MCrAlY coatings are playing an important role in protecting the substrate from oxidation and corrosion at high temperature, which can form dense and continuous oxide scales on the surface to retard the diffusion of cations and oxygen. With the depletion of Al for the growth of alumina layer, the concentration of aluminum in the coating near coating/oxide scales interface decreased, which further suppresses the consecutive Al₂O₃-scale growth, causing the formation of mixed oxide compounds and cracks as well as voids. This process can cause premature failure of the coating. Between the coatings and the superalloy, the interdiffusion process after vacuum heat treatment is beneficial, which can improve the adhesion of the coating and the substrate. However, because of its thermally activated nature, the interface diffusion process will make deleterious effects at elevated temperature. The elements of the substrate, like Ti, W, Mo, can diffuse into coating. Furthermore, the interdiffusion process in the coating/substrate will form a secondary reaction zone (SRZ), which mainly consists of high density of topologically close-packed (TCP) phases, such as σ, μ and Laves phases. These TCP phases in the matrix may reduce the high-temperature fatigue life of the superalloy.