Geoscience Reference
In-Depth Information
In contrast with HAp/bioactive glass composites, the HAp/polymer composites
are very interesting. Bonfield and coworkers
[172,327
329].
developed HAp/poly-
ethylene composites. The mechanical properties of the polyethylene-HA composi-
tion are similar to those of bone. Implant tests of the polyethylene-0.4 volume
fraction HA composites demonstrated development of bone bonding between the
natural hard tissue and the synthetic implantation. These composites have the
advantages of ease of shaping of the implant to meet the patient's needs, the time
of surgery, formation of a bioactive bond to hold the implant in place, and mechan-
ical properties that closely match those of the host tissues. HAp/polyethylene com-
posites exhibit brittle/ductile transition at a HAp volume content of 40
45%
[330]
.
Their Young's modulus is in the range of 1
8 MPa, which is quite close to the
Young's modulus of bone. Unfortunately, HAp/polyethylene composites are not
biodegradable.
There are several publications in recent years on the fabrication of HAp/collagen
composites, which are similar to bone from the point of view of chemical composi-
tion, but do not have such a complex microstructure. The composites can be pre-
pared by mixing HAp with a collagen solution with subsequent hardening by
different methods. Suchanek and Yoshimura
[265]
have reviewed all the mechani-
cal properties of various HAp/polymer composites. The hydrothermal method of
processing these composites is very useful to achieve the optimum mechanical
properties.
10.8.4 Hydrothermal Processing of Whisker Crystals
A whisker is a long filamentary crystal, often containing a single screw dislocation.
A core of the dislocation usually follows the whisker axis. Whiskers are thus single
crystals with a very low dislocation density, and are of special interest for studies
of the influence of dislocation on properties. Whiskers have unusually high tensile
properties because of their low dislocation density
[331]
.
The growth of whisker crystallites is not new. In ancient times, the use of
fire allowed the development of a ceramic industry. In the heat of pottery kilns
(pottery is an older technology, dating from about 15,000
BC
), the clays transformed
so that needle-like crystallites of aluminum silicate mullite could form as a dense
crack-free mass in the shapes of vases, lamps, amphoras, and tiles
[332]
. However,
scientific thinking on whiskers began during the nineteenth century in order to
understand their morphology, dislocation density, and other related defects. Vapor
growth is one of the favorite techniques for the formation of whiskers, followed by
growth from solutions. Whisker crystals are commonly observed in metals and
intermetallic alloys. However, interest in whiskers remained purely academic. It
was only during the 1990s that interest in whisker growth was rejuvenated, owing
to their applications. Therefore, a wide range of whisker crystals has been prepared,
even among nonmetallic, ceramic, and other inorganic compounds. For many years,
inorganic fibers and whiskers have been used mostly as reinforcements in compo-
sites and in thermal insulation. Various fibrous materials such as glass, carbon,
HAp, SiC, Si
3
N
4
,Al
2
O
3
, and ZrO
2
have been prepared for such purposes. Among
