EVect of high energy ball milling on titanium– hydroxyapatite powders

to bone are concerned are brittle materials as exempliŽ ed by hydroxyapatite and ‘Bioglass’. Bioactive coatings are This paper reports on an investigationof the potential employed on metal implants to facilitate bonding to bone use of high energy ball milling (HEBM) for the while retaining the good combination of mechanical production of homogeneous mixtures of titanium/ properties associated with the underlying metal substrate. hydroxyapatite powders. This system is of considerHowever, failure of the coating can arise due to thermal able interest for the productionof functionally graded stresses, service stresses and poor adhesion (see for example material (FGM) components for use in implants in the Ref. 1). One possible solution would be to eliminate the body. In order for these FGMs to have satisfactory abrupt bioactive material/metal interface and employ a properties homogeneousmixtures of the titanium and functionally graded material in which the bioactivematerial hydroxyapatite (HA) powders must be produced prior content increases gradually from zero in the interior of the to consolidation. In this investigation Ti/20 wt-% implant to the level required for formation of an adequate and 40 wt-%HA powder mixtures were produced bond with the bone at the surface. Such functionally graded by high energy milling for times of 15 minutes and 1 hour. Mixtures were also prepared on a convenmaterials may be produced by a powder metallurgical tional turbula powder mixer for reference purposes. route2,3 and at Imperial College preliminary research on The mixtures were consolidated by either cold the PM productionof titanium/hydroxyapatitecomponents pressing, cold isostatic pressing or hot pressing under has been undertaken.4,5 Of great importance in the successvarious conditions. Selected specimens were also subful production of these materials is the achievement of a jected to sintering over a range of temperatures homogeneous (on a very Ž ne scale) mixture of the elemental (400–1100°C). The resulting microstructures were powders prior to consolidation. This paper reports on an characterised using scanning electron microscopy, investigation of the potential use of high energy ball milling X-ray diVraction, dilatometry, diVerential thermal (mechanical alloying)5,6 to produce homogeneous powder analysis and thermogravimetry. mixtures. 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 EXPERIMENTAL PROCEDURE would produce a homogeneous product consisting of Processing HA particles evenly distributed within the Ti particles The nominal composition of the titanium powders used, and so the results were disappointing.There was some supplied by Deeside Titanium Ltd, UK, is given in Table 1. indication that the longer milling time of 1 hour proThe nominal composition of the hydroxyapatite, supplied duced a limited amount of HA surface layer breakup by Biocomposites Ltd, UK, was 0·02SiO2– 0·01TiO2– but also lead to amorphisation of the HA. A possible 0·02Al2O3–0·01Fe2O3–54·5CaO–1·07MgO–0·74Na2O– way forward may be to use extrusion as the con<0·01K2O–41·1P2O5–0·01ZnO–0·02BaO. solidation process as this would provide substantial Mixtures of 20 wt-% and 40 wt-%HA were processed in amounts of shear deformation, which in the past has a high energy ball mill, under an inert atmosphere using a been shown to be eVective in breaking up surface oxide proprietary process, at DERA, Farnborough, UK. Both layers. An additional advantage is that the relatively powders were milled for 15 minutes and an additional high speed of the extrusion process would minimise batch of the Ti–20%HA mixture was milled for 1 hour. At the time available for HA amorphisation. PM/1013 the end of ball milling the mixtures were easily removed from the mill, there was no smearing of material on the balls Dr Tsipas is in the Department of Materials and the walls or signiŽ cant agglomeration. A Ti–20 wt-% Science and Metallurgy, University of Cambridge, mixture was also produced using a Turbula 3D path Cambridge, UK. Dr Goodwin is at the Structure and rotating mixer (termed conventional mixing) for comparison Materials Centre, QinetiQ, Farnborough,Hants, UK. purposes.Particle size distributionsfor the elementalpowders Dr McShane ([email protected]) and Professor and the mixtures were obtained using a laser particle Rawlings are in the Department of Materials, Imperial size analyser (Malvern Mastersizer) and scanning electron College, London, UK. microscopy was used to conŽ rm the particle sizes and for observation of particle shape and topography. ©2003 IoM Communications Ltd. Published by Maney for the Institute of Materials, Minerals and Mining. A range of compaction methods (cold pressing, cold isostatic pressing and hot pressing) under a variety of conditions were employed. For cold pressing the powders were compacted in an 8 mm diameter, zinc stearate lubricated steel die, to a pressure of 300 MPa. The nominal height of INTRODUCTION the compacts was 12 mm. Some of the cold pressed comBioactive materials are designed to produce a speciŽ c biopacts were subsequently isostatically pressed at 350 MPa logical response in the surrounding tissue when implanted for a period of 2 minutes. Finally a number of the isoin the body, such as inducing chemical activity that leads statically pressed compacts were hot pressed at 500°C to a to a strong bond being formed with bone. The most successful bioactive structural materials as far as bonding pressure of 800MPa.