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Article

CeO2 Protective Material against CMAS Attack for Thermal–Environmental Barrier Coating Applications

by
Lei Guo
1,2,*,
Yuanpeng Wang
1,
Mingguang Liu
3,
Yuan Gao
1 and
Fuxing Ye
1,2,*
1
School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China
2
Tianjin Key Laboratory of Advanced Joining Technology, Tianjin University, Tianjin 300072, China
3
Naval Logistics Academy, Tianjin 300450, China
*
Authors to whom correspondence should be addressed.
Coatings 2023, 13(6), 1119; https://doi.org/10.3390/coatings13061119
Submission received: 7 May 2023 / Revised: 3 June 2023 / Accepted: 12 June 2023 / Published: 18 June 2023
(This article belongs to the Special Issue Advances in Thermal Barrier Coatings (TBCs): Materials, Fabrication, Corrosion and Applications)
Browse Figures
Versions Notes

Abstract

:
Calcium–magnesium–alumina–silicate (CMAS) attack is a crucial issue for thermal–environmental barrier coatings (T/EBCs) with the ever-increasing operating temperature of turbine engines. In this study, CeO2 has been demonstrated as a promising protective material for T/EBCs against CMAS attack. At 1300 °C, CeO2 powder kept excellent phase and structural stability in molten CMAS; there were some CMAS constituents dissolved into the CeO2 lattice to form a solid solution. With higher CeO2 contents and longer duration time, more CeO2 solid solution particles were formed, which acted as the nucleating agent for CMAS crystallization. As a result, apatite, anorthite and wollastonite crystalline products were easily generated. At 1300 °C for 10 h, CeO2 pellets covered with CMAS powder had limited degradation, which was attributed to the rapid crystallization of molten CMAS due to the excellent nucleating agent effect of the precipitated CeO2 solid solution.

1. Introduction

Nickel-based superalloys suffer from extremely high temperatures due to the increasing inlet temperatures of turbine engines. Their operating temperatures generally do not exceed 1177 °C, and the temperature required for engine power and efficiency has exceeded the maximum tolerance range of nickel-based superalloys [1,2,3,4,5]. Ceramic matrix composites (CMCs) have recently received significant attention because they can maintain high strength at temperatures up to 1250 °C, which is about 150 °C higher than nickel-based superalloys and have a lower density (one-third of that of Ni alloys) [6,7]. Thus, they can be used as an alternative material for nickel-based superalloys. However, CMC materials also have some shortcomings, and the most notable is the water vapor oxidation, which causes their severe recession and component failure. Therefore, it is necessary to use environmental barrier coatings (EBCs) that can block water and oxygen for improved corrosion resistance, thereby prolonging the service life of engine hot-section components. In recent years, a thermal–environmental barrier coating (T/EBC) has been an emerging system of EBCs that protects some lightweight and high-temperature SiC/SiC CMC components [3,6,8].
The concept of a T/EBC system was first proposed by NASA. It is actually a double-layer structure, the top layer being composed of a thermal barrier coating (TBC) material, the second layer being an EBC material, and there is a composition gradient layer between the TBC and EBC [9]. In this coating system, the top layer is ZrO2 (mostly yttria-stabilized zirconia, YSZ) or HfO2-based TBCs, while rare earth silicates and rare earth aluminates are used as multicomponent EBCs [10,11]. This multi-component, multi-layer coating system has low thermal conductivity, and forms no obvious cracks after 300 h gradient thermal cycling at temperatures up to 1650 °C, showing good cycle durability and resistance to water and oxygen corrosion [12]. The T/EBC system requires a proper coefficient of thermal expansion (CTE) to match the CMC matrix to ensure excellent properties such as high-temperature performance, chemical compatibility, mechanical strength (high hardness and toughness) against fracture, erosion, and impact damage [13]. Ytterbium silicate has become the most promising EBC due to its excellent high-temperature stability, good thermal shock resistance, and good coefficient of thermal expansion matching with SiC/SiC CMCs.
In addition to the performance required above, T/EBC also needs excellent resistance to high-temperature corrosion. During aero-engine operation, siliceous debris (such as sand, dust, and volcanic ash, mainly consisting of CaO, MgO, Al2O3, SiO2, CMAS) is ingested by the intake air and deposited on the surface of components, which corrodes T/EBCs, significantly decreasing their high-temperature capability [14,15,16,17]. For practical applications, T/EBCs inevitably face CMAS issues. Therefore, it is necessary to actively explore methods to prevent CMAS attack to T/EBCs [18,19,20].
CMAS penetration into T/EBCs causes a reaction between them, which largely destroys the coating microstructure and reduces the strain tolerance of the coating. Hence, suppressing the melt penetration is the key to alleviating CMAS attack to T/EBCs, and some strategies have been proposed. One is designing a coating composition which can promote melt crystallization when it comes into contact with the molten CMAS, and the other is to apply a protective layer on the coating surface to inhibit penetration [21]. Poerschke et al. prepared T/EBCs of ytterbium silicate/hafnate and found that they could promote CMAS crystallization and prevent melt infiltration [22]. Godbole et al. studied the apatite and garnet stability in the Al–Ca–Mg–Si–(Gd/Y/Yb)–O system [23]. The results showed that rare earth plays an important role in the formation of apatite and garnet crystalline phases, but no matter which crystalline phase is formed, they all have positive functions for T/EBC materials to prevent CMAS melt infiltration. However, due to the extremely high temperatures of the coating surface, the CMAS attack is severe. Thus, applying a protective layer on the T/EBC surface might be more useful [24].
Since the structure of T/EBCs takes the TBC as the top layer, the preparation method for the TBC protective layer can be referred to when preparing the protective layer. Common protective layers are made of Pd–Ag, Pt, Pd, etc., which are inert to react with CMAS, thereby preventing the melt penetration [25]. However, this method is highly limited in practice because of its high cost. Reports have shown that coatings containing large amounts of rare earth elements are highly resistant to CMAS penetration due to the rapid promotion of melt crystallization, and based on this, some researchers have proposed that applying a protective layer made of rare earth oxides on coating surfaces might be effective in inhibiting CMAS penetration [26,27,28].
Among all rare earth oxides, CeO2 is very special, with a valence of +4 rather than +3 for other rare earth elements. In this work, the interaction behavior of CMAS and CeO2 powders was first investigated to evaluate the high-temperature stability of CeO2 in the presence of molten CMAS, identifying the possible reaction products and their evolution laws with the CeO2 content and duration time. Then, CMAS powder was covered on CeO2 pellets and heat treated at 1300 °C, and the results revealed that the pellets were hardly damaged by CMAS and there was no melt penetration. This study explores the feasibility of CeO2 as a protective layer material for T/EBCs against CMAS attack.

2. Experimental Procedure

2.1. Materials

A laboratory-simulated CMAS was prepared according to our previous study and other researchers [29,30] with a chemical composition of 33CaO–9MgO–13AlO1.5–45SiO2 (mol%). Figure 1 shows the preparation process of the CMAS powder. Four raw materials with a particle size of 500 μm (CaO, MgO, Al2O3, and SiO2; Jiangtian Chemical Technology Ltd., Tianjin, China) were weighed at the appropriate molar ratio, suspended in a certain amount of deionized water, and then planetary ball milling was carried out at a speed of 400 rpm for 10 h (QM-3SP4, Nanjing University Instrument Factory, Nanjing, China). The resultant slurry was put into an oven and dried at 120 °C for 10 h, followed by heat treatment at 1550 °C at a rate of 10 °C/min and maintained in a box furnace (SX-1600 °C, Zhonghuan Electric Furnace Ltd., Tianjin, China) for 1 h. Finally, the molten CMAS was poured into a copper plate in water to obtain CMAS glass. The prepared CMAS block was hammered into small pellets with a volume of 0.4 to 0.7 cm3, ground into powder, and passed through a 200 mesh sieve. The melting point of this CMAS was measured by our previous experiments, with the value of about 1230~1240 °C [31].
About 0.5 g of the prepared CMAS powder was mixed with CeO2 of 5 × 10−4 mol and 1 × 10−3 mol in two portions, and the mixed powders were pressed into cylindrical pellets with a diameter of 10 mm. The purpose of two portions is to study the effect of CeO2 concentration on the interaction behavior with CMAS. The pellet was placed in the center of a 30 mm × 30 mm platinum crucible and then placed on the corundum sheet (Figure 1). A box furnace (SX-1300 °C, Zhonghuan Electric Furnace Ltd., Tianjin, China) was used, and the heat treatment conditions were 1300 °C for 2 h and 10 h. The temperature of 1300 °C is related to the application of T/EBCs. After the dwelling time, the samples were taken out for air cooling to preserve the state of the CMAS and CeO2 at high temperatures. Note that the temperature of 1300 °C and air cooling adopted here are to avoid CMAS self-crystallization for distinguishing whether CeO2 can cause CMAS crystallization.
As depicted in Figure 1, 1.5 g CeO2 was also weighed and pressed into a cylindrical pellet with a diameter of 15 mm. CMAS powders were uniformly covered on the surface of the pellet and kept at 1300 °C for 10 h. Then, the samples were cooled to room temperature in the furnace. The purpose of the slow cooling in the furnace is to prolong the molten state of CMAS. If CMAS cannot penetrate and corrode the CeO2 pellet after 10 h dwelling time and slow cooling in the furnace, it can be demonstrated that CeO2 has a significant effect on preventing CMAS penetration and corrosion.

2.2. Characterizations

The differential scanning calorimetry (DSC; STA 449 C, NETZSCH (Shanghai) Ltd., Shanghai, China) analysis of CMAS (0.5 g) and CeO2 (5 × 10−4 mol) powders was performed from room temperature to 1300 °C at a heating rate of 10 °C/min, and the purpose of DSC was to see whether a reaction between CeO2 and CMAS occurs. After the heat treatment, the mixed powders of CMAS and CeO2 were characterized by X-ray diffractometer (XRD; D8 Advanced, Bruker, Karlsruhe, Germany) with CuKα radiation to confirm the inference and characterize the phase composition. The cross-sectional images, EDS mapping, and element composition were characterized by a scanning electron microscope (SEM; Nova Nanosem 430, FEI, Hillsboro, OR, USA) equipped with an energy dispersive spectroscopy (EDS; Genesis XM2, EDAX, Warrendale, PA, USA).

3. Results and Discussions

3.1. Interaction Behavior of CeO2 and CMAS Powders

Figure 2 shows the morphology of the CeO2 and CMAS powder mixture after heat treatment at 1300 °C for 2 h and 10 h. The resultant samples all exhibit an opaque appearance. By comparing Figure 2a,b, it can be found that increasing the CeO2 content in the mixture from 5 × 10−4 mol to 1 × 10−3 mol leads to a decrease in the spreading area from 3.8 cm2 to 2.14 cm2. In Figure 2c,d, the spreading areas of the samples with 5 × 10−4 mol and 1 × 10−3 mol CeO2 are 1.96 cm2 and 1.2 cm2, respectively. This indicates that the spreading area of CeO2 + CMAS mixture decreases with the increase of the CeO2 content and the duration time. It is known that the spreading area of melt is related to its viscosity, i.e., a smaller spreading area means higher melt viscosity. Hence, one could conclude that increasing the CeO2 content and the duration can increase the viscosity of the molten mixture, and the detailed discussion on the viscosity will be given in the following section.
Figure 3 shows the XRD patterns of the CeO2 + CMAS mixture after heat treatment at 1300 °C. The phase mainly consists of CeO2 solid solution. In addition, some diffraction peaks ascribed to anorthite (CaAl2Si2O8) and pseudo-wollastonite (CaSiO3) phases are detected [32]. CeO2 belongs to a face-centered cubic structure, in which Ce4+ is densely packed according to the face-centered cubic structure and O2- occupies all the tetrahedral voids [33,34]. Thus, the formation of CeO2 solid solution results from the substitution of Ce4+ by the CMAS constituents, such as Ca2+, Al3+, and Si4+, which will be demonstrated by the following SEM observations.
Figure 4 is the DSC curve of the CMAS + CeO2 mixture. It is reported that CMAS does not have a specific melting point but has a certain temperature range of 1230~1250 °C [27]. In this experiment, two exothermic peaks can be found at ~1040 and 1210 °C, which may be due to the self-crystallization of CMAS leading to the formation of anorthite and wollastonite phases [31]. The melting point of CMAS used in this experiment is about 1230~1240 °C, and the absence of an endothermic peak at this temperature range may be attributed to the thermal overlap of melting and crystallization. Therefore, the endothermic peak at ~1260 °C may be formed by the thermal overlap involving crystallization and melting in the CMAS + CeO2 mixture [35]. It should be noted that the formation of self-crystallizing products does not mean that the self-crystallization behavior is dominant, but just implies that it can occur in a specific temperature region.
Figure 5a shows the cross-sectional microstructure of the mixture of CMAS + 5 × 10−4 mol CeO2 after heat treatment at 1300 °C for 2 h and the corresponding EDS results. There are many white globular particles, and the EDS mapping results demonstrate that they are composed of Ce and O elements. Previous studies [31,36,37,38,39] have shown that the crystalline products formed by the reaction or CMAS self-crystallization are generally rod-shaped (e.g., apatite), so this white globular phase is likely to be CeO2 solid solution due to reprecipitation, which is different from the original CeO2 particles, which have a strip-like appearance, as shown in Figure 5b. The remaining places where Ca, Al, and Si are gathered are the residual CMAS melt. Figure 5c is a magnification of the red rectangle in Figure 5a, and the composition positions 1 and 2 are measured. The globular phase CeO2 solid solution (position 1) can be found to contain Ca, Si, and Al. The Si content reach 18.59%, and those of Ca and Al are equivalent, which is about one-third of that of the Ce element. The large amount of Si in the globular products is attributed to the fact that Si4+ has a smaller size than Ca2+, Mg2+, and Al3+, and its valence is the same as Ce4+. Apart from these globular crystals being formed due to the dissolution and reprecipitation mechanism, no other phases could be observed, suggesting a limited reaction between CMAS and CeO2 particles under this condition.
When the CeO2 content in the mixture increases to 1 × 10−3 mol, some rod-like crystals are formed, as shown in Figure 6a. EDS analysis of regions 1, 2, and 3 suggests that the chemical compositions of these crystals are close to that of the apatite phase. Combined with their morphologies, these rod-like crystals could be determined as Ce-apatite. Figure 6b is an enlarged image of the region with a large number of globular crystals, which is similar to those observed in Figure 5, and could be confirmed as CeO2 solid solution.
Figure 5a reveals that the mixture of CMAS + 5 × 10−4 mol CeO2 can only form globular crystals of CeO2 solid solution at 1300 °C for 2 h. Here, we prolonged the duration time, and find dark contrasted and white contrasted rod-like crystals in the cross-sectional images, as shown in Figure 7. EDS mapping analysis indicates that the dark contrasted crystals are mainly composed of Ca, Si, and Al elements, which could be confirmed as the anorthite phase. The white contrasted globular crystals are rich in Ce, together with some Si, Al, and Ca, indicative of the formation of CeO2 solid solution. The rod-like crystals in the region highlighted by a yellow rectangular box in Figure 7b are apatite. The presence of apatite and anorthite phases indicates that prolonging the duration time could promote the reaction between CeO2 and CMAS, but the formation ability of apatite is much lower than that of CeO2 solid solution.
Figure 8 shows the cross-sectional SEM images of CMAS + 1 × 10−3 mol CeO2 after heat treatment at 1300 °C for 10 h. More globular crystals of CeO2 solid solution are formed, and the apatite crystals grow to more than 50 μm in length. In Figure 8b, an obvious anorthite phase can be found, and the region highlighted by a red rectangular box is enlarged in Figure 8c. The chemical compositions analysis indicates that crystals 1 and 3 are CeO2 solid solution and anorthite, respectively. In residual CMAS (region 2), very limited Ce could be detected.

3.2. Effects of the CeO2 Content and Duration Time on the Crystalline Products Formation

Based on the aforementioned observations, it can be found that some CeO2 prefers to dissolve in molten CMAS and then reprecipitate to form CeO2 solid solution, gathering some amounts of Si, Ca, and Al, among which the Si content is the highest. The reaction between CeO2 and CMAS is not vigorous, indicating that CeO2 can maintain phase and structural stability in the presence of CMAS. However, increasing the CeO2 content and duration time can promote the reaction, which leads to the formation of apatite and CMAS self-crystallization products such as anorthite and wollastonite phases.
According to our previous research, CMAS self-crystallization does not occur at fast cooling rates such as air cooling [31]. However, CMAS self-crystallization products were detected by XRD and SEM in this study. This may be because the formed CeO2 solid solution particles act as a nucleating agent for anorthite and wollastonite phases [40]. Additionally, the formation of anorthite is also affected by the Al content in the melt, and increasing the Al content can promote to produce anorthite [41,42,43]. As mentioned above, the content of Si in CeO2 solid solution is higher than that of Al; in other words, the formation of CeO2 solid solution consumes more Si, and the Al content in the CMAS is increased. As a result, the self-crystallization product of anorthite is produced. In the SEM observations, wollastonite was not found, possibly because the contrast of wollastonite is similar to that of the residual CMAS and its amount is low (its XRD peaks intensities are very low). In addition, self-crystallization products cannot be detected by XRD until the duration time is prolonged to 10 h. This indicates that CMAS self-crystallization needs time. The longer the duration of mixing CeO2 and CMAS, the more obvious the effect of CeO2 as the nucleating agent, and the more CMAS self-crystallization products produced.
In Figure 3, no apparent apatite peaks appear in the XRD patterns when the CeO2 content and duration time increase. The main reason may be related to the detection position. In this experiment, the sample is hemispherical (Figure 2), with a flat circular bottom and a concave top surface. According to the non-uniform nucleation principle, the concave surface has the highest nucleation efficiency because the smaller volume of crystal embryo can reach the critical nucleation radius. Meanwhile, nucleation also depends on defects such as interface impurities in the system, which can significantly reduce the free energy required for nucleation, so crystallization often starts from these interfaces (Figure 7b). Therefore, in the reaction between CeO2 and CMAS, apatite nucleates mainly near the concave surface and grows oriented along the maximum temperature gradient (perpendicular to the surface) [44]. However, this study selected the circular plane at the bottom for XRD measurements, which leads to the absence of apatite characteristic peaks in XRD patterns.
Increasing the CeO2 content and duration time can both promote the formation of the apatite phase, and increasing the duration time is more effective. At a lower CeO2 content, the main product is CeO2 solid solution, and the residual CMAS melt does not have enough Ce to produce apatite. Thus, increasing the CeO2 content can increase the Ce amount in the molten CMAS, which is beneficial to trigger apatite formation. Additionally, under the prolonged duration time, the action time of the nucleating agent of CeO2 solid solution particles is longer, and more Ce could be dissolved in the molten CMAS to promote the formation of the apatite phase. It has been reported that the dissolution–reprecipitation of particles in molten CMAS is a dynamic process [45,46]; thus, in this study, the Ce distribution in the melt is also dynamic. As mentioned above, in the case of a short duration time, the precipitated CeO2 solid solution particles take away a large amount of Ce. During the prolonged heat treatment, compositional fluctuations occur in the molten CMAS, which favors the nucleation and growth of the apatite phase. As a result, non-uniformly nucleated apatite is found at concave surfaces.
After clarifying the interaction behavior and product formation mechanisms, the spreading areas and viscosity changes of the molten mixture of CMAS +CeO2 could be explained (Figure 2). In silicate melts, the viscosity is determined mainly by Ca/Si, and the higher the Ca/Si, the lower the viscosity (inversely proportional) [47,48]. During the interaction of CeO2 and CMAS, with the increase of the CeO2 content and the duration time, more apatite is produced, which consumes a large amount of Ca in the CMAS melt. As a result, the Ca/Si ratio decreases, causing an increase in the melt viscosity. Therefore, the spreading areas of the molten mixture of CMAS + CeO2 decrease with the increase of the CeO2 content and duration time. Note that enhancing the viscosity could drag the melt penetration in the coatings, which is beneficial for alleviating CMAS attack to TBCs.

3.3. Interface Interaction between Molten CMAS and CeO2 Pellets

To evaluate whether CeO2 can be used as a protective layer material against CMAS attack, CeO2 pellets were used and covered with CMAS powder followed by heat treatment. This case could be approximately considered as the mixture of 1 × 10−3 mol CeO2 with CMAS in the above sections. Figure 9 is a cross-sectional view of the CeO2 pellet. It can be found that it is compact and free of microcracks but has some pores. A portion of Figure 9a is enlarged and shown in Figure 9b; some pores are found in the pellet. Since TBCs are usually designed to have some pores for stress tolerance, the pellets used for CMAS corrosion experiments need pores for more accurate simulation.
The cross-sectional view of the CeO2 pellet with CMAS after heat treatment at 1300 °C for 10 h and the corresponding EDS results are shown in Figure 10. It can be observed that the pellet microstructure has almost no change, and very limited CMAS constituent elements are detected in the pellet. On the pellet surface, a thick layer consisting of many white contrasted particles, dark contrasted strip-like crystals, and slight dark contrasted compounds can be found. According to the EDS mapping analysis, the white contrasted particles are rich in Ce and have some other elements, which could be confirmed as the CeO2 solid solution phase. The dark contrasted strip-like crystals are mainly composed of Ca and Al, which could be identified as the anorthite phase. The region highlighted by a blue rectangle box in Figure 10a was magnified and shown in Figure 10b. It is clear that no CMAS penetration could be found in the pellet, and its microstructure is not destroyed. The EDS result of the pellet (region 1) indicates that the pellet is almost pure CeO2. Region 2 is composed of Ca, Si, and O, and based on their chemical composition ratio, the compound could be determined to be CaSiO3 wollastonite phase. The EDS result of region 3 further confirms that the dark contrasted strip-like crystals are Ca2Al2Si2O8 anorthite.
The limited penetration ability of molten CMAS in the CeO2 pellet could be attributed to the rapid crystallization of the melt. At high temperatures, the interaction of molten CMAS and the CeO2 pellet at their interface causes some amount of Ce dissolution into the melt, which quickly migrates into the entire molten CMAS on the pellet surface. As mentioned above, some CeO2 solid solution particles are precipitated, which can be used as the nucleating agent for CMAS crystallization. As a result, anorthite and wollastonite phases are formed, largely increasing the viscosity of the melt. As shown in Figure 10, no CMAS glass remains after the heat treatment, and all has been crystallized, indicating that CeO2 solid solution particles have an effective function on promoting CMAS crystallization when used as the nucleating agent. Additionally, the absence of apatite phase in the crystalline layer further implies that CeO2 has a high stability in molten CMAS, which causes insufficient Ce to be dissolved in the melt to trigger the formation of the apatite phase.
Based on the investigation on the interaction behavior of CMAS and CeO2, it could be found that CeO2 is a useful material for mitigating CMAS attack to TBCs. Due to the excellent nucleating agent effect of the CeO2 solid solution, one could design a T/EBC composition doped with Ce aiming to promote CMAS crystallization and suppress the melt penetration [49]. Meanwhile, based on the theory of optical basicity (OB), CeO2 has a relatively moderate optical basicity (Λ = 1.01) compared to traditional TBC materials, which ensures that CeO2 reacts with molten CMAS to form CeO2 solid solution particles, promoting CMAS self-crystallization. Once the molten CMAS is crystallized, it is less harmful, and CeO2 can maintain high stability [50,51]. Based on the high stability of CeO2 in the presence of molten CMAS, a dense CeO2 layer could be designed on the T/EBC surface, which might have an excellent function in inhibiting CMAS infiltration.

4. Conclusions

This study investigates the interaction behavior of CeO2 powder/pellet and molten CMAS and discusses the effects of the CeO2 content and duration time on the crystalline products. The following conclusions can be drawn:
(1)
In the presence of molten CMAS, CeO2 dissolves in the melt followed by precipitating many CeO2 solid solution particles, in which Si, Ca, and Al are present and the Si content is the highest.
(2)
The reaction between CMAS and CeO2 is not vigorous at 1300 °C; only when the CeO2 content is increased or the duration time prolonged to 10 h, a reaction product of apatite and some self-crystallization products of anorthite and wollastonite phases can be formed. During this reaction process, the CeO2 solid solution particles are acted as the nucleating agent.
(3)
After heat treatment at 1300 °C for 10 h, the CMAS on the CeO2 pellet is completely crystallized due to Ce migration to the melt providing the nucleating agent. No CMAS penetration and no reaction products are found, indicating that CMAS crystallization is rapid and the Ce diffusion in the melt is limited, meaning excellent stability of CeO2 in molten CMAS.
(4)
CeO2 can be used as a dopant in T/EBC to promote CMAS crystallization, and it is also a promising protective layer material on the T/EBC surface to inhibit CMAS penetration.

Author Contributions

Conceptualization, L.G.; methodology, L.G., Y.G. and Y.W.; software, L.G. and F.Y.; validation, L.G.; formal analysis, L.G., Y.W. and Y.G.; investigation, M.L. and Y.W.; resources, L.G. and Y.W.; data curation, Y.W. and Y.G.; writing—original draft preparation, Y.W. and Y.G.; writing—review and editing, M.L. and L.G.; supervision, L.G. and F.Y.; project administration, L.G.; funding acquisition, L.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research is sponsored by the National Natural Science Foundation of China (Grants No. 51971156 and 52272070).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data presented in this article are available upon request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Hong, J.; Li, T.; Zheng, H.; Ma, Y. Applications of structural efficiency assessment method on structural-mechanical characteristics integrated design in aero-engines. Chin. J. Aeronaut. 2020, 33, 1260–1271. [Google Scholar] [CrossRef]
  2. Zhang, B.; Song, W.; Wei, L.; Xiu, Y.; Xu, H.; Dingwell, D.B.; Guo, H. Novel thermal barrier coatings repel and resist molten silicate deposits. Scr. Mater. 2019, 163, 71–76. [Google Scholar] [CrossRef]
  3. Chen, H.F.; Zhang, C.; Liu, Y.C.; Song, P.; Li, W.X.; Yang, G.; Liu, B. Recent progress in thermal/environmental barrier coatings and their corrosion resistance. Rare Met. 2019, 39, 498–512. [Google Scholar] [CrossRef]
  4. Darolia, R. Thermal barrier coatings technology: Critical review, progress update, remaining challenges and prospects. Int. Mater. Rev. 2013, 58, 315–348. [Google Scholar] [CrossRef]
  5. Bakan, E.; Vaßen, R. Ceramic top coats of plasma-sprayed thermal barrier coatings: Materials, processes, and properties. J. Therm. Spray Technol. 2017, 26, 992–1010. [Google Scholar] [CrossRef]
  6. Diaz, O.G.; Axinte, D.; Butler-Smith, P.; Novovic, D. On understanding the microstructure of SiC/SiC Ceramic Matrix Composites (CMCs) after a material removal process. Mater. Sci. Eng. A 2018, 743, 1–11. [Google Scholar] [CrossRef]
  7. Dicarlo, J.A. Advances in SiC/SiC composites for aero-propulsion. M. Ceram. Matrix Compos. 2014, 217–235. [Google Scholar] [CrossRef]
  8. Fang, G.; Gao, X.; Song, Y. A Review on Ceramic Matrix Composites and Environmental Barrier Coatings for Aero-Engine: Material Development and Failure Analysis. Coatings 2023, 13, 357. [Google Scholar] [CrossRef]
  9. Zhu, D. Advanced environmental barrier coatings for SiC/SiC ceramic matrix composite turbine components, Engineered ceramics: Current status and future prospects. M. Eng. Ceram. Curr. Status Future Prospect. 2016, 187–202. [Google Scholar] [CrossRef]
  10. Harder, B.J. Oxidation performance of Si-HfO2 environmental barrier coating bond coats deposited via plasma spray-physical vapor deposition. Surf. Coat. Technol. 2020, 384, 125311. [Google Scholar] [CrossRef]
  11. Zhu, R.B.; Zou, J.P.; Mao, J.; Deng, Z.Q.; Zhang, X.F.; Deng, C.M.; Liu, M. A comparison between novel Gd2Zr2O7 and Gd2Zr2O7/YSZ thermal barrier coatings fabricated by plasma spray-physical vapor deposition. Rare Met. 2021, 40, 2244–2253. [Google Scholar] [CrossRef]
  12. Zhu, D.; Miller, R.; Fox, D. Thermal and environmental barrier coating development for advanced propulsion engine systems. In Proceedings of the 48th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Schaumburg, IL, USA, 7–10 April 2008; p. 2130. [Google Scholar]
  13. Amer, M.; Curry, N.; Hayat, Q.; Sharma, R.; Janik, V.; Zhang, X.; Nottingham, J.; Bai, M. Cracking Behavior of Gd2Zr2O7/YSZ Multi-Layered Thermal Barrier Coatings Deposited by Suspension Plasma Spray. Coatings 2023, 13, 107. [Google Scholar] [CrossRef]
  14. Liu, S.; Liu, Q.; Hu, X.P.; Guo, J.W.; Zhu, W.; Zhang, F.; Xia, J. CMAS corrosion resistance behavior and mechanism of Hf6Ta2O17 ceramic as potential material for thermal barrier coatings. Coatings 2023, 13, 404. [Google Scholar] [CrossRef]
  15. Mehboob, G.; Liu, M.; Xu, T.; Hussain, S.; Mehboob, G.; Tahir, A. A review on failure mechanism of thermal barrier coatings and strategies to extend their lifetime. Ceram. Int. 2019, 46, 8497–8521. [Google Scholar] [CrossRef]
  16. Yang, S.-J.; Song, W.-J.; Dingwell, D.B.; He, J.; Guo, H.-B. Surface roughness affects metastable non-wetting behavior of silicate melts on thermal barrier coatings. Rare Met. 2021, 41, 469–481. [Google Scholar] [CrossRef]
  17. Guo, Y.Q.; Song, W.J.; Guo, L.; Li, X.X.; He, W.T.; Yan, X.D.; Dingwell, D.B.; Guo, H.B. Molten-volcanic-ash-phobic thermal barrier coating based on biomimetic structure. Adv. Sci. 2023, 10, 2205156. [Google Scholar] [CrossRef]
  18. Guo, L.; Gao, Y.; Ye, F.X.; Zhang, X.M. CMAS corrosion behavior and protection method of thermal barrier coatings for aeroengine. Acta. Metall. Sin. 2021, 57, 1184–1198. [Google Scholar]
  19. Mikulla, C.; Steinberg, L.; Niemeyer, P.; Schulz, U.; Naraparaju, R. Microstructure Refinement of EB-PVD Gadolinium Zirconate Thermal Barrier Coatings to Improve Their CMAS Resistance. Coatings 2023, 13, 905. [Google Scholar] [CrossRef]
  20. Hu, X.; Liu, G.; Liu, Q.; Zhu, W.; Liu, S.; Ma, Z. Failure Mechanism of EB-PVD Thermal Barrier Coatings under the Synergistic Effect of Thermal Shock and CMAS Corrosion. Coatings 2022, 12, 1290. [Google Scholar] [CrossRef]
  21. Poerschke, D.L.; Jackson, R.W.; Levi, C.G. Silicate Deposit Degradation of Engineered Coatings in Gas Turbines: Progress toward Models and Materials Solutions. Annu. Rev. Mater. Res. 2017, 47, 297–330. [Google Scholar] [CrossRef]
  22. Poerschke, D.L.; Hass, D.D.; Eustis, S.; Seward, G.G.; Van Sluytman, J.S.; Levi, C.G. Stability and CMAS Resistance of Ytterbium-Silicate/Hafnate EBCs/TBC for SiC Composites. J. Am. Ceram. Soc. 2014, 98, 278–286. [Google Scholar] [CrossRef]
  23. Godbole, E.; Von Der Handt, A.; Poerschke, D. Apatite and garnet stability in the Al–Ca–Mg–Si–(Gd/Y/Yb)–O systems and implications for T/EBC: CMAS reactions. J. Am. Ceram. Soc. 2021, 105, 1596–1609. [Google Scholar] [CrossRef]
  24. Turcer, L.R.; Krause, A.R.; Garces, H.F.; Zhang, L.; Padture, N.P. Environmental-barrier coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS) glass: Part II, β-Yb2Si2O7 and β-Sc2Si2O7. J. Eur. Ceram. Soc. 2018, 38, 3914–3924. [Google Scholar] [CrossRef]
  25. Liu, H.; Cai, J.; Zhu, J. CMAS (CaO-MgO-Al2O3-SiO2) resistance of Y2O3-stabilized ZrO2 thermal barrier coatings with Pt layers. Ceram. Int. 2018, 44, 452–458. [Google Scholar] [CrossRef]
  26. Islam, A.; Sharma, A.; Singh, P.; Pandit, N.; Keshri, A.K. Plasma-sprayed CeO2 overlay on YSZ thermal barrier coating: Solution for resisting molten CMAS infiltration. Ceram. Int. 2022, 48, 14587–14595. [Google Scholar] [CrossRef]
  27. Li, B.W.; Sun, J.Y.; Guo, L. CMAS corrosion behavior of Sc doped Gd2Zr2O7 /YSZ thermal barrier coatings and their corrosion resistance mechanisms. Corros. Sci. 2021, 193, 109899. [Google Scholar] [CrossRef]
  28. Chen, W.B.; He, W.T.; He, J.; Guo, Q.; Li, S.; Guo, H.B. Failure mechanisms of (Gd0.9Yb0.1)2Zr2O7/Yb2SiO5/Si thermal/environmental barrier coatings during thermal exposure at 1300 °C/1400 °C. J. Eur. Ceram. Soc. 2022, 42, 3297–3304. [Google Scholar] [CrossRef]
  29. Yan, Z.; Guo, L.; Li, Z.; Yu, Y.; He, Q. Effects of laser glazing on CMAS corrosion behavior of Y2O3 stabilized ZrO2 thermal barrier coatings. Corros. Sci. 2019, 157, 450–461. [Google Scholar] [CrossRef]
  30. Krämer, S.; Yang, J.; Levi, C.G.; Johnson, C.A. Thermochemical interaction of thermal barrier coatings with molten CaO-MgO-Al2O3-SiO2 (CMAS) deposits. J. Am. Ceram. Soc. 2006, 89, 3167–3175. [Google Scholar] [CrossRef]
  31. Guo, L.; Xin, H.; Li, Y.; Yu, Y.; Yan, Z.; Hu, C.; Ye, F. Self-crystallization characteristics of calcium-magnesium-alumina- silicate (CMAS) glass under simulated conditions for thermal barrier coating applications. J. Eur. Ceram. Soc. 2020, 40, 5683–5691. [Google Scholar] [CrossRef]
  32. Gao, L.; Guo, H.; Gong, S.; Xu, H. Plasma-sprayed La2Ce2O7 thermal barrier coatings against calcium–magnesium–alumina–silicate penetration. J. Eur. Ceram. Soc. 2014, 34, 2553–2561. [Google Scholar] [CrossRef]
  33. Shing, O.P.; Ping, T.Y.; Hin, T.Y.Y. Mechanochemical synthesis and characterization of calcium-doped ceria oxide Ion conductor. IOP Conf. Ser. Mater. Sci. Eng. 2011, 17, 012017. [Google Scholar] [CrossRef]
  34. Tsunekawa, S.; Sivamohan, R.; Ito, S.; Kasuya, A.; Fukuda, T. Structural study on monosize CeO2-x nano-particles. Nanostructured Mater. 1999, 11, 141–147. [Google Scholar] [CrossRef]
  35. Yang, D.; Wang, L.; Sun, Y.Z.; Zhou, K.B. Synthesis of one-dimensional Ce1−xYxO2−x/2 (0 <= x <= 1) solid solutions and their catalytic properties: The role of oxygen vacancies. J. Phys. Chem. C 2010, 114, 8926–8932. [Google Scholar]
  36. Yan, Z.; Guo, L.; Zhang, Z.; Wang, X.; Ye, F. Versatility of potential protective layer material Ti2AlC on resisting CMAS corrosion to thermal barrier coatings. Corros. Sci. 2020, 167, 108532. [Google Scholar] [CrossRef]
  37. Wolf, M.; Mack, D.E.; Guillon, O.; Vaßen, R. Resistance of pure and mixed rare earth silicates against calcium-magnesium-aluminosilicate (CMAS): A comparative study. J. Am. Ceram. Soc. 2020, 103, 7056–7071. [Google Scholar] [CrossRef]
  38. Lyu, G.; Kim, I.-S.; Song, D.; Park, H.-M.; Kim, J.S.; Song, T.; Myoung, S.; Jung, Y.-G.; Zhang, J. Sintering behavior and phase transformation of YSZ-LZ composite coatings. Ceram. Int. 2019, 46, 1307–1313. [Google Scholar] [CrossRef]
  39. Shi, Y.; Li, B.W.; Zhao, M.; Zhang, M.X. Growth of diopside crystals in CMAS glass-ceramics using Cr2O3 as a nucleating agent. J. Am. Ceram. Soc. 2018, 101, 3968–3978. [Google Scholar] [CrossRef]
  40. Wiesner, V.L.; Bansal, N.P. Crystallization kinetics of calcium–magnesium aluminosilicate (CMAS) glass. Surf. Coat. Technol. 2014, 259, 608–615. [Google Scholar] [CrossRef]
  41. Poerschke, D.L.; Levi, C.G. Effects of cation substitution and temperature on the interaction between thermal barrier oxides and molten CMAS. J. Eur. Ceram. Soc. 2015, 35, 681–691. [Google Scholar] [CrossRef]
  42. Aygun, A.; Vasiliev, A.L.; Padture, N.P.; Ma, X. Novel thermal barrier coatings that are resistant to high-temperature attack by glassy deposits. Acta Mater. 2007, 55, 6734–6745. [Google Scholar] [CrossRef]
  43. Guo, L.; Yan, Z.; Wang, X.; He, Q. Ti2AlC MAX phase for resistance against CMAS attack to thermal barrier coatings. Ceram. Int. 2019, 45, 7627–7634. [Google Scholar] [CrossRef]
  44. Hopkins, R.H.; De Klerk, J.; Piotrowski, P.; Walker, M.S.; Mathur, M.P. Thermal and elastic properties of silicate oxyapatite crystals. J. Appl. Phys. 1973, 44, 2456–2458. [Google Scholar] [CrossRef]
  45. Krause, A.R.; Li, X.; Padture, N.P. Interaction between ceramic powder and molten calcia-magnesia-alumino-silicate (CMAS) glass, and its implication on CMAS-resistant thermal barrier coatings. Scr. Mater. 2016, 112, 118–122. [Google Scholar] [CrossRef] [Green Version]
  46. Mechnich, P.; Braue, W.; Green, D.J. Volcanic ash-induced decomposition of EB-PVD Gd2Zr2O7 thermal barrier coatings to Gd-oxyapatite, zircon, and Gd,Fe-Zirconolite. J. Am. Ceram. Soc. 2013, 96, 1958–1965. [Google Scholar] [CrossRef] [Green Version]
  47. Guo, W.; Wang, Z.; Zhao, Z.; An, Z.; Wang, W. Effect of CeO2 on the viscosity and structure of high-temperature melt of the CaO-SiO2(-Al2O3)-CeO2 system. J. Non-Cryst. Solids. 2020, 540, 120085. [Google Scholar] [CrossRef]
  48. Turcer, L.R.; Padture, N.P. Rare-earth pyrosilicate solid-solution environmental-barrier coating ceramics for resistance against attack by molten calcia–magnesia–aluminosilicate (CMAS) glass. J. Mater. Res. 2020, 35, 2373–2384. [Google Scholar] [CrossRef]
  49. Guo, L.; Li, G.; Wu, J.; Wang, X. Effects of pellet surface roughness and pre-oxidation temperature on CMAS corrosion behavior of Ti2AlC. J. Adv. Ceram. 2022, 11, 945–960. [Google Scholar] [CrossRef]
  50. Krause, A.R.; Senturk, B.S.; Garces, H.F.; Dwivedi, G.; Ortiz, A.L.; Sampath, S.; Padture, N.P. 2ZrO2·Y2O3 Thermal Barrier Coatings Resistant to Degradation by Molten CMAS: Part I, Optical Basicity Considerations and Processing. J. Am. Ceram. Soc. 2014, 97, 3943–3949. [Google Scholar] [CrossRef]
  51. Chen, Q.; Qiao, Y.; Wang, H. Spectra and magneto optical behavior of CeO2 doped heavy metal diamagnetic glass. J. Non Cryst. Solids 2017, 470, 70–77. [Google Scholar] [CrossRef]
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Figure 1. The process of CMAS preparation and its covering on CeO2 pellets.
Figure 1. The process of CMAS preparation and its covering on CeO2 pellets.
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Figure 2. Morphology of the mixture of 0.5 g CMAS and different amounts of CeO2 after heat treatment at 1300 °C for 2 h (a,b) and 10 h (c,d). (a) 5 × 10−4 mol CeO2, (b) 1 × 10−3 mol CeO2, (c) 5 × 10−4 mol CeO2, (d) 1 × 10−3 mol CeO2.
Figure 2. Morphology of the mixture of 0.5 g CMAS and different amounts of CeO2 after heat treatment at 1300 °C for 2 h (a,b) and 10 h (c,d). (a) 5 × 10−4 mol CeO2, (b) 1 × 10−3 mol CeO2, (c) 5 × 10−4 mol CeO2, (d) 1 × 10−3 mol CeO2.
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Figure 3. XRD patterns of CMAS + CeO2 mixture after heat treatment at 1300 °C.
Figure 3. XRD patterns of CMAS + CeO2 mixture after heat treatment at 1300 °C.
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Figure 4. DSC curve of the mixture of CMAS + 5 × 10−4 mol CeO2.
Figure 4. DSC curve of the mixture of CMAS + 5 × 10−4 mol CeO2.
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Figure 5. Cross-sectional microstructure of the mixture of CMAS + 5 × 10−4 mol CeO2 after heat treatment at 1300 °C for 2 h (a), and the original morphology of CeO2 powder (b). The enlarged image of the region highlighted by a red rectangle in (a) is shown in (c). Corresponding EDS mapping results of Ce, Ca, Si, and Al, and elemental compositions of the marked areas (1,2) in (c) are also presented.
Figure 5. Cross-sectional microstructure of the mixture of CMAS + 5 × 10−4 mol CeO2 after heat treatment at 1300 °C for 2 h (a), and the original morphology of CeO2 powder (b). The enlarged image of the region highlighted by a red rectangle in (a) is shown in (c). Corresponding EDS mapping results of Ce, Ca, Si, and Al, and elemental compositions of the marked areas (1,2) in (c) are also presented.
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Figure 6. Cross-sectional microstructure of the mixture of CMAS + 1 × 10−3 mol CeO2 after heat treatment at 1300 °C for 2 h (a) and its enlarged view (b). EDS results of the marked areas (1–3) in (a) and (4–6) in (b) are also presented.
Figure 6. Cross-sectional microstructure of the mixture of CMAS + 1 × 10−3 mol CeO2 after heat treatment at 1300 °C for 2 h (a) and its enlarged view (b). EDS results of the marked areas (1–3) in (a) and (4–6) in (b) are also presented.
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Figure 7. Cross-sectional microstructure of the mixture with CMAS + 5 × 10−4 mol CeO2 after heat treatment at 1300 °C for 10 h (a,b). Corresponding EDS mapping of Ce, Ca, Al, Si, O, and elemental composition of marked areas are presented.
Figure 7. Cross-sectional microstructure of the mixture with CMAS + 5 × 10−4 mol CeO2 after heat treatment at 1300 °C for 10 h (a,b). Corresponding EDS mapping of Ce, Ca, Al, Si, O, and elemental composition of marked areas are presented.
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Figure 8. Cross-sectional microstructure of the mixture with CMAS + 1 × 10−3 mol CeO2 after heat treatment at 1300 °C for 10 h (a–c), where (c) is an enlarged view of (b). EDS results of marked regions (1–3) in (c) are also presented.
Figure 8. Cross-sectional microstructure of the mixture with CMAS + 1 × 10−3 mol CeO2 after heat treatment at 1300 °C for 10 h (a–c), where (c) is an enlarged view of (b). EDS results of marked regions (1–3) in (c) are also presented.
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Figure 9. Cross-sectional morphology of CeO2 pellet (a). The enlarged image of the region highlighted by a red rectangle in (a) is shown in (b).
Figure 9. Cross-sectional morphology of CeO2 pellet (a). The enlarged image of the region highlighted by a red rectangle in (a) is shown in (b).
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Figure 10. Cross-sectional microstructures of CeO2 pellet subjected to CMAS at 1300 °C for 10 h (a). The enlarged image of the region highlighted by a blue rectangle in (a) is shown in (b). Corresponding EDS mapping of Ce, Ca, Si, and O, and elemental composition of the marked regions (1–3) in (b) are also presented.
Figure 10. Cross-sectional microstructures of CeO2 pellet subjected to CMAS at 1300 °C for 10 h (a). The enlarged image of the region highlighted by a blue rectangle in (a) is shown in (b). Corresponding EDS mapping of Ce, Ca, Si, and O, and elemental composition of the marked regions (1–3) in (b) are also presented.
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MDPI and ACS Style

Guo, L.; Wang, Y.; Liu, M.; Gao, Y.; Ye, F. CeO2 Protective Material against CMAS Attack for Thermal–Environmental Barrier Coating Applications. Coatings 2023, 13, 1119. https://doi.org/10.3390/coatings13061119

AMA Style

Guo L, Wang Y, Liu M, Gao Y, Ye F. CeO2 Protective Material against CMAS Attack for Thermal–Environmental Barrier Coating Applications. Coatings. 2023; 13(6):1119. https://doi.org/10.3390/coatings13061119

Chicago/Turabian Style

Guo, Lei, Yuanpeng Wang, Mingguang Liu, Yuan Gao, and Fuxing Ye. 2023. "CeO2 Protective Material against CMAS Attack for Thermal–Environmental Barrier Coating Applications" Coatings 13, no. 6: 1119. https://doi.org/10.3390/coatings13061119

APA Style

Guo, L., Wang, Y., Liu, M., Gao, Y., & Ye, F. (2023). CeO2 Protective Material against CMAS Attack for Thermal–Environmental Barrier Coating Applications. Coatings, 13(6), 1119. https://doi.org/10.3390/coatings13061119

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