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Mechanical behavior of mother-of-pearl and pearl with flat and spherical laminations
Materials Science and Engineering C 68 (2016) 9–17
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Mechanical behavior of mother-of-pearl and pearl with flat and spherical laminations D. Jiao, Z.Q. Liu ⁎, Y.K. Zhu, Z.Y. Weng, &, Z.F. Zhang ⁎ Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
a r t i c l e
i n f o
Article history: Received 16 November 2015 Received in revised form 5 May 2016 Accepted 22 May 2016 Available online 24 May 2016 Keywords: Pearl Laminated structure Spherical laminations Nacre Kinking
a b s t r a c t Laminated structure reduces the common inverse relationship of strength and toughness in many biological materials. Here the mechanical behavior of pearl and nacre with spherical and flat laminations was investigated and compared with the geological aragonite counterpart. The biological ceramics demonstrate higher strength, better reliability, and improved damage resistance owing to their laminated arrangement. Kinking and delamination occur in pearl to resist damage in addition to the crack-tip shielding mechanisms as in nacre, such as crack deflection, bridging, and platelet pull-out. The fracture mechanisms were interpreted in terms of the stress state using finite element simulation. This study may help clarify the compressive mechanics of laminated sphere between platens and advance the understanding on the mechanical behavior of biological and bio-inspired laminated materials. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Synthetic ceramics have long been widely used in high-stress and high-temperature operating conditions, owing to their properties of high strength, high stiffness, good wear-resistance and high thermal stability [1–3]. Nonetheless, ceramics fail catastrophically without plasticity because of their less flexible covalent and ionic bonds [1,2]; such brittleness restricts the engineering applications of ceramics [3]. In comparison, natural biological ceramics synthesized by living organisms balance strength and toughness better than synthetic materials because of their hierarchical structures formed through self-assembly [4–7]. For instance, the laminated structure in natural ceramics features parallel stacking of building blocks layer-by-layer, as seen in the inner-layer shells of many mollusks, such as pearl oyster and abalone [8]. Crack deflection and bridging can be introduced by the laminated structure, contributing to improved fracture toughness [9,10]. Such structure has also been widely mimicked in a series of bio-inspired materials [11,12]. The mechanical properties of laminated materials depend strongly on their detailed structural characteristics, e.g., the orientation and thickness of constituent platelets [13,14]. For example, the compressive strength is approximately 2.3 times higher in the direction normal to the nacre layer (540 MPa) compared to the parallel direction (235 MPa) in abalone shell [14]. Understanding the structure-mechanical property
⁎ Corresponding authors. E-mail addresses: [email protected] (Z.Q. Liu), [email protected] (Z.F. Zhang).
http://dx.doi.org/10.1016/j.msec.2016.05.089 0928-4931/© 2016 Elsevier B.V. All rights reserved.
relations serves as the basis for the bio-inspired design of high-performance laminated ceramics. As a unique laminated arrangement, the spherical laminations are common in natural materials and components, ranging from asteroids to plant seeds, and in synthetic nanoparticles [15–17]. Remarkable mechanical properties can be generated in materials with spherically laminated structure or made from their precursors [18–20]. The spherically laminated carbon nanospheres exhibit superelastic deformation behavior and good structural stability that no phase transformation occurs under high pressure [18]. Unprecedented hardness and thermal stability are achieved in diamond with nano-twinned structure fabricated from the precursors of spherically laminated carbon nanoparticles [20]. However, the structure-mechanical property relations as well as the deformation and fracture mechanisms of these spherically laminated materials have rarely been explored. Besides, the compression of spheres between flat platens, as a fundamental mechanics problem, is of significance in a range of fields, such as the transportation of iron ore pellets and the production of lightweight building materials [21,22]. Pearl that is produced by living mollusks is a biological ceramic having the same component of mainly aragonite as its mother – nacre [23– 25]. Aragonite and calcite are two common mineral forms of calcium carbonate in mollusk shells with the former generally existing in nacreous layer [26,27]. The building blocks are arranged in spherically laminated architecture in pearl [23,24], which is distinct from the nacre's flat laminations. In this work, the structures and mechanical properties of nacre and pearl were investigated and compared to geological aragonite. The deformation and fracture mechanisms were clarified and correlated to their distinct structures.
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2. Materials and methods Air-dried shells in length of about 17 cm and width of approximately 13 cm of Sinanodonta woodiana (Bivalvia: Unionidae) freshwater mussels and their produced pearls in diameters of 5–10 mm were obtained from a freshwater farm in Zhuji City, Zhejiang Province, China. Geological aragonite mined in Morocco was bought from a local craft shop. The external dark brown layers of as-received shells have been removed by mechanical grinding. The longitudinal and transverse directions of shell were defined to be parallel and vertical to the growth line (Fig. 1a). For structural characterization, shells and pearls were sectioned along different directions and equatorial plane using an STX-202A water-cooled low-speed diamond saw (Shenyang Kejing, China). The sections were sequentially ground using SiC abrasive papers of grades of 1200 and 2000, carefully polished using diamond polishing paste with grit size of 0.5 μm, etched in 2 wt.% ethylenediamine tetraacetic acid (EDTA) for 1–2.5 min, and then washed with distilled water immediately and dried in air. The samples were sputter-coated with thin gold film using a 208HR sputter coater (Cressington, UK) at an electric current of 20 mA for 240 s and observed by field emission scanning electron microscopy (SEM) using an LEO Supra 55 instrument (Carl Zeiss, Germany) at an accelerating voltage of 10 kV. The fine structure was characterized by a field emission transmission electron microscopy (TEM) on a Tecnai G2F20 system (FEI, USA) at an accelerating voltage of 200 kV. For preparing the TEM specimens, thin sections were cut from nacre and pearl along different directions, manually ground using 2000-grade SiC abrasive papers to a thickness of approximately 70 μm, and then glued onto copper grids. The samples were ion-milled at an accelerating voltage of 5 kV and electric current of 2 mA with an inclination angle of 10° for 5 h and subsequently at 5° for 30 min using an EM RES101 ion beam milling system (Leica, Germany). These specimens were sputter-coated with thin carbon film in thickness of about 25 Å using a Model 681 high resolution ion beam coater (Gatan, USA) before observation.
Vickers hardness testing was performed on the polished longitudinal and transverse sections of nacre and the equatorial plane of pearl with one diagonal of indenter parallel to the laminates using an AMH43 microhardness tester (LECO, USA). The indentation load and dwelling time were 200 gf and 13 s. Geological aragonite was also tested for comparison. Rectangular specimens in dimensions of 2 × 2 × 4 mm3 were excised from nacre using low-speed diamond saw. Uniaxial compression testing was conducted at a constant strain rate of 10−4 s−1 at room temperature using an Instron 8871 machine (Instron, USA). The loading direction was perpendicular to the aragonite laminates. Geological aragonite specimens of the same sizes were prepared and compressed along the [001] crystallographic orientation. Spherical pearls in diameters of approximately 6 mm were compressed between two flat platens made of high-strength steel at a displacement rate of 0.036 mm/min. A special fixture was used to avoid the rolling of pearls on the platen. At least 11 samples were tested for each material to ensure a good reliability. The samples were sputter-coated with gold film and examined by SEM after indentation and compression. The compression of one pearl was stopped prior to the final fracture to reveal the deformation mechanisms. The unloaded sample was sputter-coated with gold and observed by SEM. Subsequently, this pearl was embedded in epoxy resin, sectioned along its meridian plane, sputter-coated with gold, and then examined by SEM to characterize the internal morphology. The deformation behavior of an ideal elastic-plastic sphere compressed between two stiff platens was computed by finite element simulation in both spherical and Cartesian coordinates. For the finite element analysis, the diameter of sphere and the applied compressive displacement were set as 6 mm and 161 μm, which were consistent with the actual tests. The strength and Young's modulus of the material were set as 793 MPa and 19.7 GPa based on the experimental results. The Poisson's ratio was chosen as 0.3 according to Ref. [28,29].
Fig. 1. (a) Macroscopic appearance of Sinanodonta woodiana shells, pearls and geological aragonite. The dashed curve indicates the growth line of shell. The arrow in the inset indicates the (001) crystallographic plane of aragonite. (b, c) SEM micrographs of the cross sections of nacre and pearl etched in 2 wt.% EDTA for 2.5 min and 1 min. Their structures are schematically illustrated in the insets. The dotted lines denote the orientations of laminates. (d) Magnified morphology of the aragonite platelets in pearl.
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3. Results and discussion
3.2. Hardness and indentation behavior
3.1. Structures of nacre and pearl
Fig. 3a shows the variations of hardness across the thickness along the inside-to-outside direction in the longitudinal and transverse sections of nacre and those along the radial direction in the equatorial plane of pearl. The hardness was nearly constant around 2.3 GPa across the thickness of nacre on both sections, which is associated with the inplane isotropy of the flat laminations. The hardness of present Sinanodonta woodiana shell is similar to that of nacre in dried red abalone (2–4 GPa) and Perna canaliculus shells (2.55 GPa) with similar structures [34,35]. In comparison, the hardness of pearl was relatively lower and more scattered on the equatorial plane. Compared to the biological ceramics, geological aragonite exhibited higher hardness of 3.26 ± 0.15 GPa along the same [001] direction. Because the indenter size is too small to capture the influence of the lamellar arrangements, no difference was observed between the indentation morphologies of nacre and pearl. Yet the damage behavior was distinct near different corners of the indenter due to the anisotropy of platelets. As shown in Fig. 3b, tiny cracks emerged around the indentation corner where the diagonal was parallel to the platelets and propagated along the platelet boundaries. The indentation morphology at adjacent corners was featured by plentiful striations that resulted from the plastic shearing between platelets (Fig. 3c). The plasticity was generated within the inter-platelet layer of organic matter [30,36, 37]. The organic matter, which consists mainly of proteins and polysaccharides, accounts for approximately 5 wt.% of nacre and plays an important role in offering toughness [36,37]. In comparison, serious damage occurred in the geological aragonite, as featured by the crushed zone around the indenter, the large fractured blocks, and the abundant long cracks (Fig. 3d). Therefore, the laminated structure effectively mitigated the indentation damage in nacre and pearl despite a moderate decrease in their hardness.
Fig. 1a shows the macroscopic appearance of the nacreous shells, pearls and geological aragonite (inset). The cross-sectional structures of nacre and pearl are presented in Fig. 1b and c with their arrangements schematically illustrated in the insets. These two biological ceramics possess “brick-and-mortar” architecture with the constituent platelets arranged in the laminated manner. The platelets are polygonal in shape and staggered with each other in adjacent layers (Fig. 1d). The thickness of individual platelets lies in the range of 0.3–1.7 μm in both materials, which is similar to the abalone (approximately 0.45 μm) and Araguaia river clam (1.5–2 μm) [30,31], with no obvious difference between them. The variation in the platelet thickness may be caused by the fluctuation of living environment. In contrast to the flat laminations of nacre, the platelets are assembled in concentric spheres in pearl, forming the spherically laminated structure. The TEM micrographs of nacre and pearl are presented in Fig. 2. The constituent platelets display similar shape and dimension in both materials. The aragonite crystalline structure of the platelets was confirmed by the corresponding selected area electron diffraction (SAED) patterns, as shown in the insets. The crystals possess the same crystallographic orientation with the [001] direction normal to the constituent layers in both materials (Fig. 2b and e). There exist abundant nano-twins within the aragonite platelets (Fig. 2c and f). The primary twin plane was indexed as (110) according to the SAED pattern (inset in Fig. 2c), which is consistent with previous reports [8,32]. The symmetric (020) crystal planes with interplanar spacing of 0.398 nm are inclined by 58° with respect to the (110) twin plane in twin and matrix, as shown by the high-resolution TEM image of the nano-twin (Fig. 2f); this is analogous to the abalone and Saxidomus purpuratus shells [32,33]. Thereupon, nacre and pearl possess similar structures at the micron and nanometer length-scales. The dominant structural difference lies in the forms of their laminated arrangements, i.e., flat for nacre and spherical for pearl.
3.3. Compressive properties and fracture behavior The compressive properties of nacre, geological aragonite and pearl are presented in Fig. 4a and b. Catastrophic fracture occurred without
Fig. 2. Bright-field TEM micrographs of aragonite platelets in (a–c) nacre and (d, e) pearl. The corresponding SAED patterns are shown in the insets. (f) High-resolution TEM micrograph of nano-twin in pearl. The inset shows the bright-field TEM micrograph of the nano-twins.
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Fig. 3. (a) Variations of Vickers hardness in nacre and pearl along different cross sections. (b, c) SEM micrographs of different corners of the indenter in pearl after indentation. (d) SEM micrograph of geological aragonite after indentation.
plastic deformation in nacre and geological aragonite after the linear elastic deformation. The nacre displayed markedly higher strength than geological aragonite. The compressive strength of pearl was
represented using the average pressure on the contact plane right before fracture, which is commonly used in the compression of spheres between flat platens [38–41]. The contact area was calculated because
Fig. 4. Ranges of the compressive (a) stress-strain curves of nacre and geological aragonite and (b) load-displacement curves of pearl. The compression of nacre and pearl is schematically illustrated in the insets. (c) Schematic diagram of a sphere compressed between two stiff platens and the definition of the spherical coordinate system. The initial sphere before compression is indicated by the dashed circle. (d) Weibull analysis of the fracture stresses of nacre, pearl and geological aragonite.
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it was difficult to be determined experimentally as the pearl ruptured into fragments at the peak load. Two calculating methods were adopted, as illustrated in Fig. 4c. One method was to assume the material to be ideally condensable; as such, the radius of the sphere remained unchanged during compression [38], as indicated by the dashed curve. Then the contact area can be expressed as A ¼ πhR0 ;
ð1Þ
where R0 is the initial radius of sphere and h is half of the compressive displacement. This method usually underestimates the contact area and hence overestimates the stress because the material expands transversely under compression. The other method assumed that the compressed material was homogeneously distributed on the free surface of sphere such that the radius of sphere increased uniformly during the deformation process [39], as indicated by the outer contour marked by the solid curve in Fig. 4c. In this scenario, the contact area can be described as n o A ¼ π 2½h þ ðR−R0 ÞR‐½h þ ðR−R0 Þ2 ;
ð2Þ
where R is the radius of sphere after compression following. R¼
1=2 2 3 1 3 : R0 þ ðR0 −hÞ =ðR0 −hÞ 3 3
ð3Þ
This method generally underestimates the stress by assuming the material to be ideally incondensable. Here, the contact area was represented using the average of the above two extremes in attempt to minimize the error. Such calculation method was validated by examining the contact area of the pearl unloaded before rupture. The calculated average area was consistent with the measured one with an error of about 0.4% (see the Supplementary material). The compressive strengths of these materials were analyzed according to the Weibull distribution function which is an effective tool to evaluate the strength of brittle materials [42]. The fracture probability
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F is described as a function of fracture stress σ following F ¼ 1‐ exp ‐ðσ=σ 0 Þm ;
ð4Þ
where σ0 and m are the characteristic strength and Weibull modulus [2, 42]. m generally lies in the range of 1.8–7.7 for mollusk shells [43–46]. The values of σ0 and m were fitted to be 849 MPa and 6.35 for nacre and 951 MPa and 6.20 for pearl (Fig. 4d). Both nacre and pearl displayed higher stress of 843 MPa than the red abalone (540 MPa) corresponding to a fracture probability of 50% [14]. The strengths of nacre and pearl were compared using t-test [47]. The calculated t-value of 1.766 was lower than the critical value of 2.086 under a significance level of 0.05, indicating no significant difference between them. Additionally, both σ0 and m of biological ceramics were markedly higher compared to the geological aragonite (σ0 = 388 MPa, m = 2.74), suggesting higher strength and better reliability in the laminated materials. The relatively harder geological aragonite displayed significantly lower strength than the softer biological ceramics, which may result from the premature fracture of the mineral in respect of its brittleness [48]. As shown in Fig. 5, the rupture of nacre was featured by macroscopic splitting in both inter- and trans-platelet modes microscopically, as proved by the rough platelet boundaries and river-like patterns. The laminated structure led to unique mechanisms, e.g., crack deflection and bridging as well as platelet pull-out, to enhance the damage resistance. In particular, the cracks were deflected along the interfaces between platelets, as evidenced by the exposed platelet surfaces. Besides, aragonite platelets that were left intact spanning the crack wake acted as bridges to carry load that would otherwise be used to extend the cracks. SEM morphologies of compressed pearls before and after rupture are shown in Fig. 6. Splitting occurred along the meridian plane and terminated towards the contact region with platen following tortuous propagation paths (Fig. 6a and b). Many wrinkles emerged at the perimeter of the contact region (Fig. 6c), showing prominent local plastic deformation. Upon further loading, the pearl ruptured into wedge-shaped fragments (Fig. 6d) with two cones formed at the contact regions (inset of Fig. 6a). The cracks propagated mainly along the platelet boundaries microscopically, i.e., in the inter-platelet manner (Fig. 6e and f). Delamination occurred along the abundant interfaces
Fig. 5. SEM micrographs of nacre after compression.
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Fig. 6. SEM micrographs of compressed pearls (a–c) before and (d–f) after final rupture. The dotted regions in (d) are magnified in (e) and (f). The inset in (a) shows the optical image of the macroscopic appearance of ruptured pearl.
Fig. 7. SEM micrographs of the meridian plane of pearl unloaded before final rupture.
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between concentric spheres. The tortuous paths for crack propagation and the interactions between the splitting and delamination cracks enhanced the strength of pearl. 3.4. Deformation and damage mechanisms The deformation and damage mechanisms of pearl were explored by examining the internal morphology of the sample unloaded before fracture. As shown in Fig. 7a, the contact region became flat after compression owing to the occurrence of local plasticity. The splitting cracks bifurcated while propagation and tended to be channeled into the delamination ones (Fig. 7b), leading to the effective arrestment of these cracks. The crack propagation was hindered by abundant bridges and pull-out of platelets or platelet bundles (Fig. 7c). Mutual sliding occurred between crack faces due to their asymmetric strain, making the pull-out process more difficult. Additionally, the organic matter between platelets was severely sheared and stretched during delamination (Fig. 7d). It has been revealed that the strength and toughness of biological ceramics benefit markedly from the viscoelastic organic matter despite its small content [36,37]. Moreover, multiple kink bands were observed at the perimeter of the contact region, as shown in Fig. 8. The kink bands varied in their sizes from several to tens of micrometers, which was associated with the varying platelet orientations and the inhomogeneous stress-strain field in pearl. The largest kink bands expanded from the contact surface towards the interior with their widths increasing. These kink bands should have a conical shape around the boundary of the Hertzian cone considering the circumferential symmetry of pearl [49]. The platelets were re-orientated at one kink boundary and recovered at the opposite through continuous local plastic deformation. Splitting also tended to occur along the kink boundaries. Kinking was expected to enhance the damage resistance of pearl in the following aspects. Firstly, kinking, as a basic mode of plastic deformation in laminated materials, helped alleviate the stress concentrations and consume mechanical energy [50,51]. Secondly, the propagation of delamination cracks, both within and outside of the kink bands, was effectively arrested at the kink boundaries, as indicated by the arrows in Fig. 8b and d. Thirdly, the platelets or platelet bundles that bridged the splitting cracks were plastically deformed through local kinking, leading to enhanced resistance against the opening or shearing between the crack faces. The deformation behavior of pearl was interpreted in terms of the stress state of a sphere compressed between flat platens. Here the stress
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state was analyzed in both spherical and Cartesian coordinates depending on the detailed deformation mode. As shown in Fig. 9a, there existed compressive stress along the φ axis near the poles in the spherical coordinates (as defined in Fig. 4c). The majority of sphere sustained an internal tensile stress σφ which was responsible for the occurrence of splitting. The tensile stress decreased gradually from the equatorial plane towards the poles; as such, the splitting cracks tended to initiate near the equatorial plane and propagate along the meridian planes, as proved by the experimental results (Fig. 6). Besides, the delamination between concentric laminations was rationalized by the internal tensile stress σx along the x axis in the Cartesian coordinates, as shown in Fig. 9b. σx increased gradually towards the core, indicating improved delamination trend at the center of pearl. In contrast to the normal stress, the shear stress was mainly concentrated around the upper and lower contact regions. As shown in Fig. 9c, the regions sustaining the maximum shear stress τrθ in the spherical coordinates displayed a cone shape similar to the cone-shaped fragments near the poles (Fig. 6a). This indicates that the final fracture near the contact regions was associated with the shearing in the r-θ plane. On the other hand, kinking should be related to the shear stress τxy in Cartesian coordinates considering the straightforward progression of kink bands in the x-y plane. As shown in Fig. 9d, the shear stress τxy decreased from the surface towards interior along gradually expanding regions of which the location and geometry were similar to those of kink bands. Such trend was also consistent with the inward propagation and broadening of the kink bands. In addition, the shearing of inter-platelet organic matter enabled the sliding between constituent layers within kink bands which was necessary for the occurrence of kinking [43,49]. Thus the kink-assisted toughening in pearl resulted from the combined effect of the composite constituents and the spherical laminations. The compressive deformation and damage behavior of pearl can be schematically illustrated in Fig. 10 based on the above analysis. Local plastic deformation occurred near the contact regions prior to macroscopic splitting in pearl, which is different from the flat laminated nacre. Microscopically, the crack-tip shielding mechanisms endowed by the laminated structure, such as crack deflection and bridging as well as pull-out of platelets, contributed to enhanced damage resistance of pearl as in nacre. Moreover, the spherically laminated structure led to the delamination along abundant platelet boundaries and the formation of multiple kink bands. These mechanisms that were unique compared to nacre and geological aragonite favored a higher resistance against damage in pearl under compression.
Fig. 8. SEM micrographs of multiple kink bands in the interior of pearl after compression. The dotted region in (a) is magnified in (b).
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Fig. 9. Finite element simulation results on the (a, b) normal and (c, d) shear stresses in spherical and Cartesian coordinates in a sphere compressed between flat platens. (a, c) show the spherical coordinates that are defined in Fig. 4c, and (b, d) present the Cartesian coordinates. In (a) and (b), the left parts present the whole ranges of stress and the tensile portions are shown in detail in the right parts.
4. Conclusions 1) Nacre and pearl possess flat and spherically laminated structures and hence exhibit unique mechanical properties and damage behavior compared to geological aragonite. Both strength and reliability were markedly higher in nacre and pearl, despite their relatively lower hardness, owing to the toughening mechanisms endowed by their laminated structures and the plasticity of the involved organic matter. 2) The compressive failure of nacre, which was mainly mediated by splitting, was resisted by a suit of crack-tip shielding mechanisms, such as crack deflection, bridging and platelet pull-out. Additionally,
kinking and delamination occurred in pearl owing to its spherical laminations to arrest the crack propagation. 3) The deformation and damage behavior of pearl was closely associated with its internal stress state. Splitting and delamination resulted from the tensile stresses σφ and σx, while the shear stress τxy was responsible for the occurrence of kinking. 4) This comparative study may help clarify the deformation and toughening mechanisms of laminated biological materials, in particular those with spherical laminations, and aid in understanding the mechanical behavior of synthetic materials mimicking them. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.msec.2016.05.089.
Fig. 10. Schematic illustrations of the deformation and damage behavior of pearl under compression.
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Mechanical behavior of mother-of-pearl and pearl with flat and spherical laminations D. Jiao, Z.Q. Liu ⁎, Y.K. Zhu, Z.Y. Weng, &, Z.F. Zhang ⁎ Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
a r t i c l e
i n f o
Article history: Received 16 November 2015 Received in revised form 5 May 2016 Accepted 22 May 2016 Available online 24 May 2016 Keywords: Pearl Laminated structure Spherical laminations Nacre Kinking
a b s t r a c t Laminated structure reduces the common inverse relationship of strength and toughness in many biological materials. Here the mechanical behavior of pearl and nacre with spherical and flat laminations was investigated and compared with the geological aragonite counterpart. The biological ceramics demonstrate higher strength, better reliability, and improved damage resistance owing to their laminated arrangement. Kinking and delamination occur in pearl to resist damage in addition to the crack-tip shielding mechanisms as in nacre, such as crack deflection, bridging, and platelet pull-out. The fracture mechanisms were interpreted in terms of the stress state using finite element simulation. This study may help clarify the compressive mechanics of laminated sphere between platens and advance the understanding on the mechanical behavior of biological and bio-inspired laminated materials. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Synthetic ceramics have long been widely used in high-stress and high-temperature operating conditions, owing to their properties of high strength, high stiffness, good wear-resistance and high thermal stability [1–3]. Nonetheless, ceramics fail catastrophically without plasticity because of their less flexible covalent and ionic bonds [1,2]; such brittleness restricts the engineering applications of ceramics [3]. In comparison, natural biological ceramics synthesized by living organisms balance strength and toughness better than synthetic materials because of their hierarchical structures formed through self-assembly [4–7]. For instance, the laminated structure in natural ceramics features parallel stacking of building blocks layer-by-layer, as seen in the inner-layer shells of many mollusks, such as pearl oyster and abalone [8]. Crack deflection and bridging can be introduced by the laminated structure, contributing to improved fracture toughness [9,10]. Such structure has also been widely mimicked in a series of bio-inspired materials [11,12]. The mechanical properties of laminated materials depend strongly on their detailed structural characteristics, e.g., the orientation and thickness of constituent platelets [13,14]. For example, the compressive strength is approximately 2.3 times higher in the direction normal to the nacre layer (540 MPa) compared to the parallel direction (235 MPa) in abalone shell [14]. Understanding the structure-mechanical property
⁎ Corresponding authors. E-mail addresses: [email protected] (Z.Q. Liu), [email protected] (Z.F. Zhang).
http://dx.doi.org/10.1016/j.msec.2016.05.089 0928-4931/© 2016 Elsevier B.V. All rights reserved.
relations serves as the basis for the bio-inspired design of high-performance laminated ceramics. As a unique laminated arrangement, the spherical laminations are common in natural materials and components, ranging from asteroids to plant seeds, and in synthetic nanoparticles [15–17]. Remarkable mechanical properties can be generated in materials with spherically laminated structure or made from their precursors [18–20]. The spherically laminated carbon nanospheres exhibit superelastic deformation behavior and good structural stability that no phase transformation occurs under high pressure [18]. Unprecedented hardness and thermal stability are achieved in diamond with nano-twinned structure fabricated from the precursors of spherically laminated carbon nanoparticles [20]. However, the structure-mechanical property relations as well as the deformation and fracture mechanisms of these spherically laminated materials have rarely been explored. Besides, the compression of spheres between flat platens, as a fundamental mechanics problem, is of significance in a range of fields, such as the transportation of iron ore pellets and the production of lightweight building materials [21,22]. Pearl that is produced by living mollusks is a biological ceramic having the same component of mainly aragonite as its mother – nacre [23– 25]. Aragonite and calcite are two common mineral forms of calcium carbonate in mollusk shells with the former generally existing in nacreous layer [26,27]. The building blocks are arranged in spherically laminated architecture in pearl [23,24], which is distinct from the nacre's flat laminations. In this work, the structures and mechanical properties of nacre and pearl were investigated and compared to geological aragonite. The deformation and fracture mechanisms were clarified and correlated to their distinct structures.
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2. Materials and methods Air-dried shells in length of about 17 cm and width of approximately 13 cm of Sinanodonta woodiana (Bivalvia: Unionidae) freshwater mussels and their produced pearls in diameters of 5–10 mm were obtained from a freshwater farm in Zhuji City, Zhejiang Province, China. Geological aragonite mined in Morocco was bought from a local craft shop. The external dark brown layers of as-received shells have been removed by mechanical grinding. The longitudinal and transverse directions of shell were defined to be parallel and vertical to the growth line (Fig. 1a). For structural characterization, shells and pearls were sectioned along different directions and equatorial plane using an STX-202A water-cooled low-speed diamond saw (Shenyang Kejing, China). The sections were sequentially ground using SiC abrasive papers of grades of 1200 and 2000, carefully polished using diamond polishing paste with grit size of 0.5 μm, etched in 2 wt.% ethylenediamine tetraacetic acid (EDTA) for 1–2.5 min, and then washed with distilled water immediately and dried in air. The samples were sputter-coated with thin gold film using a 208HR sputter coater (Cressington, UK) at an electric current of 20 mA for 240 s and observed by field emission scanning electron microscopy (SEM) using an LEO Supra 55 instrument (Carl Zeiss, Germany) at an accelerating voltage of 10 kV. The fine structure was characterized by a field emission transmission electron microscopy (TEM) on a Tecnai G2F20 system (FEI, USA) at an accelerating voltage of 200 kV. For preparing the TEM specimens, thin sections were cut from nacre and pearl along different directions, manually ground using 2000-grade SiC abrasive papers to a thickness of approximately 70 μm, and then glued onto copper grids. The samples were ion-milled at an accelerating voltage of 5 kV and electric current of 2 mA with an inclination angle of 10° for 5 h and subsequently at 5° for 30 min using an EM RES101 ion beam milling system (Leica, Germany). These specimens were sputter-coated with thin carbon film in thickness of about 25 Å using a Model 681 high resolution ion beam coater (Gatan, USA) before observation.
Vickers hardness testing was performed on the polished longitudinal and transverse sections of nacre and the equatorial plane of pearl with one diagonal of indenter parallel to the laminates using an AMH43 microhardness tester (LECO, USA). The indentation load and dwelling time were 200 gf and 13 s. Geological aragonite was also tested for comparison. Rectangular specimens in dimensions of 2 × 2 × 4 mm3 were excised from nacre using low-speed diamond saw. Uniaxial compression testing was conducted at a constant strain rate of 10−4 s−1 at room temperature using an Instron 8871 machine (Instron, USA). The loading direction was perpendicular to the aragonite laminates. Geological aragonite specimens of the same sizes were prepared and compressed along the [001] crystallographic orientation. Spherical pearls in diameters of approximately 6 mm were compressed between two flat platens made of high-strength steel at a displacement rate of 0.036 mm/min. A special fixture was used to avoid the rolling of pearls on the platen. At least 11 samples were tested for each material to ensure a good reliability. The samples were sputter-coated with gold film and examined by SEM after indentation and compression. The compression of one pearl was stopped prior to the final fracture to reveal the deformation mechanisms. The unloaded sample was sputter-coated with gold and observed by SEM. Subsequently, this pearl was embedded in epoxy resin, sectioned along its meridian plane, sputter-coated with gold, and then examined by SEM to characterize the internal morphology. The deformation behavior of an ideal elastic-plastic sphere compressed between two stiff platens was computed by finite element simulation in both spherical and Cartesian coordinates. For the finite element analysis, the diameter of sphere and the applied compressive displacement were set as 6 mm and 161 μm, which were consistent with the actual tests. The strength and Young's modulus of the material were set as 793 MPa and 19.7 GPa based on the experimental results. The Poisson's ratio was chosen as 0.3 according to Ref. [28,29].
Fig. 1. (a) Macroscopic appearance of Sinanodonta woodiana shells, pearls and geological aragonite. The dashed curve indicates the growth line of shell. The arrow in the inset indicates the (001) crystallographic plane of aragonite. (b, c) SEM micrographs of the cross sections of nacre and pearl etched in 2 wt.% EDTA for 2.5 min and 1 min. Their structures are schematically illustrated in the insets. The dotted lines denote the orientations of laminates. (d) Magnified morphology of the aragonite platelets in pearl.
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3. Results and discussion
3.2. Hardness and indentation behavior
3.1. Structures of nacre and pearl
Fig. 3a shows the variations of hardness across the thickness along the inside-to-outside direction in the longitudinal and transverse sections of nacre and those along the radial direction in the equatorial plane of pearl. The hardness was nearly constant around 2.3 GPa across the thickness of nacre on both sections, which is associated with the inplane isotropy of the flat laminations. The hardness of present Sinanodonta woodiana shell is similar to that of nacre in dried red abalone (2–4 GPa) and Perna canaliculus shells (2.55 GPa) with similar structures [34,35]. In comparison, the hardness of pearl was relatively lower and more scattered on the equatorial plane. Compared to the biological ceramics, geological aragonite exhibited higher hardness of 3.26 ± 0.15 GPa along the same [001] direction. Because the indenter size is too small to capture the influence of the lamellar arrangements, no difference was observed between the indentation morphologies of nacre and pearl. Yet the damage behavior was distinct near different corners of the indenter due to the anisotropy of platelets. As shown in Fig. 3b, tiny cracks emerged around the indentation corner where the diagonal was parallel to the platelets and propagated along the platelet boundaries. The indentation morphology at adjacent corners was featured by plentiful striations that resulted from the plastic shearing between platelets (Fig. 3c). The plasticity was generated within the inter-platelet layer of organic matter [30,36, 37]. The organic matter, which consists mainly of proteins and polysaccharides, accounts for approximately 5 wt.% of nacre and plays an important role in offering toughness [36,37]. In comparison, serious damage occurred in the geological aragonite, as featured by the crushed zone around the indenter, the large fractured blocks, and the abundant long cracks (Fig. 3d). Therefore, the laminated structure effectively mitigated the indentation damage in nacre and pearl despite a moderate decrease in their hardness.
Fig. 1a shows the macroscopic appearance of the nacreous shells, pearls and geological aragonite (inset). The cross-sectional structures of nacre and pearl are presented in Fig. 1b and c with their arrangements schematically illustrated in the insets. These two biological ceramics possess “brick-and-mortar” architecture with the constituent platelets arranged in the laminated manner. The platelets are polygonal in shape and staggered with each other in adjacent layers (Fig. 1d). The thickness of individual platelets lies in the range of 0.3–1.7 μm in both materials, which is similar to the abalone (approximately 0.45 μm) and Araguaia river clam (1.5–2 μm) [30,31], with no obvious difference between them. The variation in the platelet thickness may be caused by the fluctuation of living environment. In contrast to the flat laminations of nacre, the platelets are assembled in concentric spheres in pearl, forming the spherically laminated structure. The TEM micrographs of nacre and pearl are presented in Fig. 2. The constituent platelets display similar shape and dimension in both materials. The aragonite crystalline structure of the platelets was confirmed by the corresponding selected area electron diffraction (SAED) patterns, as shown in the insets. The crystals possess the same crystallographic orientation with the [001] direction normal to the constituent layers in both materials (Fig. 2b and e). There exist abundant nano-twins within the aragonite platelets (Fig. 2c and f). The primary twin plane was indexed as (110) according to the SAED pattern (inset in Fig. 2c), which is consistent with previous reports [8,32]. The symmetric (020) crystal planes with interplanar spacing of 0.398 nm are inclined by 58° with respect to the (110) twin plane in twin and matrix, as shown by the high-resolution TEM image of the nano-twin (Fig. 2f); this is analogous to the abalone and Saxidomus purpuratus shells [32,33]. Thereupon, nacre and pearl possess similar structures at the micron and nanometer length-scales. The dominant structural difference lies in the forms of their laminated arrangements, i.e., flat for nacre and spherical for pearl.
3.3. Compressive properties and fracture behavior The compressive properties of nacre, geological aragonite and pearl are presented in Fig. 4a and b. Catastrophic fracture occurred without
Fig. 2. Bright-field TEM micrographs of aragonite platelets in (a–c) nacre and (d, e) pearl. The corresponding SAED patterns are shown in the insets. (f) High-resolution TEM micrograph of nano-twin in pearl. The inset shows the bright-field TEM micrograph of the nano-twins.
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Fig. 3. (a) Variations of Vickers hardness in nacre and pearl along different cross sections. (b, c) SEM micrographs of different corners of the indenter in pearl after indentation. (d) SEM micrograph of geological aragonite after indentation.
plastic deformation in nacre and geological aragonite after the linear elastic deformation. The nacre displayed markedly higher strength than geological aragonite. The compressive strength of pearl was
represented using the average pressure on the contact plane right before fracture, which is commonly used in the compression of spheres between flat platens [38–41]. The contact area was calculated because
Fig. 4. Ranges of the compressive (a) stress-strain curves of nacre and geological aragonite and (b) load-displacement curves of pearl. The compression of nacre and pearl is schematically illustrated in the insets. (c) Schematic diagram of a sphere compressed between two stiff platens and the definition of the spherical coordinate system. The initial sphere before compression is indicated by the dashed circle. (d) Weibull analysis of the fracture stresses of nacre, pearl and geological aragonite.
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it was difficult to be determined experimentally as the pearl ruptured into fragments at the peak load. Two calculating methods were adopted, as illustrated in Fig. 4c. One method was to assume the material to be ideally condensable; as such, the radius of the sphere remained unchanged during compression [38], as indicated by the dashed curve. Then the contact area can be expressed as A ¼ πhR0 ;
ð1Þ
where R0 is the initial radius of sphere and h is half of the compressive displacement. This method usually underestimates the contact area and hence overestimates the stress because the material expands transversely under compression. The other method assumed that the compressed material was homogeneously distributed on the free surface of sphere such that the radius of sphere increased uniformly during the deformation process [39], as indicated by the outer contour marked by the solid curve in Fig. 4c. In this scenario, the contact area can be described as n o A ¼ π 2½h þ ðR−R0 ÞR‐½h þ ðR−R0 Þ2 ;
ð2Þ
where R is the radius of sphere after compression following. R¼
1=2 2 3 1 3 : R0 þ ðR0 −hÞ =ðR0 −hÞ 3 3
ð3Þ
This method generally underestimates the stress by assuming the material to be ideally incondensable. Here, the contact area was represented using the average of the above two extremes in attempt to minimize the error. Such calculation method was validated by examining the contact area of the pearl unloaded before rupture. The calculated average area was consistent with the measured one with an error of about 0.4% (see the Supplementary material). The compressive strengths of these materials were analyzed according to the Weibull distribution function which is an effective tool to evaluate the strength of brittle materials [42]. The fracture probability
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F is described as a function of fracture stress σ following F ¼ 1‐ exp ‐ðσ=σ 0 Þm ;
ð4Þ
where σ0 and m are the characteristic strength and Weibull modulus [2, 42]. m generally lies in the range of 1.8–7.7 for mollusk shells [43–46]. The values of σ0 and m were fitted to be 849 MPa and 6.35 for nacre and 951 MPa and 6.20 for pearl (Fig. 4d). Both nacre and pearl displayed higher stress of 843 MPa than the red abalone (540 MPa) corresponding to a fracture probability of 50% [14]. The strengths of nacre and pearl were compared using t-test [47]. The calculated t-value of 1.766 was lower than the critical value of 2.086 under a significance level of 0.05, indicating no significant difference between them. Additionally, both σ0 and m of biological ceramics were markedly higher compared to the geological aragonite (σ0 = 388 MPa, m = 2.74), suggesting higher strength and better reliability in the laminated materials. The relatively harder geological aragonite displayed significantly lower strength than the softer biological ceramics, which may result from the premature fracture of the mineral in respect of its brittleness [48]. As shown in Fig. 5, the rupture of nacre was featured by macroscopic splitting in both inter- and trans-platelet modes microscopically, as proved by the rough platelet boundaries and river-like patterns. The laminated structure led to unique mechanisms, e.g., crack deflection and bridging as well as platelet pull-out, to enhance the damage resistance. In particular, the cracks were deflected along the interfaces between platelets, as evidenced by the exposed platelet surfaces. Besides, aragonite platelets that were left intact spanning the crack wake acted as bridges to carry load that would otherwise be used to extend the cracks. SEM morphologies of compressed pearls before and after rupture are shown in Fig. 6. Splitting occurred along the meridian plane and terminated towards the contact region with platen following tortuous propagation paths (Fig. 6a and b). Many wrinkles emerged at the perimeter of the contact region (Fig. 6c), showing prominent local plastic deformation. Upon further loading, the pearl ruptured into wedge-shaped fragments (Fig. 6d) with two cones formed at the contact regions (inset of Fig. 6a). The cracks propagated mainly along the platelet boundaries microscopically, i.e., in the inter-platelet manner (Fig. 6e and f). Delamination occurred along the abundant interfaces
Fig. 5. SEM micrographs of nacre after compression.
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Fig. 6. SEM micrographs of compressed pearls (a–c) before and (d–f) after final rupture. The dotted regions in (d) are magnified in (e) and (f). The inset in (a) shows the optical image of the macroscopic appearance of ruptured pearl.
Fig. 7. SEM micrographs of the meridian plane of pearl unloaded before final rupture.
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between concentric spheres. The tortuous paths for crack propagation and the interactions between the splitting and delamination cracks enhanced the strength of pearl. 3.4. Deformation and damage mechanisms The deformation and damage mechanisms of pearl were explored by examining the internal morphology of the sample unloaded before fracture. As shown in Fig. 7a, the contact region became flat after compression owing to the occurrence of local plasticity. The splitting cracks bifurcated while propagation and tended to be channeled into the delamination ones (Fig. 7b), leading to the effective arrestment of these cracks. The crack propagation was hindered by abundant bridges and pull-out of platelets or platelet bundles (Fig. 7c). Mutual sliding occurred between crack faces due to their asymmetric strain, making the pull-out process more difficult. Additionally, the organic matter between platelets was severely sheared and stretched during delamination (Fig. 7d). It has been revealed that the strength and toughness of biological ceramics benefit markedly from the viscoelastic organic matter despite its small content [36,37]. Moreover, multiple kink bands were observed at the perimeter of the contact region, as shown in Fig. 8. The kink bands varied in their sizes from several to tens of micrometers, which was associated with the varying platelet orientations and the inhomogeneous stress-strain field in pearl. The largest kink bands expanded from the contact surface towards the interior with their widths increasing. These kink bands should have a conical shape around the boundary of the Hertzian cone considering the circumferential symmetry of pearl [49]. The platelets were re-orientated at one kink boundary and recovered at the opposite through continuous local plastic deformation. Splitting also tended to occur along the kink boundaries. Kinking was expected to enhance the damage resistance of pearl in the following aspects. Firstly, kinking, as a basic mode of plastic deformation in laminated materials, helped alleviate the stress concentrations and consume mechanical energy [50,51]. Secondly, the propagation of delamination cracks, both within and outside of the kink bands, was effectively arrested at the kink boundaries, as indicated by the arrows in Fig. 8b and d. Thirdly, the platelets or platelet bundles that bridged the splitting cracks were plastically deformed through local kinking, leading to enhanced resistance against the opening or shearing between the crack faces. The deformation behavior of pearl was interpreted in terms of the stress state of a sphere compressed between flat platens. Here the stress
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state was analyzed in both spherical and Cartesian coordinates depending on the detailed deformation mode. As shown in Fig. 9a, there existed compressive stress along the φ axis near the poles in the spherical coordinates (as defined in Fig. 4c). The majority of sphere sustained an internal tensile stress σφ which was responsible for the occurrence of splitting. The tensile stress decreased gradually from the equatorial plane towards the poles; as such, the splitting cracks tended to initiate near the equatorial plane and propagate along the meridian planes, as proved by the experimental results (Fig. 6). Besides, the delamination between concentric laminations was rationalized by the internal tensile stress σx along the x axis in the Cartesian coordinates, as shown in Fig. 9b. σx increased gradually towards the core, indicating improved delamination trend at the center of pearl. In contrast to the normal stress, the shear stress was mainly concentrated around the upper and lower contact regions. As shown in Fig. 9c, the regions sustaining the maximum shear stress τrθ in the spherical coordinates displayed a cone shape similar to the cone-shaped fragments near the poles (Fig. 6a). This indicates that the final fracture near the contact regions was associated with the shearing in the r-θ plane. On the other hand, kinking should be related to the shear stress τxy in Cartesian coordinates considering the straightforward progression of kink bands in the x-y plane. As shown in Fig. 9d, the shear stress τxy decreased from the surface towards interior along gradually expanding regions of which the location and geometry were similar to those of kink bands. Such trend was also consistent with the inward propagation and broadening of the kink bands. In addition, the shearing of inter-platelet organic matter enabled the sliding between constituent layers within kink bands which was necessary for the occurrence of kinking [43,49]. Thus the kink-assisted toughening in pearl resulted from the combined effect of the composite constituents and the spherical laminations. The compressive deformation and damage behavior of pearl can be schematically illustrated in Fig. 10 based on the above analysis. Local plastic deformation occurred near the contact regions prior to macroscopic splitting in pearl, which is different from the flat laminated nacre. Microscopically, the crack-tip shielding mechanisms endowed by the laminated structure, such as crack deflection and bridging as well as pull-out of platelets, contributed to enhanced damage resistance of pearl as in nacre. Moreover, the spherically laminated structure led to the delamination along abundant platelet boundaries and the formation of multiple kink bands. These mechanisms that were unique compared to nacre and geological aragonite favored a higher resistance against damage in pearl under compression.
Fig. 8. SEM micrographs of multiple kink bands in the interior of pearl after compression. The dotted region in (a) is magnified in (b).
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Fig. 9. Finite element simulation results on the (a, b) normal and (c, d) shear stresses in spherical and Cartesian coordinates in a sphere compressed between flat platens. (a, c) show the spherical coordinates that are defined in Fig. 4c, and (b, d) present the Cartesian coordinates. In (a) and (b), the left parts present the whole ranges of stress and the tensile portions are shown in detail in the right parts.
4. Conclusions 1) Nacre and pearl possess flat and spherically laminated structures and hence exhibit unique mechanical properties and damage behavior compared to geological aragonite. Both strength and reliability were markedly higher in nacre and pearl, despite their relatively lower hardness, owing to the toughening mechanisms endowed by their laminated structures and the plasticity of the involved organic matter. 2) The compressive failure of nacre, which was mainly mediated by splitting, was resisted by a suit of crack-tip shielding mechanisms, such as crack deflection, bridging and platelet pull-out. Additionally,
kinking and delamination occurred in pearl owing to its spherical laminations to arrest the crack propagation. 3) The deformation and damage behavior of pearl was closely associated with its internal stress state. Splitting and delamination resulted from the tensile stresses σφ and σx, while the shear stress τxy was responsible for the occurrence of kinking. 4) This comparative study may help clarify the deformation and toughening mechanisms of laminated biological materials, in particular those with spherical laminations, and aid in understanding the mechanical behavior of synthetic materials mimicking them. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.msec.2016.05.089.
Fig. 10. Schematic illustrations of the deformation and damage behavior of pearl under compression.
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