Nanocrystalline rare earth oxide coatings for improved H.T. oxidation resistance of chromia forming alloys

Abstract

Rare earths (RE) have been used to improve the high temperature oxidation resistance of chromium dioxide and alumina forming alloys. The RE are added either to the alloy or are applied as a coating to the alloy’s surface. In this study, the sol-gel technique, well known to produce fine grained ceramic oxides, was used to prepare nano-crystalline RE oxides of La, Ce, Pd, Nd, Sm, Gd, Dy, Y, Er, and Yb. These oxides were used to coat Fe-20Cr specimens. The cyclic oxidation resistance of the RE oxide coated specimens was determined. Marked improvements were observed in the coated specimens and the extent of improvement depended on the nature of the RE oxide, its ionic radius and other features of the coating. Extended oxidation tests were carried out with La2O3 and Pr2O3 coated Fe-20Cr alloy. In these tests the specimens were heated to different peak temperatures and cooled at rates of up to 1000C/s. The La2O3 coatings were more effective in increasing cyclic oxidation resistance compared to Pr2O3 coatings. The role of RE in increasing overall oxidation resistance of chromium dioxide forming alloys is discussed.