Titanium anodisation designed for surface colouration - Systemisation of parametric interaction using response surface methodology

Highlights: • Operating parameter interactions are statistically significant in colouring titanium by anodisation. • 97%, 98.8% and 99.8% of the variability in oxide thickness, lightness and hue explains deliberate parametric variation. • Variations in parametric combination skews the oxidation rate. • Oxide thickness and lightness are more sensitive to interaction than hue angle. • Multiple optimal combination of parameters exists for a desired outcome. This work statistically examines the influence of operating parameters on the thin oxide layer developed during titanium anodisation and the complexity of parameter interactions on the surface colouration. The optimal parametric combinations for obtaining desired oxide thickness and interference colours are investigated. A three parametric Doehlert experimental design was employed for the purpose. Reliable regression models were developed for oxide thickness (T_meas), and colour specification viz., lightness (L*) and hue angle (h) with determination coefficients of 0.970, 0.988 and 0.998, respectively. The three chosen parameters namely operating voltage (V), electrolyte concentration (C), and operating temperature (T) were found to have significant interactions during anodisation which are analysed through a response surface. The oxide thickness was found to be influenced by all three parameters. It was additionally found to have multiple optimal parametric combinations for a desirable outcome because of the interactions; the lightness was found to be significantly affected by the interactions of electrolyte concentration and operating temperature; the expected periodicity in hue angle, as reported in literature, persisted amidst parameter interactions with mild variations to the trend. Furthermore, interdependencies were found among the developed regression models.