Our ability to introduce color to metals lags well behind that for fabrics and textiles. Electrochemical reactions can color metals, but the chemical waste can damage the environment, and the resultant colors and patterns are limited. Printing and emulsion techniques can also produce color coatings, but these tend to wear and fade over time. A third approach—using lasers—has now advanced thanks to research by Zhongli Li and co-workers at the Singapore Institute of Manufacturing Technology and the Institute of Materials Research and Engineering of A*STAR, Singapore.
Colorless laser marking of metallic surfaces is already used widely—for example, to engrave iPod music players. Lasers can also be used to color metals by heating the metal surface enough to form thin oxide layers. Interference between reflections from the tops and bottoms of these layers generates colors in the same way as the effect of a thin film of oil on water. To achieve different and uniform colors, however, the thickness and composition of each layer must be carefully controlled.
To this end, Li and her co-workers focused on understanding the process of laser-induced oxide formation on stainless steel, which is used in products such as jewelry and souvenirs. They used optical and electron microscopes, as well as time-of-flight secondary ion mass spectrometers, to study the oxides formed as they varied laser power, the degree of laser defocusing, and the direction and speed of laser scanning across the surface.
At higher scanning speeds, they found that the chromium in the stainless steel substrate oxidized selectively, resulting in a single oxide layer. At lower scanning speeds, two oxide layers formed: a chromium oxide solution layer capped by an outer iron oxide solution layer. The researchers discovered that the thickness and composition of each of these layers changed with each pass of the laser beam scan over the substrate, as iron atoms from the metal bulk diffused outwards to form new oxide layers. In addition, the oxide layers were stable after corrosion tests and after three years of exposure to air with 80% humidity.
For Li, laser coloring represents a synergy between art and engineering, and may prove to be an interesting new tool for artists (Fig. 1). “The laser beam may become a kind of color pen, able to produce color paintings,” she says. She believes the detailed understanding of the growth of oxide layers during the laser process will make an invaluable contribution to making laser coloration of metals a practical addition to existing techniques.