Cyclic oxidation of high temperature coatings on new γ′-strengthened cobalt-based alloys

An emerging class of cobalt-based γ′-strengthened alloys promises higher temperature capabilities compared to current Ni-base superalloys commonly used in aerospace and power generation applications. As with Ni-base alloys, high temperature coatings that enhance environmental resistance are desirable. Single crystal samples of Co-9.2Al-9.0W and Co-7.8Al-7.8W-4.5Cr-2.0Ta (at.%) were coated with vapour phase nickel aluminide and MCrAlY. Samples were subjected to cyclic oxidation at 1100 °C with 300-450 1-h cycles. Compared to NiAl-based coatings, the MCrAlY coatings exhibited superior adherence and an interdiffusion zone free of detrimental intermetallic phases. Evolution of microstructure during cycling is discussed with reference to the available thermodynamic data.