Biomedical Engineering Reference
In-Depth Information
coatings resulted from the microstructural self-healing effect of hydrothermal crystal-
lization, the failure morphologies of HT-HACs represent crystallized, homogeneity, and
display a larger area fraction of cohesive failure as displayed in Figure 6.35b and c. In
contrast, the decreased area fraction of adhesive failure represents that the adhesion of
HT-HACs to the Ti-6Al-4V substrate is significantly improved, especially for the HT125
condition. Referring to the evidences demonstrated from the XPS analysis as shown in
Figure 6.18, the hydrothermal treatment effectively helps to promote the interfacial crys-
tallization through the replenished and the chemisorbed OH
−
groups, which results in a
significant chemical bonding of the HA coating to Ti substrate interface. It can be summa-
rized from the above-mentioned results that performing low-temperature hydrothermal
treatments is an available process and will allow plasma-sprayed HA-coated implants to
have desirable mechanical fixation and reliability for long-term applications without dis-
sociation problems.
Summary
HA is today a conventional applied bioactive ceramic material, which is considered
an excellent hard tissue substitute in dentistry and orthopedics due to its favorable
osteoconductivity and osseointegration properties. Plasma-sprayed HAs on metallic sub-
properties. Plasma-sprayed HAs on metallic sub-
strate exhibits enhanced interfacial strength and tends to avoid the inherent mechanical
property limitations of HA without significant loss in biocompatibility. As has been rep-
resented in this chapter, plasma-sprayed HACs have advantages and disadvantages as far
as their properties (i.e., microstructural, mechanical, biological, and material reliability),
and applications are concerned with the realization of chemical stability in solution and
thermal stability of HA.
Since the phase stability of crystalline HA is reduced during plasma spraying, a good
understanding of phase decomposition products, such as ACP, TCP, TP, CaO, and other
nonstoichiometric calcium phosphate, would contribute to the knowledge on dissolution
mechanism and stability of HACs. The identification of these phases can be achieved by
XRD and Raman spectroscopy techniques. The quantification of the crystallinity and phase
content of plasma-sprayed HACs can be obtained by the internal standard method and the
quantitative phase analysis (QPA) of Rietveld method by x-ray or neutron diffraction.
Postspray heat treatment is required and is an effective way to improve the crystalliza-
tion state and the dissolution problems of plasma-sprayed HACs. Compared with differ-
ent heat treatments, hydrothermal treatment is more favorable to eliminate the impurity
phases and ACP than high temperature heat treatments in vacuum or in the air. The deri-
vation from Arrhenius kinetics demonstrates that heating temperature is a controlling
factor for HA crystallization. A higher crystallization rate and lower activation energy of
HA crystallization for the hermetically hydrothermal treatment can be recognized as the
effect of ambient heating atmosphere with a saturated steam pressure.
Hydrothermal crystallization obviously improves the microstructural homogeneity of
plasma-sprayed HACs through the self-healing effect. The enhanced bonding strength
and erosion resistance are resulted from the increased cohesive strength of dense coat-
steoconductivity and osseointegration properties. Plasma-sprayed HAs on metallic sub-
ivity and osseointegration properties. Plasma-sprayed HAs on metallic sub-
resulted from the increased cohesive strength of dense coat-
ings and the increased adhesive strength with a significant chemical interlocking between
the hydrothermally treated HACs and substrate interface. Relative low-temperature
hydrothermal treatment can help to avoid the detrimental oxidation layer and compressive
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