Mechanical properties of modern calcite- (Mergerlia truncata) and phosphate-shelled brachiopods (Discradisca stella and Lingula anatina) determined by nanoindentation

We measured distribution patterns of hardness and elastic modulus by nanoindentation on shells of the rhynchonelliform brachiopod Mergerlia truncata and the linguliform brachiopods Discradisca stella and Lingula anatina. The rhynchonelliformea produce calcitic shells while the linguliformea produce chitinophosphatic shells. Dorsal and ventral valves, commissure and hinge of the calcitic shell of M. truncata show different nanohardness values (from 2.3 to 4.6 GPa) and E-modulus (from 52 to 76 GPa). The hardness of the biocalcite is always increased compared to inorganic calcite. We attribute the effects to different amounts of inter- and intracrystalline organic matrix. Profiles parallel to the radius of curvature of the valves cutting through the different layers of shell material surprisingly show quite uniform values of nanohardness and modulus of elasticity. Nanoindentation tests on the chitinophosphatic brachiopods D. stella and L. anatina reflect the hierarchical structure composed of laminae with varying degree of mineralization. As a result of the two-phase composite of biopolymer nanofibrils reinforced with Ca-phosphate nanoparticles, nanohardness, and E-modulus correlate almost linearly from (H = 0.25 GPa, E = 2.5 GPa) to (H = 2.5 GPa, E = 50 GPa). The mineral provides stiffness and hardness, the biopolymer provides flexibility; and the composite provides fracture toughness. Gradients in the degree of mineralization reduce potential stress concentrations at the interface between stiff mineralized and soft non-mineralized laminae. For the epibenthic chitinophosphatic D. stella the lamination is also present but less pronounced than for the infaunal L. anatina, and the overall distribution of material strength in the cross-sectional profile shows a maximum in the center and a decrease towards the inner and outer shell margins (modulus of elasticity from 30 to 12 GPa, hardness from 1.7 to 0.5 GPa). Accordingly, the two epibenthic forms, calcitic M. truncata and chitinophosphatic D. stella display fairly bulky (homogeneous) nanomechanical properties of their shell materials, while the burrowing infaunal L. anatina is distinctively laminated. The strongly mineralized laminae, which provide the strength to the shell, are also brittle, but keeping them as thin as possible, allows some bending flexibility. This flexibility is not required for the epibenthic life style.