Preparation and Characterization of Co-CuCoMnO_x Solar Selective Absorption Coatings

Abstract

The Co-CuCoMnO_x coatings with varying proportions were prepared and investigated to develop a novel metal–ceramic solar selective absorption coating employed at high temperature. The CuCoMnO_x powders were synthesized using the solid-phase reaction method. Subsequently, Co-CuCoMnO_x coatings were deposited on the surface of 316L steels utilizing the atmospheric plasma spraying (APS) technique. The results showed that the synthesized CuCoMnO_x powders were mainly composed of two phases, which were Cu_1.5Mn_1.5O₄ and MnCo₂O₄. The CuCoMnO_x powders had a solar absorptance of 0.929 and an infrared emittance of 0.862, which was considered a good solar absorbent. The synthesized Co-CuCoMnO_x coating had a typical thermal spray layered stacking structure. The chemical phases of the coatings were mainly Co, CoO, and CoCuMnO_x. Due to the addition of CuCoMnO_x inhibiting the oxidation of Co during the thermal spraying process, the 95Co-5CuCoMnO_x (wt%) coating exhibited the optimal quality factor (α/ε) of 2.184, with a solar absorptance α of 0.808 and an infrared emittance ε of 0.370, respectively. Moreover, this specific coating demonstrated a good thermal stability for up to 3 h when exposed to an atmospheric environment at 450 °C. The results indicate its significant potential for high-temperature solar selective absorption coating.

1. Introduction

In the contemporary world, numerous countries are actively engaged in research on photothermal power generation technology to effectively harness and utilize solar energy. As a critical component of solar collectors, solar selective absorption coatings play a key role [1], directly influencing the advancement of photothermal power generation technology. The principle underlying solar selective absorption coatings is their high absorptance (α) for solar radiation energy within the ultraviolet–visible near-infrared spectrum (0.3–2.5 μm), while simultaneously exhibiting low emittance (ε) in the mid–far-infrared range (2.5–25 μm) [2]. Solar selective absorption coatings can be categorized into three distinct types based on their operating temperature: low-temperature solar selective absorption coatings (operating at temperatures below 100 °C), medium-temperature solar selective absorption coatings (ranging from 100 °C to 400 °C), and high-temperature solar selective absorption coatings (exceeding 400 °C). Currently, research on solar selective absorption coating for mid- and low-temperature applications has reached a relatively advanced stage. In order to reduce costs and enhance the conversion efficiency of photothermal power generation, it is essential to continuously elevate the operating temperature of these solar selective absorption coatings. Therefore, there is a pressing need for the development of high-temperature solar selective absorption coatings that can endure the highest possible temperatures [3,4,5,6].

Metal–ceramic solar selective absorption coatings demonstrate exceptional optical performance and high-temperature thermal stability, making them promising candidates for the development of advanced high-temperature solar selective absorption coatings. Generally, metal–ceramic solar selective absorption coatings are composed of transition metals and their corresponding oxides, nitrides, or carbides. Examples include Ni-Al₂O₃ [7], Pt-Al₂O₃ [8], Ti-AlN [9], Ni/TiN-La₂O₃ [10], Co-Cr₃C₂-WC [11], and Ni/Mo-TiC [12]. Among these materials, the metal–ceramic solar selective absorption coatings based on Co-WC have been the subject of extensive investigation by various researchers. Chen et al. [13] investigated three types of WC-12Co cermet materials with micron, submicron, and nanometer structures to fabricate corresponding coatings using high-velocity oxygen–fuel (HVOF) spraying. It was observed that decarburization (the decomposition of WC into W₂C) occurred in all three structural forms during the spraying process. Notably, the nanometer structure also resulted in the formation of new phases, such as W and W₃O₄. The absorptance of the coatings across these three structures ranged from 0.78 to 0.86, while their emittances varied between 0.6 and 0.7, having a quality factor (α/ε) of 1.11~1.43. The relatively high emittance observed may be attributed to the comparatively low metal content present in these coatings. Ke et al. [14] utilized a material composition of 80% Co, 10% submicron WC, and 10% nano WC to apply a coating on stainless steel (SS) via high-velocity oxygen–fuel (HVOF) spraying. The resulting Co-WC coating exhibited an absorptance of 0.821 and an emittance of 0.434 respectively, giving a quality factor of 1.89. It is evident that the quality factor of the Co-WC type metal–ceramic solar selective absorption coating is not satisfactory.

To address the challenges associated with decarburization, oxidation, and the relatively low quality factor of Co-WC type coatings, this study contemplates the selection of a novel ceramic phase. The aim is to enhance both the optical performance and high-temperature stability of Co-containing metal–ceramic solar selective absorption coatings. Metal oxides possessing a spinel structure have demonstrated remarkable optical performance and high-temperature stability [15,16,17,18,19,20,21]. Consequently, they have frequently been employed as raw materials for the fabrication of solar selective absorption coatings in recent years. Among these materials, CuCoMnO_x with a spinel structure has garnered significant research attention [4,22,23,24,25,26,27]. Peng et al. [28] applied CuCoMnO_x coatings onto polished aluminum plates. By optimizing the coating thickness, they achieved an absorptance of 0.928 and an emittance of 0.198 for the coated surface. Rocío Bayón et al. [15] fabricated Cu_1.5Mn_1.5O₄ thin films on an aluminum substrate using the impregnation method, achieving an absorption rate exceeding 0.87. Subsequently, a SiO₂ anti-reflection layer was deposited onto the Cu_1.5Mn_1.5O₄ thin film via the sol–gel method, resulting in improved optical performance with an absorption rate of 0.94 and an emission rate of 0.06. George Kordas [29] incorporated CuO@SiO₂ microspheres into the CuCoMnO_x coating, achieving an impressive absorptance/emittance of 30. Furthermore, it was concluded that the ratio of absorptance to emittance increases linearly with the concentration of CuO@SiO₂ microspheres. Ke et al. [14] developed a composite coating on a stainless steel (SS) substrate by successively depositing layers of CuCoMnO_x, CuCoMnO_x-SiO₂, and SiO₂ atop a WC-Co coating fabricated using HVOF. This resulted in the formation of a WC-Co/CuCoMnO_x/CuCoMnO_x-SiO₂/SiO₂ composite structure. The incorporation of CuCoMnO_x was aimed at enhancing the absorption rate of the coating within the visible light spectrum while simultaneously filling in the grooves and pores present on the surface of the WC-Co layer, thereby reducing its surface roughness. The optical performance metrics for this composite coating were characterized by an absorption/emittance ratio of 0.915/0.29, demonstrating that it could maintain thermal stability for up to 50 h at 550 °C. Based on the aforementioned research, it is evident that CuCoMnO_x represents a promising ceramic phase candidate for enhancing the optical performance of Co-containing solar selective absorption coatings, while also exhibiting excellent high-temperature stability.

To obtain a solar selective absorption coating with excellent optical performance and high-temperature thermal stability, CuCoMnO_x spinel powder was synthesized using a combination of three powders—CuO, Co₃O₄, and MnO₂—in a muffle furnace. Then, various mass ratios of Co-CuCoMnO_x coatings were designed and prepared through the atmosphere plasma spraying method. The mechanism of optical property changes in plasma-sprayed coatings with different compositions and ratios was thoroughly analyzed. Finally, the coating with the best optical performance was selected to investigate its high-temperature stability, proving that it has the potential to become a candidate for high-temperature solar selective absorption coatings.

2. Experiment

2.1. Preparation of CuCoMnO_x Powders

CuO powders (with purity of 99.5%), MnO₂ powders (with purity of 99.5%), and Co₃O₄ powders (with purity of 99.9%) were sourced from Qinhuangdao Yinuo High-tech Materials Development Co., Ltd., Qinhuangdao, China. The corresponding masses of these powders were measured according to the molar ratio of Cu:Mn:Co, set at 3:3:1. Deionized water was added to the three mixed powders at a solid–liquid mass ratio of 1:0.84. Additionally, a self-prepared binder, dispersant, and several drops of defoamer (tributyl phosphate) were incorporated into the mixture. Subsequently, the mixture was introduced into a colloid mill and subjected to high-speed mixing for a duration of 2 h to ensure a homogeneous slurry. After that, the slurry was pumped into the centrifugal atomization tower to form spherical particles. The parameters for the granulation process are as follows: the air outlet temperature was set at 110 °C, while the air inlet temperature was maintained at 240 °C. The rotational speed of the atomizing disc was established at 18,000 r/min, and the feeding rate was specified as 45 mL/min. The obtained powder was sintered in a muffle furnace with a heating rate of 5 °C/min. The sinter temperature was maintained at 600 °C for 1 h to remove the binder, and at 1100 °C for 2 h to promote solid reaction. Subsequently, the material was allowed to cool within the furnace, resulting in the formation of CuCoMnO_x powder.

2.2. Preparation of Thermal Spray Powders

The prepared CuCoMnO_x powder was furtherly crushed by a ball mill to obtain tiny powders with a particle size of less than 5 μm. Co powder, with a particle size of 1.5 μm and a purity of 99.9%, was sourced from Shenzhen GeLinmei High-Tech Co., Ltd., Shenzhen, China. In this study, a total of five groups of coating materials were established, each featured different composition ratios as shown in

Table 1. Composition and proportions of the composite powder.

Group	Material Proportion (wt%)
A	100Co
B	95Co-5CuCoMnOx
C	90Co-10CuCoMnOx
D	85Co-15CuCoMnOx
E	80Co-20CuCoMnOx

. The raw powders were weight and mixed with deionized water, self-prepared adhesives, dispersants, and defoamer (tributyl phosphate) at a solid–liquid mass ratio of 1:1 though colloid milling to form a uniformly ground slurry. Then, the slurry was pumped into the centrifugal atomization tower to form spherical particles. The parameters for the granulation process are the same with those we have mentioned above. The powder obtained through granulation was subjected to high-temperature calcination in a tube furnace under an argon atmosphere. The heating rate was set at 5 °C/min, and the sintering temperature was maintained at 600 °C for 1 h to ensure complete burnout of the binder. Following this process, the binder-free powder was sieved to obtain particles within the size range of 45–75 μm, suitable for thermal spraying applications.

2.3. Preparation of Coatings

The substrate utilized for the spraying process was 316L stainless steel, with dimensions of 30 mm × 40 mm × 1 mm. The substrate underwent ultrasonic cleaning in acetone to eliminate oil contaminants, then was followed by ultrasonic cleaning in deionized water. Finally, it was dried in a drying oven to remove any residual moisture. The cleaned substrate surface was sandblasted using 100-mesh brown alumina sand, aiming to roughen the substrate surface and enhance the bonding strength between the substrate and the coating. Before the formal spraying process, the substrate was preheated in a drying oven at 120 °C. The absorption layer was fabricated using the APS-3000K plasma spraying equipment from the Beijing Institute of Aeronautical Manufacturing Engineering under AVIC in Beijing, China. The specific parameters for the spraying process are detailed in

Table 2. Process parameters used for spraying.

Factor	Parameter
Electrical current (A)	350
Voltage (V)	77
Ar gas flow (L/min)	35
H2 gas flow (L/min)	12
Spraying distance (mm)	140
Spray gun velocity (mm/s)	300
Powder feeding quantity	3.9
Spraying times	4

.

2.4. Test of High-Temperature Stability

The coated samples were placed within a tube furnace and maintained under atmospheric conditions. The heating rate was set at 5 °C/min, and the samples were held at 450 °C for a duration of 3 h before being allowed to cool alongside the furnace.

2.5. Characterization

The thermogravimetric curves of the powdered mixture comprising CuO, Co₃O₄, and MnO₂ after granulation were analyzed using TG-DTG (NETZSCH STA 2500, Netzsch Scientific Instruments Trading Co., Ltd., Shanghai, China). The phases of the synthesized CuCoMnO_x powder, the thermal spray powders, and the coatings before and after the high-temperature heat treatment were analyzed using X-ray diffraction (XRD, Spectris Group in London, UK/EMPYREAN Smart Diffractometer/Cu target, Rigaku Corporation in Tokyo Japan/Smartlab SE/Co target). The surface and cross-sectional morphologies of the coatings were analyzed using a scanning electron microscope (SEM, JEOL in Tokyo Japan, JSM-IT300). The X-ray energy dispersive spectrometer (EDS, Oxford Instruments Technology Co., Ltd., X-MaxN20 EDS spectrometer, Shanghai, China) was utilized to perform point-scanning tests on both the powder and the coating. The surface roughness of the coatings was assessed using a surface roughness measurement instrument (Beijing Time Summit Technology Co., Ltd., Beijing, China, TIME^®3221). The reflection spectra of the synthesized CuCoMnO_x powder and the coating before and after the high-temperature heat treatment were measured within the wavelength ranges of 0.3–2.5 μm and 2.5–25 μm, using a UV–visible spectrophotometer (Shimadzu, UV-3600, Kyoto, Japan) and a Fourier-transform infrared spectrometer (Bruker, Tensor27, Karlsruhe, Germany), respectively. The absorptance (α) and emittance (ε) are determined using Equations (1) and (2), where I_sol(λ) is the irradiance of the standard solar spectrum under Air Mass 1.5 (AM1.5) conditions, R(λ) denotes the spectral reflectance, and I_b(λ) indicates the radiant intensity of a blackbody at room temperature [30].

$$\alpha ={\displaystyle \frac{{\int }_{0.3}^{2.5}{I}_{sol}\left(\lambda \right)\left(1-R\left(\lambda \right)\right)\mathrm{d}\lambda }{{\int }_{0.3}^{2.5}{I}_{sol}\left(\lambda \right)\mathrm{d}\lambda }}$$

(1)

$$\epsilon ={\displaystyle \frac{{\int }_{2.5}^{25}{I}_b\left(\lambda \right)\left(1-R\left(\lambda \right)\right)\mathrm{d}\left(\lambda \right)}{{\int }_{2.5}^{25}{I}_b\left(\lambda \right)\mathrm{d}\left(\lambda \right)}}$$

(2)

3. Results and Discussion

3.1. Synthesis Mechanism and Spectral Property of CuCoMnO_x Powders

In this study, CuCoMnO_x powders were synthesized using a solid-phase reaction method. To elucidate the synthesis mechanism as well as the final sintering temperature of the CuCoMnO_x powders, we analyzed the TG-DTG results obtained from the granulated mixture of CuO-Co₃O₄-MnO₂, as illustrated in Figure 1. It can be observed from Figure 1 that the granulated powder of CuO-Co₃O₄-MnO₂ experiences mass loss at temperatures of approximately 200 °C, 531.8 °C, 619.6 °C, 1021.2 °C, and 1051.9 °C. The maximum mass loss rates corresponding to these temperature points are as follows: 0.28%/min for 200 °C, 0.62%/min for 531.8 °C, 0.25%/min for 619.6 °C, 0.46%/min for 1021.2 °C, and 0.51%/min for 1051.9 °C. It has been reported in the literature that the decomposition of MnO₂ into Mn₂O₃ occurs at temperatures of 520 °C [31], 550 °C [32], and 596.1 °C [33]. Consequently, the weight loss observed around 531.8 °C in Figure 1 can be interpreted as the decomposition of MnO₂. Based on the laboratory experience, the weight loss observed near 200 °C and 619.6 °C in Figure 1 is believed to correspond to the combustion of the binder, dispersant, and defoamer incorporated into the mixed powders of CuO-Co₃O₄-MnO₂ during the granulation process. The primary reaction equations are presented as follows:

4MnO₂→2Mn₂O₃ + O₂ (T = 531.8 °C)

(3)

Based on the TG-DTG curve, it was concluded that the mass loss kept unchanged around 1100 °C. Therefore, the final sintering temperature of 1100 °C with a duration of 2 h were selected. The X-ray diffraction (XRD) patterns of the CuO-Co₃O₄-MnO₂ powder before and after sintering at 1100 °C for 2 h are shown in Figure 2. It can be seen that the phase composition of the powder before sintering consists of CuO, Co₃O₄, and MnO₂. After sintering, the powder mainly comprises Cu_1.5Mn_1.5O₄, MnCo₂O₄, and a small amount of CuO. It was coincided with the target phases of CuCoMnO_x powder reported in other research [24], suggesting the successful synthetization of CuCoMnO_x powder. It can also be observed from Figure 2 that the diffraction peaks of Cu_1.5Mn_1.5O₄ and MnCo₂O₄ are sharp, indicating Cu_1.5Mn_1.5O₄ and MnCo₂O₄ possess good crystallinity. The weight loss observed around 1021.2 °C and 1051.9 °C in Figure 1 is deduced to correspond to the reaction of CuO, Co₃O₄, MnO₂, and Mn₂O₃ gradually synthesizing Cu_1.5Mn_1.5O₄ and MnCo₂O₄. The equation for the synthesis reactions is as follows:

6CuO + 4Co₃O₄ + 4MnO₂ + 4Mn₂O₃→4Cu_1.5Mn_1.5O₄ + 6MnCo₂O₄ + O₂ (T = 1021.2 °C, T = 1051.9 °C)

(4)

The CuCoMnO_x powder serves as a fundamental raw material for the spraying coatings, and its inherent optical performance significantly impacts the final absorptance and emittance of these coatings. To investigate the optical performance of prepared CuCoMnO_x powder, the reflectance spectra were measured in the wavelength ranges of 0.3–2.5 μm and 2.5–25 μm, and is presented in Figure 3. Based on results, the absorptance of the CuCoMnO_x powder was 0.929, while the emittance was 0.862, giving a quality factor of α/ε equaling to 1.078. Herein, it is observed that CuCoMnO_x exhibits a high absorptance, while it also has a high emittance. In other words, achieving an ideal quality factor remains impossible for single CuCoMnO_x-based ceramic coating. Composite with low-absorption and low-emission metal Co to form a metal–ceramic coating might be an effective way to obtain good solar selective absorption performance. Thus, this paper attempted to optimize the composition of the Co-CuCoMnO_x composite coatings, in order to obtain a higher quality factor of solar selective absorption performance.

3.2. Microstructure of Co-CuCoMnO_x Powders and Coatings

The microstructure of prepared Co-CuCoMnO_x powders for coating deposition was observed. As an example, the morphology and the element content of the Co-CuCoMnO_x powder in Group B was summarized and shown in Figure 4 and

Table 3. Element content in different positions of Co-CuCoMnO_x powder in Group B.

Elements	wt%
	Pattern 1	Pattern 2	Pattern 3
O	6.14	2.71	1.88
Mn	1.26	0.82	1.27
Co	91.53	94.17	95.4
Cu	1.06	2.3	1.45

, correspondingly. It can be seen that the powder for thermal spraying possessed a high degree of sphericity to ensure good flowability during the deposition. Moreover, it can be found from Table 3 that the Co-CuCoMnO_x powder in Group B comprised four elements: Cu, Co, Mn, and O. The weight percentages of each element were approximately consistent with the designed content.

To identify the phase composition of the four-groups Co-CuCoMnO_x powders, Group C (10% CuCoMnO_x) and D (15% CuCoMnO_x) were subjected to X-ray diffraction (XRD) analysis. The results are presented in Figure 5. Both groups of powders mainly contain three phases—Co, Cu_1.5Mn_1.5O₄, and MnCo₂O₄—indicating that no chemical reactions occurred within the composite powders during the processes of granulation and degumming, and confirming that the phase composition aligned with the initial design specifications.

To investigate the phase composition of the Co-CuCoMnO_x coatings fabricated by plasma spraying, X-ray diffraction (XRD) analysis was conducted on the coating for each group, and the results are illustrated in Figure 6. According to it, both Co and CoO phases were present in all the coating samples. Furthermore, the CoCuMnO_x (#47-0324) phase was also demarcated for Groups B–E. During the plasma spraying process, the temperature of plasma flame can reach as high as 10,000 °C. A portion of cobalt (Co) absorbs oxygen from the surrounding air during its transition from the spray gun to the substrate, resulting in the formation of cobalt oxide (CoO). The high-temperature flame flow inherent in plasma spraying also facilitates the mutual solubility of Cu_1.5Mn_1.5O₄ and MnCo₂O₄, leading to the generation of the CoCuMnO_x phase. By comparing the relative intensities of the two diffraction peaks at 49.64° and 60.64° in Groups A–E as illustrated in Figure 6, it is evident that the intensity ratio of CoO/Co of group A is the highest. This phenomenon suggested that oxidation within the pure Co group is most pronounced. In contrast, for the other four groups, it was inferred that the addition of the CuCoMnO_x powders may inhibit the oxidation of Co, since the intensity ratio of CoO/Co apparently decreased. It was thought that the added CoCuMnO_x powders might consume the heat for Co oxidation, which reduced the oxidation of Co.

The surface morphologies of the five groups of coatings produced by plasma spraying are illustrated in Figure 7. All the coatings exhibited similar surface morphology, which consisted of molten zones, semi-molten zones, protrusions, and pores. The molten zone came from the fully melted droplets in the flame flow flying towards the surface of the substrate and spreading out, resulting in a relatively smooth and dense area. However, there was still a portion of the composite powder that did not undergo complete heating within the plasma flame flow prior to impacting the substrate. This incomplete heating, along with some molten materials presenting at the edge of the molten zone, collectively formed the semi-molten zone. The formation of protrusions and pores may be attributed to the rapid cooling and contraction of the droplet impacted onto the substrate. The surface roughness (Ra) of these five groups of coatings was similar, which ranged from 3.19 to 4.25 μm. When observed under high-magnification views, it was found that the local spreading conditions of the coatings of Groups A–C were relatively favorable, characterized by a smoother surface with fewer protrusions and smaller pores. In contrast, the local spreading conditions of the coatings of Groups D and E were comparatively poor, exhibiting a rougher surface with more protrusions and larger pores. The surface produced by plasma spraying was a micro-uneven structure, which appeared macroscopically flat while exhibiting microscopic irregularities. Incident light with wavelengths that are less than or equal to the dimensions of the pores or the spacing between protrusions will undergo multiple reflections within these pores or among the protrusions, leading to absorption by the coating. Conversely, incident light with wavelengths exceeding the size of the pores or the distance between protrusions will be directly reflected into the surrounding environment, akin to specular reflection. Consequently, these coatings of Groups A–C might demonstrate a lower absorption rate for ultraviolet–visible near-infrared solar radiation and a lower absorption in the mid–far-infrared range, than the coatings of Groups D–E based on the surface morphology.

As an example, the cross-sectional morphology of the plasma-sprayed coating of Group A and E is illustrated in Figure 8. The coating thickness ranged from 10 to 20 μm. The interaction between the plasma-sprayed coating and the substrate manifested as mechanical interlocking, demonstrating a robust bond. The coating exhibited a layered structure, which resulted from the molten and semi-molten droplets interlacing and overlapping with one another. It could also be observed from Figure 8 that the plasma sprayed coating contained pores of varying sizes distributed throughout its structure.

To investigate the chemical composition differences among coatings containing CoCuMnO_x and those without, the corresponding EDS point analysis of the coatings of Groups A and E was selected. The results are summarized in

Table 4. Element content in different positions of Groups A and E coatings (wt%).

Elements	wt%
	Pattern 4	Pattern 5	Pattern 6	Pattern 7	Pattern 8	Pattern 9
Co	73.7	94.75	92.27	94.93	89.12	68.81
O	26.3	5.25	3.8	5.07	7.38	24.8
Fe	0	0	2.87	0	0	0
Cr	0	0	1.06	0	0	0
Cu	0	0	0	0	2.38	1.8
Mn	0	0	0	0	1.12	3.2
Cl	0	0	0	0	0	1.4

and Figure 8. In the backscattering images, the brighter the phase, the larger the corresponding atomic number. Based on the results of Patterns 5 and 6, it could be inferred that the grayish-white regions within the coating of Group A mainly consisted of cobalt (Co) along with a minor presence of cobalt oxide (CoO). In addition, A small amount of matrix elements Fe and Cr are diffused in the part of Group A coating near the substrate, and there are pores inside the coating. In contrast, according to Pattern 4, the gray areas in the coating resulted from a more extensive oxidation of Co. In conjunction with the XRD results, it was inferred that the grayish-white regions marked by Pattern 7 within the coating of Group E consist of Co and a small amount of CoO. The grayish-black areas were identified as containing Co, CoCuMnO_x, and CoO, while the black regions not only exhibited porosity but also comprised a mixture of Co, CoCuMnO_x, and CoO. In addition, the presence of Cl in Group E coating might be due to a very small amount of Cl impurity in the granulation tower, which was mixed into the spherical spray powder during the spray granulation process, resulting in the inclusion of the Cl element in the coating.

3.3. Optical Properties of Co-CuCoMnO_x Coatings

The reflectance spectra for the five groups of plasma-sprayed coatings are illustrated in Figure 9, and the optical properties derived from these reflectance spectra are summarized in

Table 5. Optical properties of Co-CuCoMnO_x coatings.

Group	Solar Absorptance (α)	Infrared Emittance (ε)	Quality Factor (α/ε)
A	0.831	0.406	2.047
B	0.808	0.370	2.184
C	0.842	0.406	2.074
D	0.881	0.494	1.783
E	0.882	0.543	1.624

. According to the results, it was found that both the solar absorptance and infrared emittance of those coatings first decreased and then increased when increasing the addition amount of CuCoMnO_x. The highest quality factor (α/ε) of 2.184 was obtained in the coating of Group B, whose solar absorptance and infrared emittance were 0.808 and 0.370, respectively.

Compared to the ceramic phase, pure cobalt exhibits a low solar absorptance as well as low infrared emittance [11]. However, the deposited cobalt coating (Group A) exhibited a solar selective absorption performance, which might be attributed to the oxidation and the formation of CoO during the plasma spraying. The t₂ state of Co²⁺ ions in CoO is characterized by unstable electrons. The interactions arising from this instability lead to intrinsic absorption, which may result in an increased absorption rate and emittance of the coating [34]. Since the two Jahn–Teller ions, Cu²⁺ and Mn³⁺, are present in Cu_1.5Mn_1.5O₄, MnCo₂O₄, and CoCuMnO_x during plasma spraying, the composite powder is rapidly heated to 10,000 °C within the plasma flame stream. The phases of Cu_1.5Mn_1.5O₄ and MnCo₂O₄ undergo mutual dissolution to form the CoCuMnO_x phase. At this juncture, alterations occur in the molecular vibration and rotational modes, which subsequently induce lattice distortion and enhance the intrinsic absorption characteristics of the material while simultaneously reducing reflectivity in the infrared spectrum [35]. Therefore, it is likely that the formation of CoCuMnO_x within the coating will further augment both absorption and emission rates compared to those exhibited by Cu_1.5Mn_1.5O₄ and MnCo₂O₄. Combining the XRD analysis of the coatings presented in Figure 6, it was evident that Group A contained cobalt (Co) and cobalt oxide (CoO), and also exhibits the highest concentration of cobalt oxide (CoO). The optical performance of the coating in Group A was a result of the combination of Co and CoO. However, the addition of 5 wt% CuCoMnO_x to Group B resulted in a hindrance of Co oxidation; which, in other words, means that the content of Co increased. Since the added content of CuCoMnO_x was relatively low, the optical performance of the coating was primarily affected by the Co content, and exhibited characteristics associated with reduced solar absorption and infrared emission. However, when a proportion of 10 wt% or more of CuCoMnO_x was incorporated into the coating, the solar absorptance and infrared emittance of Groups C–E exhibited a gradual increase. This phenomenon can be attributed to the increase in CoCuMnO_x, which possessed high solar absorptance and high infrared emittance.

3.4. High Temperature Stability of Co-CuCoMnO_x Coatings

To evaluate the high temperature stability of the Co-CuCoMnO_x coatings, the coatings of Group B underwent a high-temperature heat treatment at 450 °C in an atmospheric environment for a duration of 3 h. The optical performance prior to and after the high-temperature heat treatment are presented in

Table 6. The absorptivity and emissivity values of Group B coatings before and after the heat treatment.

	Before Heat Treatment	After Heat Treatment
Solar Absorptance (α)	0.808	0.89
Infrared Emittance (ε)	0.37	0.538

. Additionally, the X-ray diffraction (XRD) patterns of the coatings before and after the heat treatment are illustrated in Figure 10. Based on the variations in diffraction peak intensity before and after the heat treatment, it could be inferred that the content of CoO increased after the heat treatment. Consequently, the solar absorptance and infrared emittance of the coating increased after the heat treatment. The performance criterion (PC) to evaluate the high temperature stability of the coating is if the PC value described as Equation (5) [36] is below 0.05, it can be concluded that the degradation performance of the coating at elevated temperatures is not significant. Furthermore, this suggests that the coating exhibits relatively good thermal stability under such conditions [37]. After the heat treatment, it was found that no cracking or shedding happened on the coating surface. The solar absorptance of the coating reached 0.89, while its emissivity increased by 0.168 compared to pre-treatment values. The PC value of the coating after the heat treatment at 450 °C for 3 h in an atmospheric environment was calculated as 0.002, which was far below the threshold of 0.05. The attenuation of the optical performance of the coating was negligible. Therefore, it could be concluded that the coating exhibited excellent thermal stability when maintained at 450 °C in an atmospheric environment for a duration of 3 h.

PC = (α_{before heat treatment} − α_{after heat treatment}) − 0.5 × (ε_{before heat treatment} − ε_{after heat treatment})

(5)

4. Conclusions

By employing the solid-phase reaction method, CuO-Co₃O₄-MnO₂ composite materials were successfully synthesized into CuCoMnO_x powder at 1100 °C in a muffle furnace for 2 h. The CuCoMnO_x powder predominantly consisted of Cu_1.5Mn_1.5O₄ and MnCo₂O₄. Furthermore, the CuCoMnO_x powder demonstrated a solar absorptance of 0.929 and an infrared emittance of 0.862, which was considered a good absorbent. Co-CuCoMnO_x spherical spray powders were prepared using spray granulation and de-gelling processes, with the phase composition of the spray powder containing CuCoMnO_x being Co, Cu_1.5Mn_1.5O₄, and MnCo₂O₄, maintaining consistency with the initially designed components. Subsequently, Co-CuCoMnO_x coatings were deposited on the surface of 316L steels utilizing the APS technique, aiming at developing a novel metal–ceramic solar selective absorption coating employed at high temperature. The chemical phase, microstructure, and optical performance, as well as high temperature stability of the prepared coatings, were systematically studied. The results showed that the Co-CuCoMnO_x coating possessed a solar selective absorption capacity after being compound with cobalt. The synthesized Co-CuCoMnO_x coating had a typical thermal spray layered stacking structure with molten zones, semi-molten zones, protrusions, and pores distributed on the coating surface, and had a thickness of 10~20 μm. The chemical phases of the coatings were mainly Co, CoO, and CoCuMnO_x. Here, CoO was formed by the partial oxidation of Co as it absorbed oxygen from the surrounding air during plasma spraying. The inherent high-temperature flame flow in plasma spraying promoted the mutual solubility of Cu_1.5Mn_1.5O₄ and MnCo₂O₄, leading to the formation of the CoCuMnO_x phase. As the amount of CuCoMnO_x added increased, both the solar absorption rate and infrared emissivity of the five groups of coatings produced by plasma spraying showed a trend of first decreasing and then increasing, while the quality factor showed a trend of first increasing and then decreasing. Due to the addition of CuCoMnO_x inhibiting the oxidation of Co during the thermal spraying process, the 95Co-5CuCoMnO_x (wt%) coating exhibited the optimal quality factor (α/ε) of 2.184, with a solar absorptance α of 0.808 and an infrared emittance ε of 0.370, respectively. Moreover, this specific coating demonstrated a good thermal stability for up to 3 h, in which the PC value was far less than 0.05 when it was exposed to an atmospheric environment at 450 °C. This finding holds significant implications for the research and development of high-temperature solar selective absorption coatings.