromise between microstructural integrity and oxidation performance. The fifth and final phase involved the preliminary development of Pt-aluminide nanocomposite coatings, fabricated via co-deposition of α-Al₂O₃ nanoparticles within the electroless Pt layer. The best results in terms of maximum particle incorporation and minimal agglomeration were achieved with 0.5 g/L of α-Al₂O₃ in the plating bath. After aluminizing, the nanoparticles remained localized in the outermost coating region, ideally positioned to serve as epitaxial nucleation sites for α-Al₂O₃ scale formation during oxidation. The particles also induced grain size refinement via Zener pinning mechanism. Preliminary oxidation tests at 1100 °C for 500 hours showed slower oxidation kinetics for nanocomposite coatings, with better protection particularly during the transient oxidation stage. This suggests a faster transformation to α-Al₂O₃, enabled by the synergistic action of the epitaxial template effect of the particles and the enhanced Al diffusion in the fine-grained matrix, ultimately confirming improved protective behavior of nanocomposites compared to standard coatings. In conclusion, this work represents a significant advancement in the practical application of platinum-modified aluminide coatings, proposing a simpler, more controllable, and scalable Pt deposition method, while also offering new pathways for microstructural and compositional improvements. The outcomes of this research may have direct implications for the development of gas turbine and aero-engine components, where coating reliability and oxidation resistance are critical to improve performance and durability.