EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Titanium and its alloys are widely used in applications where high strength and light weight are required. Due to their biocompatibility, Ti alloys are also used in numerous medical applications like dental implant or bones prosthesis. However, Ti alloys suffer from poor tribological properties that limit their use when wear is experienced. Thermochemical treatment like nitriding or carburizing for example, and deposition of coatings are ways to overcome these limitations. Unfortunately, they often lead to a very hard and quite thin layer on top of a relatively soft substrate. The hard and typically brittle layer can fracture when loaded, the so-called eggshell effect, and produces debris that generate high wear rates. This phenomenon can be reduced drastically by the introduction of a thick diffusion zone underneath the hard surface layer. The simultaneous insertion of different interstitials, such as nitrogen, carbon and oxygen, in the structure is expected to significantly improve tribological properties and overall mechanical performance of treated parts, which potentially can expand the application range of titanium alloys within several industries.

The present contribution addresses surface hardening of titanium alloy Ti6Al4V via multi-interstitial diffusion using plasma processing. The possibility to produce thick diffusion zones containing mixed-interstitials in solid solution as function of the controlled parameters (temperature, reactive atmosphere content and pressure, duration, plasma assistance) is presented. Moreover, the response of Ti6Al4V alloy to sequential treatments adding different “loads” of interstitials is investigated. The evolution of case and core microstructures and resulting mechanical properties as a function of the interstitial (N, C, O) content are analysed by Optical and Scanning Electron Microscopy, microhardness and nanoindentation. The phase distribution is determined by X-ray diffraction. The microstructure evolution and phase distribution are correlated to the concentration profiles of the interstitials obtained by GDOES (Glow Discharge Optical Emission Spectroscopy).