Hydrothermal Crystallization with Microstructural Self-Healing Effect on Mechanical and Failure Behaviors of Plasma-Sprayed Hydroxyapatite Coatings - Biological and Biomedical Coatings: Processing and Characterization - page 245

phases is a result of the extreme temperature of plasma flame and rapid cooling and highly
reactive atmospheres that favor nonequilibrium and metastable structures.
When OA appears to be stable in the absence of water vapor environment, it will readily
retransform to HA according to the reaction [93,94]

O 2− (solid) + V (solid) + H 2 O (gas) → 2OH − (solid) (6.10)

The equilibrium temperature of the reaction shown in Equation 6.7 is determined by the
temperature of incongruent melting of HA at 1570°C [95]. Based on the decomposition
sequence, a model has been developed [91,96,97] that shows the in-flight evolution of
calcium phosphate phases, as represented in Figure 6.4. It shows that the inner core, which
is still at a temperature below 1550°C, consists of HA, OHA, and OA as stable phases
during the short residence time of HA particles in the plasma flame (reactions shown in
Equations 6.5 and 6.6). The second shell of Figure 6.2b, which is heated to temperatures
of 1360°C to 1570°C, just below the incongruent melting temperature of HA. It undergoes
a solid state decomposition to a mixture of α-TCP and TP. The third shell was heated to
temperatures above 1570°C following the reaction shown in Equation 6.7. The outer shell
is composed of CaO and a melt whose Ca/P ratio shifts by continuous evaporation of P 2 O 5
along the liquids of the phase diagram (Figure 6.2) toward CaO-richer phases following
reactions shown in Equations 6.8 and 6.9. While impacting at the substrates, this molten
phase solidifies to produce ACP with various Ca/P ratios [92,102].
X-ray diffraction (XRD) has been widely used for determining the phase composition,
phase content, and crystal structure of plasma-sprayed HACs, as well as for estimating
the index of crystallinity and identifying other calcium phosphate compounds generated
as a result of the high-temperature spraying process. Figure 6.5 shows the x-ray diffrac-
tion patterns of the well-crystallized HA powders (Figure 6.5a) and the plasma-sprayed
HACs (Figure 6.5b). The difference in the phase composition and crystallinity between
well-crystallized HA and plasma-sprayed HACs are quite evident. The peak intensity of
HA phase is significantly decreased after plasma spraying. In addition, it can be seen that
a fairly high content of ACP and other impurity phases including TCP, TP, and CaO are
represented in the plasma-sprayed HACs besides the desired HA phase. The reduction

(a) (b)

CaO + melt CaO + melt

(TCP + TP) melt

(TCP + TP) melt

HA + OHA +
OA

HA + OHA +
OA

(TCP+TP) solid

FIGURE 6.4
Schematic model of thermal decomposition of a spherical HA particle subjected to high temperature in a plasma
flame at (a) a partial water vapor pressure of about 500 mmHg and (b) a partial water vapor pressure of about
10 mmHg. (From Heimann, R.B., Surf. Coat. Technol. , 201, 2012-2019, 2006. With permission.)