Calcium Orthophosphates as Bioceramics - Calcium Orthophosphates: Applications in Nature, Biology and Medicine - page 346

of the most resistant components of the natural calcified tissues
(dental enamel: ~74 GPa, dentine: ~21 GPa, compact bone: ~18-22
GPa). Nevertheless, dense bulk compacts of HA have mechanical
resistances of the order of 100 MPa versus ~300 MPa of human
bones, diminishing drastically their resistances in the case of porous
bulk compacts [310]. Young's modulus measured in bending is
between 44 and 88 GPa. Recently, a considerable anisotropy in
the stress-strain behavior of the perfect HA crystals was found by
ab initio calculations [311]. The crystals appeared to be brittle for

tension along the

-axis with the maximum stress of ~9.6 GPa at
10% strain. Furthermore, the structural analysis of the HA crystal
under various stages of tensile strain revealed that the deformation
behavior manifested itself mainly in the rotation of PO

z

tetrahedrons
with concomitant movements of both the columnar and axial Ca ions
[311]. Vickers hardness [312] of dense HA bioceramics is within 3-7
GPa, while the Poisson's ratio [313] for the synthetic HA is about 0.27,
which is close to that of bones (~0.3). At temperatures within 1000-
1100°C, dense HA bioceramics was found to exhibit superplasticity
with a deformation mechanism based on grain boundary sliding.
Furthermore, both wear resistance and friction coefficient of dense
HA bioceramics are comparable to those of dental enamel [233].
Due to a high brittleness (associated to a low crack resistance),
the biomedical applications of calcium orthophosphate bioceramics
are focused on production of non-load-bearing implants, such
as pieces for middle ear surgery, filling of bone defects in oral
or orthopedic surgery, as well as coating of dental implants and
metallic prosthesis (see below) [110, 314, 315]. In order to improve
the reliability of calcium orthophosphate bioceramics, diverse
reinforcements (ceramics, metals or polymers) have been applied
to manufacture various biocomposites and hybrid biomaterials
[316], but that is another story (see Chapter 4). However, successful
hybrid formulations consisted of calcium orthophosphates only
should be mentioned [317-322]. For example, bulk HA bioceramics
might be reinforced by HA whiskers [318-321]. Furthermore, a
superior superplasticity of HA/β-TCP biocomposites (i.e., BCP) to
HA bioceramics has been detected [322].
Another approach to improve the mechanical properties of
calcium orthophosphate bioceramics is to coat the items by a
polymeric layer [323, 324]; however, this is still other story. Further
details on the mechanical properties of calcium orthophosphate

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