Biomimetic Hard Materials - Engineered Biomimicry - page 69

(MTM) clay, silicon carbide, aluminum oxide,
mica, and talc have been utilized [9, 46, 47] .
The selection criteria for the organic phase are
more complicated. The organic phase should
show a strong adhesion to the mineral phase in
order to prevent delamination at low levels of
stress. Concurrently, the organic phase is required
to be ductile and deformable so that it can resist
high levels of strain. Different types of polymers,
including polyelectrolytes (PEs), polyvinylalco-
hol (PVA), polycarbonate (PC), and poly(methyl
methacrylate) (PMMA) have been used as the
organic phase in biomimetic materials [46,
48-50] . Some of these polymers are attractive for
chemical assembly processes. Thus, PVA has a
good layering ability and forms hydrogen bonds
with some minerals [48, 49] , and PEs contain
electric charges that facilitate the assembly pro-
cesses [51] .
Several innovative approaches were devel-
oped recently to assemble the chosen ingredients
[52] . These methods can be classified as: (i) the
freeze-casting method, (ii) layer-by-layer (LBL)
assembly, (iii) direct-deposition techniques, and
(iv) centrifugation, sedimentation, shearing, and
gel casting. Other methods such as template-
assisted fabrication have been utilized to develop
bone-like materials. A brief review of these tech-
niques along with their aptitudes and limitations
is included in the remainder of this section.

Sublimating the ice by freeze-drying followed
by a sintering step produces a porous scaffold
composed of distinctive ceramic layers ( Figure
3.9 b). This scaffold is then filled with a tougher
second phase ( Figure 3.9 c) such as a polymer, a
metal, or a metallic alloy, resulting in a layered
ceramic/polymer (or ceramic/metal) compos-
ite [50, 53, 54] . The layered composite can then
be pressed so that the ceramic phase breaks into
tablets, resulting in segmented layered compos-
ite resembling nacre ( Figure 3.9 d). Figure 3.9 e
shows the bending stress-strain behavior of an
alumina/PMMA composite fabricated by the
freeze-casting technique compared to that of
hydrated red-abalone nacre. It shows that the
strength and area under the strain-stress curve
are higher for the alumina/ PMMA composite.
However, it is noteworthy that the constituents
and the composition are also different for the
two materials.
The advantages of this method are as
follows:

(i) Bulk-layered hybrid composites with
proper control over the thickness of layers
can be developed.
(ii) The success of the process is not depend-
ent on the interfacial compatibility of dif-
ferent phases, so that a large variety of
materials can be used.
(iii) Well-controlled interface behavior can be
induced either by changing the surface
roughness of phases or by improving chem-
ical bonds between phases (grafting) [54] .

3.4.1 Freeze - Casting Method

The freeze-casting method has recently been
utilized to develop layered nacre-like compos-
ites. This innovative technique is based on the
anisotropic growth of ice crystals by controlled
freezing. The process starts with preparation
of a suspension of ceramic particles in water.
This suspension is then frozen in a controlled
fashion so that flat ice structures with thickness
of several micrometers are generated. As the
ceramic particles are expelled from the forming
ice, they assemble to form ceramic layers con-
strained between the planes of ice ( Figure 3.9 a).

This method yields materials with micro-
structures and deformation behavior resembling
those of natural nacre, but it cannot control the
overlap between segmented minerals at neigh-
boring layers. Munch et al . [54] used this strat-
egy to fabricate artificial nacre, composed of
80 %v/v of Al 2 O 3 and 20 %v/v PMMA, which
showed 1.4% strain to failure, a strength higher
than the prediction of the rule of mixtures [55] ,
and toughness of two orders of magnitude
higher than that of Al 2 O 3 .