Phase transformation and oxidation behavior of Pt-modified MCrAlY coatings

The effects of platinum (Pt) layer on phase transformation and oxidation behavior of MCrAlY (M = Co, Ni) coatings were investigated by isothermal and cyclic oxidation tests at temperature of 1100 °C. Pt layer of about 5 μm thickness was deposited on MAR M247-DS superalloy by an electroplating method, and then the MCrAlY layers were coated with air plasma spray (APS) and vacuum plasma spray (VPS) methods. APS process creates internal oxides and interface voids, whereas the VPS process shows fewer voids in the interface between the MCrAlY layer and the superalloy used as substrate. The oxides and voids formed in the APS process deteriorate the oxidation resistance, indicating that the oxides and voids are transferred to Al₂O₃ compounds during the oxidation tests. The Pt layer shows a strong affinity with Ni element and a weak affinity with Co and Cr elements, resulting in the formation of β-(Ni,Pt)Al phase. The Pt pre-coated layer forces down for the diffusion of soluble elements to get the reduced deplete zone in the superalloy. The VPS process enhances the oxidation resistance of the MCrAlY coating and shows better bonding strength, owing to the generation of homogenous microstructure in the coating and less processing defects.