Fatigue properties of carbon- and porous-coated Ti-6Al-4V alloy

Abstract

A porous metal coating applied to a solid substrate implant has been shown, in vivo, to offer advantages over current polymethylmethacrylate cement fixation in orthopedic devices. These advantages may be lost, however, in devices requiring a sintering heat treatment to apply the coating since these treatments may have a detrimental effect on the substrate material mechanical properties. In addition, more biocompatible interface coating materials have come of interest with recent literature reports of metal ion release. These coatings may be of particular use in porous-coated systems since the surface area of implant in contact with the surrounding tissues is greatly increased. This study investigated the effects that both a porous Ti- 6Al -4V alloy coating and a ULTI carbon coating have on the fatigue properties of a Ti- 6Al -4V alloy substrate system. The fatigue properties of uncoated as-received, uncoated sinter heat treated and notched Ti- 6Al -4V material were also investigated. The results of this study revealed endurance limits for Ti- 6Al -4V alloy tested with a rotating beam system of 617 MN/m2 (uncoated as-received), 624 MN/m2 ( ULTI carbon-coated), 377 MN/m2 ( sinter heat treated), 220 MN/m2 (notched) and 138 MN/m2 (porous-coated). No effects on fatigue properties were observed when testing the material in saline compared with air. The slight increase in fatigue strength for the carbon-coated material is thought to be due to the increase in surface hardness resulting from the formation of titanium carbides on the surface. The low-endurance limit of the porous-coated material is due to both the transition from the as-received equiaxed microstructure to a lamellar microstructure upon sintering and to the notch effect created by the porous coating.