Thermal Sprayed Bioceramic Coatings: Nanostructured Hydroxyapatite (HA) and HA-Based Composites - Biological and Biomedical Coatings: Processing and Characterization - page 148

(a) (b)

20 kV ×600 20 µm 26 4 SEC 20 kV ×2300 10 µm 16 40 SEI

FIGURE 4.6
Spherical HA particles SEM micrographs of the etched cross section of HT-SHA powder (a) dense particles
showing columnar and fine equiaxed grains, (b) detail of the columnar grains in the outer layer and equiaxed
grains in the core of a newly formed solid particle, (c) detail of the columnar grains in the wall of a hollow
sphere, and (d) solid sphere retaining SDHA structure in its core. (From Espanol Pons, M., PhD thesis, Nanyang
Technological University, Singapore, 2003. With permission.)

also to increase crystal size. Further spheroidization of the heat-treated SDHA (HT-SDHA)
powder can be made using a plasma gun and quenching the product in distilled water
(Figure 4.6) (Espanol Pons 2003). The HT-spheroidized HA (HT-SHA) powder was denser
and had smoother surfaces than HT-SDHA and that markedly improved the flowability
of the HT-SHA powder (Espanol Pons 2003).
The cross-sectional view in Figure 4.5b further demonstrates the highly porous nature
of the particle and its agglomerated structure. Each powder consists of an assembly of fine
equiaxed crystals. The bond strength between the different crystals will depend on those
forces that bind them together and the ability to form interparticle necking during the
calcination process. The agglomerated powder particles were composed of nanostructures
(with rodlike grains: <230 nm in length and <70 nm in diameter).

HA-Based Composite Powder

Different approaches have been reported for synthesis of the HA-based composite pow-
der (Xu et al. 2005; Kumar et al. 2003). Figure 4.7 shows the HA/titania composite powder
made by a slurry-spray-dry approach.
The synthesis of HA/nanozirconia by an RF-plasma spray route provides a method of
producing nanozirconia-enwrapped HA particles for thermal spray deposition of the bio-
medical coatings. Fabrication of the particles has been successful (Figure 4.8) and detailed
description of the synthesis approach can be found in the paper (Kumar et al. 2003).

NanostructuredHA-BasedBioceramicCoatings
DepositedbyThermalSpray
Hydroxyapatite (HA) coatings deposited using thermal spray techniques on titanium alloy
substrates showed capability of avoiding the inherent mechanical property limitations of