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

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Mosbacher, Mike ; Holzinger, Markus ; Galetz, Mathias ; Glatzel, Uwe:
The Influence of Oxide Color on the Surface Characteristics of Zirconium Alloy ZrNb₇ (wt%) After Different Heat Treatments.
In: Oxidation of Metals. Bd. 95 (2021) Heft 5 . - S. 377-388.
ISSN 1573-4889
DOI der Verlagsversion: https://doi.org/10.1007/s11085-021-10030-1

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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).