Effect of high energy ball milling on titanium-hydroxyapatite powders

Abstract

This paper reports on an investigation of the potential use of high energy ball milling (HEBM) for the production of homogeneous mixtures of titanium/hydroxyapatite powders. This system is of considerable interest for the production of functionally graded material (FGM) components for use in implants in the body. In order for these FGMs to have satisfactory properties homogeneous mixtures of the titanium and hydroxyapatite (HA) powders must be produced prior to consolidation. In this investigation Ti/20 wt-% and 40 wt-% HA powder mixtures were produced by high energy milling for times of 15 minutes and 1 hour. Mixtures were also prepared on a conventional turbula powder mixer for reference purposes. The mixtures were consolidated by either cold pressing, cold isostatic pressing or hot pressing under various conditions. Selected specimens were also subjected to sintering over a range of temperatures (400-1100°C). The resulting microstructures were characterised using scanning electron microscopy, X-ray diffraction, dilatometry, differential thermal analysis and thermogravimetry.

Following the HEBM process, the resulting powders consisted of Ti particles coated with continuous surface layers of HA. It had been hoped that the process would produce a homogeneous product consisting of HA particles evenly distributed within the Ti particles and so the results were disappointing. There was some indication that the longer milling time of 1 hour produced a limited amount of HA surface layer breakup but also lead to amorphisation of the HA. A possible way forward may be to use extrusion as the consolidation process as this would provide substantial amounts of shear deformation, which in the past has been shown to be effective in breaking up surface oxide layers. An additional advantage is that the relatively high speed of the extrusion process would minimise the time available for HA amorphisation.