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Article

Effects of Top Ceramic Layers with an Ultrathin Dense Layer on the Thermal–Physical Properties of Thermal Barrier Coatings

by
Li Ai
1,
Xueying Wang
2,
Ming Yang
3,*,
Yuntao Lei
1 and
Yongping Zhu
2,*
1
Beijing Power Machinery Research Institute, Beijing 100074, China
2
State Key Laboratory of Multi-Phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
3
School of Material Science and Engineering, Hubei Polytechnic University, Huangshi 435003, China
*
Authors to whom correspondence should be addressed.
Coatings 2023, 13(11), 1929; https://doi.org/10.3390/coatings13111929
Submission received: 11 October 2023 / Revised: 30 October 2023 / Accepted: 9 November 2023 / Published: 11 November 2023
(This article belongs to the Special Issue Recent Advances in the Materials, Preparations, and Properties of Thermal Barrier Coatings)
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Abstract

:
Thermal barrier coatings have been used to protect superalloys under extremely harsh conditions. The durability of TBCs with a NiCoCrAlY bond layer prepared via the air plasma spray process and an ultrathin dense layer prepared via magnetron sputtering was investigated under different corrosion conditions. This paper discusses the corrosion resistance improvement mechanism of TBCs with a dense layer produced by magnetron sputtering under corrosion conditions with environmental contaminants such as calcium–magnesium–aluminum–silicon oxide systems (CMAS) at 1300 °C and NaCl-Na2SO4-V2O5 (NV) at 900 °C. The corrosion results show that CMAS will react with the stabilizers of zirconium oxide, which will change the rate of the phase transition, as determined via X-ray diffraction tests. A thermal ablation test verified that TBCs with a dense layer have a better corrosion resistance and better thermal insulation properties. All results show that preparing TBCs with a dense layer via the magnetron sputtering method will be an efficient method to improve TBCs’ properties at high temperatures in the future.

1. Introduction

Thermal barrier coatings (TBCs) were developed as a kind of thermal protection technology for some ceramic materials, with a high temperature stability, a low thermal conductivity, a high thermal expansion coefficient and corrosion resistance, and were designed to bind with metal substrates to form a coating to prolong the working time of metal substrates [1,2,3,4,5,6,7]. TBCs are mainly made from a ceramic top layer, a NiCoCrAlY bond layer and a superalloy substrate [8]. The most popular ceramic top layer material is yttria-stabilized zirconia (YSZ), which cannot work for a long time above 1200 °C as it will fail due to a phase transformation and oxidation of the bonding layer [3,9]. When oxygen infiltrates the metal bonding layer through the ceramic layer, the metal layer can be oxidized to form a thermally grown oxide (TGO) layer [10,11]. Furthermore, NaVO3 and CaO + MgO + Al2O3 + SiO2 (CMAS) corrosion will also accelerate the failure of TBCs during operation [12,13]. When CMAS reacts with stabilizers, it can produce a tetragonal to monoclinic phase transformation, which may occur upon cooling, accompanied by a 3%–5% volume expansion and a loss of coating toughness [14]. The oxygen to oxidize the TGO layer mainly comes from two places: one is the oxygen contained in TBCs during the spray process, and the other is the oxygen from the environment, which will infiltrate the top ceramic layer and enter the bonding layer at high temperatures. The large influx of oxygen will accelerate the oxidation of the metal bonding layer and increase the rate of formation of the TGO layer. Due to the difference in the thermal expansion coefficients of the TGO layer and the ceramic layer, a concentrated thermal stress area will be generated between the ceramic and metal bonding layers [15,16]. As the regional stress increases, the ceramic layer and the metal bonding layer will peel off, ultimately causing the thermal barrier coating to fail.
To solve this problem, much research has been performed to prolong the working time of TBCs. Li [17] found that the chemical reaction between wet CMAS melts and YSZ is more vigorous on the YSZ (110) lattice surface than that on other interfaces from first principles calculations. Hamide [18] found that the hot corrosion mechanism involves molten salts reacting with the stabilizers of zirconia. Shen [19] studied the thermal shock life and failure behaviors of La2Zr2O7/YSZ, La2Ce2O7/YSZ and Gd2Zr2O7/YSZ double-layer ceramic TBCs, and found that Gd2Zr2O7/YSZ TBCs have the longest thermal shock life. Wang [20] found that the high-entropy rare earth aluminate (Y0.2Yb0.2Lu0.2Eu0.2Er0.2)3Al5O12 was stable at 1400 °C. Tounsi [21] employed an efficient and simple four-variable integral high-order shear deformation theory to model the analytical solution of the boundary condition. Katea [22] found that the obtained nano-particle TBCs might be of interest for high-temperature and harsh-environment applications. Doleker [23] studied TGO layers, finding that they are predominantly composed of an alumina layer at 1000 and 1100 °C; however, an increase in mixed formations was observed in oxidation at 1200 °C. These researchers mainly reduced the oxygen permeability of coatings by changing their chemical composition, reducing their porosity and preparing gradient coatings, thereby extending their service life.
To reduce the speed of oxygen reaching the metal bonding layer and solve the above problems to improve the performance of TBCs, an efficient method has been proposed as follows: We will prepare an ultra-thin zirconia layer on the surface of plasma-sprayed thermal barrier coatings via magnetron sputtering. The ultra-thin zirconia layer can not only prevent oxygen permeating the top ceramic, but can also reduce the direct contact between both corrosive environmental contaminants and stabilizers of zirconia. Plasma-sprayed coatings contain many small pores, and these pores may form continuous free channels in the coatings. If the ultra-thin zirconia layer can seal these pores, the surface thin layer will be a good barrier to oxygen and molten salts.
The partially yttrium-oxide-stabilized zirconia powder and coating have been previously prepared. 9.5Y2O3-5.6Yb2O3-5.2Gd2O3 doped in ZrO2 (YYG) was thought to have promising applications as a ceramic thermal barrier coating material in high-temperature environments due to its low thermal conductivity and high-temperature phase stability [24,25]. Its disadvantage is its poor thermal shock resistance. If the transmission rate of oxygen can be reduced, thereby reducing the oxidation rate of the bonding layer, this will greatly improve the service life and operation temperature of thermal barrier coatings and accelerate the development of aviation engines. This study mainly focuses on the effects of a thin layer of zirconia on the thermal properties of coatings, which hinders direct contacts between corrosive substances and the coating, reduces the oxygen transmission rate and improves the service life of high-temperature components in aviation engines and gas turbines.

2. Experimental Procedures

Powder prepared by the solid reaction method was used to prepare coatings, and then the thermal properties were tested.

2.1. Specimen Preparation

Powders were prepared by the solid reaction method. Firstly, oxides of Y2O3 (Aladdin, >99.9%), Yb2O3 (Aladdin, >99.9%), Gd2O3 (Aladdin, >99.9%) and ZrO2 (Aladdin, >99.9%) were weighed according to the mass ratio of 9.5Y2O3-5.6Yb2O3-5.2Gd2O3-79.7ZrO2. All the oxides were mixed in absolute ethyl alcohol (Shandong Qilin Chemical Co., Ltd., Zibo, China, >99%) via high-energy ball milling at a speed of 400 r/min for 8 h, and then the mixture was dried in a drying box at 120 °C. Then, the mixture was placed in a high-temperature furnace and kept at 1500 °C for 8 h. Finally, the powder was used to prepare TBCs using an air plasma spray process, and the plasma spray parameters used are listed in Table 1. The corrosion testing process was consistent with the literature [26,27]. The YSZ target (AECC Beijing Institute of Aeronautical Materials, Beijing, China, 99.99%) material was used for magnetron sputtering, and the power was 200 W.

2.2. Structure Characterization and Property Measurement

Densities of the bulk samples were measured by Archimedes’ method. XRD (PANalytical, Almelo, The Netherlands) using Cu Ka radiation (k = 1.54 Å) at 5 kV and 10 mA was performed to determine the phase compositions of synthesized samples over the range of 10–90°. General Structure Analysis System X’Pert High Score Plus 3.0 software was used to analyze the XRD patterns. Morphologies and chemical compositions were analyzed using a Cold Field Emission Scanning Electron Microscope (SEM, Hitachi SU8020, Tokyo, Japan) equipped with an energy-dispersive spectroscopy (EDS) unit. The thermal shock resistance was tested using a Muffle furnace. The samples were prepared on the surface of a superalloy with dimensions of ϕ30 × 5 mm using the APS method. To examine the durability of the TBCs, a thermal cycling test was conducted at 1150 °C in a chamber furnace in an atmospheric environment, which consisted of 4 min of heating up to 1150 °C, 5 min resting at 1150 °C and air-cooling to room temperature via a deionized water-cooling process. The properties of thermal insulation and the oxidation product on coatings were measured using an air plasma spray system (APS-3000, Beijing, China) at 1500 °C, and the test details can be found in our previous work [28]. The actual thermal insulation ability was evaluated by the temperature difference between the substrate and ceramic.

3. Results and Discussion

3.1. XRD Analysis of Powders and Coatings

The YSZ and YYG XRD patterns of powders and coatings with and without an ultrathin dense layer are shown in Figure 1. The ceramic powders of YSZ and YYG have wide diffraction peaks, which might be attributed to the fine grains of the powders [29]. Some characteristic peaks of the tetragonal phase are present [30] at (101), (110), (112), etc., in the XRD patterns of the YYG powder. Figure 2 shows the X-ray diffraction patterns at high angle scans in the range of 2θ from 72.5° to 75.5°. It is obvious that the YYG coating with an ultrathin dense layer has characteristic peaks of both the YYG coating and YSZ coating at the same time, as shown in Figure 2(A,C). This proves that the dense layer is a tetragonal phase.
To further prove the phase constitution of coatings of YSZ and YYG, Figure 3 shows diagrams of the Raman shift of the coatings, respectively. The Raman signal from cubic zirconia is weaker than that from tetragonal zirconia; however, the sensitivity of the monoclinic phase in Raman spectroscopy is much higher than that in XRD [31,32]. The irreducible representation for optical and acoustical zone center phonons can be determined by a group theory analysis [33]. It can be observed that the Raman spectra of YSZ and YYG coatings contain the same peaks, which proves that they have the same phase structure. The YYG coating has broader Raman bands than those of the YSZ coating, especially around 615 cm−1.

3.2. Thermal Ablation Analysis

The thermal ablation experiment conditions are most similar to the real working conditions. Figure 4 shows typical surface photographs of the YYG coating with an ultrathin dense layer before ablation (a), after the first ablation (b) and after the second ablation (c) at 1500 °C for 1000 s. After the first thermal ablation, the coating surface color changed from white to gray, but the coating did not fail. It is possible that the bond strength between the dense layer and the top layer is not sufficient to withstand the normal working environment at 1500 °C. During the cooling process in the second thermal ablation experiment, the top layer of ceramic was partially removed, but the superalloy did not fail. The coating protected the superalloy substrate well. The YYG coating without a dense layer failed after the first thermal ablation. Thus, the dense layer can prolong the working time of TBCs. Figure 5 shows the temperature distribution of the YYG coating during the thermal ablation process. The surface temperature of the top ceramics is around 1500 °C, and the back temperature of the superalloy is around 970 °C. The total thickness of the ceramic layer is around 1 mm, and the heat insulation is around 530 °C/mm. The heating rate and cooling rate of the ceramic coating surface are higher than those of the back of the superalloy. This is mainly because the specific heat capacity of the superalloy (712 J/(kg·°C)) is much higher than that of the ceramic coating (490 J/(kg·°C)) [34,35]. This mismatch in specific heat capacity may be one of the factors which leads to the failure of TBCs [36]. The superalloy and the ceramics are under the same conditions; thus, there will be a big difference in heating and cooling rates.
Figure 6 shows the X-ray diffraction patterns of coatings after being kept at 1600 °C for 2 h and 8 h in a furnace. It is obvious that the relative peak strength of the ceramics decreases with an increase in the holding time at 1600 °C because the surface will be smoother and the coating will be denser after high-temperature treatment. This will decrease the scattering surface area. In addition, the heat transfer ability will be better at high temperatures. The relatively higher thermal conductivity will increase the failure rate of TBCs.

3.3. Corrosion Analysis

The working conditions of TBCs are extreme, and the salts and oxides in the air and fuel will react with the stabilizers of zirconium oxide. They will experience huge temperature differences in a short time. Different reactions will occur at different temperatures. Chemical reactions can increase the failure speed of TBCs. When the environmental temperature is around 900 °C, molten salts (V2O5, Na2SO4 and NaCl) in the air can influence the lifetime of TBCs [37]. Molten salts were spun on the surface of the top ceramics and then maintained at 900 °C for 4 h. The corresponding elemental mapping results for the YYG coating with an ultrathin dense layer after hot corrosion tests in V2O5-Na2SO4-NaCl molten salts at 900 °C for 4 h are shown in Figure 7. According to the elemental map of the intermixed zone, shown in Figure 7, Na, V, O and S elements were not uniformly distributed over the whole surface; S and O elements were distributed around V, which likely generates NaVO3. The EDS spectrum of the YYG coating with an ultrathin dense layer (as shown in Figure 8) proved that there were no coating elements (such as Y, Yb, Gd) on the surface. The chemical compositions of the surface micrographs of the YYG coating with an ultrathin dense layer after corrosion (as shown in Table 2) show that the surface components are Na2SO4 and NaVO3 [38]. This indicates that the surface of the ceramic layer of the thermal barrier coatings is completely covered in corrosive salts. No rare earth vanadate was found, indicating an enhanced corrosion resistance. The ultrathin dense layer can prevent molten salts from reacting with the coatings, which is an efficient method to prolong the working time of TBCs.
In the absence of any protection, the molten salt reacts with the stabilizers of zirconium oxide, resulting in the segregation failure of the coating [18,25,39]. According to previous low-temperature molten salt corrosion results, when a dense oxide film is prepared on the surface of the coating via the plasma spraying method, direct contact between the molten salt and the coating can be prevented, and the molten salt corrosion rate of the coating at about 900 °C can be slowed down. Thus, we studied whether the dense oxide film can slow down the CMAS corrosion of the coating at 1400 °C. Figure 9 shows the surface element distribution of the coatings after the CMAS attack at 1400 °C for 8 h determined by an EDS mapping test. The EDS result suggests (as shown in Figure 9 and Figure 10) that CMAS has not reacted with the coating stabilizers. From Figure 9 and Table 3, it is obvious that CMAS has reacted with ZrO2 at 1400 °C [40], and only Ca, Mg, Al, Si, O and Zr were found on the surface, but elements such as Y, Yb, Gd were not detected, indicating that CMAS may replace stabilizers and affect the service life of the coating. Figure 10 indicates that the surface of the ceramic layer of the thermal barrier coatings has been completely covered by CMAS. Oxide (CaO, MgO, Al2O3, SiO2)-stabilized zirconia may form, indicating an enhanced corrosion resistance. Thus, CMAS will increase the failure rate of TBCs when the working temperature is higher than 1400 °C.

3.4. Thermal Shock Analysis

To closely imitate the real conditions, in the thermal cycling test, the coating was first cooled in air at least 6 times, and then cooled in deionized water at room temperature until failure. The thermal cycling test results of the YYG coatings with different thicknesses are shown in Table 4. It shows that the thermal cycles increase with a decrease in thickness; the finale failure photos are shown in Figure 11. It is clear that the top of the ceramic coating with a thickness of 900 μm is completely removed from the superalloy after two air cooling cycles, but the others are only removed partly. The test results show that the coating with an ultrathin dense layer can work normally when the thickness is no less than 300 μm, which is similar to the findings in [41].

4. Conclusions

A YYG ceramic powder has been synthesized via the solid-state reaction method. Ceramic coatings with an ultrathin dense layer have been prepared via the air plasma spraying and magnetron sputtering methods. The insulation capacity of the thermal barrier coatings is nearly 530 °C/mm, which indicates that they can resist low-temperature molten salt (V2O5 + Na2SO4 + NaCl) corrosion and high-temperature CMAS corrosion and that they have a prolonged working time. The thermal shock resistance test shows that the coating with a sputtered ultrathin dense layer thickness of 300 μm performs twice as well as the coating without an ultrathin dense layer, demonstrating an improved thermal stability. The dense layer can slow down the oxidation rate of the metal bonding layer, extend the service life of the coating and greatly improve the workability of thermal barrier coatings. This can improve the service life of high-temperature components in aviation engines and gas turbines with different ceramic materials.

Author Contributions

Methodology, L.A., M.Y. and Y.Z.; Software, L.A., X.W. and Y.Z.; Validation, Y.L.; Investigation, L.A., M.Y. and Y.L.; Resources, Y.Z.; Data curation, X.W. and Y.L.; Writing—original draft, L.A. and X.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Talent Introduction Project of Hubei Polytechnic University (No:21xjz12R) and the Scientific Research Project of Hubei Provincial Education Department (No: Q20224502).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. X-ray diffraction patterns of the YSZ powder (A), YYG powder (B), YSZ coating (C), YYG coating (D), YSZ coating with an ultrathin dense layer (E) and YYG coating with an ultrathin dense layer (F).
Figure 1. X-ray diffraction patterns of the YSZ powder (A), YYG powder (B), YSZ coating (C), YYG coating (D), YSZ coating with an ultrathin dense layer (E) and YYG coating with an ultrathin dense layer (F).
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Figure 2. The X-ray diffraction patterns at high angle scans in the range of 2θ from 72.5° to 75.5° of YSZ powder (A), YSZ coating with ultrathin dense layer (B), YYG powder (C) and YYG coating with ultrathin dense layer (D).
Figure 2. The X-ray diffraction patterns at high angle scans in the range of 2θ from 72.5° to 75.5° of YSZ powder (A), YSZ coating with ultrathin dense layer (B), YYG powder (C) and YYG coating with ultrathin dense layer (D).
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Figure 3. Raman spectra of the YSZ coating (A) and YYG coating (B).
Figure 3. Raman spectra of the YSZ coating (A) and YYG coating (B).
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Figure 4. Typical surface photographs of the YYG coating with an ultrathin dense layer before ablation (a), after the first ablation (b) and after the second ablation (c) at 1500 °C for 1000 s.
Figure 4. Typical surface photographs of the YYG coating with an ultrathin dense layer before ablation (a), after the first ablation (b) and after the second ablation (c) at 1500 °C for 1000 s.
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Figure 5. Heat insulation performance of the YYG coating with an ultrathin dense layer ablated at 1500 °C for 1000 s.
Figure 5. Heat insulation performance of the YYG coating with an ultrathin dense layer ablated at 1500 °C for 1000 s.
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Figure 6. X-ray diffraction patterns of coatings after being kept at 1600 °C for 2 h (A is YSZ and B is YYG) and for 8 h (C is YSZ and D is YYG).
Figure 6. X-ray diffraction patterns of coatings after being kept at 1600 °C for 2 h (A is YSZ and B is YYG) and for 8 h (C is YSZ and D is YYG).
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Figure 7. The corresponding elemental mapping results for the YYG coating with an ultrathin dense layer after hot corrosion tests in a V2O5 + Na2SO4 + NaCl molten salt at 900 °C for 4 h, (a) is Na, (b) is S, (c) is O, (d) is V, (e) is the combination of spectra.
Figure 7. The corresponding elemental mapping results for the YYG coating with an ultrathin dense layer after hot corrosion tests in a V2O5 + Na2SO4 + NaCl molten salt at 900 °C for 4 h, (a) is Na, (b) is S, (c) is O, (d) is V, (e) is the combination of spectra.
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Figure 8. EDS spectrum of the YYG coating surface with an ultrathin dense layer after corrosion in a V2O5 + Na2SO4 + NaCl molten salt at 900 °C for 4 h.
Figure 8. EDS spectrum of the YYG coating surface with an ultrathin dense layer after corrosion in a V2O5 + Na2SO4 + NaCl molten salt at 900 °C for 4 h.
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Figure 9. The surface microstructures and EDS mapping of the YYG coating with an ultrathin dense layer after corrosion by CMAS at 1400 °C.
Figure 9. The surface microstructures and EDS mapping of the YYG coating with an ultrathin dense layer after corrosion by CMAS at 1400 °C.
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Figure 10. EDS spectrum of the YYG coating with an ultrathin dense layer surface after corrosion by CMAS at 1400 °C.
Figure 10. EDS spectrum of the YYG coating with an ultrathin dense layer surface after corrosion by CMAS at 1400 °C.
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Figure 11. Thermal shock pictures of different-thickness YYG coatings at 1150 °C for 5 min: 50 μm (a), 100 μm (b), 300 μm (c), 600 μm (d), 900 μm (e). The subscript 1 represents before ablation, and the subscript 2 represents after ablation failure.
Figure 11. Thermal shock pictures of different-thickness YYG coatings at 1150 °C for 5 min: 50 μm (a), 100 μm (b), 300 μm (c), 600 μm (d), 900 μm (e). The subscript 1 represents before ablation, and the subscript 2 represents after ablation failure.
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Table 1. Plasma spray parameters used for spraying ceramic layer powders and bond layer powders.
Table 1. Plasma spray parameters used for spraying ceramic layer powders and bond layer powders.
Plasma Spray ParametersBond LayerCeramic Layer
Argon flow (NLPM)4545
Hydrogen flow (NLPM)4.54.5
Amps (A)/volts (V)420/50620/62
Carrier gas flow (SCFH)53
Powder feed rate (g/min)2626
Cooling air pressure (bar)2.62.6
Spray distance (cm)1210.5
Gun speed (mm/s)500600
Table 2. Chemical compositions of the surface micrographs of the YYG coating surface with an ultrathin dense layer after corrosion in a V2O5 + Na2SO4 + NaCl molten salt at 900 °C for 4 h.
Table 2. Chemical compositions of the surface micrographs of the YYG coating surface with an ultrathin dense layer after corrosion in a V2O5 + Na2SO4 + NaCl molten salt at 900 °C for 4 h.
ElementsONaSV
Mass ratio wt%46.4529.0111.4413.11
Atomic percentage60.7526.47.465.38
Table 3. Chemical compositions of the surface micrographs of the YYG coating surface with an ultrathin dense layer after corrosion by CMAS at 1400 °C.
Table 3. Chemical compositions of the surface micrographs of the YYG coating surface with an ultrathin dense layer after corrosion by CMAS at 1400 °C.
ElementsOMgAlSiCaZr
Mass ratio wt%34.020.941.885.736.7350.7
Atomic percentage67.241.222.26.455.3117.58
Table 4. Thermal shock properties of the YYG coating with an ultrathin dense layer.
Table 4. Thermal shock properties of the YYG coating with an ultrathin dense layer.
50 μm100 μm300 μm600 μm900 μm
Air cooling cycles66662
Water cooling cycles6531-
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Ai, L.; Wang, X.; Yang, M.; Lei, Y.; Zhu, Y. Effects of Top Ceramic Layers with an Ultrathin Dense Layer on the Thermal–Physical Properties of Thermal Barrier Coatings. Coatings 2023, 13, 1929. https://doi.org/10.3390/coatings13111929

AMA Style

Ai L, Wang X, Yang M, Lei Y, Zhu Y. Effects of Top Ceramic Layers with an Ultrathin Dense Layer on the Thermal–Physical Properties of Thermal Barrier Coatings. Coatings. 2023; 13(11):1929. https://doi.org/10.3390/coatings13111929

Chicago/Turabian Style

Ai, Li, Xueying Wang, Ming Yang, Yuntao Lei, and Yongping Zhu. 2023. "Effects of Top Ceramic Layers with an Ultrathin Dense Layer on the Thermal–Physical Properties of Thermal Barrier Coatings" Coatings 13, no. 11: 1929. https://doi.org/10.3390/coatings13111929

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