Defect and Diffusion Forum Vols. 289-292

Abstract: Diffusion iron aluminide coatings have shown excellent resistance to high temperature oxidation in air, corrosive atmospheres and steam. A study of the diffusion behaviour of slurry applied diffusion aluminide coatings deposited on ferritic steel have been carried out under a 100% flowing steam atmosphere for up to 50,000 h at 650 °C. The results have shown that initially, the coating forms by outward growth possibly including the dissolution of the steel in molten aluminium. At later stages, during exposure to steam at 650 °C, aluminium diffuses inward and moreover, Fe also diffuses outward resulting in the progressive development of Kirkendall porosity. Results have also indicated that in order to form a pure protective Al2O3 scale the Al wt.% has to be > 4. Below this content Al-Fe mixed oxides develop exhibiting a less protective behaviour.

Abstract: The presence of hydrosoluble species, mainly chlorides and sulphates, at the metal/paint interface promotes osmotic blistering of the coating and underfilm metallic corrosion when the concentration of the soluble salts exceeds a critical level due to the accelerated entrance of water to the interface. Loss of adhesion and cathodic disbondment can be also consequences of the presence of soluble salts. These processes can lead to the deterioration of the paint system in a very short period of time. However, since the interface is not easily accessible by experiment, not much is known about interfacial structure, reactions and the processes causing degradation. Scanning Kelvin Probe (SKP) is an ideal tool for the in situ monitoring of the degradation processes at the buried interface. On the other hand, for an in-depth understanding it is important that the superposition of the effects of all different parameters involved can be disentangled. Hence, it is important to design experiments focussing on individual aspects of the complex system. A study of the fundamental aspects of the stability of the contaminated buried steel/coating interfaces, specially focused on the initial stages of blistering and underfilm corrosion, has been carried out. Results of the characterization study performed with a height-regulated SKP (HR-SKP) on surfaces contaminated at the interface by different salt concentrations with well defined crystal size and distribution are shown.

Abstract: Previous work on the oxidation of nano- and micro-sized Al particles revealed a particle size window, where no meta-stable alumina phases were observed. Depositing such particles on an austenitic substrate, diffusion layers with reduced Al contents were obtained. These findings opened new perspectives for investigating the potential impact of the Al particle size and shape on the formation of diffusion aluminide coatings. Spherical Al particles sized in the range of 2 to 5 µm were deposited with a binder by brushing on the austenitic steel X6 CrNi 18-10 (Alloy 304H). For the curing process, the samples were annealed in air at 400°C for 1h. The diffusion effect of Al into the base material was studied in isothermal experiments at 700°C and 900°C with exposure times up to 2000 h in air. The sample surfaces and the diffusion aluminide coatings in cross-section were analysed by field emission scanning electron microscopy (FE-SEM). The results show in the initial state the formation of a diffusion layer consisting of a less aluminium-rich Fe(Cr)-Al phase containing a Fe(Cr)-Al phase with higher content of Al in the region beneath the surface. On further exposure a double-layered structure is found with Kirkendall-pores between the two layers, which may lead to a complete separation of the outer layer. A thin adherent alumina scale is observed on the remaining diffusion layer after 1000 h and 2000 h at both temperatures, however overgrown by Cr2O3 at 900°C. The structure of the diffusion zone beneath agglomerates of Al particles reveals the influence of the particle size on the Al supply for the formation of the aluminide diffusion zone.

Abstract: This paper presents a numerical method to determine the composition dependent diffusivities and to predict the concentration profile during the interdiffusion process. The intrinsic diffusion coefficients in diffusion aluminide coatings (Fe-Al) were determined at 1000oC. The obtained diffusion coefficient for iron in Fe3Al or FeAl is in the range 10-10 to 10-9 cm2.s-1. The aluminum diffusion coefficient varies from 10-11 to 10-7 cm2.s-1 in the same phases.The present approach also permits to model the reactive diffusion in the Fe-Al systems.

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.

Abstract: Nitriding by microwave post-discharge process involves molecular nitrogen dissociation. It has been observed that nitrogen flux from surface to solid during the early stage does not follow a parabolic regime and that the growth rate of concomitant nitride layers is sensitive to atomic nitrogen concentration on the surface. In this work a mathematical model has been developed in order to describe the kinetics of the compound layer formation during a post-discharge nitriding process. The model is related to a moving boundary value problem and considers different stages: diffusion process, formation of the layers, layer growth and quasi-stabilization of the layer growth. Natural conditions on the nitrogen concentration consistent with the mass transfer mechanism are assumed. An analytical approximate solution of Goodman’s type is sought and numerical simulation is conducted to study the nitride layer growth.

Abstract: In this work, iron aluminide coatings were developed by Chemical Vapor Deposition in Fluidized Bed Reactor (CVD-FBR) on ferritic-martensitic steels. Small additions of zirconium powder were introduced in the fluidized bed; as a consequence, the obtained coatings are thicker than that without zirconium additions. When Zr powders are added in the fluidized bed, the deposition atmosphere drastically changes, leading to increase the deposition rate. Thermodynamic calculations were carried out to simulate the modifications in the CVD atmosphere in the Al/Zr deposition system in comparison to the single aluminization. In order to optimize the conditions of the deposition, parameters such as temperature and concentration of zirconium introduced into the bed were evaluated and compared with the results obtained for the single aluminum deposition.

Abstract: We report on the interplay between ferromagnetism and superconductivity in trilayers La0.7Ca0.3MnO3/YBa2Cu3O7/La0.7Ca0.3MnO3 made of half metallic manganite and high temperature superconductor cuprate. Samples with a fully oxygenated cuprate show a magnetic field interval where the magnetizations of the manganite are aligned antiparallel. A considerable magnetoresistance accompanies the switching between magnetization configurations (parallel vs. antiparallel) of the manganite moments. Suppression of the free carrier density of the cuprate which occurs upon oxygen depletion, results in deep modifications in the shape of the normal state hysteresis loops indicating that there may be a magnetic coupling mediated by free carrier density of the cuprate. This result outlines the importance of quasiparticle transmission in the interplay between ferromagnetism and superconductivity in this kind of samples.

Abstract: For a conductive, elastic matter in thermodynamic equilibrium the LORENTZ force, which includes the COULOMB force for charged systems, is the only macroscopic interaction between the electromagnetic field and the matter. If irreversible currents such as diffusion and ionic currents are observed, these currents and their coupling can dominate the behaviour. Then, to describe that system it is necessary to start with a macroscopic description, which remains valid outside of the equilibrium. A macroscopic description of these currents is used to deduce a very simple model for an electrolytic actuator.

Abstract: An atomic-scale graded structure has been formed by sedimentation of substitutional atoms under an ultra-strong gravitational field of 1 million G level in alloys and compounds. In this study, we investigate the sedimentation of impurity atoms in semiconductor materials under a strong gravitational field. High-temperature ultracentrifuge experiments (0.59×106 G, 400°C, 60 hours) have been performed on an InSb single crystal wafer which surface was coated with Ge by means of Physical Vapor Deposition (PVD). It was observed that the penetration depth of diffused Ge atoms under the gravitational field was several times larger than under terrestrial field at the same temperatures.