School of Engineering and Materials Science
Research Student Roll of Honours
PhD Thesis: Processing, characterisation and mechanical properties of hydroxyapatite-zirconia composites for skeletal implants
Author: TOWLER, Mark Robert
Supervisor(s): Serena Best
The objective of this research was to produce a series of HA composites, by the introduction of small amounts of zirconia (ZrO2), a relatively inert ceramic. Both commercial and laboratory synthesized HA and ZrO2 materials, with a range of particle sizes, surface areas and morphologies, were employed as starting materials, and were characterised, prior to the mixing together of the two phases. The HA-ZrO2 composites, containing a range of ZrO2 (between 0 and 50wt%) were cold pressed into discs and subsequently sintered at temperatures between 750°C and 1280°C. The composites were characterised by a series of techniques: particle size and surface area analysis, X-ray diffraction, X-ray fluorescence and density measurements. Mechanical properties (Vicker's microhardness and flexural strength) were determined and related to the sintered microstructures as established by electron microscopy. The subsequent bioactivity of selected samples was analysed to determine the effect of introduction of the second phase.
Dependent on the starting grades of ceramics employed, the mechanical properties of the HA-ZrO2 composites reached an optimum at a ZrO2 dispersion of 1-2wt%, with a flexural strength of 60MPa, twice that of pure HA, and a significant increase in Vicker's microhardness number, to 500. This enhancement was attributed to the development of a uniform microstructure, with low intergranular porosity (less than 4%) and a well dispersed ZrO2 phase. This particular microstructure increased strength through a crack deflection mechanism and also through the inhibition of decomposition of HA to -TCP, which is an inherently weaker material. Cell culture techniques indicated that bioactivity was not affected by the addition of up to 20wt% ZrO2, but at higher levels the ability of cells to proliferate on the surface of the composites was reduced.