Fabrication And Oxidation Behavior Of A Magnetron Sputtering Microcrystalline ?-NiAl Coating At1100? In Air

In this work,microcrystalline ?-NiAl coating was fabricated on a nickel-based single crystal?SC?superalloy substrate via multi-targets magnetron sputtering and subsequent annealing.It's oxidation behavior at 1100?was conducted compared with that of a conventional AIP NiCrAlY coating.The degradation and failure mechanism of the two coatings at 1100?were analyzed and discussed in detail.Besides,preliminary study of Ni-Cr-O active diffusion barrier was also carried out.By using Ni₃Al and pure Al plates as magnetron sputtering targets,microcrystalline ?-NiAl coating was successfully fabricated on a SC superalloy substrate with subsequent annealing.The grain size of the microcrystalline coating was about 300nm¹mm and the annealing process also eliminated the large flaws within the sputtered coating which homogenized and densified the coating.During isothermal and cyclic oxidation tests at 1100?in air,it can be seen that microcrystalline ?-NiAl coating displayed excellent isothermal and cyclic oxidation resistance through exclusive formation of slow-growing and adherent ?-Al₂O₃.While NiCrAlY coating has a much higher scaling rate and it underwent serious scale spallation during cyclic oxidation.After 200h cyclic oxidation at 1100?in air,the oxides formed on NiCrAlY coating consisted of outer spinel and inner alumina.Besides,Ta and Y rich oxides were included in the scale.Mixed oxides formed on the surface of single crystal superalloy substrate and inner nitride formed beneathe the oxide scale in the substrate after cyclic oxidation.Microcrystallization was considered to be the cause of the good scale adherence of microcrystallineb-NiAl coating.Ta and Y rich oxides in the scale of NiCrAlY coating may accelerate the growth rate of the scale,thus a much thicker scale which promotes the scale spallation during cyclic oxidation.Scale spallation and inter-diffusion between NiCrAlY coating and SC substrate depleted Al and Cr in the coating progressively which degraded the coating and made it unable to form exclusive alumina scale.During annealing Ni-Cr-O active diffusion barrier reacted with aluminum from both the coating and SC substrate to form an alumina/NiCr/alumina sandwich structure which effectively hampers the inter-diffusion between coating and substrate during annealing and oxidation.So the diffusion barrier completely eliminated the formation of TCP phases,inter-diffuion zone and secondary-reaction zone which greatly improved the compatibility between the coating and SC substrate.