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 253

C

N

5 µm

FIGURE 6.12
Enlarged cracks of heat-treated HACs, showing presence of crystallized (denoted by “C”) and nonstoichiomet-
ric calcium phosphate (denoted by “N”) regions that can be recognized by different contrast. (From Yang, C.W.
and Lui, T.S., Mater. Trans ., 48(2), 211-218, 2007. With permission.)

region “N” with a low Ca/P atomic ratio of 1.35 represents a nonstoichiometric calcium
phosphate region away from the “C” region. By incorporating the IOC and the thermal
dilatometry results, therefore, it can be recognized that the formation and the propagation
of enlarged cracks is due to the coating contraction during crystallization.
For biological applications, the dissolution rate of HA coatings is of interest because a
rapidly dissolving coating may not remain on the implant for a sufficient time to allow
full stabilization in bone or the desired tissue response in vivo. Ongoing development
and optimization of the plasma-sprayed HACs are aimed at higher crystallinity coatings
without sacrificing bonding strength. Although it is possible to evidently increase the crys-
tallinity of plasma-sprayed HACs by postdeposition heat treatments in vacuum and in the
air, heat treatments with temperatures over 600°C result in the degradation of mechani-
cal properties with the significant formation of crystallization-induced cracks [101,120].
Therefore, it is indispensable to develop a lower-temperature treating process [58,59,63];
for example: a hydrothermal treatment with an abundant water vapor atmosphere that
can simultaneously acquire higher crystallinity, good bonding strength, and biological
responses without microstructural deterioration [121,122].

HydrothermalSynthesisandCrystallization
The interest of synthesis of HA is linked with its importance as a major constituent of the
inorganic component in human bones and teeth, and the characteristics of HA have signifi -

he characteristics of HA have signifi
cant effects on the subsequent products with HA being in the form of dense or porous struc-
ture in coatings or in composites. Since HA has been recognized as the best biocompatible
ceramic material, there have been many reports on the biological aspects [6,7,15,123,124],
and the fabrication processing of HA has progressed with the growing importance of the
biological applications. A number of studies on the preparation of HA by various meth-

, and the characteristics of HA have signifi - the characteristics of HA have signifi -

number of studies on the preparation of HA by various meth-
ods have been reported in the literature. Yoshimura and Suda et al. have reviewed the
preparation for HA powders and grouped these methods into three categories, as listed in
Table 6.2 [125-127]. Table 6.3 illustrates the chemical reactions of these processes [128]. Two

A number of studies on the preparation of HA by various meth-