Ablative materials are widely employed to protect space vehicles from the extreme thermal conditions experienced during their flight into a planetary atmosphere. Carbon-phenolic ablators are composed of a phenolic matrix and a fibrous carbon reinforcement. In the present study, the fibrous reinforcement has been modified through the deposition of thin protective layers of zirconium oxide and aluminum oxide, with the objective of reducing fiber recession and oxidation. The depositions were carried out via atomic layer deposition (ALD), a method that allows for the controlled deposition of uniform and conformal coatings on the carbon felt fibers. The depositions were subsequently evaluated through SEM-EDS analysis. Pristine and ALD-modified felts were impregnated with a phenolic resin matrix and the ablation performance of the composite materials was evaluated through oxyacetylene flame tests. The results demonstrated that, in comparison to uncoated ablators, the ALD-modified samples exhibited enhanced performance in terms of mass loss and surface recession: compared to uncoated ablators, the former was 14% lower and the latter was diminished by 50%. Moreover, the morphological characterization of the tested specimens revealed a significantly reduced degree of oxidation of the coated fibers which were directly exposed to the flame.

High phosphorus Ni-P coatings, both unreinforced and modified by the addition of alumina (Al₂O₃) and zirconia (ZrO₂) nanoparticles, were manufactured by electroless deposition technique and heat-treated with different temperature and duration schedules. The effect of dehydrogenation (200 °C for 2 h) and its combination with crystallization heat treatment was studied in terms of microstructural changes and wear resistance. The amorphous structure of the coatings was not altered by the introduction of both Al₂O₃ and ZrO₂ nanoparticles, and the addition of 1.5 g/L of ZrO₂ yielded the highest microhardness due to better particles dispersion. Dehydrogenation improved hardness because of the early stages of grain growth; however, the greatest improvement in hardness (+120% compared to unreinforced Ni-P) was obtained after annealing at 400 °C for 1 h, because of the microprecipitation of the Ni₃P crystalline phase induced by thermal treatment. No detectable differences in hardness and microstructure were detected when annealing at 400 °C for 1 h with or without prior dehydrogenation; however, the dehydrogenated coatings exhibited a lower Young’s modulus. ZrO₂-reinforced coatings demonstrated improved wear resistance, and wear tests revealed that dehydrogenation is fundamental for lowering the coefficient of friction (−14%) and wear rate (−97%) when performed before annealing at 400 °C for 1 h. The analysis of the wear tracks showed that the non-dehydrogenated samples failed by complete coating delamination from the substrate, with abrasion identified as the predominant wear mechanism. Conversely, the dehydrogenated samples demonstrated better resistance due to the formation of a protective oxide layer, leading to an overall increase in the coating wear resistance.

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.