Influence of the substrate surface preparation on optical properties and color of anodized titanium

The study and the development of goniochromic materials remains a challenge both for industry and for scientific research. In the industry, the color quality control is rendered difficult by the complex color effects of these materials. The development of standard goniochromic materials with referenced color effects could simplify this color quality control. By comparing the manufactured materials to the reference ones by for example acquiring pictures of both materials in given geometries could be an easy process for color quality control. The challenge for scientific research consists to be able to control the color evolution with the illumination and observation conditions and define color characterization geometries in agreement with perceived color effects. A bibliographic review showed that anodized titanium is a promising candidate for reference goniochromic samples. In order to obtain samples with different levels of gloss, samples with substrate roughnesses ranging from a few to two hundred nanometers obtained through hydrofluoric etching or different mechanical and chemical polishing were prepared. Anodized titanium color is linked to the oxide layer thickness and the material refractive index. These parameters were estimated through different techniques in particular a technique based on the extrema of the material reflectance spectrum as well as ellipsometry and X-ray reflectometry. These parameters were put into an optical model in order to simulate the material specular reflectance. The diffuse and specular reflectances of the samples are also compared and a first simple model of the diffuse reflectance is presented. The Bidirectional Reflectance Distribution Function (BRDF) spectral and angular variations are used to characterize the diffuse appearance and goniochromism of the samples versus the substrate preparation and the anodizing cell potential.