Thermal Sprayed Bioceramic Coatings: Nanostructured Hydroxyapatite (HA) and HA-Based Composites - Biological and Biomedical Coatings: Processing and Characterization - page 177

3000

2500

L + CaZrO 3

F ss + L

2000 F ss

F ss + CaZrO 3 (c)
1750 ± 30ºC

T ss

T ss

+

F ss + CaZrO 3 (0)
1355 ± 15ºC

1500

F ss

T ss +
M ss

φ 2

φ 2 + CaZrO 3 (0)

1140 ± 40 ºC

φ 1

1000

φ 1 = CaZr 4 O 9

M ss

M ss + CaZrO 3(0)

φ 2 = Ca 6 Zr 19 O 44

500 0

10 20 30 40 50

CaZrO 3 ZrO 2

mol % CaO

FIGURE 4.30
ZrO 2 -CaO phase diagram. (From Stubican, V.S., in Advances in Ceramics, Volume 24A, Science and Technology of
Zirconia III , American Ceramic Society, Inc., Ohio, pp. 71-82, 1988. With permission.) T ss , tetragonal solid solu-
tion; M ss , monoclinic solid solution; F ss , cubic solid solution; L , liquid.

bonding and brittle phases that exist at the splats' interface. The increase of titania con-
tent from 10 to 20 vol.% induces a small decrease in Young's modulus. Chemical reaction
between HA and TiO2 is found to occur during coating deposition. The relatively high
fracture toughness exhibited by the composite coating compared to pure HA coating indi-
cates the improved bonding area of splats' interface (Li et al. 2002b).
It deems that the mutual chemical reaction between the two components provides higher
density, which was believed as one beneficial factor influencing fracture toughness (Callus
et al. 1999).
While reaction between HA and zirconia is also inevitable, since studies on HA/ZrO 2
coatings showed CaZrO 3 , apart from crystalline HA, α-TCP, and t-ZrO 2 (Li et al. 2004b). The
equilibrium ZrO 2 -CaO phase diagram (Stubican 1988) depicted in Figure 4.30 illustrates
possible reactions between ZrO 2 and CaO in detail. It shows that, under the equilibrium
conditions, heating of the zirconia particles up to 1140 ± 40°C caused transformation to a
mixture of t-ZrO 2 and c-ZrO 2 .
It has been found that when HA decomposition to other phases (e.g., CaO) takes place
beyond 1000°C, the gradually increased content of CaO in the present zirconai/HA system
makes the reactions between zirconia and CaO more complicated (Li et al. 2004b). The
small content change exhibited by α-TCP (increased from 6.7 to 7.5 wt.%) has indicated the
low content of CaO. It was pointed out that CaO in preference reacts with t-ZrO 2 to form
CaZrO 3 (Rao et al. 2002), which can be described as follows:

CaO + t-ZrO 2 → CaZrO 3 (4.12)