Study on the Thermal Control Performance of Mg-Li Alloy Micro-Arc Oxidation Coating in High-Temperature Environments

Abstract

This paper reports on the successful preparation of a low absorption–emission thermal control coating on the surface of LAZ933 magnesium–lithium alloy using the micro-arc oxidation method. This study analyzed the microstructure, phase composition, and thermal control properties of the coating using Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), UV–visible near-infrared spectroscopy (UV-VIS-NIR) and infrared emissivity measurements. The results indicate that the hemispherical emissivity of the coating remains unaffected with an increase in temperature and holding time, while the solar absorption ratio gradually increases. The thermal control performance of the coating after a high-temperature experiment was found to be related to the diffusion of the Li metal element in the magnesium lithium alloy matrix, as determined by X-ray photoelectron spectroscopy (XPS), flame graphite furnace atomic absorption spectrometry (GFAAS) and Glow Discharge Optical Emission Spectroscopy (GD-OES). As the holding time is extended, the coating structure gradually loosens under thermal stress. The Li metal element in the substrate diffuses outward and reacts with O₂, H₂O and CO₂ in the air, forming LiO₂, LiOH, Li₂CO₃ and other products. This reaction affects the coating’s solar absorption ratio in the end.

1. Introduction

The density of magnesium–lithium alloys ranges from 1.35 to 1.65 g/cm³, making it the lightest metal material [1]. It has attracted significant attention in the aerospace industry due to its potential for reducing spacecraft weight and advancing deep space exploration technology [2,3]. However, the high chemical activity of magnesium and lithium restricts the application of Mg-Li alloys [4,5,6]; surface treatment can enhance the corrosion resistance and provide new functions for the surface of Mg-li alloys. Common surface treatment methods for magnesium and its alloys include chemical conversion [7], electrochemical deposition [8], anodic oxidation [9] and micro-arc oxidation [10]. Micro-arc oxidation technology (MAO) stands out due to its environmentally friendly electrolytes’ ability to directly grow ceramic coatings with excellent properties in situ, which has made it a popular choice for magnesium and its alloys [11,12,13].

The hemispherical emissivity of the magnesium–lithium alloy itself is approximately 0.1 with a solar absorption ratio of around 0.5. The surface temperature can reach hundreds of degrees Celsius when exposed to sunlight in space. Therefore, in aerospace applications, the coating on the surface of magnesium lithium alloys must have high corrosion resistance and have thermal control performance. The current research primarily focuses on creating thermal control coatings on Mg-Al alloys [14,15] by the MAO process. However, research on thermal control coatings for Mg-Li alloys is limited. Previous thermal control coatings on Mg-Li alloys [16,17,18] mostly focused on the influence of relevant process parameters on the thermal control performance of the coatings, without studying the change in thermal control performance of the coatings in high-temperature environments. The lunar exploration project faces a significant challenge in the form of extreme surface temperatures ranging from −180 to +130 °C [19], which poses a great test for the high-temperature resistance of the thermal control coating. However, research on the structural changes in Mg-Li alloys in high-temperature environments mainly focuses on rolling [20,21] and hot compression experiments [22], and there is also a lack of research on the performance changes in Mg-Li alloy coatings at high temperatures. Therefore, it is crucial to analyze the properties and reasons for the thermal control coating on the surface of Mg-Li alloys in high-temperature environments.

This paper explores the microstructure and thermal control properties of the MAO thermal control coating applied to the surface of a Mg-Li alloy at elevated temperatures. The aim is to elucidate the mechanism of the effect of temperature on the performance of the coating and to provide a scientific basis for solving the problem of degradation thermal control performance in the application of Mg-Li alloy MAO thermal control coating.

2. Material and Methods

2.1. Preparation of MAO Coating

LAZ933 magnesium–lithium alloy was used as the base material, and the alloy material composition is shown in

Table 1. Chemical composition of LAZ933 magnesium–lithium alloy.

Element	Mg	Li	Al	Zn
%	BaL	8.5~9.5	2.5~3.5	2.5~3.5

. The alloy plate was processed into 40 mm × 40 mm × 2 mm plate samples by wire cutting. After being polished with 200#, 400# and 1000# SiC sandpaper, the samples were ultrasonically cleaned in acetone for 10 min and dried. The experiment utilized deionized aqueous solutions of Na₂SiO₃ and KOH as electrolytes. The micro-arc oxidation process conditions are a Na₂SiO₃ concentration of 30 g/L; a KOH concentration of 1 g/L; PH kept at 11–12; a current density of 6 A/dm²; a power supply frequency of 100 HZ; and a DC pulse power supply. The ceramic coating required for this process is prepared using multi-waveform programmed micro-arc oxidation pulse power supply equipment in the constant-current and limited-voltage power supply mode. The equipment consists of MAO power supply, an oxidation treatment tank and a water cooling system. A chiller was used for circulating cooling to maintain the electrolyte at a temperature between 20 and 40 °C. After the MAO process, the samples were cleaned with acetone and then dried in a blast drying oven.

2.2. High-Temperature Experiment

The prepared MAO coating of magnesium–lithium alloy was placed in an electric thermostatic air drying oven with two test pieces in each group, and the experiment was repeated three times. The first set of experimental conditions were as follows: the sample was placed in an electric thermostatic air drying oven for 24 h, and the temperature was set to 50 °C, 100 °C, 150 °C, and 200 °C, respectively; the second group of experimental conditions was set to 200 °C; and the sample was left for 0 H, 6 H, 12 H, 24 H, 48 H, 96 H, 144 H and 288 H, respectively, to analyze the impact of varying temperatures and holding durations on the composition and thermal radiation characteristics of the coating.

2.3. Performance Test and Tissue Observation

The phase composition of the coating was analyzed using the D2 PHASER X-ray (BRUKER AXS, Karlsruhe, Germany) diffractometer with a copper Ka target. The acceleration voltage and current were set to 40 KV and 150 mA, respectively, the scanning angle range was 10~80°, and the scanning rate was 10°/min. The reference card number in the database used for phase identification is 1557. The morphology of the coating was observed using a Magellan400 (FEI NanoPorts, Hillsboro, OR, USA) field emission scanning electron microscope with an EDS spectrometer. The hemispherical emittance of the coating was measured using an AE1-RD1 (Aoptek, Beijing, China) infrared emittance measuring instrument. The solar absorption ratio of the coating was measured using a PE950 UV-VIS (PerkinElmer, Waltham, MA, USA) near-infrared spectrophotometer with a spectral measurement range of 250~2500 nm. The surface composition of the coating was detected using ESCALAB250Xi X-ray photoelectron spectroscopy (Thermo Fisher Scientific, Waltham, MA, USA). The excitation source used was an Al Ka ray, with a target power of 250 W and an X-ray incidence angle of 45°.The analytical model was a hemispherical model and the resulting data were corrected with C1s (284.8 eV). Element distribution along the coating depth was detected using glow discharge emission spectroscopy with a discharge voltage of 550 V and a gas pressure of 2 hPa. Ar of 1 vol%H₂ (purity 6.0) was used as the analytical gas for GD-OES analysis using the GD-Profiler 2 (HORIBA, Paris, France). After the high-temperature experiment, the test piece was ultrasonically cleaned in deionized water for 5 min and the deionized water content was 150 mL. The element content of the cleaned deionized water was tested using ZEEnit 700 P(Analytik Jena AG, Jena, Germany) flame graphite furnace atomic absorption spectrometry. All tests were performed at room temperature.

3. Results and Discussion

3.1. Thermal Radiation Performance Analysis

Table 2. Test results of thermal control performance of MAO coatings after 24 h of treatment at different temperatures.

Temperature	Room Temperature	50 °C	100 °C	150 °C	200 °C
εH	0.89	0.89	0.89	0.89	0.89
αS	0.33	0.35	0.36	0.38	0.42

presents the test results of the thermal control performance of the surface coating of the Mg-Li alloy after being held at different temperatures for 24 h. As the temperature increases, the hemispherical emissivity of the coating remains constant, whereas the solar absorption ratio of the coating increases with the temperature. When the temperature is 200 °C, the solar absorption ratio of the coating increases from 0.33 to 0.42 with the greatest degree of change.

Table 3. Test results of thermal control performance of MAO coating at different holding times at 200 °C.

Time	0 h	6 h	12 h	24 h	48 h	96 h	144 h	288 h
εH	0.89	0.89	0.89	0.89	0.89	0.89	0.89	0.89
αS	0.33	0.35	0.41	0.42	0.46	0.47	0.47	0.48

displays the changes in thermal radiation properties of the coating after different holding times. It can be seen that the hemispherical emissivity of the coating does not change with the extension of holding time, while the solar absorption ratio increases. Figure 1 shows the reflectance curve of the coating at various high temperatures. It can be found from Figure 1 that the reflectance curve of the coating decreases gradually with the holding time. Consequently, the degree of reflection of the coating towards the light also diminishes, indicating an increase in the absorption rate of the film layer. This result is consistent with the thermal control performance test of the coating after different holding times.

3.2. Analysis of Structure and Composition

Figure 2 shows the XRD patterns of the coating after different durations of high-temperature exposure. The phase compositions of the coating were MgO, Li₃Mg₇, and Mg₂SiO₄, respectively. The identified crystalline phases are described separately according to card numbers 71-1176 (PAN-ICSD), 65-6742 (PAN-ICSD), and 87-0061 (PAN-ICSD). The crystallinity of the coating after different insulation times was obtained through fitting analysis using Jade software, the values of which were 18.4%, 38.45%, 20.46% and 16.95%, respectively. This indicates that the crystallinity of the coating increases first and then decreases with time in a high-temperature environment. From Figure 2, it can be seen that the phase composition of the coating has not changed, indicating that the duration of high temperature has an impact on crystal growth and has a relatively small effect on phase composition.

Figure 3 shows the SEM surface morphology results, indicating an increase in the number of protrusions on the surface of the film layer with longer holding times. The micro-arc oxide film’s porous structure allows for oxygen diffusion in high-temperature oxidation environments leading to grain oxidation near the holes and an increase in protrusions on the coating’s surface.

Figure 4 shows the cross-section morphology of the coating after different holding times. It is found that as the holding time increases, the crack size in the coating increases. This phenomenon is related to the thermal stress generated under high-temperature environments. Simultaneously, the structure of the coating loosens gradually, enhancing its sunlight absorption capacity and resulting in an increase in the solar absorption ratio of the coating.

3.3. XPS Analysis

XPS analysis was conducted on the coatings subjected to high-temperature treatments for different durations to study their chemical composition in detail. The chemical states of Si, O, Mg and Li were studied. The results are shown in Figure 5. The Mg 1 s peak can be separated into two peaks: the peak at 1303.8 eV is from MgO and the other peak at 1304.2 eV is Mg₂SiO₄ [23]. The Si 2p peak is composed of two components, attributed to spin–orbit splitting, with combined energies of 102.30 and 102.95 eV, also indicating the presence of Mg₂SiO₄ [24]. The spectrum of O1s shows three oxygen molecules, where the peak at 530.3 eV is attributed to the typical metal–oxygen bond of MgO [25], while the peak at 531.2 eV corresponds to Mg₂SiO₄ [26]. The peak at 532.6 eV may be due to oxygen adsorbed on the surface of the coating [27]. In the Li1s spectrum, the peak at 55.40 ev is attributed to Li₂CO₃ [28]. The peak at 54.70 ev is LiOH [29]. It is found that the peak strength changes significantly with the extension of holding time, with the peak intensity of Li₂CO₃ and LiOH gradually increasing. This indicates that the high-temperature process may be accompanied by the diffusion of the Li element and the formation of Li₂O on the surface, and Li₂O is unstable and easily reacts with CO₂ to produce Li₂CO₃ in high-temperature environments. LiOH may form when Li₂O reacts with water vapor in the air after the coating leaves the high-temperature environment.

Table 4. Elemental content analysis of XPS.

Atomic %	0 h	48 h	144 h	288 h
Mg1s	3.44	3.74	3.45	3.77
Li1s	14.71	16.35	18.15	22.24
Al2p	4.04	3.43	3.60	3.53
Zn2p	1.33	1.09	1.14	1.17

shows the element content analysis of XPS. It can be seen that with the increase in the high-temperature exposure time, contents of Mg, Al and Zn in the matrix elements do not change significantly, while the content of the Li element increases significantly with the extension of holding time.

3.4. Atomic Absorption Spectrometry

Under high-temperature conditions, the internal metal ions are more likely to diffuse out of the micro-arc oxide layer and be oxidized. In order to verify whether there is element diffusion inside the alloy on the coating surface, test pieces that have not been treated at high temperature and samples processed at 200 °C for different amounts of time are put into deionized water for ultrasonic cleaning. The deionized water after cleaning was tested by atomic absorption spectrometry.

Table 5. Solar absorption ratio of the coating before and after ultrasonic cleaning.

	0 h	48 h	144 h	288 h
αS (Before cleaning)	0.33	0.46	0.47	0.48
αS (After cleaning)	0.33	0.43	0.44	0.46

shows the change in the solar absorption ratio of the coating before and after cleaning. It can be found that the solar absorption ratio of the coating decreases slightly after ultrasonic cleaning with deionized water.

Table 6. Atomic absorption spectroscopy test results.

Components Analyzed (μg/mL)	Deionized Water	0 h	48 h	144 h	288 h
Mg	0.28	0.32	0.35	0.37	0.31
Li	<0.004	1.76	14.25	19.04	29.46
Al	0.06	0.04	0.05	0.06	0.04
Zn	<0.002	0.01	0.01	0.01	0.01

shows the test results of atomic absorption spectrometry. It can be seen that contents of Mg, Al and Zn ions do not change significantly with the extension of holding time at 200 °C, while the content of Li ions in deionized water gradually increases. This result shows that during the high-temperature process, diffusion of the Li metal element in the alloy from the matrix to the coating occurs. During the diffusion of Li-ions to the coating, they easily react with O₂, H₂O and CO₂ in the air to form LiO₂, LiOH, Li₂CO₃ and other products, which deepens the color of the coating surface. In the process of ultrasonic cleaning in deionized water, these products dissolve in the deionized water, increasing the Li element content. However, the dissolution of these reaction products leads to a decrease in the solar absorption ratio of the coating. Therefore, the diffusion of Li metal ions may be one of the reasons for the increase in the solar absorption ratio of the coating.

3.5. GD-OES Analysis

The distribution of coating elements at high temperature was studied by the GD-OES method. The depth profile analysis of the coating before and after exposure to high temperature for 288 h was carried out, and the results are shown in Figure 6.

As can be seen from Figure 6, the contents of Mg, Al and Zn in the coating did not change before and after high-temperature treatment, while the strength of the Li element increased significantly after high-temperature treatment. This indicates that the Li element in the substrate diffuses into the coating in high-temperature environments, which is also consistent with previous experimental findings.

4. Conclusions

Experimental analysis was conducted on the performance changes of micro-arc oxidation thermal control coating on a magnesium–lithium alloy under a high-temperature environment, and the following conclusions were drawn:

1. The solar absorption rate of the coating gradually increases with temperature and high-temperature exposure time, while the hemispherical emissivity remains unchanged.

2. As the high-temperature exposure time increases, the porous structure of the micro-arc oxidation coating becomes loose under the influence of thermal stress, leading to a gradual increase in the size of pores and cracks. Due to the small volume parameter of lithium element, when the structure of the coating is loose, lithium element is more likely to diffuse outward from the substrate, react with O2, H2O, and CO2 in the air on the surface of the coating, and generate reaction products such as Li2CO3 and LiOH. These reaction products have a high absorption rate of light energy, resulting in an increase in the solar absorption rate of the coating.

The study on the thermal control performance changes of micro-arc oxidation thermal control coatings on magnesium lithium alloys under a high-temperature environment lays the foundation for the subsequent application of magnesium lithium alloys in aerospace, which is beneficial for the development of such alloys in thermal control coatings.