Titanium nitride coatings on surgical titanium alloys produced by a powder immersion reaction assisted coating method: residual stresses and fretting behavior

Titanium and Ti–6Al–4V alloy samples were coated using a Powder Immersion Reaction Assisted Coating (PIRAC) nitriding method in order to modify their surface properties. Depending on the processing temperature, strongly adherent single(TiN)- or double(Ti2N/TiN)-layer coatings were obtained on both substrates. Several characteristics of PIRAC-coated Ti alloys relevant to their applications in total joint replacements were studied. Residual stresses in PIRAC coatings measured by sin2ψ X-ray diffraction method were found to be compressive in nature and were significantly lower than those reported for PVD TiN layers on similar substrates. In vitro fretting tests of PIRAC nitrided Ti–6Al–4V-to-Ti–6Al–4V couples simulating in vivo conditions at the interface of modular orthopedic implants demonstrated a major reduction in fretted areas, as well as a remarkable reduction of the corrosion potential drop at the initial stages of fretting as compared to the uncoated alloy. In addition, a 25% reduction of fretting-induced dissolved Ti ions concentration in testing solution was measured by EAAS. The results of the research suggest that titanium nitride PIRAC coatings can provide surgical titanium alloys with the longed-for fretting wear and corrosion resistant surface thereby minimizing the ion- and particulate-generating potential of modular orthopedic implants.