A NOVEL BLACKENING METHOD OF PREPARING ULTRA-BLACK Ni–P COATINGS: EFFECT OF PROCESS PARAMETERS ON MORPHOLOGY AND OPTICAL PROPERTY

Ni–P ultra-black coatings were prepared by a novel blackening method of anodic oxidation in nonoxidizing acid media. The effect of the applied voltage, concentration of H3PO4 and anodization time on the microstructure and optical property of blackened coatings was investigated. The results show that the applied voltage plays the most important role during the formation of the ultra-black coatings. The concentration of H3PO4 is the next-important factor, then anodization time. The optimum process parameter for obtaining ultra-black coatings is at 0.9 V in 3 mol/L for 40 min, in which the reflectance is only 0.14–0.21% in the visible region. The low reflectance is mainly attributable to the unique array structure of conical cavities ranging 1–3 μm in size, in which the innumerable tiny pits distribute on the cavity wall. In contrast, the black coatings etched by chemical etching method by strong oxidizing acid (HNO3) have larger conical cavities (10–20 μm) with smooth cavity wall, and thus lead to higher reflectance of 0.69–2.44%.