Characterization of Ti-6Al-4V modified by nitrogen plasma immersion ion implantation

T1-6Al-4V alloy is commonly used in biomedical or aerospace applications, due to its excellent combination of chemical and mechanical properties, such as bioinertness, corrosion resistance or high strength to weight ratio. The use of surface treatments or coatings has widened the application possibilities of this alloy. The often observed poor tribological performance can be overcome by the correct choice of surface engineering methods. Ion implantation is a candidate among the different available processes and excellent results have been obtained in biomedical applications. However, when complex geometries are involved, it can be a difficult and less economically effective treatment. Plasma immersion ion implantation (PIII) offers the possibility of performing three-dimensional ion beam treatments, reducing the need for manipulation under vacuum to obtain a uniform treatment of geometrically complex parts. In this work, PIII was used to implant nitrogen in the Ti-6Al-4V alloy. The nitrogen plasma was generated with a 2.45 GHz microwave excitation, and a pulsed bias voltage of 45 kV with pulse repetition rates of 50 and 400 Hz were applied. Nitrogen retained doses were evaluated by means of Rutherford backscattering spectroscopy (RBS). Pin-on-disc wear and friction tests were performed on the implanted samples. An ultra high molecular weight polyethylene pin was used as the counterface material to partially simulate the conditions encountered in biomedical applications. A reasonable improvement in load bearing capacity with respect to the unimplanted alloy was observed after these tribological tests. However, no hardening could be measured after ion implantation. The worn surfaces were observed by scanning electron microscopy and optical profilometry after the tests and the type of wear mechanism was studied. The effect of the implantation was to increase the load at which breakthrough occurred on the protective surface layer.