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ABSTRACT: Zirconia (ZrO 2) stabilized with 8 wt.% Y 2 O 3 is the most common material to be applied in thermal barrier coatings (TBC) owing to its excellent properties: low thermal conductivity; high toughness and thermal expansion coefficient similar to iron. Nevertheless, in order to increase the coatings lifetime, improvements in their thermomechanical behavior are still needed. With that purpose, we propose in this paper a graded ceramic coating. These TBC have been produced by depositing a conventional NiCoCrAlY bond coat on a Inconel 738 LC substrate followed by an atmospheric plasma sprayed top coat of ZrO 2 –8wt.%Y 2 O 3 with a porosity gradient along the cross section. The aim of the present contribution is to study residual stresses and elastic properties of the coating as a function of the porosity gradient. For the characterization of the TBCs residual stresses, we have used Raman and X-ray diffraction (XRD) in different thermal conditions: as-sprayed, after thermal shock at 1000 8C, and annealing at 1100 8C in air during 100 h. The top coatings show compressive stresses near the interface with the bond coat. A decrease of the stress level is observed along the cross section towards the surface. The residual stresses increase after annealing, however, have smaller variations after thermal shock. The elastic properties were evaluated by Brillouin scattering: the scattering of laser light by acoustic waves in the GHz frequency range. The spectra at different depths indicate that in the annealed condition the acoustic velocity increases when approaching the external surface. D 2004 Published by Elsevier B.V.
Surface and Coatings Technology 08/2004; 188–189:120–128. · 1.87 Impact Factor
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ABSTRACT: Zirconia coatings partially or fully stabilized have several high temperature applications (e.g. turbine blades and diesel engines parts). However, to increase the thermodynamic efficiency of these systems and reduce their thermal conductivity new kind of stabilizers are required. Gd 2 O 3 , among other rare earth oxides, is a potential candidate as a dopant or as a constituent together with the ZrO 2 Y 2 O 3 system. The aim of the present paper is to study the influence of the Gd 2 O 3 in the high-temperature phase stability of zirconia. Thin films of ZrO 2 Gd 2 O 3 and ZrO 2 Y 2 O 3 Gd 2 O 3 have been deposited by reactive DC magnetron sputtering with different dopant percentages and a wide range of techniques have been used for their characterization. X-ray diffraction (XRD) was used for the structural characterization of the coatings, to study the influence of Gd 2 O 3 addition on the high-temperature stabilized phases, and its influence on the microstructure and the level of stress. Energy dispersive X-ray spectroscopy (EDX) was used to assess the coating composition. The mol% of Gd 2 O 3 determined in the coatings is in the range of 2.1 to 17.5. The transition from the tetragonal to the cubic phase was clearly observed in the range of 5 to 6.5 mol% of dopant. Scanning electron microscopy (SEM) was used to determine the film thickness, to study the microstructure of the film in cross section and also the surface morphology. The surface microtopography was analyzed by atomic force microscopy (AFM) and nondestructive laser microtopography. The roughness of the coatings was evaluated and correlated with the optical parameters obtained by optical spectroscopy. A detailed description of the results and their discussion will be presented in the paper. D 2004 Published by Elsevier B.V.
Surface and Coatings Technology 07/2004; 188–189:107–115. · 1.87 Impact Factor
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ABSTRACT: Zirconia coatings partially or fully stabilized have several high temperature applications (e.g. turbine blades and diesel engines parts). However, to increase the thermodynamic efficiency of these systems and reduce their thermal conductivity new kind of stabilizers are required. Gd2O3, among other rare earth oxides, is a potential candidate as a dopant or as a constituent together with the ZrO2Y2O3 system. The aim of the present paper is to study the influence of the Gd2O3 in the high-temperature phase stability of zirconia. Thin films of ZrO2Gd2O3 and ZrO2Y2O3Gd2O3 have been deposited by reactive DC magnetron sputtering with different dopant percentages and a wide range of techniques have been used for their characterization. X-ray diffraction (XRD) was used for the structural characterization of the coatings, to study the influence of Gd2O3 addition on the high-temperature stabilized phases, and its influence on the microstructure and the level of stress. Energy dispersive X-ray spectroscopy (EDX) was used to assess the coating composition. The mol% of Gd2O3 determined in the coatings is in the range of 2.1 to 17.5. The transition from the tetragonal to the cubic phase was clearly observed in the range of 5 to 6.5 mol% of dopant. Scanning electron microscopy (SEM) was used to determine the film thickness, to study the microstructure of the film in cross section and also the surface morphology. The surface microtopography was analyzed by atomic force microscopy (AFM) and nondestructive laser microtopography. The roughness of the coatings was evaluated and correlated with the optical parameters obtained by optical spectroscopy. A detailed description of the results and their discussion will be presented in the paper.
Surface and Coatings Technology.
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ABSTRACT: In order to improve the performance of thermal solar collectors, the development of good, durable and reproducible spectrally selective solar absorber surfaces for photothermal conversion is of greatest importance. Spectrally selective cermet coatings were produced by DC magnetron sputtering using metallic chromium and titanium targets at constant target current, substrate bias and substrate temperature. These coatings are graded cermets with a metal concentration decreasing from the substrate to the coating surface. Basically, these coatings are formed with a layer structure consisting of a cermet layer enriched in metal, one or more cermet layers with less metal than the first one, and on top a pure ceramic layer as an antireflection layer. The metal layer was deposited by non-reactive DC sputtering, while the ceramic layers were deposited by DC reactive sputtering in argon–nitrogen or argon–oxygen atmosphere. The metallic and ceramic layers were deposited as a sub-layer system, consisting of alternating metallic and nitride or oxide sub-layers. The effect of the optical properties in dependence on the metallic fraction in the layers and the number of sub-layers in the film is discussed. For coatings based on chromium, the optimum selectivity achieved was a solar absorptance of 94% and a thermal emittance of 6% at 82 °C. For titanium oxynitride based coatings, the best selectivity achieved was a solar absorptance of 91% and a thermal emittance of 4%. The microstructure and thickness were studied by scanning electron microscopy (SEM). The surface microtopography was analysed by atomic force microscopy (AFM) and the metal concentration profile by Rutherford backscattering spectrometry (RBS).
Thin Solid Films.
