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Article

Micro-Nano Dual-Scale Coatings Prepared by Suspension Precursor Plasma Spraying for Resisting Molten Silicate Deposit

by
Yangguang Liu
1,†,
Yihao Wang
1,†,
Weize Wang
1,2,*,
Wenkang Zhang
1,
Junhao Wang
1,
Kaibin Li
1,
Hongchen Li
1,
Pengpeng Liu
1,
Shilong Yang
1 and
Chengcheng Zhang
3
1
Key Laboratory of Pressure System and Safety, Ministry of Education, East China University of Science and Technology, Shanghai 200237, China
2
Shanghai Institute of Aircraft Mechanics and Control, Shanghai 200237, China
3
AECC Commercial Aircraft Engine Co., Ltd., Shanghai 200241, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Coatings 2024, 14(9), 1123; https://doi.org/10.3390/coatings14091123
Submission received: 9 August 2024 / Revised: 28 August 2024 / Accepted: 31 August 2024 / Published: 2 September 2024
Browse Figures
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Abstract

:
Yb-doped Y2O3 stabilized ZrO2 (YbYSZ) coatings, developed through solution precursor plasma spraying (SPPS), are engineered to resist calcium–magnesium–alumino–silicate (CMAS) infiltration by leveraging their unique micro-nano structures. This provides superior anti-wetting properties, crucial for preventing CMAS penetration at high temperatures. The investigation focused on the structural and compositional changes in YbYSZ-SPPS coatings subjected to prolonged thermal exposure at 1300 °C. Results indicate that while the coatings undergo significant sintering, leading to densification and microstructural evolution, the elemental composition and phase stability remain largely intact after up to 8 h of heat treatment. Despite some reduction in CMAS resistance, the coatings maintained their overall protective performance, demonstrating the potential of SPPS coatings for long-term use in high-temperature environments where CMAS infiltration is a concern. These findings contribute to the development of more durable TBCs for advanced thermal protection applications.

1. Introduction

Thermal barrier coatings (TBCs) are critical in protecting high-temperature components in gas turbines and jet engines from thermal degradation [1,2]. One of the major challenges faced by TBCs is the corrosion of molten calcium–magnesium–alumino–silicate (CMAS) deposits, which can severely compromise their protective capabilities [3]. CMAS mainly comes from small particles such as dust, sand, or volcanic ash in the air (composed of CaO, MgO, Al2O3, and SiO2, abbreviated as CMAS), which are carried by the airflow and reach the surface of the hot-end components in the engine. With the improvement in engine performance and efficiency, the service temperature exceeds the melting point of CMAS [4], causing them to melt and penetrate the surface of the components. The infiltration process begins with the melting of CMAS at high temperatures, after which the molten CMAS penetrates the porous structure of the TBC, leading to chemical reactions and phase transformations that degrade the coating. This infiltration mechanism highlights the importance of developing TBCs with robust anti-wetting properties to resist CMAS penetration and maintain their thermal insulation performance.
Many scholars have conducted research on the anti-CMAS wetting performance. Dai et al. conducted surface treatment on Gd2Zr2O7 coatings using picosecond laser surface texture and introduced nanostructure on the surface, resulting in an increase in the wetting angle of coating from 16.0° to 66.3° [5]. Moreover, Zhang et al. introduced bionic structural design based on the lotus leaf structure into TBCs under different processes to explore the universality of the structure [6]. Al2O3 modification was introduced to prepare a dense Al2O3 coating with nano/micro-sized grains on the surface of TBCs. The results showed that in terms of wettability, the contact angle of Al2O3-modified APS 7YSZ TBCs (34.2°) increased by 129.5% compared to the contact angle of APS 7YSZ TBCs (14.9°); the contact angle of Al2O3 modified EB-PVD 7YSZ TBCs (11.8°) increased by 56.8% compared to the contact angle of EB-PVD 7YSZ TBCs (18.5°); and the contact angle (22.0°) of Al2O3 modified PS-PVD 7YSZ TBCs increased by 87.3% compared to the contact angle (41.2°) of PS-PVD 7YSZ TBCs. The Al2O3 overlay with a lotus leaf-like structure on the surface can effectively improve the wetting performance of coatings, regardless of various processes. The above research proves that introducing micro-nano structures can effectively increase the wetting performance of coatings. At present, surface treatment on the prepared coating to form this structure is not only complex but also costly. The use of solution precursor plasma spraying (SPPS) can more conveniently form coatings with micro-nano structures in one step.
SPPS has emerged as a promising technique for producing TBCs with enhanced resistance to CMAS infiltration [7]. SPPS technology directly uses solution precursors for spraying. The solution precursors undergo a series of processes such as solvent evaporation, droplet splitting, solute precipitation, high-temperature decomposition, sintering and melting in the plasma flame, and finally form a coating on the substrate. The technology breaks the limitation of powder particle size on the microstructure of coatings, at a relatively low cost. In addition, it solves the flowability problem of small particle size powders and can prepare TBCs with nanostructures [8,9]. Compared to the common preparation processes currently used in industrial applications, including atmospheric plasma spraying (APS) and electron-beam physical vapor deposition (EB-PVD) [10,11,12], these processes can melt powder particles to form coatings, but cannot form nanostructures on the surface, which is beneficial for hydrophobicity. The SPPS process creates unique micro-nano dual-scale structures that are crucial for the anti-wetting performance of the coatings; however, the ability of these micro-nano dual-scale structures to retain their stability under prolonged high-temperature exposure remains a significant challenge.
Low thermal conductivity materials such as 8 wt% yttria-stabilized zirconia (8YSZ) are successfully employed on the hot-section components to protect the superalloy from hot gases [13,14]; however, YSZ coatings undergo structural changes due to sintering when exposed to service temperatures close to 1200 °C, resulting in a degradation of their thermal and mechanical properties [15,16,17]. The higher operating temperatures of SPPS coatings, often exceeding 1200 °C, exacerbate these issues, limiting the application and development of biomimetic structure coatings. Sintering is an irreversible process that gradually heals the micropores within TBCs, leading to a significant increase in thermal conductivity and elastic modulus, which in turn reduces the thermal insulation performance and increases the likelihood of coating failure [18].
Despite the critical role of micro-nanostructures in SPPS coatings, research on the long-term stability and durability of these non-wetting protective methods remains limited. Most studies on TBC sintering phenomena have focused on 8YSZ materials, while research on the sintering behavior of coatings doped with rare earth elements is relatively scarce [19]. In order to improve the high-temperature sintering resistance of YSZ materials, Liu et al. [20] subjected YSZ and Yb-doped YSZ (YbYSZ) to heat treatment at 1300 °C × 32 h, indicating that YbYSZ materials had significantly improved sintering resistance due to their larger atomic mass. In addition, Fang et al. [21] subjected YSZ and YbYSZ samples to 1500 °C × 600 h heat treatment, suggesting that YSZ underwent phase transition, while YbYSZ did not undergo phase transition and exhibited good phase stability. Given that Yb-doped YSZ (YbYSZ) has also shown promise in resisting CMAS infiltration [20,22], the degradation depth of YbYSZ was improved by about 30% compared to YSZ samples under conditions of 1300 °C × 2 h and 12 h. Thus, it is necessary to investigate the structural evolution of YbYSZ-SPPS layers after sintering to ensure their performance and reliability under long-term service conditions.
This study aims to address this gap by exploring the long-term stability of YbYSZ-SPPS coatings under high-temperature conditions. Free-standing YbYSZ-SPPS coating was subjected to heat treatment at 1300 °C for 0.5 h, 1 h, 2 h, and 8 h, followed by sessile drop experiments to investigate its wettability. Through systematic experimentation and analysis, this research provides insights into the sintering behavior and structural stability of these coatings, contributing to the development of more durable and reliable TBCs for high-temperature applications. The solution, the achieved effects thanks to the applied solutions, and the practical cases of micro-nano dual-scale coatings are presented in Table 1.

2. Experimental Details

2.1. Preparation of TBCs

Yb-doped Y2O3 stabilized ZrO2 (YbYSZ) coatings were prepared using APS and the SPPS technique on carbon steel substrates, which are easily corroded by hydrochloric acid to obtain a free-standing coating. Figure 1 shows a schematic diagram of the SPPS technology. Through this process, the TBC surface was expected to achieve a lotus leaf structure as shown in Figure 2a. The substrate was pre-treated by sandblasting with 0.7 MPa pressure and 80# brown-corundum particles for 2 min. Then, the substrate was cleaned with an ultrasonic instrument at 40 KHz at room temperature for 15 min in anhydrous ethanol medium to ensure optimal surface adhesion. An atmospheric plasma spraying system (Model: F4MB-XL, Oerlikon Metro) was employed, with parameters set according to the process conditions in Table 2. A bond coat, approximately 150 μm thick, was first applied to enhance adhesion between the substrate and the ceramic layer; subsequently, a ceramic layer of about 250 μm thickness was deposited. In APS technology, the distance between the plasma flame and the powder nozzle with a diameter of 1.8 mm is 6 mm. In SPPS technology, the angle between the injector with a diameter of 1 mm and the plasma flame is 60°. The SPPS coating was formulated using a precursor solution containing zirconium nitrate (ZrO(NO3)2·5H2O, 99.5%), ytterbium nitrate (Yb(NO3)3·5H2O, 99.5%), and yttrium nitrate (Y(NO3)3·6H2O, 99.5%) dissolved in a 1:1 ethanol–water mixture, with a Yb2O3 content of 4.0 mol% and a Y2O3 content of 0.5 mol%. These reagents are all from Sinopharm Chemical Reagent Co., Ltd., (Shanghai, China). The micro-nano dual-scale layer prepared by SPPS was then applied to achieve a final thickness of approximately 50 μm. A schematic diagram of the coating structure is shown in Figure 2b. The specific process parameters are given in Table 1.

2.2. Long-Term Stability Assessment Method

The coating sample was placed in a 1300 °C ceramic fiber furnace, heat-treated for different times (0.5 h, 1 h, 2 h, and 8 h), then taken out and cooled in the air. The prepared YSZ sample was set as a control (the specific settings are shown in Table 3). Due to the significant dependence of element diffusion on the healing of micropores and phase transition in coatings, structural changes are closely related to heat treatment temperature and duration. Previous studies have shown that after 8 h of heat treatment, the penetration depth of CMAS in conventional YSZ coatings can reach 600 μm, and in YbYSZ coatings it can reach 464 μm [20]. However, the commonly used thickness of thermal barrier coatings is about 300 μm, at which point the molten CMAS has penetrated the entire coating. Therefore, the maximum heat treatment time for this experiment was 8 h. If the micro-nano dual-scale layer maintains good structure and performance after 8 h of heat treatment, it is considered that the long-term stability of the SPPS layer meets the requirements.
Due to the heat treatment temperature exceeding the maximum operating temperature of the substrate, a free-standing coating was used for relevant experiments. By using dilute hydrochloric acid to peel off the prepared ceramic coating from the substrate, a free-standing coating was obtained. Subsequently, the micro-nano dual-scale coating was subjected to heat treatment at 1300 °C (heating rate of 10 °C/min) for different durations to investigate the long-term stability of the micro-nano structure. Additionally, in order to study the wetting behavior of micro-nano dual-scale coatings, the sessile drop method was used for detection. The CMAS powders, prepared in the laboratory [22], were uniaxially pressed into CMAS cylinders of φ 5 × 4 mm and then placed on the free-standing coatings surfaces. Finally, the samples were heated to the target temperature of 1300 °C for 180 s, 300 s, 420 s, and 600 s.
The long-term stability of the micro-nano dual-scale layer was examined, including compositional stability, structural stability, and CMAS wetting performance stability. The stability of the composition was determined by energy dispersive spectroscopy (EDS) to obtain the elemental composition data for the micro-nano dual-scale layer, and the sample was measured by X-ray diffraction (XRD) with a diffraction angle range of 20°–70°. The obtained diffraction peaks were compared with the standard card in the software to obtain the phase composition data. The structural stability was observed by capturing the microstructure of the sample surface cross-section using a scanning electron microscope (SEM); The stability of CMAS wetting performance was tested by the seat drop method to determine the wetting performance of the coating sample.

3. Results and Discussion

3.1. Component Stability

The element content of the heat-treated sample was detected using EDS, and the results are shown in Table 4. The five coatings are all composed of YbYSZ, and the calculated Yb2O3 content is around 4.0 mol%. The composition of the micro-nano dual-scale layer did not show significant changes after 8 h of heat treatment. Figure 3 shows the phase component of the sample surface before and after heat treatment. Among them, JCPDS No. 50-1089 is a t-ZrO2 standard card, and JCPDS No. 37-1484 is an m-ZrO2 standard card. It can be observed that the micro-nano dual-scale layer is composed of t-ZrO2, and heat treatment at 1300 °C × 8 h did not cause a phase transformation.
The as-sprayed 8YSZ coating is mainly composed of a tetragonal phase, with a small amount of cubic phase and monoclinic phase. According to the equilibrium phase diagram, 8YSZ should be composed of monoclinic and cubic phases (c-ZrO2) at room temperature [23]; however, due to the very fast cooling rate of molten droplets during plasma spraying, phase equilibrium and eutectoid phase transformation are difficult to fully achieve, and a portion of the tetragonal phase remains in small particles. This type of tetragonal phase is commonly referred to as non-equilibrium tetragonal phase (t’-ZrO2) [24,25], which is prone to transform into t and c phases at high temperatures. When the non-equilibrium tetragonal micro-nano dual-scale layer is exposed to a high temperature of 1300 °C, the Y and Yb elements redistribute, forming equilibrium tetragonal and equilibrium cubic phases [26,27,28]. The diffusion of Y and Yb elements is essentially the thermal vibration of Y3+ and Yb3+. However, this thermal vibration requires a certain temperature as a condition, and it is difficult to achieve the redistribution of the two elements at lower temperatures. Therefore, the formation of m-ZrO2 has a significant delay (or latency), and there are reports showing that the latency of 8YSZ can be as long as 150 h [29]. In addition, the diffusion rate of Yb3+ is smaller than that of Y3+, and the heat-treatment time for this study is relatively short (≤8 h). Fang et al. confirmed that the Yb element can better ensure the thermal stability of ZrO2 at 1500 °C × 600 h. The micro-nano dual-scale layer did not undergo a phase transition, which is consistent with the expected experiment results.

3.2. Structural Stability

Figure 4 shows the surface structure of the micro-nano dual-scale layer before and after heat treatment. The micro-nano dual-scale layer has a typical cauliflower structure and is densely distributed. The droplets are broken well, mainly composed of small nanoparticles with diameters of 100~800 nm and flat noodle structures with diameters of 2~5 μm. The microstructural evolution of the micro-nano dual-scale layer subjected to various durations of heat treatment at 1300 °C was systematically analyzed. Initially, the coatings exhibited a well-defined micro-nano structure characterized by dense surface coverage of nanoparticles and lamellar formations. After 0.5 h of heat treatment, slight sintering was observed, leading to minor fusion of these structural components while largely retaining the overall integrity. However, after 1 h, significant sintering occurred, resulting in the formation of sheet-like and hollow-sphere structures, which increased the porosity of the coatings. By 2 h, widespread sintering had further replaced the original micro-nano structure, indicating a shift towards a more densified yet porous configuration. After 8 h, the coatings exhibited extensive sintering, leading to complex surface morphologies dominated by sheet-like and hollow-sphere formations, along with a marked increase in porosity, suggesting a significant degradation of the initial micro-nano structure. In addition, in the microscopic images of the surface at low magnification, the cauliflower-like structure was maintained without significant changes in shape; however, in the high magnification image after sintering at 1300 °C × 1 h, the micro-nano dual-scale structure on the surface was connected into flakes. Ultimately, the particles on the flake-like structure grew at 1300 °C × 8 h.
Figure 5 shows that the cross-sectional morphologies of the micro-nano dual-scale layer subjected to heat treatment at 1300 °C reveal a clear progression of structural changes. Initially, the coatings exhibit a dense micro-nano structure with no visible pores, which provides stable mechanical and thermal properties. However, as the heat treatment progresses to 0.5 h, initial sintering leads to minor pore formation, slightly reducing structural stability. By 1 h, significant sintering is observed, resulting in the development of porous sheet-like structures that increase the overall porosity of the coatings. This structural evolution continues, with widespread sintering after 2 h replacing most of the original micro-nano structure, further increasing porosity and leading to a marked reduction in performance. After 8 h, extensive sintering dominates the coating, characterized by complex porous structures that likely cause a significant loss of resistance to CMAS infiltration.
Figure 6 shows the fracture morphologies of the micro-nano dual-scale layer. Initially, the untreated coating exhibits well-defined, densely packed nanoparticles, characterized by a distinct nanoparticle morphology and uniform distribution; however, after 0.5 h of heat treatment, the boundaries between nanoparticles begin to blur, indicating the onset of sintering, with initial fusion leading to the formation of small pores. This suggests early structural relaxation. After 1 h, significant fusion occurs, resulting in the loss of particle interfaces and the formation of larger pores, which may lead to a decrease in the micro-nano dual-scale layer’s density and potentially compromise its mechanical strength. By 2 h, the sintering becomes more pronounced, with the complete fusion of nanoparticles, the disappearance of distinct boundaries, and the emergence of large, irregular pores. This indicates a substantial reduction in the coating’s density. After 8 h of heat treatment, the original nanoparticle morphology is entirely lost, replaced by large-scale sintered agglomerates and extensive porosity. This complete transformation highlights the serious impact of prolonged sintering, resulting in a significant decrease in coating density and thermal protection capability. These observations underscore the importance of controlling heat treatment conditions to preserve the functional properties of micro-nano dual-scale coatings in high-temperature applications.
During the heat treatment of the micro-nano dual-scale layer at 1300 °C, the structural changes in the surface, cross-section, and fracture morphologies reveal the complex evolution mechanism inside the coating. These mechanisms collectively affect the performance and stability of the coating. The essence of sintering is the process of transforming the energy of coating particles from a high-energy state to a low-energy state, manifested as the densification of particles or structures at the microscopic level [30]. The reduction in surface free energy is the driving force for sintering. Before heat treatment, the higher the excess surface energy of the coating particle system, the easier it is to transition to a low-energy state, and the greater the sintering activity [31]. Thus, the sintering ability of TBC is directly related to its inherent excess surface energy [32]. As the sintering time increases, the sintering phenomenon intensifies, and loose nanoparticles connect with each other to form more surface energy sheet-like or hollow spherical structures [33]. The porosity inside the coating increases and the microstructure is replaced. The diffusion rate of elements in the material accelerates at high temperatures, leading to the gradual disappearance of interfaces between particles. This diffusion process promotes the rearrangement of materials, ultimately forming larger aggregates and pores. The nanoparticles in the micro-nano dual-scale layer have a larger specific surface area and longer interfaces compared to other coating structures. Compared with the micro-scale splashing particles in conventional APS coatings, the excess surface energy of nanoparticles increases sharply and the diffusion length is shortened, resulting in lower sintering activation energy and a faster sintering rate [34,35]. Therefore, after 8 h of heat treatment, significant sintering phenomena can be observed in some areas of the micro-nano dual-scale layer surface.

3.3. Stability of Wetting Performance of CMAS

Figure 7 shows the macroscopic wetting morphologies of the samples at different times. Over time, all samples exhibited a progressive decrease in their anti-wetting performance. Initially, the coatings displayed relatively small wetting spots at 180 s, indicating good initial resistance to CMAS infiltration. However, as the exposure time increased to 600 s, the wetting spots enlarged further across all samples, indicating a substantial loss of anti-wetting capability as a result of prolonged high-temperature exposure. Notably, at 600 s, the YbYSZ-2, YbYSZ-3, and YbYSZ-4 samples showed gray–brown diffusion marks around the black CMAS, indicating that CMAS penetrated further into the surrounding areas. Moreover, the ring widths of YbYSZ-3 and YbYSZ-4 were larger than that of YbYSZ-2, suggesting that long-term sintering had reduced the corrosion resistance of the micro-nano dual-scale layer to some extent.
Figure 8 and Figure 9 present the time-dependent curves of wetting diameter and contact angle. The wetting diameters of all samples gradually increase with time, and the increase in wetting diameter is significant after 300 s. The wetting diameters of YbYSZ and YbYSZ-1 were relatively small throughout the entire testing time, while YbYSZ-3 and YbYSZ-4 had the worst wetting due to the longer heat treatment time. Compared with YbYSZ, the wetting diameter of YbYSZ-4 increased by 8.52%, from 3.52 μm to 3.82 μm. YbYSZ without sintering exhibited better anti-wetting performance compared with the other samples, with its wetting diameter remaining the lowest throughout the entire time, indicating that its resistance to CMAS corrosion at high temperatures is superior to those of the other samples.
The contact angle of all samples gradually decreases with time, which means that CMAS becomes easier to spread on the coating surface, and the anti-wetting performance of the coating weakens. The contact angles of YbYSZ and YbYSZ-1 were relatively large throughout the entire testing time, especially in the early stages when the contact angle decreased slowly. This indicates that YbYSZ and YbYSZ-1 have good resistance to CMAS wetting, and it is difficult for CMAS to spread on their surfaces. The contact angles of YbYSZ-2, YbYSZ-3, and YbYSZ-4 rapidly decreased in the early stages of testing, especially after 300 s when the contact angles tended to stabilize at a low level. Compared to YbYSZ, the wetting angle of YbYSZ-4 decreased by 39.65%, from 46.40° to 28.00°. This indicates that after sintering, the anti-CMAS wetting performance of these samples decreases, and CMAS is more likely to expand on the surface.
The above results are consistent with the evolution law of microstructure. At 1 h, the sintering phenomenon began to appear on the surface of the micro-nano dual-scale layer, resulting in a decrease in anti-wetting performance. The sintering phenomenon mainly occurred inside the micro-nano dual-scale layer, and the micro-nano structure on the surface did not undergo large-scale degradation. Therefore, the wetting performance of CMAS did not show a significant decrease. Heat treatment for 8 h did not cause the micro-nano dual-scale layer to lose its ability to hinder the wetting of molten CMAS. However, sintering destroys the original micro-nano dual-scale structure, and the resulting sheet-like or hollow spherical structure reduces the ability to hinder the wetting of molten CMAS. The protuberances on the micro-nano dual-scale coating grow under sintering, reducing the complex nanostructure and becoming a factor in reducing the contact angle. According to research by Guo et al. [36], the lotus-leaf-like dual-scale microstructure coating prepared by PS-PVD technology also had many micro-nano protrusions on the surface, forming discontinuous three-phase (solid, liquid, gas) contact lines on the coating surface, reducing the contact between CMAS and the coating, and helping to further reduce their adhesion. Based on the above description, it can be inferred that the coating can still maintain the three-phase contact line after sintering, but the reduction in nano protrusions on the surface leads to an increase in the contact area. Therefore, as the heat treatment time of the micro-nano dual-scale layer increases, the anti-CMAS wetting performance of the coating decreases.
Figure 10 shows variation curves of Ca content with depth after heat treatment for 600 s. Ca content is lowest in YbYSZ-1 infiltration, and 0.5 h heat treatment did not damage the protruding structure, but instead made it denser, hindering the infiltration of molten CMAS. After heat treatment times exceeding 1 h, the number of sheet-like or hollow spherical structures inside the micro-nano dual-scale layer gradually increased, and the porosity increased, leading to an increase in the penetration of molten CMAS. Similarly, due to the absence of widespread degradation of the protruding structure, the Ca element content in all samples rapidly decreased within the range of 0 to 100 μm. The micro-nano dual-scale layer after 8 h of heat treatment still had a hindering effect on the penetration of molten CMAS.
Figure 11 shows the interface between the coating and CMAS after wetting for 600 s. The as-sprayed YbYSZ underwent slight structural degradation, and the loosely bonded areas at the top of the micro-nano dual-scale layers peeled off. The other four samples subjected to 1300 °C heat treatment did not exhibit large-scale delamination of micro-nano dual-scale layers, which is attributed to the healing of small pores and tighter particle bonding caused by heat treatment. It is conducive to resisting structural degradation. Inspired by this, short-term heat treatment of the coating after as-sprayed micro-nano dual-scale layers can alleviate the drawbacks of weak cohesion and poor bonding strength to some extent. However, by observing the enlarged area in Figure 11e, it can be observed that the hollow sphere region formed by sintering inside the micro-nano dual-scale layer undergoes structural degradation, and the hollow sphere structure is filled and destroyed by molten CMAS. It can be inferred that the structural evolution caused by sintering has a destructive effect on the anti-CMAS corrosion performance of micro-nano dual-scale layers. With the extension of service time, the micro-nano dual-scale layers will lose their protective effect on the entire coating system after large-scale sintering.

4. Conclusions

In order to investigate the performance stability of the micro-nano dual-scale layer under long-term service conditions, this study adopted SPPS technology to prepare a layer of anti-CMAS wetting protective layer with a micro-nano dual-scale structure on the surface of conventional APS coating. The stability of coatings under extreme conditions is an important condition for ensuring the operation of aircraft engines. The composition stability, structural stability, and long-term wetting angle stability of the coating were evaluated by heat treatment at 1300 °C for 0.5 h, 1 h, 2 h, and 8 h. The major conclusions are as follows:
(1)
After 8 h of heat treatment, the elemental composition and phase composition of the micro-nano dual-scale layer remained unchanged, indicating good compositional stability.
(2)
After 1 h of heat treatment, the micro-nano dual-scale layer began to sinter, forming sheet-like or hollow spherical structures. As heat treatment time increased, the number of sheet-like or hollow spherical structures gradually increased, leading to an increase in the porosity of the micro-nano dual-scale layer; however, the sintering phenomenon was mainly concentrated inside the micro-nano dual-scale layer. After 8 h of heat treatment, the surface convex structure of the micro-nano dual-scale layer did not show significant degradation, indicating good stability of the micro-nano dual-scale structure.
(3)
As the heat treatment time increased, the anti-CMAS wetting performance of the micro-nano dual-scale layer decreased. The sample treated for 8 h could still resist the wetting and penetration of molten CMAS, and the micro-nano dual-scale layer had good stability with a large wetting angle over a long period of time. Interestingly, short-term heat treatment (less than 1 h) can make the micro-nano dual-scale layer denser, hinder the penetration of molten CMAS, and enhance its resistance to degradation. Therefore, the study has a guiding significance for exploring the anti-CMAS corrosion performance of nanoscale coatings, and the wetting behavior of micro-nano dual-scale coatings provides a new solution for solving CMAS corrosion.

Author Contributions

Conceptualization, Y.L. and Y.W.; Methodology, Y.L. and Y.W.; Validation, Y.L. and Y.W.; Investigation, Y.L. and Y.W.; Writing—original draft, Y.L. and Y.W.; Writing—review & editing, Y.L. and Y.W.; Resources, W.W.; Data curation, W.W.; Supervision, W.W.; Project administration, W.W.; Funding acquisition, W.W.; Writing—review & editing, W.W.; Investigation, W.Z.; Investigation, J.W.; Investigation, K.L.; Investigation, H.L.; Investigation, P.L.; Investigation, S.Y.; Investigation, C.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was sponsored by the National High Technology Research and Development Program of China (2023YFB3711200), the National Natural Science Foundation of China (52175136, 52130511), Science Center for Gas Turbine Project (P2021-A-IV-002), Shanghai Joint Innovation Program in the Field of Commercial Aviation Engines, Shanghai Gaofeng Project for University Academic Program Development, and Key Research and Development Projects in Anhui Province (2022a05020004).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Conflicts of Interest

Chengcheng Zhang is employed by AECC Commercial Aircraft Engine Co., Ltd. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results. The authors declare no conflict of interest.

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Coatings 14 01123 g001
Figure 1. Schematic diagram of SPPS technology.
Figure 1. Schematic diagram of SPPS technology.
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Figure 2. Schematic diagram of coating structure (a) and bionic structural design of TBCs based on a lotus leaf structure (b).
Figure 2. Schematic diagram of coating structure (a) and bionic structural design of TBCs based on a lotus leaf structure (b).
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Figure 3. XRD pattern of the micro-nano dual-scale layer.
Figure 3. XRD pattern of the micro-nano dual-scale layer.
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Figure 4. Surface morphologies of the samples: (a) YbYSZ; (b) YbYSZ-1; (c) YbYSZ-2; (d) YbYSZ-3; (e) YbYSZ-4.
Figure 4. Surface morphologies of the samples: (a) YbYSZ; (b) YbYSZ-1; (c) YbYSZ-2; (d) YbYSZ-3; (e) YbYSZ-4.
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Figure 5. The cross-sectional morphologies of samples: (a) YbYSZ; (b) YbYSZ-1; (c) YbYSZ-2; (d) YbYSZ-3; (e) YbYSZ-4.
Figure 5. The cross-sectional morphologies of samples: (a) YbYSZ; (b) YbYSZ-1; (c) YbYSZ-2; (d) YbYSZ-3; (e) YbYSZ-4.
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Figure 6. Fracture morphologies of samples: (a) YbYSZ; (b) YbYSZ-1; (c) YbYSZ-2; (d) YbYSZ-3; (e) YbYSZ-4.
Figure 6. Fracture morphologies of samples: (a) YbYSZ; (b) YbYSZ-1; (c) YbYSZ-2; (d) YbYSZ-3; (e) YbYSZ-4.
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Figure 7. CMAS macro-wetting morphology: (a) YbYSZ; (b) YbYSZ-1; (c) YbYSZ-2; (d) YbYSZ-3; (e) YbYSZ-4.
Figure 7. CMAS macro-wetting morphology: (a) YbYSZ; (b) YbYSZ-1; (c) YbYSZ-2; (d) YbYSZ-3; (e) YbYSZ-4.
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Figure 8. Variation curves of molten CMAS wetting diameter with time.
Figure 8. Variation curves of molten CMAS wetting diameter with time.
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Figure 9. Variation curves of molten CMAS contact angle with time.
Figure 9. Variation curves of molten CMAS contact angle with time.
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Figure 10. Variation curves of Ca content with depth after 600 s of heat treatment.
Figure 10. Variation curves of Ca content with depth after 600 s of heat treatment.
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Figure 11. Interface between TBCs and CMAS after different heat treatment times: (a) YbYSZ; (b) YbYSZ-1; (c) YbYSZ-2; (d) YbYSZ-3; (e) YbYSZ-4.
Figure 11. Interface between TBCs and CMAS after different heat treatment times: (a) YbYSZ; (b) YbYSZ-1; (c) YbYSZ-2; (d) YbYSZ-3; (e) YbYSZ-4.
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Table 1. The solution, achieved effects thanks to the applied solutions, and practical cases of micro-nano dual-scale coatings.
Table 1. The solution, achieved effects thanks to the applied solutions, and practical cases of micro-nano dual-scale coatings.
SolutionAchieved EffectsPractical Cases
Anti-CMAS attackReduce CMAS penetration and improve coating lifeHigh-temperature protection of aircraft engine turbine blades
Enhance thermal isolationReduce thermal conductivity and improve insulation performanceThermal insulation coating in gas turbines
Improve mechanical performanceImprove strain tolerance and crack resistance and stabilize mechanical performanceApplication of surface coating for high-temperature industrial equipment
Table 2. Spraying process parameters of APS and SPPS layers.
Table 2. Spraying process parameters of APS and SPPS layers.
ParametersBCCeramic LayerMicro-Nano Dual-Scale Layer
Primary gas flow rate, Ar, (SLPM)504050
Secondary air flow rate, H2 (SLPM)789.5
Current, (A)600635650
Power, (kW)404044
Spraying distance, (mm)1008550
Traverse speed of gun, (mm/s)1000500400
Liquid delivery rate, (mL/min)--30
Table 3. Long-term stability test parameters.
Table 3. Long-term stability test parameters.
SamplesHeat-Treatment TemperatureHeat-Treatment Time (h)
YbYSZ1300 °C/
YbYSZ-10.5
YbYSZ-21
YbYSZ-32
YbYSZ-48
Table 4. Element composition (mol%) of the micro-nano dual-scale layer.
Table 4. Element composition (mol%) of the micro-nano dual-scale layer.
SamplesZrYbYO
YbYSZ46.464.170.6048.77
YbYSZ-148.203.950.8746.97
YbYSZ-249.704.410.2247.68
YbYSZ-348.473.870.6746.99
YbYSZ-450.264.250.5544.94
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Liu, Y.; Wang, Y.; Wang, W.; Zhang, W.; Wang, J.; Li, K.; Li, H.; Liu, P.; Yang, S.; Zhang, C. Micro-Nano Dual-Scale Coatings Prepared by Suspension Precursor Plasma Spraying for Resisting Molten Silicate Deposit. Coatings 2024, 14, 1123. https://doi.org/10.3390/coatings14091123

AMA Style

Liu Y, Wang Y, Wang W, Zhang W, Wang J, Li K, Li H, Liu P, Yang S, Zhang C. Micro-Nano Dual-Scale Coatings Prepared by Suspension Precursor Plasma Spraying for Resisting Molten Silicate Deposit. Coatings. 2024; 14(9):1123. https://doi.org/10.3390/coatings14091123

Chicago/Turabian Style

Liu, Yangguang, Yihao Wang, Weize Wang, Wenkang Zhang, Junhao Wang, Kaibin Li, Hongchen Li, Pengpeng Liu, Shilong Yang, and Chengcheng Zhang. 2024. "Micro-Nano Dual-Scale Coatings Prepared by Suspension Precursor Plasma Spraying for Resisting Molten Silicate Deposit" Coatings 14, no. 9: 1123. https://doi.org/10.3390/coatings14091123

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