Residual stress determination in oxide layers at different length scales combining Raman spectroscopy and X-ray diffraction: Application to chromia-forming metallic alloys

In oxidizing environments, the protection of metals and alloys against further oxidation at high temperature is provided by the oxide film itself. This protection is efficient only if the formed film adheres well to the metal (substrate), i.e., without microcracks and spalls induced by thermomechanical stresses. In this study, the residual stresses at both macroscopic and microscopic scales in the oxide film adhering to the substrate and over the damaged areas have been rigorously determined on the same samples for both techniques. Ni-30Cr and Fe-47Cr alloys have been oxidized together at 900 and 1000 °C, respectively, to create films with a thickness of a few microns. A multi-scale approach was adopted: macroscopic stress was determined by conventional X-ray diffraction and Raman spectroscopy, while microscopic residual stress mappings were performed over different types of bucklings using Raman micro-spectroscopy and synchrotron micro-diffraction. A very good agreement is found at macro- and microscales between the residual stress values obtained with both techniques, giving confidence on the reliability of the measurements. In addition, relevant structural information at the interface between the metallic substrate and the oxide layer was collected by micro-diffraction, a non-destructive technique that allows mapping through the oxide layer, and both the grain size and the crystallographic orientation of the supporting polycrystalline metal located either under a buckling or not were measured

Non stœchiométrie dans le grenat de fer et d’yttrium (YIG)

Lors de l'élaboration du grenat de fer et d'yttrium, les écarts à la stœchiométrie peuvent entraîner une détérioration importante des propriétés magnétiques à cause de la présence de phases étrangères, qui, par leur caractère faiblement magnétique, jouent le même rôle que la porosité. Dans cet article, nous montrons que la synthèse du YIG par coprécipitation permet d'éviter la formation de phases étrangères lorsque le fer est en défaut par rapport à l'yttrium. Dans ce cas, les pertes magnétiques d'insertion dans le matériau ne sont pas augmentées

Grain boundary sliding in chromia thermally grown oxide

Residual macrostresses in chromia thermal oxide films which develop on a Ni-30Cr alloy have been investigated at room temperature thanks to Raman spectroscopy and X-ray diffraction. Combined with atomic force microscopy on the oxidised surface which gives morphological features at the chromia grain scale, stress release processes in the ceramic films are studied. It is evidenced that grain boundary sliding is activated as a companion mechanism of diffusion-creep. Such processes are sensitive to the oxide microstructure and the cooling conditions. © 2015 Acta Materialia Inc

Stress release phenomena in chromia scales formed on NiCr-30 alloys: Influence of metallurgical parameters

Stress release phenomena are studied for α-Cr2O 3 thermal oxide films grown on NiCr-30 alloys. The influence of specific metallurgical parameters, such as cooling rate and initial surface roughness, is investigated thanks to Raman spectroscopy. Systematic correlations are established between the residual stress level in the scales and the damage rate resulting from a delamination process by buckling. Different buckling morphologies are characterized mainly according to the cooling rate range. © 2011 American Institute of Physics

Shear strength measurements of parallel MoSx thin films

The effect of Hertzian contact pressure on the friction coefficient of parallel MoSx films was investigated by sliding sapphire and AISI 52100 steel against parallel MoSx-coated AISI 440C steel under a wide range of contact loads. Mean, initial Hertzian contact pressures in the range 0.493-1.258 GPa were created between balls and parallel MoSx-coated 440C disks by varying the applied load and the elastic moduli of the balls. The friction coefficient decreased with the increasing load. Using the Hertzian contact model, we estimated that the shear strength of parallel MoSx films was approximately 40 MPa. This value is higher than that obtained with the usual lamellar MoS2 films. An explanation in terms of an increasing reactivity of basal surfaces in parallel films is given. The effect of gaseous environment on friction measurement was also studied by varying the atmosphere conditions. For this purpose, friction tests were performed in open air and in dry Argon (Ar). The friction coefficient was shown to increase in the presence of water vapour. The shear strength increased with humidity from 40 MPa up to about 80 MPa. An explanation, taking into account the deformation microstructures that have been developed during sliding, is proposed. © 1997 Elsevier Science S.A

High resolution transmission electron microscopy study of quasi-amorphous MoSx coatings

Thin MoSx (x=1.2) films are deposited on 440C steel or Si by ion beam sputtering of an MoS2 target with (or without) high energy (160 keV) Ar+ ion beam assistance. The coating microstructures of the as-deposited films are analyzed by high resolution transmission electron microscopy (HRTEM). HRTEM images of the sputter-deposited MoSx films show domains with few fringes roughly parallel to the substrate, embedded in a typical amorphous structure. This confirms the suggestion made in a previous publication that these sputtered films are amorphous with short-range order in the form of layered clusters with layers parallel to the substrate. The HRTEM images of the ion-beam assisted films show small crystallites (with no preferential orientation) within an amorphous structure. The difference between the tribological properties of these two types of coating is explained by the difference in their microstructures. HRTEM study of the wear track microstructures shows the important role played by the layered clusters which form an interfacial film accommodating sliding motion