Nanocrystalline coatings on superalloys against high temperature oxidation: A review

To enhance oxidation resistance for industry applications, aluminides and overlaying MCrAlY or intermetallic β-NiAl coatings are the commonly used high temperature protective coatings for superalloys. However, they all face a serious problem with regard to interdiffusion with the underlying superalloy substrates. Since the sputtered microcrystalline coating of superalloy K38G for the purpose of oxidation protection was firstly reported in 1992, extensive studies have been conducted on its extension and application for different substrate materials, such as superalloys and intermetallic compounds. In addition to good oxidation protection, sputtered nanocrystalline coatings avoid interdiffusion with the alloy substrates. A more attractive merit of sputtered nanocrystalline coating is that due to the columnar structure, the thermal or growth stresses transported from oxide scale will be easily relaxed by deformation of the nanocrystalline coating, rather than by cracking of the oxide scale. This paper reviews recent progress on nanocrystalline coatings formed on substrates from poly-crystal to single-crystal alloys. The focus is to uptake the mechanisms of high temperature protective properties and to introduce microstructural and functional changes of nanocrystalline coatings.