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 267

Hydrothermal treatment
Air heating in Ref. [120]
Vacuum heating in Ref. [120]

0

R 2 = 0.99

R 2 = 0.99

-1

R 2 = 0.98

-2

0.7 1.4 2.1 2.8

1/ T x 10 -3 (K)

FIGURE 6.22
Arrhenius relationship plots for hydrothermal treatment in this study, and air and vacuum heat treatments
according to previous analysis results [120]. Slope of plot is equal to − E a / R (where R is gas constant, 8.314
J K −1 mol −1 ).

8.314 J K −1 mol −1 , and A is the Arrhenius constant). The above-calculated results are sum- above-calculated results are sum- results are sum-

marized in Table 6.5.

dIOC
d

) /

3 2 1 2 /

r = = k ( 1 − IOC P (6.22)

H O
2

t

E
RT

−

a

k A = exp (6.23)

Additional experiments show that the rate constant for 100°C atmospheric heat treatment
( k 100 ′) is about 0.011. Comparing the rate constant k 100 ′ (0.011) with k 500 , k 600 , and k 800 values
of the atmospheric heating condition listed in Table 6.5, the heating temperature can be
recognized as a factor that promotes the HA crystallization rate of the atmospheric heat
treatment. Also, comparing the rate constant k 100 ′ (0.011) with k 100 (0.294), the significantly
increased crystallization rate constant for 100°C hydrothermal treatment results from the
effect of saturated steam pressure within the autoclave. The crystallization rate constant
increased with increasing the saturated steam pressure, and the k 200 value is larger than
any other rate constant for the air heat treatment. Generally, the crystallization of hydroxyl-
deficient HA requires at least 600°C in a vacuum or in the air [61,116,120,167]. However,
high-temperature crystallization tends to undermine the structural integrity and cause
phase decomposition of crystalline HA phase, microstructural deterioration, and degrada-
tion of mechanical properties of coating layers [64,120]. Through the analysis of Arrhenius
kinetics, the hydrothermal crystallization of HA under an atmosphere of saturated steam
pressure occurs only at lower temperatures with a significantly larger crystallization rate
constant and lower activation energy than other heat treatments. This new reaction rate
information, as displayed in Table 6.5, confirms that the ambient saturated steam pressure
plays an important role in lowering crystallization temperatures.