,oO,RFAC E COATINg8 fA II#OLD ! ELSEVIER Surface and Coatings Technology 93 (1997) 305-308 Characterization of Ti-6A1-4V modified by nitrogen plasma immersion ion implantation F. Alonso a,., M. Rinner b, A. Loinaz a, J.I. Ofiate a, W. Ensinger b B. Rauschenbach b ~'INASMET, Camino de Pormetxe, 12, 20009 San Sebastidn, spain b Universitdt Augsburg, Institutfiir Physik, 86135 Augsburg, Germany Abstract 2 -- Ti-6A1-4V 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 nigh 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-6A1-4V alloy. The nitrogen plasma was generated with a 2.45 GI-Iz 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. © 1997 Elsevier Science S.A. Keywords: Friction; Load bearing capacity; Plasma immersion ion implantation; Ti-6A14V; Wear 1. Introduction Plasma immersion ion implantation (PIII) is an alter- native ion implantation technique which overcomes some of the limitations of ion beam implantation [1-3]. Contrary to ion beam implantation, PIII is not a line- of-sight process and therefore allows the treatment of complex shape components with an acceptable dose uniformity [4]. However, recent studies have demon- strated a dependence of the implanted dose on the geometry of the substrate, with differences of up to 50% in the retained doses measured in sharp edges and flat surfaces [5]. There are new facilities that allow accelera- tion voltages of 100 kV [6,7]; however, PIII treatments are normally carried out at a maximum acceleration * Corresponding author. Tel.: +34-(9)43-21 80 22; + 34-(9)43-21 75 60; e-maiI:patonso@inasmet.es fax: 0257-8972/97/$17.00© 1997ElsevierScienceS.A. All rights reserved. PII S0257-8972(97) 00065-0 voltage of 50 kV [8-10]. Moreover, when using nitrogen as the source gas, the plasma is formed by a mixture of N~- and N ÷ ions, with a N~- estimated percentage >75% [6, 11], which results in a decrease of the energy of implanted nitrogenatoms. However, despite the fact that these voltages are lower than in conventional ion beam implantation, where they can be up to 200 kV, PIII provides the potential to achieve deeper penetration. For nitrogen PIII, two mechanisms have been reported to account for the increase in depth: radiation enhanced diffusion [12, 13] and thermal diffusion, as PIII treatment can be performed at temperatures up to 500-600°C [8,141. Ti-6AI-4V is an alloy that has been successfully treated with the PIII process, earlier on. Chen and co-workers [8] showed that an increase in hardness and wear resistance was produced after nitrogen implant- ation; with selected treatment conditions the wear resis-