Effect of thermal treatment on apatite-forming ability of NaOH-treated tantalum metal

The prerequisite for an artificial material to bond to living bone is the formation of bonelike apatite on its surface in the body. This apatite can be reproduced on its surface even in an acellular simulated body fluid with ion concentrations nearly equal to those of the human blood plasma. The present authors previously showed that the tantalum metal subjected to a NaOH treatment to form a sodium tantalate hydrogel layer on its surface forms the bonelike apatite on its surface in SBF in a short period. The gel layer as-formed on the metal is, however, not resistant against abrasion, and hence thus-treated metal is not useful for clinical applications. In the present study, effects of thermal treatment on the mechanical properties and apatite-forming ability of the NaOH-treated tantalum metal were investigated. The sodium tantalate gel on the NaOH-treated tantalum was dehydrated to convert into amorphous sodium tantalate by a thermal treatment at 300 degreesC in air environment and into crystalline sodium tantalates by the thermal treatment at 500 degreesC. Resistivity of the gel layer against both peeling-off and scratching was significantly improved by the thermal treatment at 300 degreesC. The high apatite-forming ability of the sodium tantalate hydrogel was a little decreased by the thermal treatment at 300 degreesC, but appreciably decreased by the thermal treatment at 500 degreesC. It is believed that the tantalum metal subjected to the 0.5 M-NaOH treatment and the subsequent thermal treatment at 300 degreesC is useful as implants in dental and orthopaedic fields, since it shows high bioactivity as well as high fracture toughness. (C) 2001 Kluwer Academic Publishers.