Hierarchical structure and length-scale dependent structure-property correlations in the organ pipe coral (Tubipora musica).

The skeleton of Tubipora musica, also commonly known as the organ pipe coral, is made up of calcium carbonate and serves as a habitat for small sea creatures called polyps. The present paper provides a comprehensive study on the hierarchical structure and micromechanical properties of the organ pipe coral skeleton. The hierarchical structure of the coral skeleton was probed across multiple length scales using a combination of X-ray microcomputed tomography and scanning electron microscopy. At the macroscale, the structure of the coral consisted of vertical tubes connected by horizontal platforms. On the other hand, the microstructure comprises spherulites and an assembly of cells that were formed through a unique arrangement of plates of calcite. This unique arrangement of fibres and plates resulted in varying microstructural morphologies on the surface of the coral skeleton. Nanoindentation was conducted at multiple load regimes to investigate mechanical properties of coral's hierarchical structure. At smaller indentation depths, Young's modulus and hardness increased with indentation depth due to densification of the porous structure. At larger indentation depths, multiple damage mechanisms were observed, such as crack deflection and secondary crack formation.