Oxidation Behavior and Oxide Transformation of a Pt-Modified Aluminide Coating at Moderate High Temperature

Abstract

The oxidation performance of a single-phase Pt-modified aluminide coating was assessed in oxidation test at 980 °C in comparison with the single crystal superalloy. The results suggested that the Pt-modified aluminide coating exhibited superior oxidation resistance. During oxidation, the oxide scale formed on bare alloy changed constantly followed by the constitution of the multi-layer scale structure: An outer scale mainly consisted of Cr₂O₃ + NiCr₂O₄ + TiO₂ with scarce protection, and an internal scale mainly consisted of Al₂O₃. The thickness of the outer oxide scale increased with time, where the scale became looser and more porous. Meanwhile, the internal scale was discontinuous. Oxygen and nitrogen inwardly diffused into substrate, forming Ta₂O₅ and TiN particles. In contrast to the complex constitution of oxide scale, a uniform and continuous Al₂O₃ scale formed on Pt-modified aluminide-coated samples after oxidation at 980 °C for 1500 h, which showed no spallation and cracking. Interestingly, θ-Al₂O₃ and α-Al₂O₃ phases remained after such a long oxidation time. It is the relatively lower temperature and the presence of Pt retarded θ-α transformation. The degradation rate from β-NiAl to γ′-Ni₃Al was very slow in the coating. The various development of oxide scale on the coating and substrate was individually studied.

1. Introduction

Nickel-base single crystal (SX) superalloy series have been widely used as structural material in the hot-section components of aero, gas turbines, small type turbines and turboshaft engines [1,2], because of its satisfactory mechanical property (e.g., creep-resistance and rupture life) at high temperature. Albeit holding excellent mechanical property, the oxidation rate of superalloy increases significantly once the temperature exceeds 900 °C. As serving in harsh oxygen-rich environment, the high-temperature oxidation performance plays a critical role in determining the service life of SX superalloys, which depends on the formation of a dense and protective oxide scale (mainly refers to Al₂O₃ or Cr₂O₃) on the surface. A continuously complete oxide scale can effectively inhibit continued ingress of oxygen to the substrate alloy [3,4,5]. Upon exposure, if the critical Al/or Cr concentration required to maintain a protective oxide scale is not retained by the alloy, the alloy will quickly be failed [6]. Because the SX superalloys contain limited Al (<7 wt.%), it is essentially necessary to apply a high temperature protective coating on the surface of the alloys.

Pt-modified aluminide coatings have been proved to be satisfactory because of its superiority of resisting both high temperature oxidation and hot corrosion for protecting SX superalloys [7,8]. Amazingly, the refurbished Pt-modified aluminide coating also exhibited superior isothermal and cyclic oxidation resistance at 1100 °C [9]. Several studies [10,11,12,13] have shown that the presence of Pt can promote uphill diffusion and selective oxidation of Al, improve the stability of alumina and decrease the oxidation rate. Pt can be acting as a ‘cleanser’ at the oxide scale/coating interface by restraining the formation of voids or quickly filling in the voids, which enhances adhesion of alumina scale and lowers the tendency of scale spallation. A conventional preparation process of a Pt-modified aluminide coating consists of a pre-deposition of a Pt layer and a successive aluminization using either pack cementation, slurry, composite electrodeposition techniques, ‘above-pack’ or chemical vapor deposition (CVD) methods. The high-temperature-low-activity (HTLA) above-pack treatment can produce the exclusive β-(Ni,Pt)Al phase as the CVD technique does.

In this paper, a single-phase Pt-modified aluminide coating was prepared on a first-generation SX superalloy by Pt-electrodeposition and subsequent aluminization using the ‘above-pack’ treatment. As the servicing temperature of the electric gas turbine or small type turbine engine and turboshaft engine is generally 950–1000 °C, we investigate the isothermal oxidation performance of the Pt-modified aluminide coatings at 980 °C in static air, in comparison with the bare SX superalloy. The purpose of carrying out this study is to observe microstructural evolution of oxide scale during long-term oxidation and to discuss the various oxidation behavior of the coating and the SX substrate.

2. Materials and Methods

A Ni-base single crystal superalloy with nominal composition of Ni-12Cr-9Co-3.7W-1.9Mo-5Ta-3.6Al-4Ti (wt. %) was used as the substrate. Cylindrical samples with dimensions of Φ17 × 2 mm were cut from the SX superalloy rod using a wire cutting machine (EDM, DK7720, Founder CNC Machine Tool Co., Jiangsu, China). The samples were ground up to 400 mesh SiC sandpaper, and then sand-blasted with 300 mesh alumina grits at 0.3 MPa. Before Pt electroplating, the samples were degreased by two-step processing: (1) Boiling in NaOH solution (10 g/L) for 10 min; (2) ultrasonic cleaning in acetone and ethanol for 20 mi, respectively. A basic Pt-plating solution was utilized to deposit the Pt layer with a thickness of ~5 μm. The details of electroplating parameters can be referred to our recent publication [14]. After that, the samples with Pt layer were annealed in a vacuum tube furnace at 1 × 10⁻³ Pa at 1050 °C for 1 h. Eventually, an aluminizing treatment was performed using the above-pack manner at 1060 °C for 5 h to obtain the single-phase (Ni,Pt)Al coating. The heating rate was about 10 °C/min. The aluminizing powder mixture was composed of FeAl powder (96 wt. %) and NH₄Cl (4 wt. %) activator. The surface and cross-sectional morphologies of the single-phase (Ni,Pt)Al coating prepared on SX substrates are shown in Figure 1. A typical morphology of the outwardly-grown coating that consists of (Ni,Pt)Al outer zone and underlying inter-diffusion zone (IDZ) is observed. Pt is present in the solid solution in β-NiAl, as the crystal lattice of NiAl has been enlarged to a greater extent due to the effect of Pt [1,11,15].

Isothermal oxidation test of the SX alloy samples with and without Pt-modified aluminide coating was performed in a muffle furnace at 980 °C for 1500 h in static air. The samples were taken out from the furnace after 300, 500, 1000 and 1500 h, respectively. X-ray diffraction (XRD, X’Pert, Cu Kα, PANalytical Co., Netherlands) was used to analyze the phase constitution of samples with and without (Ni,Pt)Al coating after different oxidation times. The microstructure and chemical composition of the samples was analyzed using a scanning electron microscope (SEM, Inspect F50, FEI Co., Hillsboro, Oregon) equipped with X-ray Energy Dispersive Spectroscopy (EDS, X-Max, Oxford Instruments Co., Oxford, UK). To protect oxidation products from possible spallation, the oxidized samples for cross-sectional observation were deposited a thin layer of electroless Ni-plating, then mounted in epoxy resin and polished to a mirror.

3. Results

3.1. Phase Composition

Figure 2 shows the XRD patterns of the substrate alloy after oxidation at 980 °C for 300 h, 500 h, 1000 h and 1500 h, respectively. The predominant phases on the surface after 300 h are Cr₂O₃, TiO₂ and NiCr₂O₄ with weak Al₂O₃. In addition, the γ/γ′ matrix phase can be detected. The XRD results after oxidation for 300, 500, 1000 and 1500 h are similarly identical. When the oxidation time was extended to 1500 h, the peaks of the γ/γ′ phase became weak, which is due to increase of oxide thickness (the depth of X-ray is limited).

For the (Ni,Pt)Al coating, the XRD results (Figure 3) reveal that the oxidation product during the isothermal oxidation at 980 °C consists of exclusive Al₂O₃ and the coating phase of β-(Ni,Pt)Al can be detectable throughout the entire oxidation test. No peak of the γ/γ′ phase can be identified, indicating that the (Ni,Pt)Al coating exhibited good oxidation resistance and capability of extending service life.

3.2. Surface Morphology

The surface morphologies of the SX superalloy substrate after isothermal oxidation at 980 °C for different times are shown in Figure 4. The average compositions of the marked regions are summarized in

Table 1. Average composition of the regions marked in Figure 4 (EDS, at.%).

	O	Al	Ti	Cr	Ni
1	66.64	5.58	4.87	15.25	7.57
2	58.76	19.62	4.11	8.17	9.34
3	64.23	10.58	10.00	14.47	0.72
4	56.29	25.19	1.88	5.81	10.83
5	69.06	6.02	10.62	12.44	1.86
6	58.59	20.02	6.40	8.63	6.36
7	68.21	0.65	2.76	20.20	8.18
8	59.49	28.57	0.70	3.78	7.46

. After oxidation for 300 h (Figure 4a), the thick inhomogeneous oxide is clear to observe. The EDS analysis of the cauliflower-like island (Label 1) in Figure 4a shows relatively high contents of Cr and Ti. The combination of XRD and EDS results reveals that NiCr₂O₄, TiO₂ and Al₂O₃ are present in this region. The flat area (label 2) mainly comprises Al₂O₃, in addition to minor TiO₂, NiCr₂O₄ and Cr₂O₃. It can be seen from the high magnification morphology that the oxide scale in the flat area with a high Al content is dense to provide a barrier for the diffusion of oxygen. After 500 h, the surface morphology is shown in Figure 4b. The composition of both the coarse region and the flat region is not different from that in 300 h. After isothermal exposure for 1000 h, the coarse region enlarges and contains large particles (Figure 4c), which suggests that Al-concentration decreases and accordingly the concentration of Cr and Ti increases (Table 1). When oxidation prolongs to 1500 h (as shown in Figure 4d), the coarse region almost covers the entire surface of the substrate alloy that is rich in Cr and Ni, conforming to the XRD analysis. The oxide product is primarily constituted of non-protective oxide mixture of Cr₂O₃, TiO₂ and NiCr₂O₄, which increases the possibility of internal oxidation.

Figure 5 illustrates the surface variation of the (Ni,Pt)Al coating during isothermal oxidation at 980 °C. After 300 h oxidation, whisker-shaped oxide grains are observed on the surface, presenting the characteristics of transient oxide θ-Al₂O₃ (Figure 5a) [16,17,18]. The shape of oxide changes into longer whisker-like oxide after 500 h oxidation at 980 °C, as shown in Figure 5b. Figure 5c shows surface images after exposure for 1000 h, in which alumina grows continuously and gets coarser. After 1500 h, the oxide scale is still completely intact (Figure 5d), but the morphology of Al₂O₃ transforms from whisker to blade. It is found that although whisker or blade morphology appears on the outer surface after exposure for 1500 h at 980 °C, the spallation of the thin oxide scale cannot be observed and the oxide scale maintains compact and uniform, which can effectively protect the substrate alloy from oxygen invasion.

3.3. Cross-Sectional Morphology

The cross-sectional morphologies of the SX superalloy oxidized at 980 °C for different times are illustrated in Figure 6.

After oxidation for 300 h, zone A and zone B are observed obviously (Figure 6a). Zone A reveals three distinct layers, as shown clearly at high magnification in the inserted image. The combination of EDS and XRD results suggests that the outer scale comprises a mixture of Cr₂O₃, TiO₂ and a few NiCr₂O₄. The dense internal layer is rich in Al, showing that the Al₂O₃ layer formed. Ti-rich precipitates are visible beneath the internal layer, according to the study of Pfennig et al. [19], it is TiN. The enlarged image of zone B is shown on the right of Figure 6a, which corresponds to the cauliflower-shape region shown in Figure 4a. Along the surface to the substrate, zone B is composed of a relatively thick outer scale (mainly Cr₂O₃ + NiCr₂O₄ + TiO₂), white Ta-rich oxide layer (mainly Ta₂O₅) [20] close to the outer scale, internal Al₂O₃ layer and short rod-like TiN. Ti and Ta exist in the oxide layer in a unique way. Compared with zone A, zone B with relatively thick oxide scale reflects a combination of internal oxidation and external oxidation. The uneven distribution of Al in the substrate alloy plays an important role in the formation of Al₂O₃, leading to Al₂O₃ layer with different thickness.

After 500 h, from the magnified image of zone C (right of Figure 6b), it can be seen that in oxide scale a few Ta₂O₅ formed in between the loose outer layer and the internal oxide layer, followed by an oxidation-free zone with emergence of some TiN precipitates. After 1000 h, the amount of oxidation products increase with oxidation time, as shown in Figure 6c. Zone D presents a four-layer structure: The outer and internal layers remain continuous, but have been thickened with obviously loose features. When the substrate alloy was exposed for 1500 h, zone E and zone F emerged out (Figure 6). Zone E with a triple-layer structure has no difference to zone A. For the four-layer structured zone F, it can be seen that a thick and porous outer layer forms accompanied with a Ta-rich oxide product, a continuous layer of Al₂O₃ has not yet been established, and the adjacent layer is enriched with TiN (right of Figure 6d).

Figure 7 shows the cross-sectional morphologies of the (Ni,Pt)Al coatings after oxidation at 980 °C for 300, 500, 1000 and 1500 h. After 300 h, as shown in Figure 7a, the continuous Al₂O₃ scale reveals a dual-layered structure, which is composed of a whisker-like top layer and dense internal layer, with a total thickness of ~2.1 μm. In the case of oxidation for 500 h, the oxide layer is still composed of Al₂O₃ and the total oxide scale thickness is about ~3.1 μm, as presented in Figure 7b. The average content of the coating is listed in

Table 2. Average contents of the marked regions of (Ni,Pt)Al coating after isothermal oxidation at 980 °C for different times, showing the main elements of Ni, Pt and Al (at.%).

Oxidation Time	Pt	Ni	Al
300 h	3.45	54.42	42.13
500 h	3.87	56.63	39.50
1000 h	3.11	58.62	38.27
1500 h	4.14	59.59	36.27

, where the content of Al is reduced by the sustaining consumption. Meanwhile, a few β phases in the outer zone of the coating have been transformed to γ′ phases. Figure 7c shows that the oxide scale grows to the thickness of ~4.2 μm after 1000 h. The fraction of γ′ phase gets increased slightly in the outer zone. As the oxidation time prolonged to 1500 h, there is neither a significant change of the morphology nor of the composition of the oxide scale. A continuous Al₂O₃ scale has formed with no cracking or void formation (Figure 7d) and the scale thickness increases to ~5 μm. The Al-rich β phase in the outer zone is gradually converted to the γ′ phase due to Al depletion induced by oxidation. However, the degradation rate from β to γ′ is slow, which is able to form a compact Al₂O₃ scale for future long term. It is confirmed that the (Ni,Pt)Al coating can efficiently protect the substrate from aggressive oxidation attack.

4. Discussion

4.1. Oxidation Behavior of SX Superalloy at 980 °C

When the oxygen partial pressure pO₂ on the metal surface is higher than the equilibrium value of pO₂, the metal oxidation reaction will occur:

M+ O₂ = MO₂

(1)

Following this reaction for M/MO₂ the equilibrium value of pO₂ is given by,

$$\mathrm{p}{\mathrm{O}}_2=\exp \int (\frac{\Delta {\mathrm{G}}_{\mathrm{MO}2}^{\mathsf{\theta }}}{\mathrm{RT}})$$

(2)

According to Wagner′s theory [21], the critical content N_B in the alloy for the formation of external oxide scale can be expressed as following:

$${\mathrm{N}}_{\mathrm{B}}> {\left[\frac{\mathsf{\pi }\mathrm{g}*}{2\mathrm{b}}·{\mathrm{N}}_{\mathrm{O}}\frac{{\mathrm{D}}_{\mathrm{O}}·{\mathrm{V}}_{\mathrm{m}}}{{\mathrm{D}}_{\mathrm{B}}·{\mathrm{V}}_{\mathrm{OX}}} \right]}^{1/2}$$

(3)

where π and g* (close to ~0.3) are constants, N_oD_o is the diffusion coefficient for oxygen in alloy, V_m is the molar volume of metal, D_B is the diffusivity of species B, and V_ox is the molar volume of oxides.

Combining surface and cross sectional morphologies together, the oxidation procedure of the SX superalloy at 980 °C can be illustrated, as in Figure 8. The inhomogeneity of the structure and composition for the substrate alloy promotes the production of the complex oxides during oxidation, which is responsible for the varied oxide microstructure at surface. At the initial stage of oxidation, the concentration of Cr and Ti in SX alloy is greater than their critical content N_B to generate a continuous outer layer dominated by Cr₂O₃ + NiCr₂O₄ + TiO₂. Nevertheless, the concentration of the Al content is below its critical value required for forming external A1₂O₃ scale. The pO₂ is reduced beneath the outer oxide, which enhances the selective growth of Al₂O₃ below the non-protective outer oxide mixture of Cr₂O₃ + NiCr₂O₄ and TiO₂, leading to the lateral growth and coalescence of the Al₂O₃ precipitates. After a period of oxidation, a continuous Al₂O₃ internal layer has already formed, as shown in Figure 8a.

As the oxidation time increases, the content of Al in local substrate is reduced by spallation of oxide scale, leading to incapability of supplying Al required to maintain a continuous internal Al₂O₃ scale. This allows the outward diffusion of Cr, Ti, Ni and Ta, and the inward diffusion of nitrogen and oxygen, which cause the phenomenon that the non-protective outer layer grows and Ta₂O₅ embeds between the outer layer and internal layer. Furthermore, the aggravation of internal oxidation degree and formation of TiN can be observed (Figure 8b). This process is thermodynamically feasible, because the Gibbs free energy of TiN is more negative than that of AlN, and also Al has been depleted underneath the outer layer in the alloy due to the internal oxidation. As a result, more thermodynamically stable TiN formed instead of AlN. With further oxidation, the thickness of porous non-protective oxide layer increases, and Ta₂O₅ dispersed and approached to the surface due to outward diffusion of Ta and the growth of outer layer. Moreover, the regions of internal oxidation and TiN both expanded, as shown in Figure 8c. In conclusion, the oxide product formed on the surface of bare SX superalloy cannot prevent the substrate from oxidation invasion.

4.2. Oxidation Behavior of (Ni,Pt)Al Coating at 980 °C

The results have demonstrated that better oxidation performance is observed on SX specimens deposited with (Ni,Pt)Al coating than the substrate alloy. The protection against high-temperature oxidation is provided by the (Ni,Pt)Al coating, which relies on the formation of a continuous Al₂O₃ scale on the surface. The Al-rich β-NiAl phase enhances the selective growth of thermodynamically stable Al₂O₃. Normally, the effective diffusion rates of oxygen and metal ions through Al₂O₃ scale are relatively low, which retards the growth rate of the oxide scale [22]. With addition of Pt, it is not prone to cracking and spalling of alumina scale. [1,2,23,24]. The oxide scale thickness as a function of oxidation time for the samples that coated with (Ni,Pt)Al are shown in Figure 9, where the kinetic curve basically follows a parabolic law. It can be seen from Figure 7 that the Al-rich β phase gradually transformed into the Al-poor γ′ phase due to Al diffusion and depletion [7]. Generally, Al in the (Ni,Pt)Al coating is consumed gradually over time, which acts as one of the reasons for the eventual failure of the coating [21]. However, during isothermal oxidation at 980 °C for 1500 h, the (Ni,Pt)Al coating has shown relatively slow degradation rate and sufficient Al concentration to ensure the compact and self-healing formation of exclusive Al₂O₃ scale.

Figure 7 shows the Al₂O₃ scale is dual-layered with a whisker-like top layer followed by a dense internal layer. It is reported [25] that, in stress-free condition, α-Al₂O₃ produces luminescence peaks located at 14,402 and 14,432 cm⁻¹, and θ-Al₂O₃ located at 14,575 and 14,645 cm⁻¹. Figure 10 shows the luminescence spectra of the Al₂O₃ scale formed on the (Ni,Pt)Al coating. Both θ-Al₂O₃ phase and α-Al₂O₃ phase co-existed from 300 h to 1500 h oxidation at 980 °C. As suggested by Young et al. [16], at 980 °C the oxide scale with existence of θ-Al₂O₃ is formed on the surface of NiAl coating. At the initial oxidation stage, whisker-like θ-Al₂O₃ grains grew outwardly from the original coating surface and covered the surface rapidly. However, α-Al₂O₃ grains only formed beneath the outer θ-Al₂O₃ grains and then grew into a dense and continuous layer [17,26], which is consistent with the structure of the surface scale formed on the NiAl coating, as shown in Figure 7. Generally, the growth of θ-Al₂O₃ is predominantly controlled by the outward diffusion of Al, in which the fast growth requires massive transport of Al. The θ-Al₂O₃ contains a high amount of defects, causing the outward diffusion of Al through the whiskers to the outer surface possible. At the same time, a small amount of θ-Al₂O₃ is transformed to α-Al₂O₃. However, the morphologies of α-Al₂O₃ still retained whisker- or blade-like oxides after this transformation [25,26]. Therefore, the outer layer probably also contains a certain amount of α-Al₂O₃ phase in addition to θ-Al₂O₃ phase.

During the initial oxidation period and at lower temperature (such as 800 °C), metastable alumina oxide series grow, such as γ-, δ- and θ-Al₂O₃ [27]. Brumm et al. [28] found that the transformation from θ-Al₂O₃ to α-Al₂O₃ could be observed in the temperature range 950–1050 °C. Normally, α-Al₂O₃ grows by both outward diffusion of aluminum and inward diffusion of oxygen, whereas θ-Al₂O₃ grows mainly by outward diffusion of aluminum reacting with oxygen at the upmost surface [29,30]. The θ-Al₂O₃ that transforms to α-Al₂O₃ varies significantly under different experimental conditions. Either addition of elements, oxidation temperature and even environment would play a role in influencing metastable θ-Al₂O₃ growth and transformation [31]. Cr in the β-NiAl coating can accelerate the transformation from θ-Al₂O₃ to α-Al₂O₃, because the initial formation of Cr₂O₃ increases nuclei sites for α-Al₂O₃ formation. The addition of larger ions of Y or Zr inhibits the outward transport of Al, which tends to delay θ-Al₂O₃ to α-Al₂O₃ transition. At higher temperature, the metastable alumina phases are just present for a short time, while they exist for a long-term at lower temperature. In the present study, the moderate-high temperature of 980 °C permits the existence of θ-Al₂O₃ till 1500 h (see Figure 10).

Numerous studies have reported that the addition of Pt can significantly increase the adhesion of the oxide scale, improve oxidation performance and prolong the service life of the alloy at elevated temperatures. Once oxidized at even higher temperature (over 1100 °C), whisker- or blade-like θ-Al₂O₃ were not observed for the Pt-modified aluminide coating due to the extremely short transformation time that cannot be perceived [28,29,30,31,32,33]. Chen et al. [32] reported that a combination process of nano-powder mixing and SPS approach was used to produce a Pt-modified β-NiAl coating, in which Pt may favor the θ-to-α transformation, but the main contributing factor should be given to the higher oxidation temperature of 1150 °C. Liu et al. [34] concluded that Pt addition in γ-Ni/γ′-Ni₃Al coatings can slow down the θ-Al₂O₃ to α-Al₂O₃ transformation and thus extend the metastable θ-Al₂O₃ duration. Wit [33] et al. found that the oxide scale on the PtNiAl sample is grown mainly by aluminum transport at 900 °C. The θ-Al₂O₃ with needle and blade structure formed on top of a compact layer, which still existed after 50 h at 1000 °C. The more Pt addition results in more θ-Al₂O₃ in the oxide scale [34].

The presence of a large amount of Pt in β-NiAl will change the oxidation mechanism. On one hand, Pt can hinder the diffusion of oxygen through oxide scale, whereas promote the diffusion of Al. On the other hand, high content of Al is unfavorable with growth of α-Al₂O₃. A metastable θ-Al₂O₃ grows mainly through the outward diffusion of Al and it contains a high amount of defects promoting Al diffusion through the oxide to the outer surface conveniently, which will lead to a continuous growth of θ-Al₂O₃. Furthermore, the moderate-high temperature retards θ-to-α transformation, resulting in long-term existence of metastable θ alumina particles. In addition, orientation plays part of a role in θ-Al₂O₃ to α-Al₂O₃ transformation as well [35]. The θ-Al₂O₃ was retained in the oxide scale after 1500 h at 980 °C, as shown in Figure 10, which showed an effect of delaying θ-to-α Al₂O₃ phase transformation. This seems to be disadvantageous to the scale spallation resistance. However, after long term oxidation, the oxide scale still keeps intact and dense (Figure 7). The addition of Pt can significantly improve spallation resistance of the oxide scale and, consequently, contribute to the superior protection for long-term oxidation. That is to say, the dual-layer structure of alumina scale applied no deterioration to the oxidation resistance at all, which derived from the ability of forming exclusively pure alumina scale.

5. Conclusions

The microstructural evolution and isothermal oxidation behavior of Pt-modified aluminide coatings were investigated at 980 °C in static air in comparison with bare SX superalloy. The following conclusions can be drawn from the above experimental results:

• Building of layered oxide on SX superalloy changed constantly. The multi-layered scale mainly consisted of a thick and porous outer layer, which was composed of the non-protective oxide mixture of Cr₂O₃ + NiCr₂O₄ + TiO₂, and an internal layer of Al₂O₃. As the oxidation time prolonged, the thickness of the loose outer layer increased and the internal alumina scale became discontinuous, which allowed the inward diffusion of nitrogen and oxygen to form Ta₂O₅ and TiN, resulting in the aggravation of internal oxidation.
• During oxidation, a continuous Al₂O₃ scale formed on the surface of the Pt-modified aluminide coating, which held a dual-layer structure with a whisker/blade-like outer layer and a dense internal layer. The long-term existence of θ-Al₂O₃ whiskers was attributed to the moderate-high temperatures and the addition of Pt. The Pt-modified aluminide coating confirmed its competence in service of a relatively long lifetime.
• In the oxidation process, the β-phase in the outer zone of the Pt-modified aluminide coating gradually converted to the γ′ phase due to aluminum depletion induced by oxidation.
• In short, the Pt-modified aluminide coating showed the best oxidation resistance by forming a continuous Al₂O₃ scale and possessing a slow degradation rate from β to γ′. It can be used for protecting the hot components of electric gas turbines, small type turbine engines and turboshaft engines.