Influence of high-temperature protective coatings on the mechanical properties of nickel-based superalloys

The low-pressure plasma spray coating process has been established in the field of gas turbines and is used for hot parts, such as turbine blades and duct segments, which are exposed to corrosive gases at high temperatures. Overlay coatings based on the MCrAlY alloy system (M is Ni, CO or both) are commonly employed as oxidation- and corrosion-resistant coatings. Mechanical properties, such as creep and fatigue lives, of various MCrAlY coating systems were investigated at high temperature as compared with the uncoated substrates, such as eqiaxis IN738LC, directional solidified CM247LC and single-crystal CMSX-2. It was clear that the MCrAlY coatings had no significant influence on the creep lives of substrates for the sake of superior ductility of MCrAlY coatings at high temperature. The low-cycle fatigue lives of MCrAlY coated superalloys at high temperature showed only a little superior performance in comparison with the uncoated results. However, the high-cycle fatigue lives of MCrAlY coated superalloys at high temperature showed inferior performance in comparison with the uncoated results. It was because that the low-cycle fatigue cracks initiated at casting cavities inside the substrate in both the coated and the noncoated cases. However, the high-cycle fatigue cracks initiated at interface defects, such as small pores and grit residues, between the MCrAlY coating and the substrate and grew into the MCrAlY coating, and then into the substrate.