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 251

and CaO, are obtained. The phase decomposition during plasma spraying is quantitatively
demonstrated and the amorphous content increased substantially at higher powers.
Because the crystallization of plasma-sprayed HACs is usually accompanied with a
change in the coating dimensions, the variation in microstructure during heat treatments
can be clarified through thermal dilatometry. In order to exclude the influence of metallic
substrate and interfacial reaction between HA/Ti of the HA-coated Ti-6Al-4V specimens,
only plasma-sprayed HAC test pieces of dimensions 20 ( l ) × 3 ( w ) × 3 ( t ) mm 3 without Ti
substrate are used for thermal dilatometry analysis by a dilatometer. The heating condi- . The heating condi-

tions are the same as the above-mentioned temperatures with a heating rate of 10°C/min,
followed by furnace cooling after holding for 3 h. The dilatation curve of HACs during
heating process is shown in Figure 6.10, which can be divided into three regions of mark-

are used for thermal dilatometry analysis by a dilatometer. The heating condi- used for thermal dilatometry analysis by a dilatometer. The heating condi- analysis by a dilatometer. The heating condi-

is shown in Figure 6.10, which can be divided into three regions of mark-
edly different slopes. The increased dilatation approaches linearity in the temperature
intervals of 100°C to 400°C (region I). The linear increased dilatation represents that just
a thermal expansion effect on the HACs during heating in temperature interval 100°C to
400°C, and the coefficient of thermal expansion (CTE, α) of sprayed HA coating layer can
be suggested about 15.6 × 10 −6 °C −1 calculated from the slope.
Besides thermal expansion effect in region II, however, the dilatation decreases in a non-

process is shown in Figure 6.10, which can be divided into three regions of mark-

non-
linear manner at heating temperatures from above 400°C to 700°C should be correlated
with the main temperature interval for the crystallization of plasma-sprayed HACs. When
the HA crystallization effect with a fair amount of volume contraction is larger than the
thermal expansion of coatings in this region, the dilatation δ L / L o (or the slope of the dila-
tation curve) decreased and even becomes a negative value, especially during the 500°C
to 700°C heating interval. Combined with quantitative analysis as shown in Figure 6.8,
the evidence is that an apparent change in phase content and crystallinity within this
temperature interval. Although the investigations by Feng et al. [61] and Gross et al. [116]
indicated that the crystallization has been identified as taking place at about 400°C, but the
main HA crystallization region of the HACs should be suggested over 500°C heating tem-
peratures. As the heating temperature approaches to 700°C, there is also a dilatation bal-

a non-

As the heating temperature approaches to 700°C, there is also a dilatation bal-
ance between crystallization and thermal expansion of HACs. Thus, the dilatation curve
displays a linear thermal expansion larger than crystallization contraction over 700°C
heating temperatures in region III.
Figure 6.11 also illustrates the thermal dilatometry curves for the HACs without
Ti-6Al-4V, heating at 400°C, 500°C, 600°C, 700°C, and 800°C, respectively. They all show

As the heating temperature approaches to 700°C, there is also a dilatation bal-

0.6

I II III

0.4

0.2

0.0

0 100 200 300 400 500 600 700 800

Temperature ( º C)

FIGURE 6.10
Dilatation curve of plasma-sprayed HACs for a single heating process through thermal dilatometry.