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Synthèse et résistance mécanique des couches d'oxyde de zirconium

Abstract : MOCVD deposition, air thermal oxidation and thermal oxidation followed by MOCVD deposition were used to produce the zirconia layers. First, in order to obtain films of controlled thickness (a few microns), we determined the optimal MOCVD deposition conditions on the 304L and Zy-4, at 400°C and 500°C. The two other techniques allowed us to obtain thicker layers of different tetragonal phase fractions. The volume fraction of tetragonal phase of zirconia and the residual stresses in the layers was characterized by DRX. Multiple cracking tests were performed inside a SEM through bending or tensile tests. On the Zy-4, we obtained the classic damage scheme: cracks perpendicular to the extension direction appear and multiply until saturation. On the 304L, cracks were localized for the thinner layers; this may be due to the grain size of the substrate with respect to the layer thickness. For the layers obtained on Zy-4, the minimal distance between the cracks does not depend neither of the tetragonal phase fraction nor of the residual stresses. It varies linearly with the layer thickness. Using the finite element method, the stress at fracture initiation was obtained. It decreases with the increase of the layer thickness independently of the tetragonal phase fraction and of the residual stresses. To take into account the residual stresses in assessment of the layer strength, high residual stress gradient have to be supposed.
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Submitted on : Thursday, March 8, 2018 - 11:35:07 AM
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Josiane Christelle Djuidje Dzumgam. Synthèse et résistance mécanique des couches d'oxyde de zirconium. Matériaux. Université Paris-Saclay, 2018. Français. ⟨NNT : 2018SACLS021⟩. ⟨tel-01726383⟩

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