High Temperature Oxidation Behavior of Nano-Alumina–Modified NiAl Coating

Abstract

In this article, the nickel aluminide coating prepared by the chemical vapor deposition method has adhered deliberately with nano-alumina film on the surface by the electrophoresis method. The analysis results of oxidation behavior in the air at 1,000°C reported that the nano-alumina particles adhering to the nickel aluminide coating surface appear to be effective in facilitating the θ-Al₂O₃ to α-Al₂O₃ phase transformation. The fast θ to α phase transformation helps reduce the oxidation rate of the nickel–aluminum coating and prevents the cracking and peeling in the oxide scale. The research addressed a novel method to improve the high-temperature oxidation resistance of nickel aluminide coatings.

Introduction

Nickel aluminide coatings, such as NiAl and Ni₂Al₃, are generally applied to the surfaces of engine turbine components to lengthen their service life (Goward, 1970; Goward and Boone, 1971; Leng et al., 2020). The common preparation methods of nickel aluminide coatings include slurry, powder embedding, physical vapor deposition, and chemical vapor deposition (CVD) (Nicholls, 2003; Bouchaud et al., 2013; Mollard et al., 2013; Montero et al., 2013). Compared with the other methods, the coating prepared by the CVD method has a stronger bonding force with the substrate and is denser, so its high-temperature oxidation resistance is better (Warnes and Punola, 1997; Yavorska et al., 2011; Goral et al., 2020).

The surface of the nickel aluminide coating will form protective thermally grown oxide (TGO)-Al₂O₃ at high temperatures (Swadba et al., 2011). Several types of Al₂O₃ have been found on alloys forming aluminides, such as γ-Al₂O₃, δ-Al₂O₃, θ-Al₂O₃, and α-Al₂O₃. α-Al₂O₃ is a good protective oxide scale because of its thermodynamic stability and slow growth rate (An et al., 2000). During the oxidation at a temperature above 1,100°C, the nickel aluminide coating quickly forms the thermodynamically stable α-Al₂O₃ scale, while the metastable alumina variants scales (normally γ and θ) are formed after a oxidation for a long time (Khana et al., 2019) at a temperature below 1,100°C, during which they always experience the phase transformation of metastable-to-stable alumina (Peng et al., 2003; Huang and Peng, 2016), such as θ-to-α. This phase transformation accompanies a volume shrinkage of about 10%, sometimes resulting in the generation of micro-cracks in the alumina scale (Peng et al., 2003; Veal et al., 2006; Wang and Zhou, 2012; Huang and Peng, 2016).

As a result, prompting the conversion of metastable-to-stable alumina is advantageous for improving the oxidation resistance of nickel aluminide coatings at high temperatures (Khana et al., 2019). Moreover there is a well-known fact that a rising temperature causes a faster phase transformation of the metastable-to-stable alumina, dopants or template effects have been found to change this process (Tolpygo and Clarke, 2000; Huang et al., 2019). For instance, Pint and his co-workers (Pint et al., 1997; Huang et al., 2019) found that Y₂O₃, La₂O₃, ZrO₂, and HfO₂ dispersions in β-NiAl tend to retard the θ-to-α phase transformation, whereas TiO₂ dispersion appears to promote this process. Lately, Peng and his co-workers (Xu et al., 2010; Huang and Peng, 2016; Wang et al., 2016; Khana et al., 2019) reported the promoted formation of an α-Al₂O₃ scale on nickel aluminide with surface Cr₂O₃ particles. On these bases, we have a concept that spreading oxide particles (e.g., Cr₂O₃) with the same crystallographic structure as α-Al₂O₃ to aluminide can promote the growth of an α-Al₂O₃ scale at a high temperature (Yin and Zhen, 2015). Valet et al. (Liu et al., 2007; Huang and Peng, 2016; Khana et al., 2019) found that the remaining fine α-Al₂O₃ grits, used for the polishing surface of the intermetallic sample, appeared to promote the nucleation of α-Al₂O₃.

In this article, nano-alumina particles were deposited on the surface of aluminide coatings by electrophoresis, and their effects on the phase transition and oxidation kinetics of the nickel aluminide coatings were investigated. It is aimed to better understand the phase transitions of the oxidative properties of nickel aluminides, and find a more effective way to improve the oxidation resistance of these nickel aluminide oxides by promoting the growth of α-Al₂O₃.

The Experiment

The pure nickel samples with dimensions of 10 mm × 10 mm × 2 mm were cut from an electrolyte pure nickel plate. After being abraded to a final 2,000 grit SiC paper, they were ultrasonically cleaned in acetone. By using the CVD method, the Ni₂Al₃ coating was developed on the pure nickel surface after aluminizing at 750°C × 200 torr for 3 h. The preparation process is as follows: high-purity Cl₂ was injected into the reaction chamber containing high-purity aluminum particles (purity: 99.99%, size: 3 mm) to generate gas-phase halide AlCl_x. AlCl_x was carried into the deposition chamber by Ar, and then was reduced to activity [Al] by H₂. The [Al] deposited on the pure nickel, and reacted with nickel to form a nickel aluminum coating. After aluminizing for 3 h, the samples were cooled to room temperature with the furnace. The main equations involved in the reaction are as follows:

The nano-Al₂O₃ film was made to adhere to the nickel aluminum coating surface by electrophoresis technology. High temperature oxidation experiments were performed using the SETSYS evolution thermo-gravimetric analyzer (for in situ recording oxidation curves) in the air at 1,000°C for 50 h. A nickel layer was coated on the oxide scale surface using electroplating in the acidic electroless nickel plating mixed solution (containing 25 g/L C₃H₆O₃, 6.25 g/L Na₂CO₃, 43 g/L NiSO₄·6H₂O, and 25 g/L NaH₂PO₂·H₂O) at 98°C for 5 h. Scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) were used to investigate the surface and cross-section morphologies, structure, and the phases of various nickel aluminide coatings.

Results and Discussion

The cross-sectional morphology (Figure 1A) shows that a single layer of nickel aluminide coating (∼30 µm thickness) was deposited on pure nickel after aluminizing. The coating was dense and well adherent to the pure Ni substrate. The Ni content and Al content of the coating were 39.3 at.% and 58.9 at.%, respectively. The XRD result (Figure 1C) characterized them as deposited coating as a δ-Ni₂Al₃ phase. Backscattered SEM images of the polished cross-section of a nano-Al₂O₃ film sample are shown in Figure 1B. The as-electrophoresis Al₂O₃ film was continuous with a thickness of around 3 µm. However, the Al₂O₃ film was partially detached from the coating due to grinding and polishing during the preparation process.

FIGURE 1

Figure 2 shows the dependence on the time of mass grain and the square of mass grain of naked and Al₂O₃ film-covered Ni₂Al₃ coatings oxidated at 1,000°C for 50 h. The mass grain of Al₂O₃ film-covered Ni₂Al₃ coating is less than that of the naked one as shown in Figure 2A. In the early stages of oxidation , the surface of the two types of coatings quickly formed an aluminum oxide scale, and the oxidation weight gain rate was high. Due to the presence of alumina, the contact surface between oxygen and the coating gradually decreased, and the alumina scale gradually stabilized, so the oxidation weight gain curve tended to be flat. As shown in Figure 2B, during the 50-h oxidation, the oxidation processes of both coatings can be divided into two stages: In the initial stage (<3 h), the parabolic oxidation rate constant (Kp^I) of the Al₂O₃ film–covered Ni₂Al₃ coating is 3.2 × 10^–13 g²/cm⁴s lower than that of the naked coating with parabolic oxidation rate constant Kp^I of ∼4.4 × 10^–13 g²/cm⁴s. In the steady stage (>3 h), the parabolic oxidation rate constant (Kp^Ⅱ) of the Al₂O₃ film–covered Ni₂Al₃ coating is 1.2 × 10^–13 g²/cm⁴s, also lower than that of the naked coating with parabolic oxidation rate constant Kp^Ⅱ of ∼2.4 × 10^–13 g²/cm⁴ s.

FIGURE 2

The Al₂O₃ film-covered coating macroscopic surface after oxidation is shown in Figure 3A, and then the electrophoretic alumina film was removed using alcohol, as shown in Figure 3B. The surface characteristics of the naked Ni₂Al₃ coating changes with the increasing oxidation duration , as illustrated in Figures 4A,C,E,G. Fine alumina crystals developed on the surface at the early stage of high-temperature oxidation (30 min), and the color depth difference was caused by the distinct growth directions. The size of thin needle-like alumina crystals grew bigger after 2 h oxidation, and the oxide scale got thicker and denser. After being oxidized for 5 h, some acicular alumina became stagnant, cracks appeared on the surface, and the acicular alumina crystals were rapidly regenerated in the spalling area. After 50 h of oxidation, the oxide scale is mostly composed of acicular alumina and round alumina. According to the above morphologies and the characteristics of the alumina phase structure (Swadba et al., 2011), it can be judged that θ-Al₂O₃ is rapidly formed in the early stage of oxidation, and then the θ phase is partially converted into the α phase. The growth of α-Al₂O₃ will inhibit the growth of the θ phase. The lateral growth direction of α-Al₂O₃ resulted in surface unevenness. The conversion of θ phase to α phase will cause the microcracks or even local spalling because of volume shrinkage (the transformation from metastable alumina to α-Al₂O₃ is a reconstructive phase transition, which is a first-order phase transition and the process is irreversible). The spalling area will quickly generate θ-Al₂O₃, so both θ-Al₂O₃ and α-Al₂O₃ phases can be seen on the surface after high-temperature oxidation for 50 h. For the Al₂O₃ film–covered Ni₂Al₃ coating, an α-Al₂O₃ scale was formed rapidly in the early stage of oxidation. With the prolongation of oxidation time, the oxide scale gradually thickens and becomes flat (Figures 4B,D,F,H). After 5 h of oxidation, the scale appeared to slightly peel. After 50 h of oxidation, a dense and stable alumina scale has been formed. Therefore, there is no phase change in the oxide scale of the Al₂O₃ film–covered coating during the whole oxidation process. It can be seen in Figure 5A that the coating degradation completely conforms to the mechanism of the nickel–aluminum coating: δ-Ni₂Al₃→β-NiAl→γ′-Ni₃Al. At the beginning of oxidation, the generated alumina is too thin and its X-ray diffraction peak intensity is weak, so the original diffraction peak pattern is partially enlarged as shown in Figures 5B′,C′, from which it is known that the α phase is formed on the surface of the electrophoretic alumina film sample from the beginning of oxidation. Based on the XRD, SEM, and TGA analysis, the early oxidation progress of aluminide in the absence and presence of the surface Al₂O₃ film is shown in Figure 6. At the beginning of oxidation (Stage Ⅰ), θ-Al₂O₃ is rapidly formed on the naked aluminide surface, while α-Al₂O₃ is formed on the surface of the aluminide coating with electrophoretic Al₂O₃ particles (An et al., 2000). Then both increase to form an alumina scale (Stage Ⅱ); the alumina is much denser as shown in Figures 4C,D. In stage Ⅲ, generated α-Al₂O₃ grows inward and outward at the same time, during which the most electrophoresed Al₂O₃ film on the surface is pushed outward, and a small part is wrapped by the outwardly growing α-Al₂O₃ (Huang and Peng, 2016). At this time, the exposed θ-Al₂O₃ of the naked sample transforms into α-Al₂O₃ (Figure 4E). With α-Al₂O₃ gradually increasing, an α-Al₂O₃ scale was formed at the interface between NiAl coating and θ-Al₂O₃ (Stage Ⅳ). During this phase transition, volume shrinkage will occur, resulting in cracks or even peeling of the scale (Tolpygo and Clarke, 2000), which are visible on the surface topography (Figure 4).

FIGURE 3

FIGURE 4

FIGURE 5

FIGURE 6

The nickel aluminide thermally grows θ-Al₂O₃ before α-Al₂O₃ because the former has lower surface energies and thus a much lower nucleation barrier ΔG*. This is the direct origin of why the metastable alumina phases rather than the most stable ones usually form during the initial oxidation of β-NiAl (Huang et al., 2019). Therefore, during the subsequent oxidation, the undesirable metastable-to-stable phase transformation of alumina becomes unavoidable. The direct α-Al₂O₃ formation without metastable-to-stable alumina phase transformation on typical oxide particles (e.g., α-Al₂O₃, Cr₂O₃, and Fe₂O₃) means that the template effect of the oxides isostructural to α-Al₂O₃ intrinsically reduces ΔG* of α-Al₂O₃ down to such a level in favor of its thermal nucleation (Pint et al., 1997; Wang and Zhou, 2012; Yin and Zhen, 2015; Huang et al., 2019). The template effect has been widely studied by revealing some oxides (e.g., Cr₂O₃ and Fe₂O₃), which are isostructural to α-Al₂O₃, and may serve as the nucleation sites facilitating the growth of the α-Al₂O₃ (Pint et al., 1997; Wang and Zhou, 2012; Yin and Zhen, 2015; Huang et al., 2019). This interpretation coincides well with a promoted θ-to-α alumina phase transformation on β-NiAl when it has been embedded with the fine particles of α-Al₂O₃ (Veal et al., 2006). The electrophoretic α-Al₂O₃ particles have the same HCP crystal structure as the α-Al₂O₃ grown from the coating and can act as an epitaxial growth platform for α-Al₂O₃ during oxidation, reducing the nucleation energy of the latter. Therefore, when the surface is completely covered with alumina nanoparticles, the θ-to-α phase transition can be skipped when the coating is oxidized at 1,000°C, and α-Al₂O₃ is directly generated.

The Al₂O₃ particle pretreatment significantly decreases the oxidation rate, leading to a slower degradation rate of the nickel aluminide. As seen in the cross-sectional characteristics in Figure 7, after 50 h of oxidation, the phase structures of both types of coatings are NiAl + Ni₃Al phases, indicating that the unstable δ-Ni₂Al₃ phase degenerates into the NiAl phase at a high temperature of 1,000 °C, and then unceasingly degenerates into the Ni₃Al phase with oxidation time prolongation. In Figure 7, the thickness of the naked coating and the Al₂O₃ film–covered coating after oxidation is thicker than that before oxidation, indicating that both coatings have a certain degree of Al elements internal-diffusion during the high-temperature oxidation process. It can be seen from Figures 7A,B that the alumina scale thickness of the naked coating is about 7.9 µm, which is twice that of the Al₂O₃-scale–covered coating (∼3.5 µm). This is consistent with the analysis result of the oxidation growth curve. During the oxidation process, aluminum ions moving from the inside of the coating to the coating surface meet with oxygen atoms to form an aluminum oxide scale, so there will be a γ-Ni₃Al phase formed due to the Al rapid consumption between the aluminum oxide film and the nickel–aluminum coating. Since the naked coating needs to consume more Al elements to form the aluminum oxide scale, the thickness of the γ-Ni₃Al phase coating is significantly thicker than that of the Al₂O₃ film–covered coating, which is near the aluminum oxide scale. The reason for the difference in the thickness of the two coatings is that electrophoretic alumina can promote the formation of α-Al₂O₃ on the coating surface, which can effectively prevent the oxygen element from entering the coating, reducing the consumption of the Ni₂Al₃ coating. Therefore, promoting the formation of stable-phase α-Al₂O₃ is an effective way to improve the high-temperature oxidation resistance and prolong the life of the coating. The large difference in coating consumption in the same sample is due to the consumption of aluminum elements again to form an aluminum oxide scale after the local scale peels off. With the prolongation of oxidation time, the cracks increase and cause local peeling of the aluminum oxide scale. The spalled area will quickly deplete the nickel–aluminum coating to form a new aluminum oxide scale.

FIGURE 7

Conclusion

The Ni₂Al₃ coating with a thickness of 35 μm was prepared on the pure Ni matrix using a chemical vapor deposition method at 750°C × 200 T for 3 h, and then the Al₂O₃ film was made to adhere to the Ni₂Al₃ coating surface by electrophoresis technology. Because of the lower power required to form the critical nuclei, the θ-Al₂O₃ formed earlier on the naked Ni₂Al₃ coating and then undergo a θ-to-α Al₂O₃ transition process during the oxidation process at 1,000 °C. The electrophoretic Al₂O₃ film adhering to Ni₂Al₃ coating can skip this phase transition because the Al₂O₃ film on the coating surface can provide a crystal template to promote the nucleation of α-Al₂O₃ at the beginning of oxidation. The rapid nucleation of α-Al₂O₃ significantly reduces the oxidation rate of NiAl coating and prevents microcracks and voids caused by phase transformation.

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