Cyclic oxidation resistance of CoCrAl alloys and their sputtered microcrystalline coatings with and without yttrium

Abstract

The cyclic oxidation of the normal-grained CoCrAl alloys and their sputtered microcrystalline coatings with and without yttrium was investigated at 1000°C in air. The results indicated that the normal-grained Co30Cr5Al alloy exhibited very poor resistance against cyclic oxidation. After only one or two cycles, the initially formed Al 2O 3 scale severely spalled off, and then Cr 2O 3 and CoCr 2O 4 spinels were formed as a surface layer, while Al 2O 3 only existed in the form of internal oxides. In contrast, the normal-grained CoCrAl alloy with yttrium and the two kinds of sputtered microcrystalline coatings with and without yttrium showed excellent cyclic oxidation resistance at 1000°C. Neither scale spallation nor obvious internal oxidation was observed. The scales formed on the latter three kinds of samples were mainly composed of Al 2O 3. The reasons for the beneficial effects of reactive elements on the oxidation resistance have been well documented in the literature. In this paper, the mechanisms of the beneficial effect of microcrystallization on the oxidation resistance have been proposed.