Researching On High-temperature Oxidation Property Of Ni-base Single Crystal Super-alloys/NiCoCrAlYTa Coatings

Ni-based single crystal super-alloys have been widely used in various fields likeaerocraft, aerospace and energy fields against high temperature oxidation and corrosion in theextremely severe environment. In order to improve high temperature properties and theservice lives of the hot components, a protective coating must be applied.In this paper, the NiCoCrAlYTa coatings were obtained on the single crystal super-alloysubstrate by air-plasma spraying (APS), low-pressure plasma spraying (LPPS), highvelocity oxygen-fuel spraying (HVOF), and two kinds of low temperature high velocityoxygen fuel (LT-HVOF) methods. The phase compositions and microstructure for all thecoatings were characterized by using the XRD, SEM and EDS and the high temperatureoxidation resistance were mainly studied in this paper.The results showed that the oxidation kinetic of Ni-based single crystal super-alloyfollows parabolic rate law after oxidation at900℃and1000℃for200h, while observeslinear equations at1100℃. Protective oxide film including Al2O3and slight TiO2wereobserved at900℃, while the NiO and spinel-type oxides found at1000℃(Al2O3inside) andmany spinel-type oxides formed at1100℃.The LPPS coating possesses dense and uniform micro-structure and the as-sprayed andheat treated coatings are composed by γ-Ni、γ’-Ni3Al and β-NiAl. The oxidation kinetic ofLPPS coating follows parabolic rate law at the three testing temperatures for200h oxidationtest. The thickness of the oxide layer was about1-3μm and thickness of oxidatoion layer wasincreased with temperature. The oxide scales are composed mainly of α-Al2O3and spinel-typeoxides.The APS coating features high levels of porosity and oxygen content. The as-sprayedcoating was mostly composed of γ-Ni、γ’-Ni3Al and β-NiAl,while Al2O3was observed afterheat treatment. The oxidation kinetic of the APS coating follows parabolic rate law at thethree temperatures for200h of oxidation. The continuous and dense oxide film can not beformed during oxidation. The oxide are composed mainly of Cr2O3and spinel-type oxides.HVOF, LT-HVOF-1, LT-HVOF-2coatings possess dense micro-structure except highoxide content found in HVOF coating. All the sprayed and heat treated coatings are composedby γ-Ni、γ’-Ni3Al and β-NiAl, whileAl2O3appeared for heat treated HVOF coating.The oxidation kinetic of HVOF coatings follows parabolic rate law after200h oxidation at900℃and1000℃and linear equations at1100℃. The oxide is mainly composed ofα-Al2O3at900℃, other oxide like NiCr2O4observed at1000℃. However, the oxides areAl2O3and mixed spinel-type oxides at1100℃.The oxidation kinetic of two different kinds of LT-HVOF (LT-HVOF-1andLT-HVOF-2) coating follows parabolic rate law at the three temperatures after200h oxidation.Both coatings exhibited excellent oxidation resistant because a dense protective α-Al2O3filmis formed at900℃and1000℃, even though few spinel-type oxide is observed in LT-HVOF-2coating. The oxide are composed mainly of Cr2O3and spinel-type oxides in LT-HVOF-1coating, α-Al2O3and spinel-type oxides in LT-HVOF-2coating at1100℃.High temperature oxidation at different temperatures for200h for all the coatings revealthat the oxidation resistance of LT-HVOF-1coating is slightly superior to that of HVOF,LT-HVOF-2and LPPS coating, while APS coating offers the worst oxidation resistance.