Erica Scrinzi

The slurry aluminizing process is widely employed to enhance the oxidation and corrosion resistance of nickel-based superalloys used in high-temperature environments such as gas turbines and aerospace engines. This study investigates the effects of the concentration of Al vapors in the reactor chamber and the initial slurry layer thickness on the microstructure, chemical composition, and phase composition of aluminide coatings. Coatings were manufactured on Ni-based superalloy substrates using CrAl powders as an aluminum source and chloride- and fluoride-based activator salts. The effect of the initial thickness of the slurry layer was studied by varying the amount of deposited slurry in terms of mg_slurry/cm²_sample (with constant mg_slurry/cm³_chamber). The microstructure and phase composition of the produced aluminide coatings were evaluated by SEM, EDS, and XRD analysis. Slurry thickness can affect concentration gradients during diffusion, and the best results were obtained with an initial slurry amount of 100 mg_slurry/cm²_sample. The effect of the Al vapor phase in the reaction chamber was then investigated by varying the mg_slurry/cm³_chamber ratio while keeping the slurry layer thickness constant at 100 mg_slurry/cm²_sample. This parameter influences the amount of Al at the substrate surface before the onset of solid-state diffusion, and the best results were obtained for a 6.50 mg_slurry/cm³_chamber ratio with the formation of 80 µm coatings (excluding the interdiffusion zone) with a β-NiAl phase throughout the thickness. To validate process flexibility, the same parameters were successfully applied to produce platinum-modified aluminides with a bi-phasic ζ-PtAl2 and β-(Ni,Pt)Al microstructure.

This study aims to develop Ni-P coatings with high P content (≥11 wt.%) reinforced with WC nanoparticles on F22 steel substrates. The introduction of conductive WC in the plating solution dramatically increases reactivity of the plating solution, and consequently a tuning of deposition parameters, in terms of temperature and WC concentration, is required to obtain nanocomposite coatings with improved mechanical properties. The coatings’ porosity and incorporation and dispersion of the reinforcing phase as a function of temperature and WC concentration were analyzed by quantitative image analysis from Scanning Electron Microscopy (SEM) micrographs. Increasing the temperature and concentration of nanoparticles leads to a faster plating rate and a dramatic increase in both porosity and agglomeration of the reinforcing phase, with detrimental effects on the coatings’ microhardness. The best compromise between coating parameters was obtained by deposition at 70 °C and 6.5 g/L of WC, with a plating rate ≈ 12 μm/h, porosity lower than 1.5%, and a good combination between particle incorporation and agglomeration. In these conditions, a hardness increase by 34% is achieved in comparison to standard Ni-P. Coatings were then heat treated in air at 200 °C for 2 h, to induce growing stress relaxation, or 400 °C for 1 h, to study effects of crystallization and precipitation. X-Ray Diffraction (XRD) analysis demonstrated that WC introduction does not alter the microstructure of Ni-P coatings, but delays grain growth coarsening of precipitates. Hardness improvement by 6.5% and 45% is registered after treatment at 200 °C and 400 °C, respectively. An increase in elastic modulus, measured by instrumented indentation, was found in WC-reinforced coatings compared with Ni-P. Potentiodynamic polarization tests revealed that both introduction of WC nanoparticles and heat treatment also enhance corrosion resistance.