The main focus of this study is sealed zirconia-based thermal barrier coatings produced by plasma spraying. AEM examinations conducted on phosphate-sealed Y 2 O 3 -ZrO 2 coatings and orthophosphoric acid sealed MgO-ZrO 2 coatings provide detailed information on the phases in the sealed coatings and the bonding mechanism of phosphate-based sealants on zirconia. In the case of aluminum phosphate sealed alumina and chromia, the primary objective is to add clarity on the bonding mechanism by means of transmission electron microscopy. Investigations of the sealing-layer interface indicate that the sealing of the layer lamellas is due to chemical reactions. Paper includes a German-language abstract.

This paper examines plasma-sprayed alumina and chromia coatings, with and without aluminum phosphate sealing, in order to assess the stability of the as-sprayed layers and phosphate reaction as a function of temperature during the sealing process. Thermogravimetric analysis showed no changes in mass in the alumina layers, an increase of 1.7% in the mass of the chromia layers due to oxidation, and a 14% loss of mass in the sealed layers, which agrees well with the evaporation behavior of a pure seal. Paper includes a German-language abstract.

Microstructural study of plasma sprayed chromia coatings sealed with aluminum phosphate, was carried out for determining strengthening mechanisms of the sealant. Characterization was accomplished by X-ray diffractometry, scanning electron microscopy, and analytical transmission electron microscopy. The main phase in the coating is the eskolaite type α-Cr 2 O 3 . The overall structure of the coating is lamellar with columnar grains parallel to the lamella thickness. Amorphous aluminum phosphate sealant has penetrated into the coating filling the structural defects such as cracks, gaps and pores between the lamellas. The average composition of the sealant in the coating is 25 at% aluminum and 75 at% phosphorus giving the molar ratio P/Al of 3, that corresponds to metaphosphates Al(PO 3 ) 3 . The aluminum phosphate sealing in the chromium oxide coatings is based on adhesive binding due to the attractive forces between the condensed phosphates and the coating. There were no indications about chemical binding due to reactions between the sealant and the coating in the sealing treatment for chromia coatings.