The preparation and hot corrosion resistance of gradient NiCoCrAlYSiB coatings

An Al and Cr gradient NiCoCrAlYSiB coating was prepared by a two-step deposition: (i) deposition of a NiCoCrAlYSiB layer onto Ni-based superalloy surface by arc ion plating (AIP); and (ii) co-deposition of NiCoCrAlYSiB and Cr–Al by AIP and magnetron sputtering (MS), respectively. The results of EPMA show that Al and Cr have a gradient distribution in the surface layer of the gradient coatings. After annealing, the NiCoCrAlYSiB coatings consist of γ/γ′ and β-NiAl, while the gradient coatings consist of σ-(Cr, Co), AlCr2, and Al8Cr5, besides γ/γ′ and β-NiAl. The phases of σ-(Cr, Co), AlCr2, and Al8Cr5 can act as reservoir phases of Cr and Al to improve the hot corrosion resistance and lifetime of the coatings. In order to investigate the effect of Al and Cr on the hot corrosion of coatings, NiCoCrAlYSiB coatings and the gradient coatings were coated with 0.5 mg (Na2SO4+K2SO4 20 wt.% salt mixture)/cm2 and oxidized at 900 °C for 20 h. XRD and EDS results show that there are only the alumina formed on the NiCoCrAlYSiB coatings, so the basic fluxing of Al2O3 occurred, which caused an accelerated hot corrosion after 10 h. For the gradient coatings, there formed Al2O3 and Cr2O3 at the primary stage of hot corrosion. Cr2O3 is easier to react with Na2SO4 than Al2O3, which reduces the oxygen ion activity of Na2SO4, so Al2O3 was protected. The formation of the continual and protective layer of Al2O3 and Cr2O3 leads to the hot corrosion kinetics of gradient coatings similar to oxidation kinetics.