B. Bouchaud

pH-distribution of cerium species in aqueous systems

Abstract Cerium-based oxide coatings can be obtained through either chemical or electrochemical processes on various conductor and semiconductor substrates. In both cases the films develop through a precipitation mechanism, which strongly depends on the solution chemistry. In the particular case of the electrolytic approach, the elaboration parameters play a key role on the interfacial pH modification thereby leading to an indirect precipitation mechanism. Indeed, the nucleation and growth mechanisms of crystallites and the composition of the resulting layers have been shown to be also strongly affected by the deposition conditions as well as by the substrate composition, which could in turn modify the protectiveness provided by such coatings. Therefore a better fundamental understanding of the system is required, in particular of the distribution of cerium-containing species in aqueous solution. To this end, the present work intended to develop a diagram showing the distribution as well as the relative amount of Ce(III)/Ce(IV) species in aqueous media as a function of the pH range. The resulting pH-distribution diagram turned out to be a useful tool to predict the relevant precipitation mechanisms and species involved during the growth of cerium-containing films and to draw correlations with the characteristics of the as-deposited films.

Electrosynthesis of Rare Earth Oxide Coatings for High Temperature Applications

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.

On the Development of a Protective Oxide System in Rare Earth Oxide Coated Nickel Superalloy under Isothermal Oxidation Conditions

Isothermal oxidation experiments at 1100°C in air were carried out to evaluate the protective capability of a new rare earth oxide coating realized by electrodeposition onto a Ni-base single crystal superalloy. A subsequent heat treatment of the RExOy coating already allowed the establishment of a very thin and discontinuous inwardly grown alumina scale. Under isothermal conditions at 1100°C in air a fully parabolic regime installed from 25h leading to parabolic rate constants of 2.5 10-7 mg2.cm-4.s-1 after 200h, similar to those of conventional β-NiAl coatings. The initial, transition and parabolic regimes were ascribed to the major development of NiAl2O4/Al2O3 mixed oxides by in situ high temperature X-ray diffraction (HT-XRD). No major transient alumina was observed. The α-Al2O3 scale intensity increased with increasing oxidation time, in particular with respect the rare earth oxide coating signal. The scanning electron microscopy (SEM) images showed an oxide system consisting on a top NiAl2O4 oxide and a bottom α-Al2O3 scale underneath the RExOy coating. Alumina grew within the substrate surface. After 500 and 1000h of oxidation, very scarce nodules grew between the alumina and the rare earth oxide deposit. Despite the thermodynamic calculations suggested a REAlOy perovskite at the alumina-RExOy interface, this was not observed experimentally either by XRD or scanning electron microscopy (SEM).

Evolution of Oxide Scales on Aluminide Coatings under Isothermal and Cyclic Conditions

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.

Enhanced Cyclic Oxidation Resistance of a Single Crystal Superalloy with an Electrodeposited Reactive Element Oxide Coating

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.

On the Influence of a Heat Treat for an Aluminizing Progress Based on Al Microparticles Slurry for Model Ni and Ni20Cr. Experimental and Theoretical Approaches.

The use of thermal barrier coating systems allows superalloys to withstand higher operating temperatures in aeroengine turbines. Aiming at providing oxidation protection to such substrates, an aluminum-rich layer is deposited to form the α-Al2O3 scale over which a ceramic layer (i.e. YSZ layer) is applied to provide thermal insulation. A new approach is now being investigated within the FP7 European project « PARTICOAT », in which a single step process is employed by applying micro-sized aluminum particles. The particles are mixed in a binder and deposited by brushing or spraying on the substrate surface. During a heat treatment, the particles sinter and oxidize to form a top coat composed of hollow con-joint alumina spheres and simultaneously, an Al-rich diffusion zone is formed in the substrate. For a better understanding of the diffusion / growth processes, preliminary tests were carried out on pure nickel and Ni20Cr model alloys prior to further application on commercial superalloys. The effect of the heat treatment on the coating characteristics (number of layers, thickness, composition, homogeneity, etc.) was particularly investigated to emphasize the mechanisms of diffusion governing the growth of the coatings. The establishment of the diffused layers occurred very readily even at intermediate temperatures (650 and 700°C). However, the layers formed did not match perfectly with the thermodynamic modeling because of the quick incorporation of Ni into molten Al at intermediate temperatures (650°C). In contrast, at higher temperatures (700 and 1100°C) the phases predicted by Thermocalc are in good agreement with the observed thickness of the diffused layers. The incorporation of Cr as an alloying element restrained Al ingress by segregation of Cr even at very low temperatures aluminizing temperatures (625°C).

Diffusion Enhanced Rumpling Associated with Martensitic Transformation upon Cycling of Aluminide Bond-Coats

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.

Establishment of Thermally Grown Oxides upon the Early Oxidation Stages of Ni20Cr and Ni30Cr

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.