Development of a Recyclable High-Entropy Alloy Coating System for Turbine Blades

Thesis Abstract

The escalating demand for turbine efficiency and environmental compliance necessitates the development of protective coatings that combine high-temperature performance with recyclability to reduce lifecycle costs and material waste. This study addresses the challenge of designing a recyclable high-entropy alloy (HEA) coating system capable of withstanding extreme turbine operating conditions while enabling straightforward end-of-life material recovery. The aim is to design, fabricate, characterize, and evaluate a HEA coating system for turbine blades that balances high-temperature oxidation resistance, mechanical robustness, and recyclability. Specific objectives include (i) synthesizing a family of Al-Fe-Co-Cr-Ni-Cu-based HEA coatings with controlled microstructure, (ii) optimizing deposition parameters via pulsed laser deposition and magnetron sputtering to achieve dense, adherent coatings on nickel-based superalloy substrates, (iii) assessing high-temperature oxidation, creep, and wear resistance up to 1050°C, (iv) evaluating recyclability through thermomechanical processing routes and traceable alloy recovery, and (v) developing a predictive framework linking compositional design to performance and recyclability. A mixed-methods approach integrates experimental materials science with materials informatics. The population comprises turbine-grade nickel-base superalloy coupons and turbine blade segments (IN738LC equivalent). Sample sizes include 60 coated coupons for initial screening, 20 accelerated oxidation-creep-wear tests at 900–1050°C, and 10 blade-scale segments for field-simulated testing. Data collection instruments include scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) for microstructural analysis, X-ray diffraction (XRD) for phase identification, transmission electron microscopy (TEM) for interfacial characterization, nanoindentation for hardness and modulus, gravimetric oxidation/fatigue tests, and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for traceability of alloying elements. Recyclability assessment employs differential scanning calorimetry (DSC) to monitor phase stability during recycling cycles and compositional analysis post-reprocessing to quantify segregation or loss, supported by thermodynamic modeling with CALPHAD databases. Data analysis proceeds through a hierarchical framework. Descriptive statistics summarize coating thickness, adhesion, and microhardness. Inferential analyses include ANOVA to evaluate the effects of deposition parameters on coating adhesion and oxide scale adherence, regression analyses to correlate HEA composition with creep-rupture life and wear resistance, and multivariate principal component analysis (PCA) to identify dominant factors driving performance. A finite element-based thermo-mechanical model assesses residual stresses and thermal gradients in coated blades under turbine-stage loading. The study adopts a design-for-recyclability lens underpinned by the theory of materials circularity and the durable yet repairable materials paradigm, with the multi-criteria decision analysis (MCDA) used to balance performance and recyclability outcomes. The conceptual framework integrates these theories with empirical results to guide alloy-composition selection and process parameters. Expected findings include (i) a coating microstructure featuring a coherent HEA matrix with nano-precipitates that enhance high-temperature strength and oxidation resistance, (ii) deposition parameter windows yielding coatings with superior adhesion, reduced spallation, and maintained integrity after thermal cycles, (iii) oxidational and creep performance meeting or exceeding benchmark coatings at 1050°C with acceptable wear resistance, and (iv) a replicable recycling pathway enabling recovery of constituent elements with minimal degradation (<5% loss in critical elements after three recycling cycles). The study anticipates establishing quantitative relationships between specific element ratios (e.g., Al, Cr, Ni, Co, and Ti) and performance-recyclability metrics, enabling predictive design of recyclable HEA coatings. The contribution to knowledge includes advancing the design of recyclable HEA coatings tailored for turbine blades, establishing a validated methodology for integrating recyclability metrics into high-temperature coating development, and delivering a transferable modeling framework that links composition, processing, performance, and end-of-life recovery. The main conclusion posits that a carefully engineered Al-Fe-Co-Cr-Ni-Cu HEA coating can simultaneously deliver enhanced high-temperature performance and recyclability, with the recommended strategy emphasizing controlled nano-precipitation, optimized deposition to ensure strong interfacial bonding, and a standardized recycling protocol that preserves critical alloying elements with minimal degradation. Policy and practice recommendations include adopting the proposed design-framework in industrial coating development and integrating recyclability assessments early in material selection to reduce lifecycle environmental impact.