The effect of nitrogen diffusion hardening on the surface chemistry and scratch resistance of Ti-6Al-4V alloy

Modular, head-stem, mixed-metal connections are susceptible to mechanically mediated electrochemical interactions. Any attempt to improve the performa nce of these connections should center around increasing their resistance t o mechanical damage, particularly the titanium alloy (Ti64). This study inv estigated the effect of a nitrogen-diffusion-hardening process on Ti64, wit h specific reference to changes in composition, chemistry, electrochemistry and its ability to resist and/or repassivate scratch damage. The nitrogen- diffusion-hardened Ti64 alloy had TiN and TiNO complexes at the immediate s urface and sub-surface layers. The diffusion-hardened samples also had a de eper penetration of oxygen compared to regular Ti64 alloy samples. The elec trochemical impedance spectroscopy data corroborated the increased thicknes s of the barrier oxide on the diffusion-hardened samples. The nitrogen-diff usion-hardened samples were more resistant to scratch damage and repaired/r epassivated faster after such damage. The results suggest that the nitrogen -diffusion-hardened titanium alloy should exhibit increased resistance to m echanical-electrochemical interactions in mixed-metal modular interfaces in total hip prostheses. (C) 2000 Elsevier Science Ltd. All rights reserved.