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Xi'an University of Electronic Science and Technology: Composition Modification of Ni₃Al Thermal Barrier Coatings

September 26, 2025 Materials 27

γ'-Ni₃Al is one of the most promising bond coat materials for industrial application in thermal barrier coatings (TBCs)

γ'-Ni₃Al is one of the most promising bond coat materials for industrial application in thermal barrier coatings (TBCs), with key improvement targets being plasticity, oxidation resistance, and adhesion. Researchers from Xi'an University of Electronic Science and Technology, China State Shipbuilding Corporation (CSSC), and Shanghai Jiao Tong University have achieved performance enhancements through composition modification.

What are the main materials used in thermal barrier coatings?
Thermal barrier coatings (TBCs) are widely used to lower the surface temperature of turbine blades, thereby enhancing their performance in high-temperature environments such as aircraft engines and gas turbines. A typical TBC system consists of an outer ceramic top coat (TC) and an inner metallic bond coat (BC).

Commonly used TC layer material:

Yttria-stabilized zirconia (YSZ)

Commonly used BC layer materials:
Bond coat materials, which connect the ceramic layer to the metal substrate, mainly fall into two categories:

MCrAlY (where M = Ni and/or Co)

Ni₃Al (Note: The standard notation is Ni₃Al. The subscript is omitted in the original text for ease of reading.)

In γ'-Ni₃Al, the aluminum content is lower than in traditional γ+β-phase or β-phase bond coats. As the oxidation process continuously consumes aluminum, once the aluminum activity drops below the critical level required to form a stable Al₂O₃ scale, nickel begins to participate in the oxidation, forming brittle spinel phases (such as NiO and NiAl₂O₄). These phases lack excellent oxidation resistance, leading not only to rapid thickening of the thermally grown oxide (TGO) layer—significantly shortening the service life of the TBC—but also reducing the adhesion of the outer ceramic layer, ultimately causing spallation failure.

In this study, Ni₃Al alloy was modified by the addition of a trace amount of Fe. Samples were fabricated via spark plasma sintering (SPS) using elemental powders of nickel (20 μm), aluminum (45 μm), and iron (5 μm) as raw materials.

SEM Analysis
SEM images indicate that both the undoped Ni₃Al sample and the sample doped with 1 wt% Fe exhibit a single-phase structure. This suggests that no secondary phases formed during the ball milling and sintering processes. The atomic percentage of Ni to Al remained strictly at a 3:1 ratio.

EBSD Analysis
EBSD results reveal that the addition of Fe effectively inhibits grain growth or promotes nucleation. As the Fe content increases from 0 to 0.1 wt%, the twin frequency rises significantly. Grain refinement leads to a reduction in the number of anti-phase domains (APDs). Since a single grain contains only one APD, this structural change results in a notable improvement in the alloy's plasticity. Twin formation enables sustained deformation, preventing brittle fracture, while dislocations can also promote twinning, creating a positive feedback loop that enhances plasticity.

TEM Analysis
TEM images show that dislocations are present in both Fe-doped and pure Ni₃Al samples, whereas stacking faults are observed only in the doped samples. This indicates that Fe doping may reduce the stacking fault energy (SFE) of Ni₃Al. The introduction of Fe causes slight lattice distortion in Ni₃Al, altering the quantitative interatomic interactions and bonding energies within the crystal structure. This reduces the lattice stability of Ni₃Al, thereby decreasing the energy required for the formation of stacking faults.

The presence of stacking faults promotes cross-slip during dislocation movement, consequently enhancing the material's plasticity.

Hardness of Ni₃Al Alloy
The Vickers hardness of the samples decreases from 302 HV to 220 HV as the iron content increases.

Oxidation Resistance of Ni₃Al Alloy
After 10 hours of isothermal oxidation in air at 1000°C, the sample doped with 0.5 wt% Fe exhibited the smallest spallation area of the oxide scale. This indicates that an appropriate level of Fe doping can improve the adhesion of the oxide scale to the Ni₃Al substrate.

As the Fe doping level increases, the intensity ratio of the Al₂O₃ peak to the NiO peak gradually rises. This ratio reaches its maximum in the Ni₃Al sample containing 1 wt% Fe, suggesting that Fe doping suppresses the growth of nickel oxides during isothermal oxidation.