MATERIALS FOR ORTHOPEDIC APPLICATIONS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 62

Another study 31 revealed that HAp in HAp-ZrO 2 - Al 2 O 3 nano - composites
formed biphasic calcium phosphate (BCP) when hot pressed at 1400 °C. BCP is
reported to have high biocompatibility. Recently, BCP ceramics have received
increased attention as an ideal bone substitute due to its controlled degra-
dability 32 . Any composite that shows superior mechanical and physical properties
and is also biodegradable would be the material of choice in near future. The lit-
erature report indicates that it is possible to alter HAp :TCP ratio to form BCP of
desired properties 32 . In one of the earlier studies, it was observed that 10 wt%
phosphate glass additions to HAp-composites leads to partial dissociation of
HAp to TCP 33 . By varying the glass addition, the resorption of TCP can be con-
trolled. Suchanek and co-workers 34 developed HAp - HAp w (whisker) composite,
which was hot pressed at 1000-1100 °C. Despite the use of whiskers as a toughen-
ing agent, the toughness of HAp-HAp w (10% addition) could be increased to
1.1 MPa m 0.5 under optimal processing conditions. It is, however, possible that
toughness could be further improved with increase in whisker addition, but such
experiments are not conducted as yet.
The following discusses the processing-property results, obtained with sev-
eral CaP-based biocomposites. In the authors' recent investigation 35 , they synthe-
sized HAp-Mullite composite, which was densifi ed successfully by pressureless
sintering and without any sintering additives. Detailed analysis revealed that dis-
sociation of HAp to TCP depends on sintering temperature as well as mullite
content. Higher mullite containing (

> 20 wt%) samples are more prone towards

dissociation. -TCP are the main phases along with mullite for higher mull-

ite content samples. Both solid state and liquid phase sintering guided the den-
sifi cation mechanism towards dense, crack, pore free body. Limited reaction
between Mullite and CaO produce grain boundary alumino-silicate phases. Mull-
ite grains grow anistropically (needle shaped) because of liquid phase sintering.
The transmission electron microscopy (TEM) images in Figure 3.5, show the pres-
ence of mullite needle in HAp/TCP matrix. X-ray diffraction (XRD) patterns of
HAp - 30 wt% mullite and HAp - 10 wt% mullite, sintered at 1350 ° C along with
ball-milled powder are shown in Figure 3.6, which shows that HAp is predomi-
nantly present along with

β and α

-TCP in case HAp-10 wt% mullite. In contrast, in case
of 30 wt% mullite, the major phase is

α

-TCP. This newly developed material could
be appropriate for bone replacement material.

β

3.4.3 Hydroxyapatite-Titanium Composites
Besides the development of HAp-based bioceramic composites, efforts have
been put forward to fabricate a HAp-metal particulate composite with better
mechanical properties. Wataria et al. 36 fabricated Ti/HAp functionally - graded
materials (FGM) specimens using Spark Plasma Sintering (SPS). The optimiza-
tion of both mechanical and biological properties was also pursued. SPS pro-
cessed Ti/HAp FGM showed that the maturation of newly formed bone was
preceded in the HAp-rich region. The gradient functions in the biochemical
affi nity to osteogenesis and the mechanical properties with stress relaxation in