Sealing of coatings for service at high temperatures

Sealing is one of the most common post-treatment procedures for thermal spray coatings, directed mostly to prevention of attack of corrosive media to the substrate. Organic sealants are dominating but are limited to service at room or slightly elevated temperature. Technical solutions for sealers allowing coating service up to 800 °C are seldom, and have been focused on aluminum orthophosphate in the past. Possible changes in the coating microstructure at temperatures up to 800 °C are neglected so far. In this study we studied three new commercial silicate-based sealers in comparison with aluminum orthophosphate and own developed glass sealer. The penetration and curing of these sealers into selected oxide coatings (alumina, alumina-titania, spinel, zirconia and chromia) sprayed by APS as well as HVOF-sprayed Cr3C2-NiCr was studied, including determination of the wetting behavior of the sol-gel sealers and the molten glass to the coatings. Alumina free-standing deposits sprayed by water stabilized plasma were studied as well. The changes of the coating microstructures after treatment at 800 °C (changes of the coating microstructures and phase compositions as well as the interaction between coating material and the sealer were taken into consideration. Extensive microstructural studies were carried out. The success of the sealing operation was estimated using corrosion tests or dielectric breakdown tests in the case of insulating coatings.