Effect of Isothermal Oxidation on the Structural Properties of (Ni,Pt)Al Coatings Doped with Zr at 1150 °C

Abstract

The fabrication of a single-phase (Ni,Pt)Al coating, doped with zirconium, was achieved via a method that included the simultaneous electroplating of a Pt-Zr layer, followed by a process of gas-phase aluminizing. The Zr-doped (Ni,Pt)Al coating was then subjected to an evaluation of its isothermal oxidation resistance at a temperature of 1150 °C in static air compared with a conventional (Ni,Pt)Al coating. The findings indicated that the incorporation of zirconium into the (Ni,Pt)Al coating led to a marked escalation in the rate of oxidation and a worse-scale spallation resistance, which was totally opposite to the results obtained at 1100 °C. The harmful effect of Zr on the oxidation resistance of the coating is discussed in this paper.

1. Introduction

Thermal barrier coatings (TBCs) are mainly applied to the hot-end components of aero-engines, including the high-pressure turbine guide vane, turbine rotor blade, and combustion chamber. The application of TBCs on gas turbine engines can increase the turbine inlet temperature, thereby improving the engine efficiency. Meanwhile, it can significantly reduce the surface temperature of the hot-end components, thus improving the service life and reliability of the hot components. In addition, TBCs also play a role in reducing fuel consumption and improving the aerodynamic performance of the engine.

TBCs generally have three structural forms: double-layer, multi-layer, and gradient. The most widely used is the double-layer structure. The surface layer of double-layer TBCs is a ceramic top coat (TC), and the bottom layer is a metallic bond coat (BC) [1,2]. The ceramic layer mainly serves as a thermal insulator, but it also has anti-corrosion, erosion, and abrasion properties. The bond coat alleviates the thermal expansion mismatch between the substrate and the ceramic top coat. Furthermore, it offers the high-temperature oxidation and corrosion resistance of the substrate alloy. In practical service circumstances, a thermally grown oxide (TGO) layer is often formed at the interface between the bond coat and the top coat, which is usually composed of α-Al₂O₃. A thin, continuous and dense TGO layer can prevent further inward diffusion of oxygen and protect the substrate alloy [3,4].

Pt-modified NiAl coatings (β-(Ni,Pt)Al) present a promising bond coat for thermal barrier coatings (TBCs). Despite the benefits of Pt-modified NiAl coatings, they are susceptible to spalling due to surface undulation during cyclic oxidation, potentially causing the detachment of the ceramic top layer in TBC systems [5,6,7,8]. To mitigate this, the integration of reactive elements like zirconium (Zr), hafnium (Hf), and yttrium (Y) into NiAl-based alloys and coatings has been investigated [9,10,11,12]. These elements are supposed to bolster the adherence of the TGO layer and to decelerate the oxide scale’s progression over numerous thermal cycles. Although some researchers have attempted to interpret the influence of these reactive elements on the oxidation behavior of NiAl alloys and coatings, the precise mechanisms remain a contentious topic among researchers [13,14,15,16,17].

Zirconium, a prominent RE, has been recognized for its role in enhancing the oxidation and hot-corrosion resistance of certain alloys. Studies such as those by Hong et al. [13] have shown that Zr inclusion in a two-phase Pt-modified aluminide coating can lead to the formation of oxide pegs that improve resistance to spalling. The high chemical reactivity of Zr also enables it to sequester sulfur beneath the oxide scales, preventing its detrimental segregation [13]. The improved scale adhesion due to Zr is largely due to its influence on the oxide scale microstructure. Nevertheless, the body of work examining the integration of zirconium within platinum-modified aluminide coatings remains sparse. A recent investigation [18] detailed the process of infusing zirconium into a single-phase (Ni,Pt)Al coating, which involved a simple electroplating technique succeeded by a vacuum annealing stage and a gaseous aluminization process. This approach is more cost-effective and versatile for complex-shaped components compared to electron-beam physical vapor deposition [13]. Our previous findings [19,20,21] also indicated that the inclusion of Zr not only significantly improved the Type-I hot-corrosion resistance of the coating but also enhanced its isothermal and cyclic oxidation resistance at 1100 °C. Temperature significantly influences the efficacy of reactive element effects. To understand the role of Zr comprehensively in single-phase platinum-modified nickel aluminide coatings, this research extends the analysis to include a comparative study of the isothermal oxidation characteristics between zirconium-doped β-(Ni,Pt)Al and conventional (Ni,Pt)Al coatings at 1150 °C. The effect of zirconium on the kinetics of oxidation and the evolution of the microstructure of the single-phase (Ni,Pt)Al coating at elevated temperatures are discussed.

2. Materials and Methods

2.1. Materials and Coating Preparation

A second-generation nickel-based single-crystal superalloy, characterized by its composition of 7% chromium, 7.5% cobalt, 5% tungsten, 1.5% molybdenum, 6.5% tantalum, 6.2% aluminum, 3% rhenium, and trace carbon, with the remainder being nickel, was utilized as the substrate material. Cylindrical specimens, measuring 15 mm in diameter and 2 mm in thickness, were crafted from crystal bars aligned in the [001] crystallographic direction through the process of electrical discharge machining. These samples were then polished to achieve a 400-grit surface finish and subjected to an abrasive blasting process with 300-mesh alumina particles. Following this, the samples underwent degreasing in a boiling solution of 50 g/L sodium hydroxide for 10 min, followed by ultrasonic cleansing with a mixture of acetone and ethanol for 30 min each.

Figure 1 shows a flowchart of a Zr-doped single-phase Pt-modified aluminide coating. Given its inert chemical nature, Zr remains stable in most acids except hydrofluoric and nitrohydrochloric acids. A suspension was prepared with Zr particles ranging from 0.1 to 10 µm and an acidic platinum coating solution for the application of a Pt-Zr composite coating. A magnetic stirrer operated at 10–20 rpm was utilized to maintain the suspension of the Zr particles. Subsequently, the samples coated with the Pt-Zr composite were treated for homogenization at a temperature of 1343 K under a vacuum environment, with a pressure of less than 6 × 10⁻³ Pa, for a duration of sixty minutes. This process aimed to remove hydrogen and ensure an even dispersion of platinum across the surface. Subsequently, the aluminization procedure, which is characterized by its low-activity gas phase, took place in a vertical furnace filled with argon gas at a temperature of 1348 K, lasting for 4.5 h, as per the methodology outlined in [7]. The final product was a Zr-enriched single-phase (Ni,Pt)Al coating, with a thickness of about 50 µm, encompassing the interdiffusion zone adjacent to it. For comparative purposes, the normal (Ni,Pt)Al coating was also fabricated under identical deposition parameters and compositional standards.

2.2. Isothermal Oxidation Test

Isothermal oxidation test was conducted within a muffle furnace under atmospheric conditions (78% N₂, 21% O₂, 0.94% rare gasses, and 0.03% CO₂, vol%.) at a constant temperature of 1150 °C, extending over a period of 500 h. Prior to the tests, alumina crucibles were heated to 1150 °C and maintained at this temperature until their weight stabilized, ensuring that any mass fluctuations were negligible. The combined mass of the samples and the pre-heated crucibles was then precisely recorded after oxidation for 20 h, 50 h, or 100 h, taking into account any detached oxide particles. A high-precision electronic scale—the BP211D model (by Sartorius, Göttingen, Germany), known for its accuracy to 0.01 milligrams—was utilized to measure the average weight gain in the samples tested in triplicate at various intervals throughout the oxidation procedure. The oxidation rate was calculated based on the square of mass gain vs. oxidation time.

2.3. Characterization

X-ray diffraction (XRD) was performed utilizing a state-of-the-art X’Pert PRO instrument, equipped with a Cu Kα X-ray source operated at 40 kilovolts, supplied (by PANalytical, Almelo, The Netherlands), to ascertain the phase composition within the coated samples. For examining the cross-sectional microstructure, the samples were first plated with a layer of nickel using a chemical process, followed by encapsulation within resin. The cross-sectional microstructure of the samples was examined using a field-emission scanning electron microscope (FE-SEM), namely model Inspect F50 (from FEI Company, Hillsboro, OR, USA). This microscope was complemented with an energy-dispersive X-ray spectrometer (EDS), the X-Max model (from Oxford Instruments, Abingdon, UK).

3. Results and Discussion

3.1. Oxidation Kinetics

As previously documented, both Zr-doped and normal (Ni,Pt)Al coatings are made of an outer zone (OZ) comprising β-(Ni,Pt)Al and a substrate-adjacent interdiffusion zone (IDZ). The as-deposited microstructure of the pristine coatings has been detailed in our prior study [18].

Figure 2 illustrates the graphs of mass gain and squared mass gain for both coatings throughout the duration of the isothermal oxidation experiment. As depicted in Figure 2a, the doped (Ni,Pt)Al coating shows an increase in mass compared to the standard coating throughout the entire test period. It should be noted that the mass gain curve of the Zr-doped (Ni,Pt)Al coating increases sharply after oxidation for 300 h due to some reasons, while the weight gain for the normal (Ni, Pt)Al coating follows a roughly parabolic trend. The total mass gains for the Zr-doped (Ni,Pt)Al coating and the normal one were 2.0 mg/cm² and 0.5 mg/cm², respectively, after oxidation for 500 h. As depicted in Figure 2b, the square of the mass gain for both the normal and Zr-doped (Ni,Pt)Al coatings are closely aligned with the fitting curve. The oxidation rate constants (Kp) derived from the regression analysis of the data are presented in Figure 2b. Before 300 h, the Kp value for the Zr-doped (Ni,Pt)Al is calculated to be 2.91 × 10⁻³ mg²cm⁻⁴h⁻¹, while it raises sharply to 1.24 × 10⁻² mg² cm⁻⁴ h⁻¹ after oxidation for 300 h. For the normal one, the Kp value remains at 4.86 × 10⁻⁴ mg² cm⁻⁴ h⁻¹.

3.2. Microstructure and Phase Evolution

Figure 3 shows the surface morphologies, cross-sectional morphologies, and XRD patterns of the two coatings after oxidation for 300 h. The normal (Ni,Pt)Al displays an entirely crack- and spallation-free surface (Figure 3a), highlighting the benefits of Pt in bolstering oxide scale adhesion and crack resistance. For the Zr-doped (Ni,Pt)Al, the surface features reveal significant oxide scale detachment on the surface (Figure 3b), suggesting weak interfacial bonding with the underlying coating. Correspondingly, the cross-sectional morphologies of the two coatings are presented in Figure 3c and d. The normal (Ni,Pt)Al coating formed a uniform and intact oxide scale on the surface, while the oxide scale formed on the surface of the Zr-doped one appears to exhibit some cracks and spallation. Additionally, some internal oxidation happened in the Zr-doped (Ni,Pt)Al coating. A phase transformation from β to γ’ took place in the two coatings. Round and needle-shaped topologically close-packed (TCP) phase precipitation appeared in the two coatings. Based on XRD patterns, the oxide scale formed on the normal (Ni,Pt)Al coating sample is composed of α-Al₂O₃ exclusively, while that formed on the Zr-doped (Ni,Pt)Al coating consists of α-Al₂O₃ and NiAl₂O₄ spinel.

Figure 4 shows the surface morphologies, cross-sectional morphologies and XRD patterns of the two coatings after oxidation for 500 h. Both coatings exhibit more serious oxidation with longer exposure times. It can be observed from Figure 4a that the normal (Ni,Pt)Al exhibits an undulated surface with some cracks and spallation. For the Zr-doped (Ni,Pt)Al, the surface reveals numerous cracks and spallation (Figure 4b). Correspondingly, the cross-sectional morphologies of the two coatings are presented in Figure 4c and d. The normal (Ni,Pt)Al coating still formed a uniform and intact oxide scale on its surface, with some cracks located in a small area. In contrast, the oxide scale formed on the surface of the Zr-doped one appears to show some cracks and spallation. A large amount of internal oxidation happened in the Zr-doped (Ni,Pt)Al coating. The TCP phase precipitation disappeared in the IDZ of the Zr-doped (Ni,Pt)Al coating due to dissolution again in the γ phase. According to the XRD patterns, the oxide scale formed on the normal (Ni,Pt)Al coating sample is still composed of α-Al₂O₃ exclusively, while that formed on the Zr-doped (Ni,Pt)Al coating consists of α-Al₂O₃ and NiAl₂O₄ spinel.

In order to further analyze the internal oxidation products, the magnified cross-sectional morphology of the Zr-doped (Ni,Pt)Al coating sample after oxidation at 1150 °C for 500 h is presented in Figure 5. It can be seen that the internal oxidation products are distributed along the grain boundary of the coating. Based on the EDS analysis (

Table 1. EDS results for the tagged area in Figure 5 (at.%).

Area	O	Al	Ni	Cr	Co	Zr
1	56.3	27.9	7.9	0.6	0.7	6.6
2	64.6	32.5	0.5	/	/	2.4

), the white and bright precipitates are rich with Zr, implying that this sample mainly consists of zirconium oxide. At 1100 °C, the Zr-doped (Ni,Pt)Al coating exhibited superior isothermal oxidation resistance compared to the normal (Ni,Pt)Al; however, the exact opposite was observed at 1150 °C. The temperature has a great effect on the RE effect. The reason for this is that as the temperature rises, the solubility of the zirconium within the NiAl crystal structure is reduced [10], so the appropriate doped content of Zr in the (Ni,Pt)Al at 1100 °C is over-doped at 1150 °C. Zr atoms present a higher affinity with oxygen. Oxygen tends to diffuse inward through the grain boundary and combine with Zr to form an oxide product. These oxide products may become pathways for crack propagation and reduce the adhesion of the oxidation scale, thereby promoting the spalling of the oxide scale.

4. Conclusions

A Zr-doped (Ni,Pt)Al coating was fabricated via a combination of Pt-Zr composite electroplating and subsequent processes including vacuum annealing and gaseous aluminization. The isothermal oxidation performance of this Zr-doped coating and a Zr-free (Ni,Pt)Al coating were evaluated at 1150 °C, and the conclusions can be drawn as follows:

1. The Zr-doped (Ni,Pt)Al coating showed inferior resistance to oxidation compared with the conventional (Ni,Pt)Al coating at 1150 °C;

2. The oxidation rate of the Zr-doped (Ni,Pt)Al coating was faster than that of the normal (Ni,Pt)Al coating, with an oxidation weight gain that was four times higher and an oxidation kinetic constant Kp value that was an order of magnitude higher;

3. Internal oxide products consisting of ZrO₂ were formed along the grain boundaries within the coating due to over-doping at 1150 °C.