Abstract
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).
Highlights
Oxidized zirconium alloys are widely used in joint prostheses such as artificial knee joints because of their favorable properties
Oxidation of Metals (2021) 95:377–388 this work, alloys like ZrNb7 are oxidized in air between 500 and 700 °C to generate an oxide scale of about 5–7 μm thickness. This oxide scale is chemically bonded to the substrate via a thin oxygen diffusion zone, as zirconium has the ability to solve up to 30 at% of oxygen in its lattice at temperatures around 700 °C
The basic layer formation is the same in all three process routes: the oxide scale is chemically bonded to the substrate by an oxygen diffusion zone
Summary
Oxidized zirconium alloys (mostly ZrNb2.5) are widely used in joint prostheses such as artificial knee joints because of their favorable properties. Oxidation of Metals (2021) 95:377–388 this work, alloys like ZrNb7 are oxidized in air between 500 and 700 °C to generate an oxide scale of about 5–7 μm thickness This oxide scale is chemically bonded to the substrate via a thin oxygen diffusion zone, as zirconium has the ability to solve up to 30 at% of oxygen in its lattice at temperatures around 700 °C. Previous works have focused on enlarging the diffusion zone via a solid state reduction [6,7,8] During this process, oxygen from the oxide scale diffuses into the metal substrate and thereby reduces the hardness gradient between surface and bulk material. This in return can enhance coating adhesion [6,7,8]
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