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 268

On the basis of the above analytical results and discussion, the saturated steam pres-
sure is a crucial factor for HA crystallization in addition to the heating temperatures
during the autoclaving hydrothermal treatment. The heat treatment in a high moisture
atmosphere with abundant H 2 O molecules and a saturated steam pressure can further
promote and accelerate the HA crystallization (larger rate constant k and lower activation
energy E a ) because H 2 O molecules effectively replenish the missing OH − groups within
the dehydroxylation state to improve the structural integrity of plasma-sprayed HACs.
Experimental evidence also confirmed that the ambient saturated steam pressure plays an
important role in lowering heating and reaction temperatures. This results in a significant
microstructural self-healing effect through the grain growth of HA nanocrystallite [171],
as shown in Figures 6.16 and 6.17, within the hydrothermally crystallized HACs, which
also shows a statistically higher extent of new bone apposition [121] essential in the initial
fixation of implants in clinical applications.

CrystallizationEffectonMechanicalPropertiesofPlasma-SprayedHACs
Since the use of plasma-sprayed HA-coated implants for biomedical applications is strongly
influenced by the long-term stability of the coating system, the coating/substrate adhesion,
which can be represented as the interfacial bonding strength, and interlamellar cohesion
are of great concern. The adhesion of plasma-sprayed coatings is not only an interfacial
problem of each lamella within coatings, but involves the microstructural integrity,
crystallinity, residual stresses, crack propagation, defects content, and defects distribution
of coatings. Therefore, the evaluation of the interfacial bonding strength between coatings
and substrates is quite important to nearly all branches of the surface engineering.
According to the definition of the American Society for Testing and Materials (ASTM),
adhesion is the state in which two surfaces are held together by interfacial forces that may
consist of valence forces or interlocking forces or both (ASTM D907-70). To determine the
adhesive bonding strength of plasma-sprayed HACs, scratch testing [183-189], in which
a sharp needle under a given weight necessary to reach the underlying metallic surface
is an indication of the comparative bonding strength. Push-out tests have been used to
measure the ability of the implant to resist forces tending to shear the bonding between
the implant surfaces and surrounding bone [190-196]. Among these methods, the most
commonly used method of determining tensile strength involves the coating of a metallic
disk followed by testing utilizing a variation of ASTM C633. To test the adhesive bonding
strength of HA coatings to an implant such as titanium alloy, the surface of the cylindrical
substrate fixtures is first prepared as the actual surface on an implant using all cleaning
and grit-blasting steps. After plasma-spraying HA coatings on substrates, the loading fi x- substrates, the loading ix-

tures are also grit-blasted and bonded to the surface of the HA coatings using a heat-cured
adhesive glue. Then the assemblies are subjected to tensile tests after curing. It is noted
that the roughness of the substrate is important in achieving high bonding strength of
plasma-sprayed HACs. The bonding of the plasma-sprayed HACs to metallic substrates
appears to be mostly mechanical in nature, as there is a lower degree of chemical bonding
in the as-deposited coatings. In addition, the crystallization of plasma-sprayed HACs with
performing postheat treatments can also help to improve the bonding strength and the
effect of crystallization on bonding strength, wear resistance, and failure properties will
be presented in the following sections.

-blasting steps. After plasma-spraying HA coatings on substrates, the loading fi x- blasting steps. After plasma-spraying HA coatings on substrates, the loading fi x- -spraying HA coatings on substrates, the loading fi x- spraying HA coatings on substrates, the loading fi x-