High-intensity plasma ion nitriding of orthopedic materials: Part I. Tribological study

Abstract

In this paper, we describe a newly developed technique—high-intensity plasma ion nitriding (HIPIN), which takes the advantages of low energy beam ion implantation at elevated temperatures that generates thick nitrided layers compared to conventional plasma nitriding and ion implantation, but overcomes the inherent line-of-sight shortcoming of beam implantation. In this technique, a low-pressure nitrogen plasma (<1 mTorr) is used. With an electron source (filament) to emit electrons and by drawing them to the anode (vacuum chamber), the plasma is generated and maintained due to electron-neutral gas collisions. With the application of a low voltage on the parts that are immersed in the plasma, ions are drawn and implanted to the surface of the parts. At elevated temperatures, nitrogen diffusion occurs, thus forming deep nitride layers. Using this technique, we have processed orthopedic materials made of Ti-6Al-4V and CoCrMo at various temperatures. It has been observed that for Ti-6Al-4V, nitriding at higher temperatures generate deep nitrided layers that are quite hard and wear resistant. For forged CoCrMo, nitriding at both low and high temperatures show much better performance than at medium temperatures. For comparison purposes, nitrogen plasma immersion ion implantation has also been performed. However, nitrided samples using the HIPIN technique out perform the one using implantation. A reduction of wear over 30 times has been observed. In contrast to the forged CoCrMo, nearly no improvement has been observed on cast CoCrMo possibly due to the microstructure effect.