Influence of a Sealing Treatment on the Behavior of Plasma-Sprayed Alumina Coatings Operating in Extreme Environments
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JTTEE5 17:410–419 DOI: 10.1007/s11666-008-9184-2 1059-9630/$19.00 ASM International
Influence of a Sealing Treatment on the Behavior of Plasma-Sprayed Alumina Coatings Operating in Extreme Environments Geoffroy Berard, Patrice Brun, Jacques Lacombe, Ghislain Montavon, Alain Denoirjean, and Guy Antou (Submitted October 8, 2007; in revised form February 7, 2008) In numerous applications developed at the Commissariat a` lÕEnergie Atomique, Direction de lÕEnergie Nucle´aire (CEA-DEN, French Atomic Agency, Atomic Energy Department), particularly those encountered in the processing of nuclear wastes, metallic components are subjected to extreme environments in service, in terms, for example, of ageing at moderated temperature (several months at about 300 °C) coupled to thermal shocks (numerous cycles up to 850 °C for a few seconds and a few ones up to 1500 °C) under a reactive environment made of a complex mixture of acid vapors in the presence of an electric field of a few hundred volts and a radioactive activity. Alumina plasma-sprayed coatings manufactured with feedstock of different particle size distributions, graded alumina-titania coatings, and phosphate-sealed alumina coatings were investigated to improve the properties of metallic substrates operating in such extreme environments. The effects of particle size distribution, phosphate sealant, and graded titania additions on the dielectric strength of the as-sprayed, thermally cycled and thermally aged coatings were investigated. Thermal ageing test was realized in furnace at 350 °C for 400 h and thermal shocks tests resulted from cycling the coating between 850 and 150 °C using oxyacetylene flame and compressed air-cooling. Alumina coating structures and phase content were characterized in parallel by scanning electron microscopy (SEM) coupled to image analysis and stereological protocols and X-ray diffractometry (XRD). The dielectric strength was assessed by measuring the breakdown voltage at 50 Hz during and after the thermal tests.
Keywords
dielectric coating, diffusion barrier, extreme environment, plasma spraying, porosity, thermal shock
1. Introduction The increase in the lifetime of AISI 304L stainless steel structures, which are widely used in the nuclear industry, can enable new applications in extreme environments, particularly in the ones encountered in nuclear waste processing, where components experience long-term ageing (about 300 C for several months) coupled to thermal shocks (numerous cycles up to 850 C for a few seconds and a few ones up to 1500 C) under a reactive environment made of a complex mixture of acid vapors in the presence of an electric field of a few hundred volts and a Geoffroy Berard, Patrice Brun, and Jacques Lacombe, Commis sariat a` lÕEnergie Atomique, Direction de lÕEnergie Nucle´aire, VALRHO – Centre de Marcoule, 30207, Bagnols-sur-Ceze, France; and Ghislain Montavon, Alain Denoirjean, and Guy Antou, SPCTS – UMR CNRS 6638, Faculty of Sciences, University of Limoges, 123 Avenue Albert Thomas, 87060, Limoges Cedex, Fran
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