Wear resistance of plasma immersion ion implanted Ti6Al4V

The plasma immersion ion implantation (PI3 TM) process has been employed in the treatment of the Ti6Al4V alloy in order to improve its notoriously poor tribological properties. In particular, this study was undertaken with a view to its potential application for the surface engineering of orthopaedic implants. PI3 has been developed over recent years at the Australian Nuclear Science and Technology Organisation (ANSTO). The hybrid nature of this technique combines elements of both ion implantation and plasma nitriding, and has been shown to produce components with unique surface properties and optimum performance characteristics. A detailed study of the PI3 process on the Ti6Al4V alloy has been undertaken. Treatment was carried out in a pure nitrogen atmosphere at temperatures of 350, 450 and 550 degrees C. In each case, specimens were treated for 5 h, with a high voltage pulse (typically 40 kV) applied directly to the workpiece. Wear resistance of the treated samples 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 diffraction (GAXRD) was employed to determine the phases present in the surface modified layer. These findings were then compared to those achieved from parallel work with conventionally ion implanted and low temperature plasma nitrided samples. It was established that a high treatment temperature of 550 degrees C was necessary for substantial improvements in the properties of the Ti6Al4V material. Under these conditions the PI3 technique promoted significant increases in Knoop hardness, and wear resistance an order of magnitude greater than conventional ion implantation. Wear rates were typically reduced by four orders of magnitude 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.