Abstract
Thermal control ceramic coatings on Mg–Li alloys have been successfully prepared in silicate electrolyte system by plasma electrolytic oxidation (PEO) method. The PEO coatings are mainly composed of crystallized Mg2SiO4 and MgO, which have typical porous structure with some bulges on the surface; OES analysis shows that the plasma temperature, which is influenced by the technique parameters, determines the formation of the coatings with different crystalline phases and morphologies, combined with “quick cooling effect” by the electrolyte; and the electron concentration is constant, which is related to the electric spark breakdown, determined by the nature of the coating and the interface of coating/electrolyte. Technique parameters influence the coating thickness, roughness and surface morphology, but do not change the coating composition in the specific PEO regime, and therefore the absorptance (αS) and emissivity (ε) of the coatings can be adjusted by the technique parameters through changing thickness and roughness in a certain degree. The coating prepared at 10 A/dm2, 50 Hz, 30 min and 14 g/L Na2SiO3 has the minimum value of αS (0.35) and the maximum value of ε (0.82), with the balance temperature of 320 K.
Highlights
As one of the lightest structural material[1,2], Magnesium-Lithium alloys show great application prospect in the fields of aeronautics and astronautics
Various thermal control coating on Al9–11, Mg12,13 and Ti14–16 have been successfully prepared by Plasma electrolytic oxidation (PEO) method
Wu et al.[10] prepared the thermal control coatings with low ε/αs on Mg alloy and Al alloy, respectively and found that the ε of coatings were related with surface morphology
Summary
The formation of the bulges on the coating surface and the increase of the coating thickness and roughness are attributed to the enhancement of the heat effects by the gradual increase of the plasma temperature with the reaction time and current density and the quick-cooling effect of the electrolyte as well. The proper enhancement of the electric parameters or the extending of the reaction time, which means that the increase of the applied energy by the power source, does not change ibreak, but it is helpful for the increase of igrowth and the improvement of the plasma temperature (Te) of the micro reaction zones, which influences the growth rate, the crystallization and phase transformation and the structure and morphology of the coatings. Current density and reaction time presents the larger adjustment ability than concentration of NaSiO3.9H2O and working frequency
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