Composition and structure of coatings based on rare-earth zirconates

Abstract

Coatings based on lanthanum, neodymium, samarium, and gadolinium zirconates have been grown by atmospheric plasma spraying of powders, and their composition and structure have been investigated by scanning electron microscopy, chemical analysis, and X-ray diffraction. The results demonstrate that the chemical composition of the coatings differs from the compositions of the powders they were prepared from. During plasma spraying of Ln₂Zr₂O₇-based powders, the rare-earth oxide vaporizes more rapidly than zirconia. The difference in composition between the powders and coatings decreases as the atomic number of the rare earths increases in going from La to Gd. We have studied the processes that take place in the coatings during heat treatment at a temperature of 1250°C. It has been shown that the major phase in the La₂Zr₂O₇-, Nd₂Zr₂O₇-, and Sm₂Zr₂O₇-based coatings undergoes a defect fluorite–pyrochlore structural phase transition. The La₂Zr₂O₇-based coatings have been found to contain ∼5 wt % t-ZrO₂ in addition to the major phase. The Gd₂Zr₂O₇-based coating retained a defect fluorite structure.