Microstructural study of plasma sprayed chromium oxide coatings:: Effect of aluminum phosphate sealing

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 alpha-Cr2O3. 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(PO3)(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.