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Mechanical Properties of Hydroxyapatite using a Dispersal Phase of Nano-
Zirconia and Sintering using Conventional and Microwave Methods
Declan J Curran
Thomas J Fleming, Mark R Towler, Stuart Hampshire
declan.curran@ul.ie
Abstract
Laboratory synthesized hydroxyapatite (HA) and
commercial nano-sized zirconia (ZrO2) were
comparatively sintered using conventional and
microwave methods at temperatures of 700, 1000, and
1200 o C. The microwave sintered (MS) samples show
less decomposition than their conventionally sintered
(CS) counterparts. Stabilisation of the ZrO2 phase
occurs in small amounts in both the CS and MS samples
at 1200 o C. Increasing sintering temperature increases
density, with no discernable difference between regimes
at 1200 o C. The change in relative density is determined
as the main controlling factor over the mechanical
properties. The nano-sized ZrO2 has no strengthening
effects. Instead it works to hinder densification.
1. Introduction
Due to its resemblance to the mineral phase of bone,
hydroxyapatite (HA) [Ca10(PO4)3(OH)2] is used in an
increasing number of medical applications. HA has been
shown to be both bio-compatible and osteo-conductive
[1], allowing it to promote new bone growth in-vivo
without eliciting an immune response. The inherent low
mechanical strength and brittleness of HA have
excluded its employment as a load bearing implant.
Sintering at high temperatures can result in the
formation of calcium phosphate based decomposition
products that, in certain instances, have been reported to
adversely affect biological response [2]. Zirconia (ZrO2)
has a high fracture toughness and is relatively bio-inert,
and as such provides an ideal toughening material for
implant applications. However, high sintering
temperatures are required to fully densify ZrO2. In this
work HA-ZrO2 composites have been fabricated from
nano-sized powders to reduce the activation energies
necessary to cause densification. In addition,
microwave sintering is used to reduce sintering
temperatures.
2. Materials & Methods
Laboratory synthesized HA, which was evaluated for
quality, was ball-milled with 0, 1, 2, 3, 4 and 5wt%
commercial nano-sized ZrO2 powder to ensure
homogeneity and to break up any soft agglomerates.
Cylindrical green bodies ~3mm thick & ~19mm Ø were
uniaxially pressed (5000 Kg , 20 seconds). Comparative
microwave and conventional sintering regimes were
done on the samples. The physical and mechanical
properties were then determined.
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3. Results & Discussion
Increasing sintering temperature increases relative
density over all compositions. Relative density is the
major controlling factor over BFS, Figure 1.
Figure 1: Average BFS vs. relative density.
Hardness shows very similar trends to relative
density over the sintering range of 700-1200 o C. Grain
size has no effect on the mechanical properties.
Increasing ZrO2 content does not increase BFS or
hardness; instead it reduces these properties.
It is hypothesized that the ZrO2 particles impede
densification by segregating at the grain boundaries,
hence increasing porosity.
Figure 2: Fracture surface SEM of HA (grey) with
5wt% ZrO2 (white) CS at 1200 o C.
4. Conclusion
Microwave sintering has no advantage over
conventional sintering in HA-ZrO2 composites. Nanosized
ZrO2 impedes densification, reducing the
mechanical properties.
8. References
[1] K. Anselme, B. Noel, B. Flautre, M.C. Blary, C.
Delecourt, M. Descamps, et al. “Association of porous
hydroxyapatite and bone marrow cells for regeneration”,
Bone, 1999, pp. 51S-4S.
[2] K.A. Hing, I.R. Gibson, L. DiSilvio, S.M. Best, W.
Bonfield, “Effect of variation in Ca:P ratio on the cellular
response of primary human osteoblast-like cells to
hydroxyapatite-based ceramics”, Bioceramics, 1998, pp. 293-
6.