Role of Hafnium (Hf) in the Oxidation Behaviour of Nickel Aluminide (β-NiAl) in Thermal Barrier Coatings

UoM administered thesis: Phd

  • Authors:
  • Lin Qiu

Abstract

Failure associated with oxidation of the Ni-based metallic bond coat has been a major problem of thermal barrier coatings (TBCs). β-phase nickel aluminide (β-NiAl) is widely used as bond coat materials in thermal barrier coatings. The additions of reactive elements improve the oxidation performance of β-NiAl alloys and coatings, in which Hf is found to be the most effective dopants. The underlying mechanisms behind this benefit, however, are not well understood. In this thesis, the effect of Hf composition on the oxidation of β-NiAl alloys and coatings is studied, with focus on the oxide scale spallation, creep during oxidation and surface rumpling of β-NiAl coatings. The oxidation of β-NiAl alloy affected by Hf additions was studied by fabricating and thermal cyclic oxidizing bulk β-NiAl alloys with various Hf additions (0-0.9 at.%) at 1200 °C in laboratory air. The progressive increase of Hf additions in the alloy results in (1) significantly improved durability of the oxide scale when the Hf addition is less than 0.1 at.%, which is attributed to reduced oxide growth, improved adherence of alumina scale to the alloy due to formation of alumina inclusions and inhibited rumpling (2) decreased durability of the oxide when Hf additions increasing from 0.1 at.% to 0.9 at.%, because of increment of void formation at hafnium oxide/alumina interface. The creep of bond coat during oxidation promotes its rumpling, resulting in tensile stress at peaks of the undulations and further the failure of the thermal barrier coatings. The creep of the above β-NiAl materials with Hf additions during oxidation is studied by measuring the elongation of the thin alloy plates using digital image correlation (DIC) technique coupled with optical microscopy. Alloys were processed to thin plates and cyclic oxidized at 1200 °C in air. The results show that creep of β-NiAl alloys reduces with Hf additions, especially in the range of 0.02-0.05 at.%, which is probably attributed to solution strengthening of Hf in the alloy; also creep is inhibited by increasing the thickness of alloy plates, which is caused by the reduced constrain induced by the oxide scale during oxidation. The influence of Hf additions on the oxidation performance of the β-NiAl coatings with low Al composition (35 at.%) and high Al composition (50 at.%) is studied. β-NiAl coatings with various Hf additions (0, 0.02 and 0.15 at.%) were fabricated by pack cementation. The oxidation of the coatings was carried out in air at 1100 °C. The Hf addition results in (1) improved durability of the oxide scales with 0.02 at.% Hf addition, owing to the inhibited rumpling of the bond coat due to increased creep resistance of the bond coat (2) decreased durability of the oxide scale with 0.15 at.% Hf addition due to increasing voids in the oxide scale and increasing rumpling amplitude caused by promoted phase transformation of β-NiAl to γ’-Ni3Al. Al composition also plays an important role in the failure of coatings by affecting creep of the bond coat.

Details

Original languageEnglish
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Award date31 Dec 2019