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Article

Interfacial Structure and Physical Properties of High-Entropy Oxide Coatings Prepared via Atmospheric Plasma Spraying

by
Tae-sung Park
1,
Nana Kwabena Adomako
2,
Andrews-nsiah Ashong
1,
Young-kuk Kim
3,
Seung-min Yang
4 and
Jeoung-han Kim
1,*
1
Department of Materials Science & Engineering, Hanbat National University, Yuseong-gu, Daejeon 34158, Korea
2
School of Materials Science & Engineering, UNSW Sydney, Sydney, NSW 2052, Australia
3
Magnetic Materials Department, Korea Institute of Materials Science, Seongsan-gu, Changwon 51508, Korea
4
Functional Materials and Components R&D Group, Korea Institute of Industrial Technology, Gangneung 25440, Korea
*
Author to whom correspondence should be addressed.
Coatings 2021, 11(7), 755; https://doi.org/10.3390/coatings11070755
Submission received: 9 May 2021 / Revised: 8 June 2021 / Accepted: 21 June 2021 / Published: 24 June 2021
(This article belongs to the Special Issue Surface Modification and Functionalization of High-Temperature Alloys)
Browse Figures
Versions Notes

Abstract

:
The feasibility of using a high-entropy rare-earth oxide (REO) as a top coating material for thermal barrier coatings was explored using the atmospheric plasma spray technique. The microstructure and Vickers hardness of the coating layer were compared to those of an 8 mol % yttria-stabilized zirconia (8YSZ) top coating material. Macroscopic observations revealed the formation of a well-coated surface with no surface defects or delamination. Scanning electron microscopy images showed the presence of several parallel and vertical microcracks in the REO and 8YSZ coating layers. The origin of these cracks is attributed to differences in the coefficient of thermal expansion, very fast cooling, and process parameters. X-ray diffraction demonstrated the high phase stability and excellent thermal properties of REO due to the absence of phase transformation after plasma spray processing. The measured Vickers hardness of REO was 425 HV, which is lower than that of sintered REO powder and the 8YSZ coating.

1. Introduction

Thermal barrier coatings (TBCs), which are protective ceramic coatings applied to the surfaces of hot metallic sections, are widely used in gas turbines and aircraft engines, among others. The main function of TBCs is to reduce the heat load and ensure oxidation resistance in high-temperature conditions, thus improving the operating temperature and efficiency [1,2,3]. TBCs usually consist of three layers: substrate, bond coating, and top coating. In general, the top coating layer is formed on a substrate composed of a super heat-resistant alloy [4,5]. The bond coating is used to accommodate differences in the thermal expansion coefficient between the ceramic coating and the metallic substrate under practical conditions, and to protect the substrate from harsh environments. One of the most popular bond coatings is NiCrAlY as it is compatible with almost all types of coatings, substrates, and methods [6,7].
The top coating is composed of ceramic powder, which acts as a thermal shield. A typical ceramic material for the top coating is 8 mol % yttria-stabilized zirconia (8YSZ), which has excellent thermal shielding properties. 8YSZ has been widely used as a TBC material over recent decades owing to its low density and thermal conductivity, high thermal stability, and appropriate thermal shock and corrosion resistance, rendering satisfactory performance under harsh industrial conditions [8,9,10,11]. The excellent properties of 8YSZ are attributed to its columnar microstructure with perfect strain tolerance and adhesion, prevention of phase transformation of ZrO2 by yttrium, and reduction in thermal conductivity by oxygen vacancies and yttrium. However, despite these impressive properties, YSZ coatings exhibit certain disadvantages, such as a limited operating temperature (1200 °C), hot corrosion, and phase transformation (~1200 °C) [12,13,14]. Hence, 8YSZ is less stable above 1200 °C, and its thermal conductivity should be lowered to withstand high temperatures [15].
Currently, extensive studies are being conducted to develop a suitable alternative to 8YSZ [2,16,17]. These research efforts have led to the discovery of new materials with enhanced thermal barrier properties [18]. For practical applications, TBCs should exhibit a high melting point, high thermal stability, low thermal conductivity, chemical compatibility, a high thermal expansion coefficient, and sintering resistance [19]. Mullite (3Al2O3-2SiO2) and alumina (α-phase Al2O3) are two promising candidates for TBCs; however, their high thermal conductivity and failure at high temperatures limit their applications. On the other hand, metal–glass composites, perovskite structure oxides (ABO3), magnetoplumbite compounds, and rare-earth materials, including rare-earth zirconates, oxides, tantalate, and silicates, are at the cutting edge of development of superior-performance TBCs [20,21,22,23,24].
During the last decade, the design and development of multicomponent materials called high-entropy alloys and high-entropy ceramics (HECs) with remarkable characteristics and applications have become more significant in the field of TBCs [25]. High-entropy materials have higher configurational entropy (ΔSconf) than conventional materials, promoting the formation of a single-phase solid solution, which leads to promising mechanical properties at elevated temperatures [26,27]. New research studies have revealed that formation of a single-phase solid solution in high-entropy materials is an effective method for enhancing the thermal properties of TBCs [28,29]. For instance, Rost et al. [30] developed a high-entropy oxide (HEO), consisting of five different cations, which exhibited a single-phase structure. Subsequently, other HEOs were developed by adding more components to obtain materials exhibiting a combination of beneficial properties [31].
HEOs have been demonstrated to possess many exotic properties including thermoelectric properties, wear resistance, and corrosion resistance, and their possible applications include catalysts and batteries, among others. In particular, recent studies have demonstrated that forming a high-entropy solid solution is an effective method for improving the thermal insulation properties of thermal barrier materials [16,32,33]. For example, high-entropy (La0.2Ce0.2Nd0.2Sm0.2Eu0.2)2Zr2O7 exhibits a low room-temperature thermal conductivity of 0.76 W/m⋅K [16]. Wright et al. [34] investigated 22 compositionally complex rare-earth zirconates and proposed size disorder as a descriptor for predicting thermal conductivity in these ceramic materials.
An important factor affecting the properties of TBCs is the fabrication method. With the advancement of TBCs over recent decades, various methods have also been developed to synthesize new TBCs with more promising properties and applications. These methods are broadly divided into several categories, including the use of lasers, thermal spraying, vapor deposition, powder metallurgy, and chemical-based methods [35,36]. Among these, thermal spraying is the most widely used method because of its efficiency, accessibility, and practicability over a wide range of materials. During thermal spraying, raw stock particles are melted or heated using a flame and then deposited onto a substrate surface. Several thermal spraying techniques have been developed based on energy input, carrier gas, and flame type. Atmospheric plasma spray (APS) is the most flexible spray technique with regard to bond coating, coating, and substrate materials [36]. Applicability, the capability to process different kinds of materials with different shapes and sizes, and the ability to apply the coating on a variety of substrates and produce coatings with different thicknesses distinguish the APS technique from other thermal spray methods. Additionally, APS can be used to deposit coatings with fine and columnar microstructures, which not only demonstrate superior mechanical properties but also increased resistance against thermal stresses [37,38,39].
As discussed earlier, research on applying high-entropy materials to the top coating of TBCs has recently been initiated. Most studies have attempted to improve the physical properties of existing ceramics by adding some elements to increase the configuration entropy [16,19,32,40,41]. We carried out a preliminary study to explore the feasibility of using HEO as a TBC material. In particular, this research focused on the use of a high-entropy rare-earth oxide (REO), fabricated via ball milling, as a top coat material. Material selection and design were performed based on minimum differences in ionic size to ensure easy formation of a solid solution, low thermal conductivity, and high thermal stability. Specifically, the phase compositions, microstructures, and microhardnesses of the TBC were investigated. Furthermore, the thermal conductivity and thermal stability of the REO coating at elevated temperatures, cross-sectional microstructure, and the phase of the coating layer were analyzed and evaluated. The overall results were then compared to those of commercial 8YSZ.

2. Experimental Method

2.1. Thermal Barrier Coating

Figure 1 shows the schematic diagram of the experimental methods and materials used in this study. A commercial powder of composition Co–Ni–Cr–Al–Y (Amdry 365-2) was used for the bond coating layer, whereas 8YSZ (8–9 mol % Y2O3-doped ZrO2) and REO were utilized for the top coating layer. The REO powder was synthesized via planetary ball milling. Five rare-earth oxides, i.e., Y2O3, La2O3, CeO2, Nd2O3, and Gd2O3 (Sigma-Aldrich, St. Louis, MO, USA, 99.9%), were used as precursors. The typical characteristics of the REOs used in this study are listed in Table 1. The oxides were mixed for 24 h using a 3D blending machine (model) according to the stoichiometric ratio. The powder mixture was then put in a zirconia pot together with a zirconia ball (diameter. 9.525 and 4.762 mm) at a powder/ball mass ratio of 1/10 and ball milled for 8 h at a rotational speed of 600 rpm.
After ball milling, the synthesized REOs were subjected to heat treatment at 1500 °C in air for 10 h, followed by furnace cooling. Powders less than 75 µm in size were sieved using a 200-mesh screen. Afterward, the Co–Ni–Cr–Al–Y (Amdry 365-2) and REO powders were plasma sprayed onto the surface of a metallic substrate (coin-shaped specimen composed of Inconel 625). Before spraying, the substrates were cut into pieces with diameter of 12.7 mm and thickness of 1.5 mm. The specimens were also polished using SiC papers, down to 150 grit, and alumina solution. The depositions of both the coatings were carried out using APS under an argon (Ar) + hydrogen (H) atmosphere. By using Ar and H mixture as the carrier gas, the oxidation rate of the coatings can be significantly reduced and a more stable plasma can be obtained. In fact, the quality and power of plasma are directly related to the interaction between electric discharge and gas flow, which converts electrical energy to thermal energy by stabilizing high-enthalpy plasma. This is a thermodynamic process affected by mass and heat transfers and electro-physical phenomena [39,42,43]. The process parameters are listed in Table 2.

2.2. Characterization

The phase compositions of the raw REO powder and both REO and 8YSZ top coatings were analyzed by X-ray diffraction (XRD, Rigaku SmartLab) using Cu Kα radiation (1.541 Å). The 2θ scanning range and step intervals were 20°–80° and 0.02°, respectively. The cross-sections of the coated specimens were characterized via field-emission scanning electron microscopy (FESEM, HITACHI SU5000, Tokyo, Japan) with energy-dispersive X-ray spectroscopy (EDS, Oxford X-MAX).
The microhardnesses of the as-milled REO powder and the REO and 8YSZ top coating layers were measured using a Vickers hardness tester (Struers Duramin-40, Westlake, OH, USA) at an indenter load of 0.2 kg and a holding time of 50 s. Afterward, the results were compared with the previously reported hardness value of 8YSZ. The hardness was measured at 14 points in the horizontal direction, and the average was calculated, excluding the lowest and highest values. A dilatometer (NETZSCH DIL 402 C, Selb, Germany) was employed to determine the coefficient of thermal expansion (CTE) of the specimens. The measurement was performed under an argon atmosphere from 25 to 1300 °C at a heating rate of 5 °C/min.

3. Results and Discussion

3.1. Phase Analysis

X-ray diffraction analyses of the REO powder and the top coating layers were carried out to study their phase stabilities. Figure 2a shows the XRD pattern of the raw REO powder, which indicates a bixbyite structure with space group Ia3. The diffraction peaks of the raw oxides are not discernible, owing to the reduction in particle size during ball milling (which leads to peak broadening). The characteristic peaks of the fcc phase are distinctly observed compared to other peaks, which implies that the REO was synthesized via solid-state reactions. Additionally, a few minor peaks are observed at 2θ values between 30° and 75°, which are attributed to substructures or metastable phases. The origin of these minor peaks could not be identified.
The XRD pattern of the REO coating shows a similar bixbyite phase structure (Figure 3b). This indicates that there was no phase transformation of REO after melting and rapid cooling to ambient temperature. Rapid cooling may lead to a very uniform, fine, and crystalline microstructure. The similar XRD patterns of the ball-milled and coated REO also indicate that REO did not undergo oxidation during the APS process. HEOs have excellent thermal stabilities even at high temperatures. In fact, the sluggish diffusion effect is related to the solid solution and multicomponent structures of these materials [44].
The XRD pattern of plasma-sprayed 8YSZ is shown in Figure 2c. It shows the presence of tetragonal and cubic ZrO2 and is similar to the previously reported XRD pattern of 8YSZ [45]. A slight broadening of the main diffraction peaks may be due to the formation of a fine-grain structure. The tetragonal phase is believed to have formed from the cubic phase; very fast solidification after APS provides a driving force for a diffusionless phase transformation from cubic to tetragonal [3,16,21]. Although the chemical composition is the same, the tetragonal phase, which has a more complicated microstructure, has higher thermal stability and better resistance against crack propagation than the cubic phase [46,47,48,49].

3.2. Microstructure

The photographs of the REO- and 8YSZ-coated specimens shown in Figure 3a,b, respectively, reveal the formation of well-coated surfaces with no observable defects in both the coatings. This result demonstrates that the process parameters used for plasma coating were optimal. Generally, crack initiation or failure in TBCs occurs at edge regions where thermal stresses concentrate during thermal spraying [50]. Plasma-sprayed 8YSZ coatings are widely used for commercial applications [51], and thus process optimization is available. On the other hand, process optimization for plasma spraying of REO is not available because this is the first time this material has been used in this type of application. Nevertheless, the well-coated surface indicates the feasibility of using REO as a top coating material. The difference in the surface color originated from the colors of their respective powders: REO is olive, whereas 8YSZ is white.
Figure 3c shows the cross-sectional SEM image of the TBC specimen with REO. The thickness of the REO top coating was measured to be 65 μm. The cross-section reveals good interfacial bonding without any delamination between the REO, bond coating, and IN625 substrate. This shows high compatibility between the layers. However, vertical and parallel microcracks are observed inside the REO coating layer. These cracks are randomly distributed in the layer, suggesting that the strength of the coating is considerably low. For comparison, Figure 3d shows the cross-section of the TBC specimen with 8YSZ. The thickness of the coating layer is approximately 130 μm, which is twice that of REO. The large difference in thickness is because of the large amount of thermal-sprayed 8YSZ powder compared to the REO. Similar to the REO specimen, the cross-section reveals good interfacial bonding between the 8YSZ, bond coating, and IN625 substrate. However, a small number of continuous cracks are observed inside the 8YSZ coating layer, which are larger than those in REO.
Crack formation is attributed to a number of factors. As the coating thickness increases over time, the heating capability of the plasma jet declines. Hence, non-melted or partially melted particles accumulate and produce microcracks [52]. The bond coating is another significant factor. For the NiCrAlY bond coating, the concentrations of elements are very important. If the Al content is higher than normal, the coating becomes unstable because of the formation of LaAlO3. Furthermore, high temperatures within the bond coating and at the bond coating/coating interface may accelerate oxidation and other adverse reactions [53,54]. Oxidation causes many defects, especially interlamellar gaps, porosity, and delamination [55]. Grain growth can also affect microcrack formation. The system thermodynamically tends to decrease its surface energy through the reduction in grain boundaries. High temperatures during ASP provide favorable conditions for rapid grain growth of the bond coating or top coating. This restricts grain boundary migration because atoms need to pass through a long diffusion path. Therefore, gaps are formed between neighboring grains, leading to crack initiation [44,49,52].
The differences in the cracks observed in the two coatings may be attributed to other factors. High-entropy materials sometimes show strange crack propagation modes. In these materials, the crack propagation is controlled by crack length, crack deflection, and plastic relaxation [56]. Crack formation is also controlled by residual stresses. Residual stresses are mostly generated during cooling from plasma to ambient temperature. Hence, stresses are produced across the coating thickness. These residual stresses mainly result from the difference in the thermal expansion of the materials. A higher amount of residual stresses can be observed at the TGO/top coating interface, causing crack initiation. Owing to the higher thermal shock at the TGO/top coating interface, the residual stresses are compressive near the interface, while they are tensile in farther regions with lower thermal shock [57,58]. These results show that suitable heat treatments in terms of time and temperature can effectively influence the cracking mode in TBCs by decreasing and changing the residual stress [59]. Simulation results also revealed that the formation of TGO is the main reason for the increase in the internal stresses in the top coating, the TGO/top coating interface, and even the bond coating/top coating interface [60].
The elemental maps of the REO coating are shown in Figure 4. These maps indicate that Ce, Gd, La, Nd, and Y are uniformly distributed in the coating. O is the main element in the coating. The EDS map of the bond coating shows a homogeneous distribution of Al, Co, Cr, and Ni without any trace of the rare-earth elements. However, O exists on both sides of the top coating/bond coating interface, indicating partial diffusion of O due to the high temperature of APS. Additionally, crystallographic defects, such as pores, vacancies, voids, and dislocations, act as fast diffusion pathways for O. Al is also found on both sides of the interface. In fact, O and Al play key roles in the formation of thermally grown oxide (TGO) layers. The quality of TGO is very important for enhancing the thermal barrier properties. A uniform and continuous TGO layer restricts more internal oxidation and hence improves the performance of TBCs [61,62]. It is worth mentioning that one of the most significant features of high-entropy materials is their higher oxidation resistance governed by sluggish diffusion compared to other conventional materials [63]. This characteristic may lead to the formation of a more uniform and continuous TGO layer, as illustrated in Figure 5. The line scan shows the diffusion of O and Al at the interface where TGO is formed.

3.3. Microhardness

Figure 6 shows the average Vickers microhardness values of the as-milled REO powder, REO coating, and 8YSZ coating [64]. The hardness of as-milled REO is 521 HV, which is higher than that of the REO coating (425 HV). Although the lower hardness of the REO coating compared to that of the as-milled REO is unexpected (because melting and solidification lead to the formation of a denser solid [65]), the numerous cracks observed in the REO coating may be a possible reason. As shown in Figure 3, porosities, crystalline defects, and probable oxide components resulted in a heterogeneous microstructure with a non-uniform microhardness distribution. Similar phenomena have been observed in many other research studies [66]. The 8YSZ coating exhibited a microhardness of 741 HV, which is higher than that reported previously (580 HV) [64]. This shows that 8YSZ has a very high overall hardness compared to the REO coating. Studies have revealed that the mechanical properties of YSZ coatings strongly depend on the ASP process conditions. For example, a prolonged process or higher temperatures than normal may cause decreases in the hardness and other mechanical properties. This is a direct consequence of the cubic-to-tetragonal phase transformation [67].
The phase analyses, microstructure observations, and hardness measurements confirm that the large cracks formed in the 8YSZ coating are due to rapid cooling of the layer after plasma spraying. The rapid cooling process easily generates cracks in the very hard (brittle) layer. Highly brittle materials are known to be prone to cracks owing to sharp thermal variations. The hardness of the REO layer was extremely low to permit the initiation of such large cracks.

3.4. Thermal Behavior

Thermal conductivity and thermal expansion are two significant factors that affect the insulation properties of TBCs. Figure 7 shows the measured thermal properties of the materials involved in the fabrication of both the TBCs. According to Figure 7a, there are large differences between the CTEs of the REO coating, bond coating, and Inconel 625 substrate. The REO coating has the lowest CTE among the tested materials. The change in the thermal conductivity of the REO coating with respect to temperature is illustrated in Figure 5b. High entropy values and lattice distortions are two extraordinary characteristics of high-entropy ceramics that provide low thermal conductivities [68]. In REO, cations with different masses and ionic radii may randomly occupy equivalent lattice crystallographic sites. The disordered chemical composition and mass difference result in strong lattice distortion and local strain distribution, which lead to accelerated phonon scattering from disordered oxygen vacancies in HEOs [33,69,70].
Closely matched CTEs of the TBC components and lower thermal conductivity are the two most important features of a promising TBC. Hence, REO is not a good choice for coating Inconel 625 [32]. Additionally, the numerous small cracks observed in the REO layer can be attributed to large differences in the CTEs of REO and the bond coating, as observed in Figure 7a. Figure 7b shows the change in thermal conductivity with temperature for both REO and 8YSZ coatings. The thermal conductivity of the REO coating decreases from 0.891 to 0.66 W/m⋅K with the increase in temperature from 100 to 500 °C, while that of 8YSZ increases from 1.83 to 1.99 W/m⋅K [71]. The difference between the thermal conductivities of REO and 8YSZ coatings is ascribed to microstructural defects that change the heat transfer pattern through the TBCs [72]. According to Figure 7b, REO exhibits a superior performance compared to 8YSZ with the increase in temperature. On the other hand, the difference in the CTE between the 8YSZ coating and the bond coating is small, and thus the formation of cracks cannot be attributed to CTE differences. However, the main drawback of 8YSZ is its thermally activated phase transformation, which leads to volume changes during thermal cycles. Further, 8YSZ exhibits a high rate of oxygen conductivity and thus may undergo severe oxidation [73].

3.5. Factors Affecting Coating Properties

As mentioned in the former sections, the properties of TBCs are influenced by several factors, which are summarized here. The thickness of the top coating plays a key role in the failure and cracking of TBCs. As the thickness of the top coating increases, thermal gradients and residual stresses increase. Thus, cracking is promoted when the top coating is thick [74]. Density and porosity have direct effects on the performance of TBCs. Pores help to reduce the thermal conductivity and enhance the performance of TBCs at elevated temperatures. Additionally, pores can relieve internal stresses by compensation of thermal expansion differences during thermal cycles [75]. However, pores are preferred sites for crack nucleation, growth, coalescence, and consequent fracture. Similarly, the large quenching stresses usually cannot be retained within the individual splats as residual stresses, and hence the splats shrink and form microcracks that help relieve these internal stresses [76]. Phase transformation is a thermal phenomenon in TBCs, especially in YSZ coatings, that has a bilateral effect. On one hand, phase transformation causes volume expansion, leading to the formation of a fine microcrack network; this can relieve internal stresses and thermal shocks. On the other hand, volume changes during cooling and heating resulting from phase transformation can cause the coating to crumble and finally fracture [77,78,79]. Other factors that affect the lifetime of TBCs include impurities, roughness, chemical composition, and substrate; they mostly affect TGO formation through production of internal stresses [80].

4. Conclusions

TBCs with REO as a top coating material were successfully prepared using the plasma spray technique. SEM analysis demonstrated that REO could be coated without delamination or surface defects, similar to 8YSZ. XRD analyses revealed that the powder phase is not transformed after coating, which shows that REO has sufficient phase stability and is expected to have excellent thermal properties owing to the structural properties of bixbyite. Several parallel and vertical microcracks were observed in the REO and 8YSZ coating layers. Although these cracks were attributed to CTE differences, the effects of very fast cooling and process parameters require further investigation. The REO coating exhibited a Vickers hardness value of 425 HV, which is lower than that of sintered REO powder and the 8YSZ coating. The comparison between the thermal properties of the TBC materials revealed that differences in the CTE between the materials are the main reason for crack initiation and failure.

Author Contributions

Conceptualization, J.-h.K.; methodology, Y.-k.K.; formal analysis, N.K.A.; investigation, A.-n.A.; investigation, S.-m.Y.; writing—original draft preparation, T.-s.P.; writing—review and editing, J.-h.K.; correspondence, J.-h.K. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financially supported by Hanbat National University Financial Accounting Research Fund, 2019 academic year.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Coatings 11 00755 g001 550
Figure 1. Schematic diagrams of the experimental methods.
Figure 1. Schematic diagrams of the experimental methods.
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Figure 2. XRD patterns of: (a) REO powder; (b) REO coating; and (c) 8YSZ coating.
Figure 2. XRD patterns of: (a) REO powder; (b) REO coating; and (c) 8YSZ coating.
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Figure 3. Macroscopic and microscopic images of the thermal barrier coating specimens: (a,b) photographs; (c,d) cross-sectional SEM images.
Figure 3. Macroscopic and microscopic images of the thermal barrier coating specimens: (a,b) photographs; (c,d) cross-sectional SEM images.
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Figure 4. Elemental maps obtained from the cross-section of the REO coating.
Figure 4. Elemental maps obtained from the cross-section of the REO coating.
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Figure 5. Line scans obtained from the cross-section of the REO coating.
Figure 5. Line scans obtained from the cross-section of the REO coating.
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Figure 6. Comparison of Vickers hardness values of as-milled REO, REO coating, and 8YSZ coating.
Figure 6. Comparison of Vickers hardness values of as-milled REO, REO coating, and 8YSZ coating.
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Figure 7. (a) CTEs of REO and other comparative materials; (b) thermal conductivities of REO and 8YSZ [71,72] with respect to temperature.
Figure 7. (a) CTEs of REO and other comparative materials; (b) thermal conductivities of REO and 8YSZ [71,72] with respect to temperature.
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Table
Table 1. Properties of the precursor oxides used in this study.
Table 1. Properties of the precursor oxides used in this study.
PrecursorSpace GroupValenceIonic Radius (nm)
CeO2fluorite (Fm3m)4+0.97
Y2O3bixbyite (Ia3)3+0.9
La2O3trigonal (P3m1)3+1.1
Nd2O3trigonal (P3m1)3+1.048
Gd2O3cubic (Ia3)3+0.938
Table
Table 2. Air plasma spraying parameters for REO and 8YSZ coatings.
Table 2. Air plasma spraying parameters for REO and 8YSZ coatings.
ParametersValue
Powder feed rate (g/min)20
Gas flow (L/min)Hydrogen (22.5)
Argon (100)
Gun speed (mm/s)1000
Gun to work distance (mm)100
Thickness after 1 pass (µm)6.5
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Park, T.-s.; Adomako, N.K.; Ashong, A.-n.; Kim, Y.-k.; Yang, S.-m.; Kim, J.-h. Interfacial Structure and Physical Properties of High-Entropy Oxide Coatings Prepared via Atmospheric Plasma Spraying. Coatings 2021, 11, 755. https://doi.org/10.3390/coatings11070755

AMA Style

Park T-s, Adomako NK, Ashong A-n, Kim Y-k, Yang S-m, Kim J-h. Interfacial Structure and Physical Properties of High-Entropy Oxide Coatings Prepared via Atmospheric Plasma Spraying. Coatings. 2021; 11(7):755. https://doi.org/10.3390/coatings11070755

Chicago/Turabian Style

Park, Tae-sung, Nana Kwabena Adomako, Andrews-nsiah Ashong, Young-kuk Kim, Seung-min Yang, and Jeoung-han Kim. 2021. "Interfacial Structure and Physical Properties of High-Entropy Oxide Coatings Prepared via Atmospheric Plasma Spraying" Coatings 11, no. 7: 755. https://doi.org/10.3390/coatings11070755

APA Style

Park, T.-s., Adomako, N. K., Ashong, A.-n., Kim, Y.-k., Yang, S.-m., & Kim, J.-h. (2021). Interfacial Structure and Physical Properties of High-Entropy Oxide Coatings Prepared via Atmospheric Plasma Spraying. Coatings, 11(7), 755. https://doi.org/10.3390/coatings11070755

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