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Theses

Des particules revêtues aux matériaux massifs : synthèse par mécanofusion et dépôt chimique en phase vapeur, caractérisation et étude de l'oxydation à haute température.

Abstract : Protection against oxidation of materials has promoted extensive research activity.
It exists various methods, most of them are based on the formation of oxidation-resistant surface coatings.
Any deterioration of the surface coating by mechanical strain can be prejudicial for the life span of the material because it is in direct contact with the oxidizing atmosphere.
To avoid this problem, this work is devoted, in a first step, to the formation of oxidation-resistant coatings on the surface of metallic particles.
The ultimate objective is to use powder metallurgy, and more precisely coated particles sintering, in order to synthesize a bulk material. The as-elaborated composite material is thus protected against oxidation at the particle scale.
The mechanofusion process has been successfully applied to coat iron particles with alumina.
The fluidized-bed metal-organic chemical vapour deposition (FB-MOCVD) process has been applied to coat iron particles with aluminium. By this last method, a dual-layer Al/gamma-Al2O3 is formed around iron particles, the metastable alumina being in contact with the surrounding atmosphere.
From oxidation kinetics study, it is demonstrated that Al/gamma-Al2O3-coated Fe particles do not oxidize, whereas Al2O3-coated Fe particles oxidize slower than uncoated ones.
At low temperatures (T < 600°C), the internal oxidation of Al2O3-coated Fe particles results from the incursion of oxidizing gas through the cracks or porosities of the alumina coating.
At high temperatures, oxidation mechanisms are governed by Fe cations' diffusion through the alumina layer.
Bulk materials sintered by spark plasma sintering using Al2O3-coated Fe particles preserve the microstructure of coated particles. Concerning the bulk materials sintered by hot isostatic pressing, the coating is gathered in small islands dispersed in an iron matrix. During both sintering processes, alumina is transformed to hercynite FeAl2O4.
Oxidation tests carried out at 720°C on sintered samples show that hercynite slows down the oxidation process.
Moreover, the bulk materials which have conserved the microstructure of coated particles show the slowest oxidation rate.
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Submitted on : Monday, March 30, 2009 - 2:59:32 PM
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Frederic Jay. Des particules revêtues aux matériaux massifs : synthèse par mécanofusion et dépôt chimique en phase vapeur, caractérisation et étude de l'oxydation à haute température.. Matériaux. Université de Poitiers, 2008. Français. ⟨tel-00371807⟩

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