Relation between Mechanical Hardening and Nitrogen Profile of PBII Nitrided Titanium Alloy

Surface treatments of Ti-6Al-4V alloys are of utmost importance for biomedical applications since they allow for tribological gain. Here, Ti-6Al-4V disks have been PBII nitrided at either 500, 600, 700 and 800 °C. A set of techniques (XRD, SEM-EDS, EBSD and GDOES) was used to characterize the surface microstructural and chemical changes. Nanoindentation was used to assess the induced changes in terms of mechanical properties. Two types of nitrided domains are revealed. Starting from the surface, a nitride bilayer composed of δ-TiN/ϵ-Ti2N with enhanced surface resistance is supported by an α-Ti(N) solid solution formed at depth. Hardness values peak at 12-14 GPa at the surface, which is almost twice as large as the bulk value (about 7 GPa). For the moderate temperatures used here, a deep (10-15 µm) and strong hardness (14 GPa) enhancement together with a smooth gradient can be achieved.