Journal of Materials Science

, Volume 48, Issue 13, pp 4549–4556 | Cite as

Surface hardening of biocompatible ultrafine-grained niobium zirconium alloy by two-stage oxidation treatment

  • F. Rubitschek
  • T. Niendorf
  • I. Karaman
  • H. J. Maier
Nanostructured Materials


The present study reports on an optimized surface hardening process for biocompatible ultrafine-grained (UFG) niobium 2.3 wt% zirconium (NbZr) alloy, a promising candidate implant material. The as-received material of conventional grain size (CG) was processed using multipass equal channel angular processing at room temperature to obtain an UFG microstructure featuring high strength and ductility. Subsequent surface hardening was performed by a heat treatment leading to internal oxidation. Using a thermogravimetric system, the influence of temperatures, time, and partial pressure of oxygen (\( p_{{\text{O}}_2} \)) on the oxidation kinetics were investigated. Metallographic and microscopic analyses and hardness measurements were employed to evaluate maximum hardness, penetration-depth and scale formation under various conditions. Heat treatment at 620 °C for 6 h at a \( p_{{\text{O}}_2} \) of 0.2 hPa led to sufficiently rapid oxidation kinetics yielding a relatively high depth of penetration without formation of loose Nb2O5 on the surface, which was observed at higher \( p_{{\text{O}}_2} \). As compared to CG material, improved hardness profiles were reached using the same heat treatment parameters, since the UFG structure significantly changes diffusion conditions and therefore oxidation kinetics. After a second heat treatment in high vacuum the high maximum hardness of 820 HV0.01 in the UFG material was reduced effectively and a less steep hardness gradient was achieved, both contributing to a less brittle behavior under mechanical loading. High-cycle fatigue tests performed on surface-hardened UFG NbZr samples showed a substantial improvement of fatigue life in tests conducted near the endurance limit. Especially when high fatigue and wear resistance are key issues for a given application, the internal oxidation process offers an effective way to further improve the properties of UFG NbZr.


Internal Oxidation Hardness Profile Electro Discharge Machine Solute Oxygen Equal Channel Angular Processing 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.



The help of Mr. Eugen Itermann with the thermogravimetric setup and with the experiments is gratefully acknowledged.


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Copyright information

© Springer Science+Business Media New York 2013

Authors and Affiliations

  • F. Rubitschek
    • 1
  • T. Niendorf
    • 1
  • I. Karaman
    • 2
  • H. J. Maier
    • 1
  1. 1.Lehrstuhl für Werkstoffkunde (Materials Science)University of PaderbornPaderbornGermany
  2. 2.Department of Mechanical EngineeringTexas A&M UniversityCollege StationUSA

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