Powder processing methodology for fabrication of Copper/Graphite composite materials with enhanced thermal properties

International audienceThis study focuses on the powder processing methodology (PPM) for the fabrication of metal matrix composite using Graphite flakes (Grf) reinforced Copper (Cu). The manufacturing route involved (1) a reductive treatment of Grf powder to purify and increase its quality, (2) the mixing of the Cu and the Grf (0–40 vol%) powders with a fast and efficient Resonant Acoustic (RA) mixer, and finally (3) the cold and hot-pressing of composite powders. Comparison of PPM is made with literature and a usual method used in our laboratory. The quality of Grf after different steps was analyzed by Raman spectroscopy and XRD. Microstructure of the composite materials were analyzed by SEM. It was shown that this new PPM demonstrated better composite structural and thermal properties even at low volume fraction of Grf with a maximum of 630 W·m−1·K−1 obtained with 40 vol% of Grf at 70 °C

Determination of residual stress fields in a thermally grown oxide under thermal cycling loadings, using XRD and Raman spectroscopy. Correlations with microstructural states

Abstract. The presence of residual stresses in thermal oxide layers has been recognized for a long time. In the present work, the mechanical fields for chromia oxide are determined either by XRD or Raman spectroscopy. In addition, the microstructure of the chromia films is investigated ant its influence on the evolution of the stress release processes is analyzed. Introduction NiCr alloys are currently used at high temperatures because it develops a dense chromia surface oxide film which slows down the oxidation process. In turn, the material durability depends on the ceramic film integrity. The isothermal oxide layer growth or the cooling steps usually induce the development of high residual stress in the ceramic film. The determination of such growth or residual stress has already been undertake

Determination of residual stress fields in a thermally grown oxide under thermal cycling loadings, using XRD and Raman spectroscopy. Correlations with microstructural states

Abstract. The presence of residual stresses in thermal oxide layers has been recognized for a long time. In the present work, the mechanical fields for chromia oxide are determined either by XRD or Raman spectroscopy. In addition, the microstructure of the chromia films is investigated ant its influence on the evolution of the stress release processes is analyzed. Introduction NiCr alloys are currently used at high temperatures because it develops a dense chromia surface oxide film which slows down the oxidation process. In turn, the material durability depends on the ceramic film integrity. The isothermal oxide layer growth or the cooling steps usually induce the development of high residual stress in the ceramic film. The determination of such growth or residual stress has already been undertake

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

Stress analysis of local blisters coupling Raman spectroscopy and X-ray diffraction. Correlation between experimental results and continuous damage modelling for buckling in an iron oxide/phosphated iron system

In this present work, local stress development in the iron oxide layers growing on phosphated α-Fe at 400 °C in ambient air is investigated by Raman spectroscopy. Coupled with X-ray diffraction it enables to obtain directly local stresses' maps in the oxide layers. Use of Raman spectroscopy allows obtaining better accuracy on mechanical behaviour at local scale. This characterisation technique is very useful to study systems developing mechanical heterogeneities on surface, especially in case of buckling phenomenon. Investigations on particular local blisters have been done to measure some characteristic lengths at local scale. From local measurements, we are able to evaluate general effect of buckling from simplified scale transition. So, a macroscopic approach has been performed to calculate global stress evolution of the oxide layer, based on continuous damage mechanics. Consequently, it leads to good comparison between modelling and experimental values (global stresses versus oxidation time) in α-Fe 2 O 3 oxide. © 2010 Elsevier B.V. All rights reserved

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

Competition between stress generation and relaxation in iron oxide films. Experiments and modelling

International audienceIn the present work, the calculated growth stresses arising from iron oxide layers are compared to measurements of the macroscopic residual stresses. High temperature x-ray diffraction was used for determining the stress created in thin iron oxide layers formed on phosphated α-Fe, as a function of the oxidation time. Experiments are conducted both in situ during oxidation and after cooling, allowing us to derive respectively, growth and thermal stresses. A numerical model was developed to calculate the growth stress and the relaxation phenomena occurring during isothermal oxidation. This model takes into account the mechanical behaviour of both the substrate and the oxide, and parameters such as oxide growth kinetics, substrate thickness, etc. The elasto-viscoplastic model correlates the experimental results with the various parameters of the study. © 2004 Taylor & Francis Ltd

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

On the growth strain origin and stress evolution prediction during oxidation of metals

International audienceHigh temperature oxidation of metals leads to residual stresses in the metal and in the oxide. In this work, we try to predict the evolution of the residual stresses in the growing oxides layers, during isothermal oxidation. The origin of these stresses is based on the microstructural model of Clarke, however, another justification is proposed, assuming a proportional dependence of the growth strain with the oxide layer thickness. Using the mechanics of thin layers, as well as the analysis proposed to describe the growth strain, a system of equations are deduced that predict the stresses evolution with oxidation time. Numerical analysis is performed, leading to a set of theoretical curves. © 2005 Elsevier B.V. All rights reserved