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Phase Decompositions of Bioceramic Composites
Colin Reidy,
Stuart Hampshire, Mark Towler, Thomas Fleming
Materials Surface Science Institute, University of Limerick
colin.reidy@ul.ie
Abstract
The effects of increasing zirconia (ZrO2) content on the
phase stability of microwave sintered Hydroxyapatite
(HA) were examined. An increase in the amount of
zirconia content led to a substantial increase in the
decomposition of HA to Tricalcium Phosphate (TCP).
The decomposition was also found to be temperature
dependant, with greater temperatures leading to
increases in the amount of decomposition.
1. Introduction
Calcium phosphate based ceramics, in particular
hydroxyapatite [Ca10(PO4)6(OH)2, HA], have generated
a significant amount of attention as replacements for
hard tissue due to their biocompatibility, bioactivity,
osteoconductivity and direct bonding to bone tissue [1].
However, HA is generally limited to non-load bearing
applications and metal implant surface coatings due to
its limited mechanical properties [2]. This has led to
attempts at using ZrO2 as a reinforcing phase. In
particular, Yttria doped tetragonal zirconia polycrystals
(Y-TZP) tend to be the most widely used zirconia
ceramic for biomedical applications due to the retention
of the “metastable” tetragonal phase, thus maximising
the toughening mechanism [3]. Studies on the formation
of HA-ZrO2 composites suggest that ZrO2 can lead to
decomposition of HA using conventional sintering
techniques [4]. This study examines the effect of a
microwave sintering technique on the phase
compositions of HA-ZrO2 composites.
2. Materials and methods
Laboratory synthesized HA and composites
containing 0, 5 and 10 wt% ZrO2 (3Y-TZP, Tosoh,
Japan) were milled and uni-axially pressed to form
pellets ~ 2mm thick and 20 mm in diameter. Pellets
were sintered using a hybrid microwave furnace at
temperatures of 1000-1300°C. X-ray diffraction (XRD,
Philips X’Pert) was performed on the sintered pellets.
The XRD patterns were then matched to patterns in the
JCPDS database, using the X’Pert software, to
determine the phases present. Quantitative analysis was
performed using Reitveld analysis.
3. Results & Discussion
In terms of phase stability it was found that the
decomposition of HA to α and/or β-TCP increased with
increasing amounts of ZrO2. This decomposition
175
increased with increasing temperature and was found to
increase in microwave sintered samples at 1300°C,
Figure 1.
Relative intensity
(a) MS HA+5 wt % ZrO 2
HA c-ZrO 2
TCP CaZrO3 TCP
1300 C
1200 C
1100 C
1000 C
26 28 30 32 34
theta
36 38 40
Figure 1. Phase composition of composites
containing 5 wt % ZrO2
It was also found that CaO released through the
decomposition of HA reacted with the ZrO2 reinforcing
phase to form either a c-ZrO2 solid solution and/or
CaZrO3 above temperatures of 1100°C.
4. Conclusion
Microwave sintering of HA-ZrO2 compositions
resulted in the decomposition of HA to TCP. The
amount of degradation was found to be dependent on
both the quantity of ZrO2 present and the sintering
temperature, with increases in either leading to
increased decomposition.
5. References
[1] M. Vallet-Regi and J.M. Gonzalez-Calbet. “Calcium
phosphates as substitution of bone tissues”, Progress in Solid
State Chemistry, 2004, pp 1-31.
[2] L.L. Hench, “Bioceramics: from Concept to Clinic”,
Journal of the American Ceramic Society, 1991, pp. 1487-
1510.
[3] R.H.J. Hannink, P.M Kelly and B.C. Muddle,
“Transformation toughening in Zirconia-Containing Ceramics”,
Journal of the American Ceramic Society, 2000 pp 333-340.
[4] A. Rapacz-Kmita, A. Slosarczyk, Z. Paszkiewicz and C
Paluszkiewicz, “Phase stability of hydroxyapatite-zirconia
(Hap-ZrO 2) composites for bone replacement”, Journal of
Molecular Structure , 2004, pp 333-340.