Investigation on the Microstructures and Mechanical Properties of the Shells of Tridacna crocea

Researchers have discovered that the aragonite nanofibers are packed as highly ordered cross-lamellar structures in the shells of Giant clam Tridacna gigas and have excellent mechanical properties. However, differences in the microstructures and mechanical properties between different locations of the shells of Giant clams have not been reported. The microstructures of the shells and the relationship of the micro- and nanostructures and the mechanical properties of the different parts of the shells (main part and rib part) of another kind of Giant clam Tridacna crocea have not been reported yet. In this work, the multilevel ordered micro- and nanostructures in the main part and rib part of the shells of T. crocea were investigated in detail. It shows that both the main part and the rib part are composed of cross-lamellar structures, and the third-order structure (building units) are aragonite nanoribbons, which are densely packed as a self-locking mosaic. This is the first time to report that the building units for the aragonite cross-lamellar structures are nanoribbons instead of nanofibers, while the nanoribbons have particular structural features such as self-locking mosaic packing and dense packing phenomenon on the fractured section. Furthermore, we find that the angles between the long axes of the nanoribbons of the two sets of lamellar structures of the main part and the outer layer of the rib part vary from 120 to 100°, a novel structural feature for cross-lamellar structures. In addition, the mechanical properties such as flexural strength, compressive strength, hardness, and elastic modulus for the different cross sections of different parts of T. crocea were found to be different, and their relationship with the microstructures was studied and discussed. The flexural strength values of the horizontal planes of the main part and the outer layer rib part of the shells of T. crocea are 114.3 ± 2.49 and 118.0 ± 3.74 MPa, respectively, much higher than that of the longitudinal cross sections (79.0 ± 4.55 and 58.7 ± 1.25 MPa, respectively). The compressive strength tests show that the transverse cross sections of the main part and outer layer of the rib part have the highest compressive strength values (221.7 ± 4.82 and 153.3 ± 5.91 MPa, respectively), medium values for the longitudinal cross section (189.0 ± 7.87 and 126.3 ± 9.50 MPa, respectively), and the lowest values for the horizontal plane. The average hardness and elastic modulus values of the transverse and longitudinal cross sections of the rib part gradually increase from the outer to the inner layers. In this work, we show that the mechanical properties are strongly related to the different multilevel ordered assembly modes of micro- and nanostructures.