目的 研究镁合金不同微弧氧化时间黑色热控膜层的状态及性能。方法 在镁合金表面制备出不同时间微弧氧化黑色热控膜层,采用粗糙度测试仪、SEM（含EDS）、XRD、分光光度计、电化学工作站分别对膜层粗糙度、微观形貌、相组成、热控性能以及腐蚀性能进行检测。结果 随着氧化时间增加,膜层宏观颜色逐渐变深,且粗糙度和厚度逐渐增大;表面孔隙率逐渐降低,但是孔洞尺寸及裂纹宽度逐渐增加。膜层内存在大量封闭孔洞,且孔洞尺寸自基体向膜层表面呈现增加趋势。不同氧化时间,膜层表面Fe元素含量相近,但是Fe3O4的含量存在差异。从氧化初期到末期,太阳吸收率从80%提高至93%,红外半球发射率从0.78提高至0.85。相对于镁合金基体,微弧氧化后腐蚀电流降低了2个数量级,但是随着氧化时间的增加,略有上升。结论 氧化后期氧化电压和溶液温度上升,加速了成膜反应和熔融物冷却速率下降,同时铁氧化物和MgO共结晶,致使熔融物结晶尺寸增大,膜层粗糙度增加。Fe3O4含量及膜层表面状态与热控性能密切相关。微弧氧化热控膜层提高了基体的耐蚀性,但是氧化后期膜层微观孔洞和裂纹,导致耐蚀性能有所下降。
Abstract
Metal-modified micro-arc oxidation (MAO) coatings on magnesium alloys demonstrate exceptional stability in space environments and have been extensively utilized in aerospace electronic components. The properties of MAO-derived thermal control coatings are critically dependent on element doping levels, phase composition, and microstructural morphology of the coatings. Investigating the effects of varying MAO duration on the growth behavior of thermal control coatings on magnesium alloy surfaces offers theoretical foundations for optimizing process parameters and facilitating engineering applications.
In this work, the micro-arc oxidation (MAO) coatings on ZK61M magnesium alloy are fabricated in a sodium phosphate solution system containing ferric citrate under varying oxidation duration (5, 15, 25, and 30 min). This study systematically investigates the coating status, thermal control properties, and corrosion resistance of these MAO coatings, providing crucial insights for optimizing process parameters in practical applications. Coating thickness and roughness are analyzed with a roughness tester and an eddy current thickness gauge. Microstructure, elemental distribution, and phase composition are characterized via scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) and X-ray diffraction (XRD). Solar absorptance (αs) and infrared hemispherical emissivity (εH) are evaluated by spectrophotometry and radiometry, while corrosion resistance is assessed through potentiodynamic polarization and electrochemical impedance spectroscopy (EIS).
The results show that prolonged oxidation progressively darkens the coating color from light gray to deep black. Coating thickness increases from 10 μm (initial stage) to 45 μm (30 min), with surface roughness rising from 1.0 μm to 2.7 μm. The oxidation voltage rapidly ascends to 500 V during the initial phase, promoting molten oxide generation and deposition. Subsequent voltage stabilization combined with continuous electrolyte temperature elevation reduce molten material cooling rates, leading to coarse crystallized particles and increase surface roughness. SEM reveals high porosity with uniform pore distribution during early oxidation, while prolonged oxidation induces molten oxide stacking and Fe3O4 enrichment, elevating internal stress and crack width. The coating contains a significant number of closed pores within its structure, with pore sizes ranging from less than 1 μm to over 2.5 μm, and locally elongated pores exceeding 10 μm in length are observed. These features are primarily attributed to voltage variations between the initial and final stages of oxidation, as well as the coverage of molten oxide. XRD confirms MgO, Mg3(PO4)2, and Fe3O4 as primary phases, with Fe exhibiting gradient enrichment from the substrate to the surface. Although surface Fe content remains constant across different duration, Fe3O4 concentration varies as evidenced by XRD peak intensities.
Thermal control performance improves significantly: αs increases from 80% (5 min) to 93% (30 min), and εH rises from 0.78 to 0.85. These enhancements are attributed to microprotrusions, open pores, increased Fe3O4 content (enhancing blackness), and sufficient thickness (>30 μm) to suppress substrate interference. Electrochemical tests demonstrate optimal corrosion resistance at 5 min (Jcorr=0.18 μA/cm², |Z|=431 kΩ·cm2), two orders of magnitude lower than bare substrate (17.7 μA/cm2). However, prolonged oxidation degrades corrosion resistance (Jcorr=0.23 μA/cm2, |Z|=281 kΩ·cm2 at 30 min) due to enlarged pores and microcrack propagation.
In conclusion, the synergistic regulation of oxidation voltage, electrolyte temperature, and Fe doping governs coating thickness, roughness, and coloration. Thermal control properties positively correlate with Fe3O4 concentration, surface roughness, and thickness. However, accumulated internal stress and interfacial stress concentration at Fe3O4/MgO heterophases exacerbate microcracking, compromising corrosion resistance during extended oxidation. These findings provide critical insights for optimizing MAO parameters and advancing engineering applications of magnesium alloy thermal control coatings.
关键词
镁合金 /
微弧氧化 /
氧化时间 /
微观形貌 /
热控性能 /
耐蚀性
Key words
magnesium alloy /
micro-arc oxidation /
oxidation duration /
microstructure /
thermal control /
corrosion resistance
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