Microstructural characteristics and oxidation behavior of the modified MCrAlX coatings: A critical review

Abstract

Currently, the service temperature of the MCrAlX coating (M = Ni, Co, or NiCo; X = minor elements such as Y, Ce, Si, Ta) either as an overlay or as a bondcoat in some critical engineering parts for gas turbine and aero-engine applications is more than 1000 °C. Therefore, the improvement of thermal stability and service lifetime of the MCrAlX coatings are the key factors that need to be emphasized. Among them, the high-temperature oxidation resistance is one of the key properties of MCrAlX coatings. The key modification processes can be employed to improve the structural properties and oxidation behavior of MCrAlX coatings. These methods mostly include techniques such as vacuum heat treatment, nano-crystallization, pre-oxidation, alloying with the reactive element or rare earth elements, dispersion of oxide particles, laser treatment, spark plasma sintering, hot isostatic pressing, and using multilayered/graded coating systems. In recent decades, investigations on the increasing of the oxidation resistance of MCrAlY coatings were mostly focused on the modification of these coatings using reactive elements, laser treatment, and development of advanced multilayered or gradient coatings. Recent investigation exemplified that reactive elements (or their oxides( as well as nano-crystallization can increase high-temperature corrosion and oxidation resistance of MCrAlX coatings. This enhancement may be associated with the structure refinement, grain boundary purification, and adhesion improvement of the Al 2 O 3 oxide scale on the coating surface. However, there are conflicting opinions regarding the effect of reactive elements on the oxidation behavior of the MCrAlY coatings. The controversial findings in the literature indicated that the dominant mechanism of the enhancement of oxidation resistance for the reactive element or rare earth element modified MCrAlY coatings is still unclear and complicated. Therefore, a basic conception of the mechanism of the oxide scale growth in MCrAlY coating under short- and long-term oxidation is still far from obvious. The present work is a review with critical viewpoints in the case of the improvement of MCrAlY coatings and investigation of their structural characteristics and oxidation behavior, focused on different types of modification techniques. • Different modification techniques can be used to improve the structural characteristics of the MCrAlX coatings. • Reactive element modified MCrAlX coatings have higher oxidation resistance than conventional coatings. • The enhancement of oxidation properties may be attributed to the higher oxide scale adhesion in modified MCrAlX coating.