High intensity plasma ion nitriding of orthopedic materials: Part II. Microstructural analysis

Orthopedic materials (Ti-6Al-4V and CoCrMo) were nitrided at temperatures up to 850 °C using a newly developed technique—high intensity plasma ion nitriding (HIPIN)—in order to enhance lifetime wear properties. Near surface microstructure and phase formation of both nitrided Ti-6Al-4V and CoCrMo alloys were evaluated using cross-sectional SEM and X-ray diffraction techniques. Nitrogen was found to most readily incorporate into the Ti-6Al-4V structure at around 750 °C (slight expansion of the lattice) with additional formation of TiN and Ti2N at higher temperatures. In the case of the CoCrMo alloys (forged and cast), a metastable, high nitrogen phase was also found to form at between 300 and 600 °C along with the formation of CrN/Cr2N/σ phases above 700 °C. It was concluded that increased wear properties, as a result of HIPIN processing, can be related to both the formation of deep nitrided layers and the formation of specific microstructures at the surface (for both alloy types).