NANO LETTERS
In Situ Observation of Nanograin Rotation and Deformation in Nacre
2006 Vol. 6, No. 10 2301-2304
Xiaodong Li,*,† Zhi-Hui Xu,† and Rizhi Wang‡ Department of Mechanical Engineering, UniVersity of South Carolina, 300 Main Street, Columbia, South Carolina 29208, and Department of Materials Engineering, UniVersity of British Columbia, 309-6350 Stores Road, VancouVer, British Columbia V6T 1Z4, Canada Received July 28, 2006; Revised Manuscript Received August 30, 2006
ABSTRACT Nacre is a natural nanocomposite material with superior mechanical strength and toughness. What roles do the nanoscale structures play in the inelasticity and toughening of nacre? Can we learn from this to produce nacre-like nanocomposites? Here we report in situ dynamic atomic force microscope observations of nacre with aragonite nanograins (nanoparticles) of an average grain size of 32 nm, which show that nanograin rotation and deformation are the two prominent mechanisms contributing to energy dissipation in nacre. The biopolymer spacing between the nanograins facilitates the grain rotation process. The aragonite nanograins in nacre are not brittle but deformable.
Nature has evolved complex bottom-up methods for fabricating ordered nanostructured materials that often have extraordinary mechanical strength and toughness.1,2 One of the best examples is nacre (mother-of-pearl) that is found in the shinny interior of many mollusk shells and consists of about 95% brittle inorganic aragonite (a mineral form of CaCO3) and a few percent of organic biopolymer.3 This material has a brick-and-mortar-like structure with highly organized polygonal aragonite platelets of a thickness ranging from 200 to 500 nm and an edge length about 5 µm sandwiched with a 5-20 nm thick organic biopolymer interlayer, which assembles the aragonite platelets together.4 The combination of the soft organic biopolymer and the hard inorganic calcium carbonate produces a lamellar composite with a 2-fold increase in strength and a 1000-fold increase in toughness over its constituent materials.2 Such remarkable properties have motivated many researchers to synthesize biomimetic nanocomposites that attempt to reproduce nature’s achievements5,6 and to understand the toughening and deformation mechanisms of natural nanocomposite materials.7-14 The structure of nacre has evolved through millions of years to a level of optimization not currently achieved in engineered composites.15 It has been long thought that the toughening of nacre originates from its unique lamellar structure. The hierarchical structure of aragonite platelets can deflect the crack to a direction with an unfavorable stress state and prolong the crack propagation.7 This implies more energy absorption during the crack travel. The organic matrix * Corresponding author.
[email protected]; http://www.me.sc.edu/ research/nano/. † University of South Carolina. ‡ University of British Columbia. 10.1021/nl061775u CCC: $33.50 Published on Web 09/14/2006
© 2006 American Chemical Society
composed of proteins, such as Lustrin A, which has a highly modular structure, i.e., a multidomain architecture with folded modules, serves as an adhesive binding the platelets together. In order to break the molecule chain with folded modules, significant energy is required to unfold each individual module, which also contributes to the high toughness of nacre.11 Moreover, the unique lamellar structure also allows nacre to deform inelastically despite the brittle nature of its predominant constituent: CaCO3. This inelastic deformation enables nacre to redistribute stress around strain concentration sites and eliminate stress concentration, which renders nacre “notch insensitive”.16-18 The mechanisms responsible for the inelastic deformation of nacre were proposed to be interlamellae slip upon shear loading and formation of dilation bands at interplatelet boundaries accompanied by interlamellae sliding upon tensile loading.9,10 The aragonite platelets were thought to be brittle single crystals. However, our recent studies on the nanostructure and nanomechanics of nacre using atomic force microscopy (AFM) and nanoindentation techniques reveals that the individual aragonite platelets in fact consist of millions of nanosized grains (particles) with an average grain (particle) size of 32 nm and the aragonite platelets are not brittle but ductile.4 Photoacoustic Fourier transform infrared spectroscopy studies of nacre also indicate that water present at nanograin interfaces contributes significantly to the Viscoelasticity of nacre.19,20 The exact roles of these nanograins in the inelastic deformation and toughening of nacre remain entirely unknown. In this Letter, natural nacre materials from California red abalone (Haliotis rufescens) that belongs to the class of gastropoda were studied. The shells were collected alive in
Figure 1. Schematic of tested nacre specimens and their dimensions: (a) tensile specimen; (b) three-point bending specimen. Red circles in the specimens denote the positions of AFM observation. Units in mm.
Santa Barbara, CA. To minimize the detrimental effect of drying on the structure of shells, they were cleaned and airdelivered in ice to the laboratory where the experiments were conducted. Specimens were cut from the nacre layer of the shells with a water-cooled, low-speed diamond saw. Then they were mechanically ground and polished using abrasives and powders down to 50 nm in size and finally rinsed thoroughly with distilled water prior to testing. The shape and dimension of tensile and bending specimens are shown
in Figure 1. These specimens were then mounted to a customdesign micromechanical tester which was integrated with an AFM (Veeco Dimension 3100 AFM, Veeco Metrology Group, Santa Barbara, CA) to perform three-point bending and tensile tests in situ where the nacre sample surfaces were imaged simultaneously by AFM. Controlled bending deflection or tensile displacement was gradually applied to the tested specimen until fracture occurs. In situ observations of the deformation behavior of nacre nanograins during bending or tension were made at a fixed position (denoted by red circles in Figure 1) with the tapping mode AFM. During the testing process, water was intermittently applied to the specimen surface to keep it wet. AFM phase images of the nanograins in aragonite platelets subjected to different tensile strains are shown in Figure 2. Figure 2a shows a typical AFM image of the nanograins without any deformation (0% applied tensile strain). To deform the nacre sample, a tensile displacement was gradually applied to the nacre specimen along the direction pointed out by the arrows in Figure 1a. Deformation of the nanograins highlighted by the rectangle with tensile strain from 1% to 5% is shown in parts b-f of Figure 2. Compared with no applied tensile strain (Figure 2a), the spacing between nanograins increases with an increase in the applied tensile strain. This indicates that the viscoelastic-plastic biopolymer between the nanograins acted as “rubber-bands” and was stretched upon tension. This spacing behavior during tension results in an expansion of aragonite platelets along the direction perpendicular to the tensile direction. The width variation of the observed platelet at positions A and B with
Figure 2. AFM phase images of nanograins in nacre aragonite platelets under different tensile strains. (a) 0%, arrows under picture denote the tensile direction. (b-f) Images of the rectangle area in (a) with different applied strains: (b) 1%; (c) 2%; (d) 3%; (e) 4%; (f) 5%. (g) Variation of the width of the platelet at positions A and B with the applied strain. 2302
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Figure 3. High-magnification AFM images of the nanograins in the rectangle (blue) areas with a dimension of 364 nm × 293 nm in parts a and f of Figure 2: (a) 0% applied tensile strain; (b) 5% applied tensile strain. The green arrows in the pictures highlight the rotation of individual nanograins. The blue arrows under the pictures denote the tensile direction.
applied tensile strain is plotted in Figure 2g, where width of the aragonite platelet upon tension l is normalized by its original width l0. As can be seen, the width of the aragonite platelet increases with increasing tensile strain when the applied tensile strain is below 0.7%. With further tension, the width of aragonite platelet fluctuates from about 1.1 to 1.3 where failure occurs. High-magnification AFM images of the nanograins in the blue rectangle areas with a dimension of 364 nm × 293 nm in parts a and f of Figure 2 are shown in parts a and b of Figure 3, respectively. Comparison of the nanograins in part a (0% applied tensile strain) and part b (5% applied tensile strain) of Figure 3 shows expansion of the aragonite platelet, especially in the direction perpendicular to the tensile direction. The spacing and rotation of individual nanograins under tension are highlighted by the green arrows. This indicates that, unlike the common materials, nacre has a negative Poisson’s ratio. Detailed discussion about this spacing behavior will be given later in this paper. Figure 4 shows the in situ deformation of the nanograins inside and between aragonite platelets at the tensile part of a nacre sample subjected to three-point bending. An AFM image of the observed area of an aragonite platelet is shown in Figure 4a. The inside area is highlighted by rectangle A while the area between the two aragonite platelets is highlighted by rectangle B. The rigid rotation of the sample caused by bending was corrected by rotating each image to the same orientation as that of Figure 4a. Parts b-d of Figure 4 show the deformation of the nanograins highlighted in rectangle A. As can be seen, with increasing bending deflection, an individual nanograin in circle 1 rotates and deforms from a straight bar (Figure 4b) to an S shape (Figure 4d) to accommodate the applied bending deformation. In addition to the rotation and deformation of individual nanograins, spacing between the nanograins highlighted in circle 2 is also observed (See panels b-d of Figure 4). The biopolymer spacing between the nanograins facilitates the grain rotation. Parts e-g of Figure 4 show that the grain rotation and deformation occur between two aragonite platelets. Grain rotation and deformation were also clearly observed, as shown in circle 3. With an increase in the Nano Lett., Vol. 6, No. 10, 2006
Figure 4. Deformation of the nanograins inside and between the aragonite platelets at the tensile part of a nacre sample subjected to three-point bending. (a) An image showing the observed area. (b-d) The nanograins inside the aragonite platelet highlighted by rectangle A in (a) with different bending deflections: (b) 50 µm; (c) 350 µm; (d) 460 µm. In (b-d) rotation of an individual nanograin is highlighted by circle 1 while spacing between the nanograins is highlighted in circle 2. (e-g) Nanograins between two aragonite platelets highlighted by rectangle B in (a) with different bending deflections: (e) 50 µm; (f) 350 µm; (g) 460 µm. In (e-g) rotation of an individual nanograin is highlighted in circle 3. Sliding of nanograins occurs at circle 4. Spacing between the platelets is highlighted in circle 5. The arrows next to (a) denote the bending tensile strain direction.
bending deflection, spacing between the two platelets increases (highlighted in circle 5). This allows a relative sliding between the two nacre platelets, which can be clearly observed by comparing the regions highlighted by circle 4 in parts e-g of Figure 4. The grain rotation and deformation mechanisms in nacre aragonite platelets can be summarized by Figure 5. With no external applied strain/stress, nanograins with irregular shapes are originally packed very closely by the biopolymer adhesives to form a robust structure (as shown in Figure 5a). Upon tension, the biopolymer between the nanograins is stretched in the tensile direction, which allows enough space for certain grains to rotate. Since the shape of these nanograins is normally irregular, the rotation of individual nanograins will push their neighbor grains apart, thereby resulting in an increase in the spacing between the rotated nanograins and their neighbor grains (as shown in Figure 5b). The spacing behavior between the nanograins within an aragonite platelet causes the aragonite platelet to expand in the direction perpendicular to that of the applied strain/ stress (Figure 5b). This reflects in fluctuation of the width of an aragonite platelet under tension over its original width (Figure 2g). Meanwhile, during grain rotation, the contact and shear between adjacent grains lead to grain deformation. The aragonite nanograins are not brittle but ductile. A similar characteristic has also been observed in Cu2O nanocubes that 2303
of the Government and no official endorsement should be referred. References
Figure 5. Schematics of grain rotation and deformation mechanisms in an aragonite platelet. Dashed lines denote the original width of the platelet. (a) Closely packed nanograins without external applied strain/stress. (b) Grain rotation, grain deformation, and biopolymer spacing between the grains under external applied strain/ stress. Arrows denote the rotation direction of grains. D in the figure denotes grain deformation. The blue arrows under the pictures denote the tensile direction.
exhibit a high deformability while its bulk form is brittle.21 The grain rotation and deformation are responsible for the high deformability of individual aragonite platelets discovered in literature4 and are the two important mechanisms contributing to energy dissipation in nacre. Acknowledgment. This work was supported by the National Science Foundation (Grant No. EPS-0296165), the South Carolina Space Grant ConsortiumsNASA, the South Carolina EPSCoR Grant, the ACS Petroleum Research Fund (ACS PRF# 40450-AC10), and the University of South Carolina NanoCenter Seed Grant. The content of this information does not necessary reflect the position or policy
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NL061775U
Nano Lett., Vol. 6, No. 10, 2006
