Influence of nanoporous aluminum oxide interlayer on the optical absorptance of black electroless nickel–phosphorus coating

• Nickel nanowires were synthesized in porous template by electroless plating.
• Nickel formed a network of nanowires on the surface.
• Effect of porous structure on light absorption of Ni–P coating was studied.
• Duplex coating showed better optical properties compared to other methods.

This paper introduces a technique to make an ultra-black surface by employing nanoporous anodized aluminum oxide as a template and deposition of nickel–phosphorus nanowires by the electroless process. The optical properties were compared with two other processes; a conventional black Ni–P deposition and a nickel electro-coloring process, on aluminum substrate. Surface morphologies of the samples were examined by field emission scanning electron microscope and elemental analysis of the coatings was performed by the energy dispersive spectroscopy method. Optical properties of surfaces were determined by spectrophotometry and infrared spectroscopy techniques. In addition, optical characteristics of the coated surfaces were evaluated by calculation of absorption and emission coefficients of the surfaces. The results showed that ultra-black duplex coating possessed an absorption coefficient higher than 99%, while emission coefficient decreased about 6% compared with simple black electroless Ni–P. Calculation of ξ factor indicated that a value of 5.1 proved that optical properties in the duplex coated sample had a significant improvement.