"Oxygen Diffusion Hardening" (ODH) of titanium group elements and their tribological properties

• Applicants: Privatdozent Dr.-Ing. Mathias Galetz; Professor Dr.-Ing. Uwe Glatzel
• Subject Area: Coating and Surface Technology ; Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: from 2015 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 270293189

• Final Report Year: 2024

Final Report Abstract

Oxygen diffusion hardening is a promising approach to improve the wear properties of biocompatible alloys like the Zirconium alloy ZrNb7, as examined in the previous project. The goal of this project was to investigate the impact of alloy composition for several Titanium alloys Ti6Al4V, Ti6Al4V+Zr, and the high entropy alloy TiZrNbHfTa, and to compare the resulting surface properties. To achieve this, various oxidation processes were implemented for Ti6Al4V: • a single-step process (680 °C, 16 h, air), leading to the formation of a TiO 2 oxide layer with only a marginal O diffusion zone. • a two-step process (680 °C, 16 h, air + 850 °C, 6 h, vacuum), in which the oxide layer is reduced, and a pronounced O diffusion zone is formed. • a three-step process (680 °C, 16 h, air + 850 °C, 6 h, vacuum + 680 °C, 16 h, air), resulting in a TiO2 oxide layer and a pronounced O diffusion zone. • Oxidation at reduced O-partial pressure (850 °C, 2 h, PO2=10^-13 Pa), leading to a TiO2- Al2O3-TiO2 oxide layer and a pronounced O diffusion zone. • Oxidation in dry air and humid air with 10 vol.% H2O (500-700 °C, 100-500 h), resulting in a thicker oxide scale and O diffusion zone with increasing temperature and time, but also severe oxide spallation in the presence of water vapor. • a novel slurry process (1150 °C, 5 h, Ar), enriching Ti6Al4V with Zr, such that ZrO2-enriched TiO2 is formed on the surface during subsequent oxidation (680 °C, 16 h, air) due to the higher oxygen affinity of Zr compared to Ti. Oxidation at reduced O-partial pressure proves to be particularly promising. Unlike the singlestep and three-step processes, the obtained hard oxide layer does not have pores at the oxidesubstrate interface, leading to good layer adhesion. Also the slurry-coated substrate offered an exceptional wear behavior, not only due to the oxygen diffusion hardening but also microstructural changes, known as Widmannstätten structure. For TiZrNbHfTa, single-step oxidation (600°C, 24 h, air) was successfully realized for the first time. For ultrafine-grained TiZrNbHfTa, an adherent surface oxide layer of 1-2 µm thickness forms on an oxygen-rich zone with internal Hf and Zr oxides, which increased the surface hardness by more than four times (1522±64 HV0.5). However, the single-phase nature of TiZrNbHfTa is altered during the oxidation process. Two-step oxidation (550°C, 24 h, air + 1200 °C, 1 h, vacuum, quenching) restores the single-phase nature in the bulk material, with hexagonally close-packed precipitates forming in the oxygen-rich zone subsurface region.

Publications

• Improving the Adhesion of a Hard Oxide Layer on Ti6Al4V by a Three‐Step Thermal Oxidation Process. Advanced Engineering Materials, 24(3).
  Dickes, Daniel; Öztürk, Beyza; Völkl, Rainer; Galetz, Mathias C. & Glatzel, Uwe
  
• Influence of Water Vapor and Temperature on the Oxide Scale Growth and Alpha-Case Formation in Ti-6Al-4V Alloy. Oxidation of Metals, 97(3-4), 241-260.
  Öztürk, B.; Mengis, L.; Dickes, D.; Glatzel, U. & Galetz, M. C.
  
• Thermal oxidation of TiZrNbHfTa high entropy alloy at 600 °C; Mendeley Data (2022)
  Dickes D. & Öztürk B.
  
• Oxide layer and diffusion zone formation during oxidation of Ti6Al4V and TiZrNbHfTa and their influence on tribological properties; Berichte aus der Materialwissenschaft; Shaker, Düren, (2023) ISBN:9783844092936. Dissertation
  Dickes, D.
  
• Surface hardening of TiZrNbHfTa high entropy alloy via oxidation. Corrosion Science, 217, 111159.
  Dickes, Daniel; Öztürk, Beyza; Baier, Felix; Berger, Pascal; George, Easo P.; Völkl, Rainer; Galetz, Mathias C. & Glatzel, Uwe
  
• Improved tribological behavior of Ti-6Al-4V through a novel zirconium diffusion coating. Surface and Coatings Technology, 487, 130993.
  Öztürk, B.; White, E.M.H.; Dickes, D.; Glatzel, U.; Mengis, L. & Galetz, M.C.
  
• Influence of oxygen ingress on microstructure and phase formation during two-step oxidation surface hardening of TiZrNbHfTa high entropy alloy. Applied Physics Letters, 124(7).
  Dickes, Daniel; Zhao, Yujun; Baier, Felix; Öztürk, Beyza; Völkl, Rainer; Li, Tong; Galetz, Mathias C. & Glatzel, Uwe
  
• Influence of Water Vapor on the Oxidation of Pure Titanium. High Temperature Corrosion of Materials, 101(6), 1341-1351.
  Öztürk, B.; Rudolphi, M.; White, E. M. H.; Dickes, D.; Glatzel, U. & Galetz, M. C.