Pervasive nanoscale deformation twinning as a catalyst for efficient energy dissipation in a bioceramic armour

Journal name:
Nature Materials
Year published:
Published online


Hierarchical composite materials design in biological exoskeletons achieves penetration resistance through a variety of energy-dissipating mechanisms while simultaneously balancing the need for damage localization to avoid compromising the mechanical integrity of the entire structure and to maintain multi-hit capability. Here, we show that the shell of the bivalve Placuna placenta (~99 wt% calcite), which possesses the unique optical property of ~80% total transmission of visible light, simultaneously achieves penetration resistance and deformation localization via increasing energy dissipation density (0.290 ± 0.072 nJ μm−3) by approximately an order of magnitude relative to single-crystal geological calcite (0.034 ± 0.013 nJ μm−3). P. placenta, which is composed of a layered assembly of elongated diamond-shaped calcite crystals, undergoes pervasive nanoscale deformation twinning (width ~50 nm) surrounding the penetration zone, which catalyses a series of additional inelastic energy dissipating mechanisms such as interfacial and intracrystalline nanocracking, viscoplastic stretching of interfacial organic material, and nanograin formation and reorientation.

At a glance


  1. Microstructural/crystallographic features and mechanical behaviour of biogenic calcite in Placuna placenta in comparison to single-crystal geological calcite.
    Figure 1: Microstructural/crystallographic features and mechanical behaviour of biogenic calcite in Placuna placenta in comparison to single-crystal geological calcite.

    a, Schematic diagram (not to scale) of the foliated microstructure in P. placenta. ‘L’ refers the longitudinal direction of the laths. b, Tilting angles of c axes of the calcitic laths with respect to the surface normal in the shell of P. placenta (black, as reported in ref. 14). A single-crystal calcite sample was sectioned and polished along one of the {108} planes (red). Standard interplanar angles between {001} and {104}, and {001} and {108} planes in calcite are also indicated. c, Loading portions of multiple individual load–depth curves (conospherical diamond tip, semi-angle = 30°, tip radius = ~1μm, maximum load = 10 mN).Fi anddi are the load and depth corresponding to the initial fracture event detected, and Δd represents ‘pop-in’ depth. d,e, SEM images of indentation residues of P. placenta (d) and calcite (e). Three-dimensional parameters are defined: Ri, the radius of the inner indentation crater;Ro, the radius of the entire fracture pattern by fitting it with the smallest circle; C, the distance between the centres of the two fitted circles. f, Comparison of the mechanical parameters of P. placenta and calcite. The values corresponding to calcite are normalized to 1. EO−P, 1 = 73.42 ± 1.74 GPa;HO−P, 1 = 2.34 ± 0.10 GPa;hmax, 1 = 1.53 ± 0.13μm;hdef, 1 = 6.96 ± 1.29μm;Ro, 1 = 5.94 ± 0.88μm; C/Ro, 1 = 0.54 ± 0.06;Fi, 1 = 5.34 ± 1.35 mN;di, 1 = 675.2 ± 167.0 nm;Δd, 1 = 129.0 ±  192.1 nm; ΔEdiss, 1 = 8.77 ± 1.71 nJ;Vdef, 1 = 257.2 ± 94.2μm3;ediss, 1 = 0.034 ±  0.013 nJ μm−3. Avg. s.d., average standard deviation; hmax, maximum depth.

  2. Nanoscale deformation twinning in P. placenta.
    Figure 2: Nanoscale deformation twinning in P. placenta.

    a, TEM image of the entire cross-section of the indentation zone (conospherical tip; semi-angle = 30°; tip radius = ~1μm; maximum load = 10 mN). The yellow dashed line marks the boundary between the plastically deformed region close to the indentation tip and surrounding undeformed regions. White arrows indicate the location of deformation twins. Inset: Top-view SEM image of the original indentation residue. The yellow solid line indicates the location and orientation of the TEM sample prepared by FIB. Pt, protective platinum layer. b, TEM image showing deformation twinning bands with parallel boundaries running across the laths. White arrows indicate the interfacial openings associated with the twinning bands. c, Corresponding SAED patterns in the matrix (top) and twinned (bottom) regions with zone axis = [8̄81̄]. ‘Matrix’ in b,c refers to untwinned regions of the calcitic laths that maintain the original crystallographic orientation19, 20. d, HRTEM image of the twinning boundary (TB) of {018}. e,f, Top-view (e) and cross-section-view (f) SEM images of microindentation residue (conospherical tip; semi-angle = 30°; tip radius = 2μm; maximum load = 500 mN). g, TEM image of multiple deformation twinning bands within the deformed zone shown in f. h,i, SEM images of deformation twinnings in multiple orientations induced by manually compressing the shell with a mortar and pestle. j, Schematic model of the three crystallographically equivalent {018} twinning systems in calcite. ‘L’ refers to the longitudinal direction of the laths. In all figures the symbol ⊗ indicates the orientation of L is into the page.

  3. Nanoscopic inelastic deformation in individual calcitic layers of P. placenta.
    Figure 3: Nanoscopic inelastic deformation in individual calcitic layers of P. placenta.

    a, Bright-field TEM image of the permanently deformed region close to the indentation crater. Inset: Top-view SEM image of the original indentation residue. The yellow line indicates the location and orientation of the TEM sample. Pt, protective platinum layer. b,c, SAED patterns acquired in the deformed (b) and surrounding undeformed (maintaining original single crystal structure, c) regions indicated by the circles in a. d, Dark-field TEM image of the deformed region (corresponding to the region in the white box in a) with the corresponding selected diffraction spots indicated in b with the red circle. e, HRTEM image of misoriented calcite nanograins in the permanently deformed region. f, Tapping mode AFM height image of an indent corner (Berkovich tip) showing the flattening of nanoscopic asperities within an indentation crater. g,h, Bright-field TEM images showing crack deflection within individual laths (white arrows).

  4. Nanoscale deformation mechanisms in P. placenta and single-crystal calcite under indentation.
    Figure 4: Nanoscale deformation mechanisms in P. placenta and single-crystal calcite under indentation.

    a,b, Schematic of the deformation zone close to the indenter for P. placenta (a) and single-crystal calcite (b; conospherical tip; semi-angle = 30°; tip radius = ~1μm; maximum load = 10 mN). The diagrams are drawn to scale based on microscopic dimensional measurement. Dislocation arrays in calcite are not shown in b. c, TEM images illustrating the progression of nanoscale deformation mechanisms of P. Placenta(listed below the images) with decreasing distance from the indenter. All scale bars, 100 nm.


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  1. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Massachusetts 02139, USA

    • Ling Li &
    • Christine Ortiz


L.L. and C.O. designed the research, analysed the data and wrote the manuscript. L.L. conducted the experiments

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