The Influence of Oxide Color on the Surface Characteristics of Zirconium Alloy ZrNb7 (wt%) After Different Heat Treatments

Oxidized zirconium alloys can appear as black, white and all gray shades depending on the heat treatment process. The black color results from a high amount of oxygen vacancies in non-stoichiometric zirconia (ZrO2−x) that effectively reduces the band gap of the material. In this work we compare the surface properties of black and white zirconia on ZrNb7 substrate. An oxidation in air at 600 °C for 1 h results in a dark-gray oxide with only a few micro cracks. Oxidation at low oxygen partial pressure at 600 °C for 8 h (pO2 = 10–19 Pa) generates a dense, totally black oxide scale. A three step heat treatment process, that was introduced for better coating adhesion, results in a white oxide layer with many micro cracks parallel to the surface. From the results of various microscopy and spectroscopy techniques, we derive a model of the layer formation of zirconia on ZrNb7 and give reason why black zirconia, and therefore the oxidation at low oxygen partial pressure, is favorable for tribological applications (e.g., artificial joint replacements).