Diffusion dans les couches d'oxyde en cours de croissance

Though the growth of oxide scales on metallic substrates can be limited by many processes, which are described in a first part, it is possible to analyse the parabolic growth laws by diffusionnal processes. Such an analysis needs diffusion data in oxides, preferentially obtained on oxide scales. In order to obtain an available analysis of the diffusionnal phenomena in polycrystals or oxide scales, it is necessary, at first, to consider that the first part of the penetration profiles corresponds to an apparent diffusion and the second part to intergranular diffusion. The apparent diffusion coefficient must be taken into account for the calculation of the oxidation constant. Secondly, the roughness of the oxide scale surface must be evaluated and it induces a modification of the $f$ value, fraction of sites associated with the grain boundaries, compared to the traditionnal value taken for smooth surfaces. These two points have been considered in the diffusionnal study in Cr$_2$O$_3$ scales presented in this paper. Finally, the compilation of the results indicate that the microstructure and the microchemistry of grain boundaries must differ in massive polycrystals and in scales.Bien que la croissance de couches d'oxyde sur des substrats métalliques à haute température mette en jeu de nombreux processus compétitifs, décrits dans une première partie, il est possible d'analyser les lois de croissance paraboliques par des processus diffusionnels. Cette analyse nécessite des données de diffusion sur les oxydes en question, et de préférence directement sur couches d'oxyde. Une analyse correcte des phénomènes diffusionnels dans les couches d'oxyde ou les polycristaux nécessite en premier lieu de considérer que le premier domaine des profils de pénétration correspond à une diffusion apparente et le second est relatif à la diffusion intergranulaire. C'est le coefficient de diffusion apparente qui doit être pris en compte pour calculer la constante d'oxydation. En second lieu, il faut prendre en considération la rugosité de la couche, ce qui entraîne une modification de la valeur de $f$, fraction de sites associés aux joints de grains, par rapport à la valeur traditionnellement prise. C'est ce qui a été fait dans l'étude de diffusion sur couches de Cr$_2$O$_3$ présentée dans cet article. Enfin, l'ensemble des résultats compilés montre que la microstructure et la microchimie des joints doit être différente dans les polycristaux et dans les couches

Répercussion de la ségrégation de l'yttrium sur la résistance à l'oxydation d'alliages chromia ou alumino formeurs

Un certain nombre de travaux du laboratoire ont porté sur l'étude de la ségrégation intergranulaire de l'yttrium dans des oxydes, l'alumine en particulier. Parallèlement, les coefficients de diffusion intergranulaire dans les oxydes tels que Al2O3, Cr2O3, soit massifs polycristallins, soit obtenus en couches par oxydation ont été déterminés, en présence ou non d'un dopage par l'yttrium. Par ailleurs, la résistance à l'oxydation de matériaux métalliques susceptibles de former des couches de Al2O3 ou de Cr2O3 a été largement étudiée, montrant qu'un élément tel que l'yttrium améliore le caractère barrière de diffusion de telles couches et surtout leur adhérence, en relation avec les modifications du caractère élasto-viscoplastique du système métal-oxyde. L'ensemble de ces travaux permet de cerner l'effet de la ségrégation intergranulaire de l'yttrium sur les propriétés de transport et les propriétés mécaniques de couches de Al2O3 ou Cr2O3

Recent improvements in the understanding of alumina film formation and durability

High Temperature Corrosion and Protection of Materials 6 Les Embiez (France) 16-21 mai 2004 ed. P. Steinmetz, I.G. Wright, G. Meier et al... Trans Tech PublicationsInternational audienceThe nucleation and growth of alumina layers on two different FeCrAl alloys were compared, taking into account the presence of alloying elements and impurities. Moreover, the oxygen pressure effect on the nucleation, formation and transformation of transition aluminas was considered. Using an "in situ" deflection test, it was possible to determine the kinetics of the alumina transformation. It was found that impurities and oxygen pressure play important roles on the alumina morphology, on the growth mechanism and on the oxide film mechanical resistance. The long-term oxidation resistance is also related to morphological and chemical aspects of the substrate/alumina interfac

Détermination de la distribution des contraintes dans un matériau oxydé par une méthode de la courbure

A new curvature method was developed for determining the stress distribution in an oxidized material. This technique consists in measuring the evolution of the curvature of a previously oxidized thin sample during successive sputterings of one of its large faces. It was applied to the NiCr-Cr2O 3 system and allowed to plot the stress distribution curves in both the oxide scale and the substrate.Pour déterminer les contraintes dues à l'oxydation d'un matériau, une nouvelle méthode de courbure a été mise au point. Elle consiste à mesurer l'évolution de la courbure d'un échantillon mince préalablement oxydé au cours d'abrasions successives de l'une de ses grandes faces. Appliquée au système NiCr-Cr 2O3, elle a permis de tracer les courbes de distribution des contraintes dans la couche oxydée et dans le substrat

Recent improvements in the understanding of alumina film formation and durability

High Temperature Corrosion and Protection of Materials 6 Les Embiez (France) 16-21 mai 2004 ed. P. Steinmetz, I.G. Wright, G. Meier et al... Trans Tech PublicationsInternational audienceThe nucleation and growth of alumina layers on two different FeCrAl alloys were compared, taking into account the presence of alloying elements and impurities. Moreover, the oxygen pressure effect on the nucleation, formation and transformation of transition aluminas was considered. Using an "in situ" deflection test, it was possible to determine the kinetics of the alumina transformation. It was found that impurities and oxygen pressure play important roles on the alumina morphology, on the growth mechanism and on the oxide film mechanical resistance. The long-term oxidation resistance is also related to morphological and chemical aspects of the substrate/alumina interfac

Effect of impurities on the oxidation mechanism of nickel at 800°C

International audienceThe effect of impurities on the oxidation mechanism of nickel was studied on commercial nickel grades compared to a pure nickel. On the basis of oxidation kinetics, SEM and STEM microstructural and analytical investigations allowed us to identify the oxidation mechanism for both types of nickel at 800C. The morphology of the oxide scale notably differs according to the purity of the nickel. For oxidised commercial grades, a duplex structure was observed with an outer columnar layer and an inner layer made of equiaxed grains. The inner NiOyouter NiO interface is planar without any segregation, while the NiOyNi interface is convoluted with large cavities. Mn, Ti and sometimes also silicon impurities were detected at this latter interface. Below the NiOyNi interface, in the underlying nickel, large internal oxidation was observed. The observed microstructure was quite different for the pure nickel. A single porous NiO layer, composed of equiaxed grains, was observed. The NiOyNi interface was facetted and no porosity was detected. The presence and localisation of impurities, as well as morphological changes through the scale in the nickel grades, were taken into account to explain the modification of oxidation kinetics with substrate purity

Relation between the oxidation mechanism of nickel, the microstructure and mechanical resistance of NiO films and the nickel purity. II. Mechanical resistance of NiO films

International audienceThe differences in the nickel oxidation mechanism and the microstructure of the NiO films depending on the nickel purity were described in the preceding paper (part I). In this second part, the mechanical characteristics were determined mainly by three-point bending tests performed “in situ” in a scanning electron microscope. In relation with the nature of the oxidation mechanism and the film microstructure differences, specific mechanical behaviours were observed. Nevertheless, regardless of the nickel grade, the oxide toughness evolves with the thickness of the scale and tends towards the value given for bulk NiO. Amongst the differences related to Ni purity, one consists in the fact that spalling occurred mainly at the oxide/oxide interface for the industrial grades, while it appeared at the metal/oxide interface for pure Ni. This fact was related to the scale morphology, i.e. to the oxidation mechanism

Relation between the oxidation mechanism of nickel, the microstructure and mechanical resistance of NiO films and the nickel purity, I. Oxidation mechanism and microstructure of NiO films

International audienceThe effect of impurities on the oxidation mechanism of nickel and on the mechanical characteristics of NiO films was studied on two industrial nickel grades compared to a pure nickel. In this part, influence of impurities on the oxidation mechanism and on the NiO film microstructure will be detailed. The oxidation mechanism, especially studied at 800 °C in air, was clarified using complementary techniques, SEM and STEM microstructural and analytical investigations, XPS analyses, profilometry, oxygen isotopic exchange and SIMS. Depending on the Ni purity, the morphology of the oxide scale and the porosity amount differs. Duplex oxide layers and internal oxidation are observed on industrial grades, while a simple NiO film grows on pure nickel without internal oxidation. An oxidation mechanism, which differs according to the presence or not of impurities in the nickel substrate, is proposed. The mechanical characteristics will be characterised and discussed in the following paper (part II) in relation with the microstructural modifications induced by changing the nickel purity