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 degreesC 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 degreesC (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 degreesC along with the formation of CrN/Cr2N/sigma phases above 700 degreesC. 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). (C) 2004 Elsevier B.V. All rights reserved.