WEAR-RESISTANCE OF PLASMA IMMERSION ION-IMPLANTED TI6AL4V

The plasma immersion ion implantation (PI3 TM) process has been employ ed in the treatment of the Ti6Al4V alloy in order to improve its notor iously poor tribological properties. In particular, this study was und ertaken with a view to its potential application for the surface engin eering of orthopaedic implants. PI3 has been developed over recent yea rs at the Australian Nuclear Science and Technology Organisation (ANST O). The hybrid nature of this technique combines elements of both ion implantation and plasma nitriding, and has been shown to produce compo nents with unique surface properties and optimum performance character istics. A detailed study of the PI3 process on the Ti6Al4V alloy has b een undertaken. Treatment was carried out in a pure nitrogen atmospher e at temperatures of 350, 450 and 550 degrees C. In each case, specime ns were treated for 5 h, with a high voltage pulse (typically 40 kV) a pplied directly to the workpiece. Wear resistance of the treated sampl es was assessed using a standard CSEM pin-on-disc wear machine, with a single crystal ruby ball as the contact lip. Glancing angle X-ray dif fraction (GAXRD) was employed to determine the phases present in the s urface modified layer. These findings were then compared to those achi eved from parallel work with conventionally ion implanted and low temp erature plasma nitrided samples. It was established that a high treatm ent temperature of 550 degrees C was necessary for substantial improve ments in the properties of the Ti6Al4V material. Under these condition s the PI3 technique promoted significant increases in Knoop hardness, and wear resistance an order of magnitude greater than conventional io n implantation. Wear rates were typically reduced by four orders of ma gnitude compared to those of the untreated Ti6Al4V. This is thought to be associated with the increased mobility of nitrogen in alpha-Ti at these temperatures, producing a deeper, hardened case. The presence of TiN was observed in the microstructure of PI3 Ti6Al4V samples at all temperatures in the range.