Thermal Sprayed Bioceramic Coatings: Nanostructured Hydroxyapatite (HA) and HA-Based Composites - Biological and Biomedical Coatings: Processing and Characterization - page 181

structure.) The difference in lattice parameter between HA and oxyapatite is reported to
be so small that structural changes arising from loss of water are not obvious from XRD
patterns. OH − -depleted HA has been shown to be very reactive, absorbing H 2 O at ~600°C
even in a vacuum of 1.33 × 10 −2 Pa (MacPherson et al. 1995).
Generally, the phase transformation may induce the following problems (Ogiso et al.
1998b): (1) stress accumulation in the crystal phase due to volume differences, (2) forma-
tion of CaO and TTCP, (3) higher solubility, (4) easier diffusion migration of atoms due to
irregular distribution of Ca, and (5) diffusion migration of OH − group into TCP in vivo.
Therefore, the decomposition of HA during coating deposition should be avoided.
The extent of HA decomposition could be examined through determining the Ca/P
ratio of the coatings (Gross et al. 1998b). The amount of ACP in most plasma-sprayed HA
coating was detected in a range up to 70%. The ACP phase has considerable dissolution
ability, which makes the phase dissolve into the tissues quickly and consequently reduces
the reliability of the implants. Therefore, the existence of amorphous phase at the coating/
substrate interface could weaken the adhesive strength of the coating (Gross et al. 1998a).
Furthermore, it is believed that the content of amorphous phase near the coating-substrate
interface is more than that within the coating due to the low thermal diffusivity of HA.
This phenomenon has been revealed through morphological analysis (Gross et al. JBMR
1998; Ashroff et al. 1996).
The rapid cooling of a splat on substrate surface is the fundamental phenomenon of
thermal spray process. Gross et al. (1998b) summarized the factors that mainly influenced
the formation of ACP. Dehydroxylation of the molten particle during flight, rapid cooling
as the particle impinges onto the metal substrate, and the low substrate temperature are
the main variables. They suggested the viscosity of the melted particle as an important
variable that influenced the amorphicity. Overheating of the molten particle decreased its
viscosity, hence a larger flattened splat would be obtained, which resulted in better cooling
and this was beneficial to the formation of the amorphous phase. This was confirmed by
experiments through alternative spray angles and spray parameters (Gross et al. 1998b).
The effect of dehydroxylation is believed to have a great influence on ACP formation in
comparison to the increase in cooling rate.
The presence of water vapor, hydrostatic pressure, or a combination of these could pro-
mote the crystallization of hydroxyapatite, which is composition-dependent (Gross et al.
1998c; Yamashita et al. 1994). Furthermore, heat treatment in vacuum at a very high tem-
perature such as 950°C can induce the chemical reaction at the HA/Ti-6Al-4V interface
with a titanium phosphide as the resultant compound detected by transmission electron
microscope analysis (Ji et al. 1993). This phenomenon can be the proof of the diffusion of
Ti or P elements at the coating/substrate interface. Meanwhile Filiaggi et al. (1993) believed
that increases in mechanical properties after heat treatment on the as-sprayed coatings can
be attributed to diffusion at the interface. Furthermore, the crystallization temperature of
the as-sprayed HA coatings differs to some extent depending on the characteristics of the
raw HA powder and the spray system employed. Even though the transformation point
from amorphous phase to crystalline HA is generally determined, there are still reports
that suggest that the crystallization can occur at low temperatures if very long isothermal
treatment condition is satisfied (Vogel et al. 1996).
The postspray annealing treatment at 750°C has achieved a complete crystallization of
the ACP in the thermal sprayed HA coating (Figure 4.34) (Li et al. 2002a). It is clear that the
amorphous phase transformed only to crystalline HA. Through the investigation of the
crystallization of amorphous calcium phosphate by Vogel et al. (1996), it was postulated
that the thermal decomposition of HA during plasma spraying is reversible when annealed