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Etude de la projection plasma sous très faible pression - torches et procédé de dépôt

Abstract : During the last decade, very low pressure plasma spraying (VLPPS) technology(below 10 mbar) attracted attention because it could allow to produce coatings with astructure similar to that of vapor deposited materials (PVD) with kinetics close to thatof thermal spray. This technology aims to fully or partially evaporate the feedstockmaterials in order to build rapidly dense, thin, and columnar coatings.The work during this thesis preparation was thus devoted to the study anddevelopment of tools and techniques allowing fusion and evaporation of ceramicmaterials in order to obtain high quality deposits with new performance and then tocharacterize the properties of those deposits. In a first approach dense and homogeneous yttria-stabilized zirconia (YSZ) coatingswere deposited successfully on a stainless steel substrate using “classical” plasmaspray torches such as F100 and F4 under very low pressure (1 mbar) by means ofusing an original way of introducing the feedstock material in the core of the plasmajet via the plasma gas port. Optical emission spectroscopy was used to analyze theproperties of the plasma jet and especially to observe the feedstock materialevaporation rate. The phase composition and the microstructure of the coatings werecharacterized by X-ray diffraction and scanning electron microscopy. Results showedthat the YSZ powder was partially evaporated and that the coatings possessed aduplex microstructure which was composed of splats formed by the impingement ofmelted particles and a little amount of a matrix formed by the condensation of thevapor emitted by overheated particles.In order to try and increase the evaporation rate, a home-made transferred arc nozzlewas made and mounted on a F100 plasma torch in order to enhance the energy levelof the plasma jet and then to increase thermal exchanges. The effects of thetransferred arc nozzle on the plasma jet properties were evaluated by optical emissionspectroscopy and electron temperature calculation. YSZ and alumina (Al2O3) coatingsWere elaborated using such a nozzle below 1 mbar. It was found that the YSZ coatingsdisplayed a duplex microstructure similar to that obtained in the previous experiments.However, no vapor condensation could be observed in the case of the Al2O3 coatingsindicating that the evaporation capacity of the system remained limited.The microstructure, the mechanical properties and the thermal shock resistance of theYSZ coatings were studied in more details and compared to that of deposits madeusing classical thermal spray routes. An enhanced resistance to thermal shock couldthus be observed for the coatings with a duplex structure.In order to find a solution for a substantial increase in the energy density of theplasma jet, the laboratory commissioned a novel experimental tri-cathode plasmatorch made to the AMT Company. This new torch was modeled and first testedunder atmospheric conditions, which revealed a poor spray yield. Following thosefirst experimental results, a modified nozzle was designed. As a result, the sprayefficiency was considerably increased and the coating fabricated by the tri-cathodetorch displayed a better microstructure. Now this work has to be pursued with theintegration of this torch in the low pressure spray tank.
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Submitted on : Friday, June 1, 2012 - 12:07:28 PM
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  • HAL Id : tel-00703247, version 1



Lin Zhu. Etude de la projection plasma sous très faible pression - torches et procédé de dépôt. Alimentation et Nutrition. Université de Technologie de Belfort-Montbeliard, 2011. Français. ⟨NNT : 2011BELF0169⟩. ⟨tel-00703247⟩



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