The preparation and hot corrosion resistance of gradient NiCoCrAlYSiB coatings

Abstract

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), AlCr 2, and Al 8Cr 5, besides γ/γ′ and β-NiAl. The phases of σ-(Cr, Co), AlCr 2, and Al 8Cr 5 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 (Na 2SO 4+K 2SO 4 20 wt.% salt mixture)/cm 2 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 Al 2O 3 occurred, which caused an accelerated hot corrosion after 10 h. For the gradient coatings, there formed Al 2O 3 and Cr 2O 3 at the primary stage of hot corrosion. Cr 2O 3 is easier to react with Na 2SO 4 than Al 2O 3, which reduces the oxygen ion activity of Na 2SO 4, so Al 2O 3 was protected. The formation of the continual and protective layer of Al 2O 3 and Cr 2O 3 leads to the hot corrosion kinetics of gradient coatings similar to oxidation kinetics.