Morphology of Al 2O 3 scales on doped CoCrAl coatings

Abstract

Adhesion between surface scale and protective coating is an essential requirement of a material possessing good overall oxidation resistance. Poor adhesion can lead to exfoliation of the oxide in response to thermal cycling or mechanically applied stresses, resulting in enhanced oxidation rates. Marked improvements in scale-substrate adhesion can be obtained by means of the formation of protrusions of oxide growing into the substrate. These act to key the protective scale to the surface and are more effective when a uniform distribution of small oxide pegs can be achieved. Thus the structure of the coating, the type of “rare earth addition” and its concentration are all important and this has been examined using electron beam physical-vapor-deposited CoCrAl coatings containing various elemental additions. The coatings were subjected to both isothermal and cyclic oxidation in the temperature range 1000–1200°C. The coating-scale interface and the development of pegs growing into the coating was examined in detail by dissolving away the underlying metal and coating using a bromine-methanol solution. The distribution of the pegs provides a good qualitative assessment of the oxide scale adherence. Results are compared with earlier studies on cast CoCrAl alloys containing various additions.