Interstitial Oxygen diffusion hardening — A practical route for the surface protection of titanium

Commercially pure Titanium grade 2 and Ti–6Al–4V alloy (grade 5) are tested for interstitial Oxygen diffusion hardening (ODH). A two-step process for ODH which allows the reproducible deep case hardening of large batches of small and medium sized components is presented. First, the metal surface is thermally oxidized (TO) in air. Then, the metal is sealed in a depressurized quartz tube. During the following vacuum heat treatment (VHT) Oxygen is liberated through dissociation of the TiO2 scale and diffuses into the bulk metal. Thickness gradients of the hardened cases are measured in terms of Vickers hardness and interstitial Oxygen concentration as it is shown by wavelength dispersive X-ray analysis (WDX) line-scans. ODH yields tailored hardness gradients whereby the maximum surface hardness is mainly a function of the TO time and the gradientʹs depth a function of VHT time and temperature. Cases of 16–56 μm depth are realized for cp-Ti grade 2, and 32–72 μm for Ti–6Al–4V alloy. A maximum hardness increase of ~ 240 HV and ~ 275 HV is measured for these two materials, respectively. Severe grain coarsening is observed during the VHT of the grade 2 material, whereas grade 5 maintains a fine structure due to the presence of β-Ti. Strategies to limit grain coarsening as well as potential biomedical applications are discussed.