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

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Oxidized zirconium and titanium alloys are used in tribological applications such as endoprosthetics. The advantage of this material class lies in the ceramic surface, which provides hardness, corrosion and abrasion resistance, and the metallic substrate, which ensures good ductility and toughness. A disadvantage is the sometimes poor coating adhesion, which arises during the conventional oxidation of the metallic alloys.In the DFG predecessor project, various heat treatment processes for the zirconium alloy ZrNb7 were examined in more detail, which significantly improve the layer adhesion. These methods are based on an oxygen diffusion zone which lies below the oxide layer and improves the chemical bonding of the oxide to the metallic substrate. This is described in the literature as Oxygen Diffusion Hardening (ODH). In addition to zirconium, the metals titanium and hafnium have a pronounced oxygen solubility in the lattice and are therefore also suitable for this heat treatment process. A problem with oxidized titanium alloys, however, lies in the high defect density of the oxide layers, which occur mainly parallel to the metal surface.The aim of this project is to investigate three systems for their suitability for oxygen diffusion hardening and to improve the mechanical properties of the oxide layers to this end. The three systems are 1) Ti6Al4V, 2) Ti6Al4V coated with Zr (via powder packing) and 3) TiZrNbHfTa (20 at.% each). Ti6Al4V is by far the most commonly used Ti alloy. A suitable oxygen diffusion hardening could significantly improve this alloy for use in tribological applications. Since thermally grown titanium oxides generally have a more complex structure than zirconium dioxide and form many substoichiometric oxides, another promising approach is to coat Ti6Al4V alloy with zirconium using a powder packing process. Oxygen diffusion hardening should then be applicable analogous to the investigated alloy ZrNb7 from the current project Gl 181/41-1. The third system, the highly deformable TiZrNbHfTa high entropy alloy, contains all three elements with pronounced oxygen solubility. The near-surface layers that form during the oxidation of this alloy are of particular interest.For all three systems suitable heat treatment parameters should be developed to improve both the layer adhesion and the wear resistance. The latter is evaluated in pin/disc and cylinder/flat test benches. The data from the heat treatment processes will be used to develop a mathematical model that can predict the layer structure and the thicknesses of the individual layers at a given temperature and heat treatment time.