Preparation of high absorbance and high emittance coatings on 6061 aluminum alloy with a pre-deposition method by plasma electrolytic oxidation

With Sm _0.5 Sr _0.5 CoO ₃ microparticles as the raw material, high solar absorptive and high infrared emissive coatings were successfully prepared on 6061 aluminum alloys using a new method including firstly microparticle pre-deposition by gravity and then plasma electrolytic oxidation (PEO) technique. All four coatings were characterized by XRD, SEM and EDS respectively. The solar absorbance (α _s ) and infrared emittance (ε _T ) were also investigated. The results showed that all four coatings were black and rough; the Na and P elements from the electrolyte were incorporated into the coatings; amorphous compounds containing Na and P also existed in the coatings. All four coatings had a complex multiphase composition mainly consisting of Sm ₂ O ₃ , SrAl ₄ O ₇ , AlPO ₄ and CoAl ₂ O ₄ phases, which might derive from drastic reactions under high temperature and high pressure during the PEO process. The surface morphology and the relative content of main crystal phases changed with increasing current density or time. The cross-section of the coatings exhibited a layer structure. As current density or time increased, the α _s decreased but ε _T increased. All four coatings showed a high solar absorbance value ≥ 0.85 and high infrared emittance value ≥ 0.90, which indicated that the coatings might have promising applications in solar and infrared heating for aluminum alloys.