Communications in Information Science and Management Engineering
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The Study on Toughening Mechanism and
Mechanical Properties of Stacked Microstructure of
Shell
Jinbo Zhang1,2, Jin Tong* 1, Caihua Li 2, Yunhai Ma 1, Xue Di2
1. The Key Laboratory of Bionic Engineering (Jilin University), Ministry of Education, Changchun 130022, China.
2. College of Mechanical Engineering, Jiamusi University, jiamusi 154007, China.
zhangjinpo9872@sina.com
Abstract-The stacked microstructure of shell was analyzed by
SEM, the results showed that the structure of shell is a kind of
biological ceramic composite, which is composed of calcite,
aragonite and collagen. The calcite layer has irregular laminated
structure, and the aragonite layer has regular laminated
structure which are composed of staight aragonite strips. The
results of nanoindentation test showed that both the hardness
and elastic modulus of aragonite layer are higher than those of
calcite layer. It was illustrated that the carrying capacity of
aragonite layer is higher than that of calcite layer under the same
conditions. According to the further observation of
nanoindentation apperance of shell, the crack propagation modes
of two materials are different. The crack shape of calcite is
irregular but the aragonite layer`s is relative straight. The
aragonite layer has excellent mechanical properties.
Keywords-chlamys farerri shell; stacked
mechanical properties; ceramic composite
I.
microstructure;
INTRODUCTION
During the evolution for long period in the nature,
biologies have grown up such biological materials as
mammalian teethridge, skeleton and molluscan shell[1], which
have perfect structures and excellent properties. The molluscs
is a kind of typical biological ceramic composite which is
composed of crystalline inorganic minerals (calcite and
aragonite) of 95%-99% and organic collagen of 1%-5%[2].
Although both the mineral and protein are low-intensity
materials, but in the shell, they formed the highly ordered
hiberarchy, which consist of primary structure (mainly
aragonite platelets and organic layer) and subprime
microstructure (consist of platelets, organic interlayer
boundaries and organic layers), makes it present exellent
mechanical properties of even man-made materials cannot
reach such a high level[3]. Usually, it is considered that the
superior mechanical properties of the structure is attributed to
its unique microstructure.In last several decades, a lot of
researches have been carried out on the microstructure
observations,experimental measurement and mechanical
behavior modeling for the shell. Currey first observed the
staggered ammgement of shell platelets and presented a
“Brick and Mortar” model to describe the mechanical
behavior of the shell platelets[4]. wang et al.[5,6] observed the
nano-asperities on the platelets and developed a finite element
(FE) model based on the friction mechanism. Song et al. [7-9]
found the mineral bridges (which are mineral connections
crossing protein layer between adjacent mineral platelets) and
proposed a "Brick, Bridge and Mortar" model to interpret the
strengthening mechanism of shell structures. Katti et al. [10]
developed a platelet interlock model. Barthelat et al. [11]
observed a wavy structure and presented an FE model, etc. All
above researches are on the sea-shell structures. However,
researches on the mechanical property of the limnetic shells
are very few. Wang et al. [12] first studied a kind of limnetic
shell (Cristaria plicata) using nano-indentation and proposed
several strengthening mechanisms based on their observations.
They found that there were some distinctions between the seashell and limnetic shell structures. The mechanical prosperities
were not investigated in their study. Therefore it is significant
things to systematically investigate the microstructure feature
and the mechanical properties of the limnetic shell.
Although such biological ceramic composite is composed
of brittle calcium carbonate over 95%, it has a fracture
toughness of 3000 times higher than common calcium
carbonate[13,14]. The molluscs possess such high fracture
toughness due to the compound mode of inorganic minerals
and organic collagen, including the shape of microstructures,
size, distribution and relationship of combination[15,16]. The
microstructures of shell will have significant difference
because its species and living environment[13,15]. The
characteristics of microstructure are different at the different
positions although for a same shell. But many shells can be
divided into three layers: corneum, prismatic layer and nacre
from outside to inside along the direction of thichness[16,17]. The
corneum which is composed of conchiolin mainly, is the
outermost layer of shell. The prismatic layer which is primary
composed of calcite crystal, is the middle layer of shell. The
nacre which is composed of lamellar aragonite crystal mainly,
is the innermost layer of shell[18]. In the past three decades, the
researches on molluscs shells generally focus on such
fields[19,20,21]: (1) the research on microstructure, including the
crystal structure of mineral phase, components and its
structures of organic matrix layers, the connective relationship
between the aragonite platelets and subprime microstructure of
organic interlayer boundaries; (2) the experimental researches
of mechanical property include the measurements of modulus,
stiffness, strenght and toughness of shell; (3) the relevant study
of microstructure and mechanical properties, including all the
phases coupling and response, scale, shape, structural effect
and the strengthening and toughening mechanisms of single
phase under the different stress state. In addition, also includes
the simulations of structural mechanics response of shell; (4)
the on biomineralization and the forming of shell shape,
including the confirmation of active substance in the organic
matrix layers, the aggregation mechanism of organic
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macromolecules, the mechanisms of forming and growing of
crystal nucleus, ranked distribution of structures and the
growing mechanisms of shell; (5) bioautograghy of shell,
including the separation, refinement, synthesis, and structure
confirmation with each macromolecule in the organic matrix
layer, the researches on cloning, copy and self-assembly with
macromolecule, the design of new materals using
macromlecules based on biomineralization and morphological
theory[3]. The research on the microstructure and fracture
mechanism of shell is helpful to improving the brittleness of
existing ceramic materials and developing new bionic ceramic
materials which have excellent strength and toughness, and this
work will make the bionic ceramic materials bing applied
widely to the engineering. The chlamys farreri shell was
determined as research object in this paper, and the
microstructures of fracture was observed using scanning
electronic microscope, and its microstructures and mechanical
properties were researched in-depth.
II.
MATERIALS AND METHODS OF TEST
As research object in this paper, chlamys farreri shell was
collected from Dalian, as shown in Fig.1.
Fig.1 The Chlamys Farreri Shell and the Position of Sampling
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Fig.3 Nanomechanical Test System (Hysitron Plus)
Firstly, the internal soft tissues of shell were cleared away,
and then naturally dried in the air for 3 hours after washing by
distilled water completely. The shell was breaked into pieces
along growth direction and perpendicular to growth direction
respectively by mechanical means, then those pieces were
manufactured into SEM samples for observating with the size
of 3mm× 3mm× 2mm. Because of poor conductivity, the
samples must be sputtered gold powder using ion sputtering
instrument (JEOLJFC-1600) for 60s. Firstly, the samples were
fastened to the platform by electroconductive tape, the
samples room keeps high vacuum state, the maximum current
is 20A. The microstructures of samples were observed by
SEM (JSM-6360LV,fig.2) with amplification in the rang of
5000 times under the 20kv of accelerating voltage. The pieces
were cut off with the size of 3mm×5mm from the position as
shown in Fig.1 by precision cutter, then inlaid them into the
resin as the test samples after polishing with inside surface and
outside surface by polisher. The two polished surfaces of
samples were measured by nanomechanical test system
( Hysitron Plus, fig.3 ). This equipment is a modern instrument,
its maximum indentation depth less than 100nm and minimum
depth is 40um, the temperature drift is 0.05nm/s and the
indentation resolution is 0.2nm. Using diamond indenter with
triangular pyramidal shape, the temperature is 23℃, relative
humidity is 45%, and the maximum load is 2500μN. The loaddisplacement curve and photographs of nanoindentaion
apperance of the samples were obtained by the system after
testing, in addition, the hardness and modulus of elasticity of
samples can be worked out automatically by the system
according to the load-curve.
III.
Fig.2 SEM (JSM-6360LV)
RESULTS AND DISCUSSION
The fracture of chlamys farreri shell was observed by SEM
(Fig.4), the results shown that the chlamys farreri shell is a
kind of lamellar biological ceramic compsite, which is
composed of inorganic calcite and aragonite that is mixed up
with organic collagen. The calcite which has rhombohedral
system structure, lies in the outside layer of shell, and
metastable aragonite which has orthorhombic structure, locate
in the inside layer of shell, aragonite will be transform into
stable calcite when the conditions are suitable [22]. And such
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structure can ensure stability of shell in the natural
environment.
The aragonite layer which is filled by collagen, grows up
parallel to the the surface of shell and collagen serve as a
connection role in each layers. The thickness of aragonite
layer is different in the each positions of shell. The thickness
of aragonite layer which close to the outside surface, is small,
but the thickness which close to the inside surface, is large.
The difference of microstructure at the different positions of
shell can satisfy its need of structure and function. Through
further observation of the mcrostructure of aragonite layer of
chlamys farreri shell, it was found that such aragonite layer is
composed of long thin aragonite pieces (as shown in Fig.5),
and each aragonite piece is consisted of many columnar
aragonite strips (Fig.6). Such interlocked lamellar structure
which is filled by collagen, makes shell has excellent fracture
toughness.
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aragonite layer when the load approach 2500μN, it was
demonstrated that the carrying capacity of aragonite layer is
larger than that of calcite layer under the same conditions.
In order to ensure the authenticity and availability of data,
the nanoindentation measurment were performed with 20
points, which selected singly 10 points from each of surfaces
in this test. Finally, the hardness and modulus of elasticity of
two surfaces of chlamys farreri shell were obtained, as shown
in Fig.8 and Fig.9. According to the results which showd in
the Fig.8 and Fig.9, the hardness of aragonite layer and calcite
layer of chlamys farreri shell are 4.53Gpa and 4.31Gpa
respectively, and the modulus of elasticity are 53.57Gpa and
42.34Gpa respectively. All the above values are averages of
10 times measurment. According to the results of measurment,
both the modulus of elasticity and hardness of aragonite layer
are higher than those of calcite layer, but the difference is not
significant for hardness of the two layers. The reasons which
led to the value difference of the modulus of elasticity and
hardness, lie in the age and positions of indentation of chlamys
farreri shell.
Fig.4 The Fracture of Chlamys Ferreri Shell （SEM）
Fig.7 The Load-Displacement Curves of Nanoindentation of Calcite Layer
and Aragonite Layer
Hardness/GPa
4.70
Fig.5 The Lamellar Structure of Calcite Layer
Aragonite plan
Calcite plan
4.60
4.50
4.40
4.30
4.20
1
2
3
4
5
6
7
8
9
10
Fig.8 Hardness of Chlamys Farreri Shell
Modulus/GPa
65.00
Fig.6 The Boxed Area of Figure 3
In general, the fracture toughness of composite due to the
internal fiber reinforced structures of its matrix to a great
degree. Such interlocked microstructure of chlamys farreri
shell, which is composed of inorganic calcium carbonate
matrix and organic collagen fiber, just corresponds with the
reinforce mechanism of materials, so the chlamys farreri shell
has excellent mechanical properties. From the results of
nanoindentation test (Fig.7), it was found that the load curve
(rising segment) of aragonite is above the calcite`s. The
nanoindentation depth of calcite layer is larger than that of
Aragonite plan
Calcite plan
60.00
55.00
50.00
45.00
40.00
35.00
30.00
1
2
3
4
5
6
7
8
9
10
Fig.9 Modulus of Elasticity of Chlamys Farreri Shell
The deformation of such materials as shells is different to
that of metals, the former is due to crack propagation, but the
latter is led to dislocation motion [23]. In addition, the
interaction between the inorganic phases and organic phase,
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such as fiber reinforce and interlocking, will significant affect
to the deformation of shells [20].
Through the observation of nanoindentation appearence of
chlamys farreri shell (Fig.10 a, b), it was found that both the
edges of nanoindentation of calcite layer and aragonite layer
appeared obvious cracks which even possess the trend of
extending. It can be seen from the three-dimensional
photograpgs (Fig.10 c, d) of nanoindentation of two surfaces
that there are stacking phenomenon of material around the
edges of indentation of calcite layer and aragonite layer, it was
demonstrated that the two materials generated plastic
deformation to a certain degree although they are biological
ceramic composite, and the two materials have significant
anisotropy [23].
（b）Nanoindentation of Aragonite Layer
The direction of crack propagation of calcite layer
appeared multidirectional feature, and the edges of
nanoindentation is irregular. The reason of features mentioned
above is that calcite is composed of irregular calcium
carbonate crystals, the crystal boundaries are connected by
relative weak macromolecules, when external fore was loaded
on them, the cracks will appear firstly around the boundaries
of load point, and then the racks will extend rapidly along
irregular boundaries of calcite crystal [24]. So the calcite is a
brittle material, and its cracks appear multidirectional feature.
（c）3-D Nanoindentation of Calcite Layer
The cracks of aragonite layer have comparative regular
shape with straight and clear edges. It is different from the
cracks feature of calcite layer, the cracks of aragonite layer
appear firstly in the organics boundaries which lie in the strips
or lamellar structures near the load point, and cracks extend
along the interlaminar boundaries, then will stopped at the
boundary of next layer. Therefore, both amount and length of
cracks will be reduced. In addition, the generation of cracks
absorb a part of external energy, therefore the damage and
deformation of shell can be limited within a least range, and
the cracks will be gathered to the boudary of lamellar structure,
so the fracture toughness of materials is improved.
Such deformation characteristic of chlamys farreri shell is
close related to the internal crystal type and structure [25].
Meanwhile, it is important for anisotropy of internal and
external materials to reinforce the mechanical properties of
materials and protect the inside soft tissue.
（D）3-D Nanoindentation of Aragoniter Layer
Fig.10 Nanoindentation of Chlamys Farreri Shell
IV.
CONCLUSIONUSING
The chlamys farreri shell is composed of calcite, aragonite
and collagen. The calcite has irregular columnar crystal
structure, and the aragoniter has regular strip or lamellar
crystal structure.
The test results of nanoindentation demonstrated that the
hardness and modulus of elasticity of aragonite layer are larger
than those of calcite layer, and the carrying capacity of
aragonite layer is also larger than that of calcite layer under
the same conditions. It was shown that the aragonite layer has
excellent fracture toughness.
（a）Nanoindentation of Calcite Layer
The difference of crystal structure of aragonite layer and
calcite layer lead to the different generation mechanism and
extending mechanism of crack under the external load. The
crack propagation of calcite layer not only has multidirectional
and irregular characteristics, but also has a large quantity. And
the crack propagation of aragonite layer has straight clear
features and the amount is less.
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ACKNOWLEDGMENT
This project was supported by Jilin Provincial Science and
Technology Department of China (Grant No. 20100711,
20091013), by “985 Project” of Jilin University, by National
Key Technology R&D Program (Grant No. 2011BAD20B09),
by Innovation Project of Scientific Frontier and
Interdisciplinary of Jilin University (Grant No. 200903264),
by National Natural Science Foundation of China (Grant No.
51075177) and by The Science and Technology Research
Project of Jiamisi University (Grant No. L2009-112).
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