Papers by Author: F. Pedraza

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Authors: F.J. Pérez, F. Pedraza, M.P. Hierro, M.C. Carpintero, C. Gómez
Authors: B. Bouchaud, J. Balmain, F. Pedraza
Abstract: In this work, β-NiAl aluminide coatings (cubic B2 structure) deposited on a DS substrate have been isothermally as well as cyclically oxidised at 1100°C for up to 240 h to study the diffusion mechanisms associated with the growth of the oxide scales. A 24 h cycle has been shown to promote enhanced Al depletion, thus requiring a sufficient Al flux to maintain a protective oxide scale. Glancing incidence X-ray diffraction (GI-XRD) combined to electron microscopy (FEG-ESEM / EDS) has been carried out to characterize the evolution of the phases induced by the progressive Al depletion into the coating. The results show that upon cycling, specimens undergo significant oxide scale spallation and increased roughness that can be ascribed to both the growth stresses and the phase transformation contribution whereas the coating has barely evolved after 240 h of isothermal exposure. In particular, the martensitic transformation (tetragonal L10 structure) that accompanies thermal cycling was found to be much more significant than the evolution of the γ’-Ni3Al (cubic L12 structure) phase over the same thermal cycle and therefore the B2 to martensite transformation could originate the progressive roughening of the surface. Conversely, upon isothermal exposure, the coating exhibited a typical alumina scale with almost no spallation and the appearance of rumples.
Authors: S. Poupard, F. Pedraza, J. Creus
Abstract: Among the new environmentally friendly alternatives to improve the corrosion resistance of metallic substrates, cerium oxide based electrodeposited layers have raised particular attention. In this work, various electrodeposited layers on zinc substrate have been elaborated at different conditions to produce tailored compositions, microstructures and different degrees of porosity. The use of electrochemical techniques (polarisation resistance measurements, electrochemical impedance spectroscopy) has allowed to ascertain the porosity, which coupled to Scanning Electron Microscopy (FEG-SEM) and Energy Dispersive Spectrometry (EDS) studies along the cross sections have revealed the inward and/or outward diffusion of the different elements through the film till the metallic surface. The resulting corrosion mechanisms of these coatings immersed in 3% NaCl are finally discussed.
Authors: F. Pedraza, Baptiste Bouchaud, J. Balmain, G. Bonnet, Justine Menuey
Abstract: Rare earth oxides are commonly employed as dopants or coatings to improve the development and adherence of alumina scales. However, for practical applications, doping is difficult to control and the use of coatings is preferred. Nevertheless the thickness of such coatings is relatively limited for long term exposures at high temperatures and thicker coatings are hence required. With this in mind, the cathodic electrodeposition technique has been investigated in this work. The results show that deposits of about 20 µm RExOOHy coatings can be obtained on a Ni superalloy in 20 min. The applied current density and time significantly influence the microstructure, thickness, crystallite size and number of oxygen vacancies of the coatings. Their needle-like microstructure is indicative of non negligible amounts of rare earth hydroxides. However, the hydroxide peaks overlap with the oxide peaks in the X-ray diffraction (XRD) patterns. XRD also suggests that the coatings are either amorphous or of nanocrystalline nature, as supported by Raman spectroscopy. Their multicracked morphology is related to the shear stresses between the coating and the substrate, hydrogen bubbling and mostly by drying of the coatings in air. The number of cracks is increased after a heat treatment which also allows full crystallization of the RExOy coating and pre-oxidation (α-Al2O3) of the superalloy. The combined effect of both oxides results in an improved oxidation resistance of the Ni-base superalloy at 1100°C in air.
Authors: F. Pedraza, Baptiste Bouchaud, J. Balmain, G. Bonnet, Justine Menuey
Abstract: Cathodic electrodeposition was used to generate a rare earth (RE)-containing deposit on a single crystal Ni-based superalloy. The deposition parameters were optimised in order to get a RE oxy-hydroxide coating with a “well-fitted” dry-mud like morphology, i.e. presenting a multi-cracks network. A further thermal treatment was applied to dehydrate the deposit to obtain a well crystallised oxide coating (RExOy). The uncoated and RExOy-coated substrates were then submitted to cyclic oxidation tests at 1100°C in laboratory air. They demonstrated the efficiency of the coating as uncoated samples severely spalled after a few cycles whereas the coated ones did not lose their protective oxide layer even after 2000 cycles. This result was attributed to the formation of a duplex oxide scale very similar to that obtained on g/g’ coatings, to the presence of nanograins at the RExOy/scale interface and to the Hf-rich oxide pegs at the scale/substrate interface.
Authors: B. Bouchaud, Ludovic Douminge, F. Pedraza
Abstract: The performance of Ni-based alloys at high temperature strongly depends on the formation of a protective chromia scale which prevents from fast growth NiO scale. The evolution of the oxide scales will be shown to be strongly dependent on the chromium content especially for the shortest oxidation times. Indeed, transient stages will be reported to condition the subsequent oxidation behavior at longer times until the oxidation rate constant becomes invariant. To this end, a comparative study on the outward and inward diffusion of species upon oxidation at 800°C for short times (0.5 / 1 / 2 / 4 / 8 h) has been evaluated on commercial available Ni20Cr and Ni30Cr alloys through in-situ Raman spectroscopy and supported by post-mortem X-ray diffraction (XRD) as well as scanning electron microscopy coupled to energy dispersive spectrometry (FEG-ESEM / EDS) investigations.
Authors: B. Bouchaud, J. Balmain, F. Pedraza
Abstract: The oxide scale evolution with high temperature on CVD aluminide coatings deposited on a Directionally Solidified (DS) Ni-base superalloy is studied in this work. High temperature oxidation was carried out at 1100°C in air for 240 h under isothermal conditions and for 10 cycles (1 cycle = 24 h at 1100°C). The morphological and microstructural characterisation of the coatings has been performed using optical and electron microscopy as well as X-ray diffraction. Contrary to most of the results published in the literature, the rumpling phenomenon appears on the isothermally oxidised specimens whereas spallation, nodule formation and wrinkling of the oxide scale occur on the cyclically oxidised samples. The results are discussed in terms of the β-NiAl to γ’-Ni3Al phase transformation, the likely associated volume changes and of the growth stresses at high temperature.
Authors: F. Pedraza, Jean Luc Grosseau-Poussard, J.F. Dinhut, J. Balmain, G. Bonnet
Abstract: Nitriding by low energy high flux processing has been carried out at about 400°C in fcc metal substrates (pure Ni, Ni-20Cr model alloy and a conventional AISI 304L stainless steel). The gradual ingress of this element into the structures will be shown to depend markedly on the chemical composition of the substrate. The associated expansion of the fcc lattices and surface roughness will be discussed in this work with the support of X-ray diffraction, atomic force, scanning and transmission electron microscopy techniques. In light of the resulting composition, microstructures and thickness of nitrided layers, some preliminary results of their behaviour under isothermal oxidation conditions at high temperatures will be discussed. The high temperatures will provoke decomposition of the expanded austenite into a conventional gamma phase and some chromium nitrides. Trapping of chromium therefore shall explain a reduction of the high temperature oxidation resistance against the untreated substrates.
Authors: F. Pedraza
Abstract: State-of-the-art aluminide coatings for high pressure aeroturbines rely on their chemical stability through the addition of platinum. In this work, the influence of diffusion on the composition and microstructure of Pt/Al coatings is investigated upon the different processing steps. Upon heat treatment of the electrodeposited Pt layers, the ready solubility of Pt in the ’-Ni3Al and its concentration gradient allow fast ingress of this element in the coating in spite of the higher diffusion coefficients of the substrate elements. The relatively high Pt contents at the surface thereafter promote Al ingress upon aluminisation by creating a greater concentration gradient between the coating atmosphere and the coating. This results in a -NiAl based coating grown by both Ni outward and Al inward diffusion. The diffusivity of Pt, Ni and Al being dependant on the stoichiometry of the -NiAl coating, different metal particles may precipitate mainly at the NiAl grain boundaries. In addition, the diffusion of the refractory elements towards the interdiffusion layer seems to be hurdled by increasing the Pt content. As a result of the countercurrent diffusion of species, different microstructures and compositions will appear across the coatings.
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