Biomedical Engineering Reference
In-Depth Information
7.2 Ti-HA Nanocomposites
Hydroxyapatite is considered as the most promising biomaterial
for clinical use. However, it is well known that its poor mechanical
properties compared to those of living bone are one of the most
serious obstacles for wider applications, especially for load bearing
implants. Intensive research is ongoing to produce HA composites
with improved mechanical properties [4−7, 27]. Thus, it is necessary
to introduce some matrix materials — in our case, titanium. Current
research on the synthesis of nanoscale metallic and composite bio-
materials, shows that Ti-bioceramic nanocomposites posses better
mechanical and corrosion properties than microcrystalline titanium
[8−15, 19−24].
7.2.1
Microstructure and Phase Constitution
Titanium-hydroxyapatite nanocomposites can be produced by
mechanical alloying method [19]. The typical XRD patterns of
titanium (ICDD: 5-682) and hydroxyapatite (ICDD: 9-432) before
mechanical alloying are shown in Figs. 7.1a,b, respectively. During
MA process, the original sharp diffraction lines of the Ti and HA
gradually become broader and their intensity decreases with milling
time (not shown). The peak broadening represents a reduction in the
crystallite size and increase in the internal strain in the mechanically
alloyed materials. After 44 h of MA, the amorphous phase forms
directly from the starting mixture, without the formation of other
phases (Fig. 7.1c). But differentiation between a “truly” amorphous,
extremely ine grained material and a material in which very
small crystals are embedded in an amorphous matrix in produced
materials has not been easy on the basis of diffraction basis.
During the mechanical alloying process, the powder Ti and
HA particles are periodically trapped between colliding balls and
are plastically deformed. Such a feature occurs by the generation
of a wide number of dislocations as well as other lattice defects.
Furthermore, the ball collisions cause fracturing and cold welding of
the elementary particles, forming clean interfaces at the atomic scale.
Further milling leads to an increase of the interface number and the
sizes of the elementary component area decrease from millimeter
to submicrometer lengths. Concurrent to this decrease of the
elementary distribution, some nanocrystalline intermediate phases
