Preparation of black PEO layers on Al–Si alloy and the colorizing analysis

• An uniform black PEO coating is fabricated on hypoeutectic Al–Si alloy successfully.
• A segmented voltage is used in the PEO processing.
• The colorants are a combination of NH4VO3 and Na2WO4.
• The black PEO coating is described quantitatively with a CIELAB color space model.
• The colorizing mechanism is analyzed in detail.

Uniform black ceramic coatings were successfully fabricated on the surface of hypoeutectic Al–Si alloy by a plasma electrolytic oxidation (PEO) technology. The coloring additives were NH4VO3 and Na2WO4. The positive voltage was controlled to increase from 0 to 400 V by nine steps. SEM, EDS, XPS and XRD were used to study the characteristics of the black coatings. It is found that the distribution of the pores left by the discharge channels became more and more uniform with the processing time. The color of the layer changed from gray to brown gradually, and finally turned to black. The layer looks black mainly for the existing of vanadium oxides and tungsten oxides on its surface, and VO3− plays a more important role than WO42− in the colorizing.