•A duplex nanocrystalline coating is designed and prepared.•The duplex coating shows high oxidation and hot corrosion resistance.•The duplex coating displays high spallation resistance.•The duplex coating avoids element interdiffusion.A new duplex nanocrystalline coating is designed for high temperature oxidation and hot corrosion protection. This coating combines the advantages of traditional NiCrAlY and nanocrystalline coatings, i.e., providing high resistance to oxidation and hot corrosion simultaneously, while avoids any disadvantages that the traditional coatings have suffered from, such as scale spallation, element interdiffusion (along with the formation of harmful TCP phases). It gives a good choice as the bond coating of a TBC system.

•Enamel–alumina composite coating improves oxidation resistance of K444 alloy.•The enamel coating changes the oxidation mode of the K444 alloy.•Its protection mechanism relies on the retarding of oxygen diffusion.•Protection mechanism is associated with the alumina content.•Higher content of alumina favors its oxidation protection.As the service environment has become increasingly harsh, high-temperature protective coatings were widely researched and applied in industrial fields. K444 is a typical Cr-rich Ni-based superalloy used in power system. Its protection at high temperatures relies on the formation of chromia which is not alone but accompanied by the growth of other oxides or compounds thereby decreasing the oxidation resistance of chromia. In this paper, an enamel–alumina composite coating was prepared on the K444 alloy. Its oxidation behavior at 900 °C was investigated and the oxidation mechanisms were carefully elucidated. Results indicate that oxidation resistance of the K444 superalloy has been increased for more than six times by the enamel–alumina composite coatings. As the traditional theory suggested, the enamel composite coatings retard oxygen diffusion to the superalloy surface, leading to the decrease of oxidation rate. However, it is found that the protection mechanism is more associated with the change of oxidation mode of the alloy substrate, i.e. preventing Cr oxidation while promoting Al and Ti oxidation by the enamel coatings. Furthermore, this protection depends strongly on the alumina content of the enamel composite coating. With higher alumina content, the enamel composite coating retards oxygen diffusion more effectively, and the oxidation mode with respect to the selective oxidation of Al keeps unchanged for longer times.

•Obvious IDZ and SRZ form for MCrAlY/N5 system after long term oxidation.•No notable interdiffusion occurs for nanocrystalline coating.•Nanocrystalline coating has a better oxidation resistance than NiCrAlY.•Nanocrystalline coating has a better spallation resistance than NiCrAlY.The oxidation and interdiffusin behavior of NiCrAlY and sputtered nanocrystalline coatings on a single-crystal superalloy were studied. Results indicated that the superalloy substrate already possessed a high oxidation resistance. However the spallation resistance of oxide scale was very low. NiCrAlY coating enhanced the spallation resistance to some extent of the superalloy substrate at the expense of elevating oxidation rate and occurrence of severe interdiffusion. Sputtered nanocrystalline coating enhanced substantially the spallation resistance of the superalloy substrate meanwhile avoided elements interdiffusion. Its oxidation rate was lower than the superalloy substrate at 1000 °C but a little higher at 1100 °C.

In order to extend the use of a SiO2–Al2O3–ZnO–CaO based glass coating for higher temperature applications on Ni-base superalloy substrates, its interfacial reaction with the K38G superalloy substrates at 1000 °C was investigated. Results indicated that the interfacial reaction resulted in bubbling of the glass coatings due to Zn gas volatilization and formation of a loose TiO2 interlayer which finally accounted for the peeling of the coatings. With a pre-crystallization process of the glasses, its interfacial reaction was controlled and the bubbling of the coatings was avoided. After the heat-treatment at 1000 °C for 20 h, the glass coatings were still very dense and bonded well to the K38G superalloy substrates. Instead of a loose TiO2 interlayer, an Al-riched layer formed at the interfaces.Highlights► Interfacial reaction between glass coating and superalloy substrate is considered. ► Zinc gas products resulted in bubbling of glass coating. ► Loose TiO2 interlayer leads to spalling off of glass coating. ► Pre-crystallization of glass coating suppressed interfacial reaction.