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2931 |
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Wednesday, September 28, 2011, Saal B2.2 12:15 PM New Processes 1 |
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Comparison of electrical properties of Al2O3 sprayed coatings from feedstock powders and aqueous suspensions |
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Filofteia-Laura Toma* / Fraunhofer IWS, Germany Stefan Scheitz / Fraunhofer Institute IWS, Germany Lutz-Michael Berger/ Fraunhofer Institute IWS, Germany Viktar Sauchuk/ Fraunhofer Institute IKTS, Germany Conny Rödel/ Fraunhofer Institute IKTS, Germany Annegret Potthoff/ Fraunhofer Institute IKTS, Germany |
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Thermally sprayed alumina coatings with suitable dielectric properties have been prepared and tested for manufacturing of electrostatic chucks, discharge devices, or insulating coatings for high-temperature heaters. Compared with sintered alumina (corundum), thermally sprayed coatings consist predominantly of metastable phases (gamma-, delta-alumina), even though all commonly used feedstock powders consisted of corundum (alpha-Al2O3). The properties of metastable phases differ significantly from those of corundum; i.e. g-alumina shows higher hygroscopicity which is deleterious to the dielectric properties of the material. In the most of cases, there is a great need to obtain coatings consisting of alpha-Al2O3. This paper proposes a comparative study on the phase stability and electrical properties of thermally sprayed alumina coatings starting from alpha-Al2O3 spray powders and aqueous suspensions. APS and HVOF processes were used to produce the conventional sprayed coatings. Suspension sprayed coatings have been obtained by internal injection of the suspensions into the modified combustion chamber of the HVOF Top Gun. The electrical characteristics i.e. dielectric strength and electrical resistance (electrical resistivity) were investigated using dielectric breakdown test, and electrochemical impedance spectroscopy (EIS). The electrical resistance was measured at room temperature at different relative humidity (RH) levels, from 6% RH to 97% RH. The coating microstructure, phase composition were studied. Differences in the electrical insulating properties due to the different coating system characteristics are discussed. Of the coatings and conditions investigated in this work, the suspension-sprayed Al2O3 coatings showed superior dielectric breakdown strength and electrical resistance stability at high humidity levels which could be explained by a denser microstructure and retention of a higher content of alpha-Al2O3. |