Degradation of nonmodified and rhodium modified aluminide coating deposited on CMSX 4 superalloy

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Abstract

Summary

The Ni-base superalloy CMSX 4 used in the turbine blades of aircraft engines was coated with rhodium layer (0.5-μm thick). Next coated CMSX 4 superalloy was aluminized by the CVD method. The rhodium modified aluminide coating and nonmodified aluminide coating were oxidized at 1100°C at the air atmosphere. The rhodium modified aluminide coating showed about twice better oxidation resistance than the nonmodified one. The spallation equal 62% of the total area was observed on the surface of the nonmodified coating whereas only 36% spallation area was observed on the surface of the rhodium modified aluminide coating after the oxidation test. The oxide layer formed on the surface of the nonmodified coating was composed of nonprotective (Ni,Cr)Al2O4 and (Ni,Cr)O phases. Aluminium in the coating reacts with oxygen, forming a protective α-Al2O3 oxide on the surface of the rhodium modified aluminide coating. When the oxide cracks and spalls due to oxidation, additional aluminium from the coating diffuses to the surface to form the oxide. The presence of protective Al2O3 oxide on the surface of the rhodium modified aluminide coating slows coating degradation. Therefore, rhodium modified aluminide coating has better oxidation resistance than the nonmodified one.

Lay description

The CMSX 4 superalloy was coated with 0.5-μm thick rhodium layer and aluminized by the CVD method. The rhodium modified aluminide coating showed about twice better oxidation resistance than the nonmodified one. The 62% spallation area was observed on the surface of nonmodified coating whereas only 36% spallation area was observed on the surface of rhodium modified aluminide coating after the oxidation test. Oxide layer formed on the nonmodified coating was composed of (Ni,Cr)Al2O4 and (Ni,Cr)O oxides. A protective α-Al2O3 oxide was on the surface of rhodium modified aluminide coating. Moreover, Al-depleted β-NiAl phase remained in the modified coating, whereas γ′-Ni3Al phase was observed in the nonmodified one.

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