Crystal interface mechanics of cold-water coral skeletons

Cold-water coral (CWC) skeletons utilise aragonite crystals, which possess exceptional strength (4-6 GPa), to construct a composite skeletal framework that is significantly less strong (~0.5 GPa) yet remarkably resilient. The interfacial processes governing this transition from strong crystal to tough composite remain unclear, suggesting that the organic interfaces between crystals, rather than crystal defects, control the macroscopic mechanical behaviour. We systematise the investigation of this phenomenon by employing Molecular Dynamics simulations, which reproduce experimental elastic constants within 5%. We characterise three limiting interface compositions: dry twin aragonite boundaries, hydrated protein-mediated interfaces, and water interfaces - by applying tensile and shear loading and quantifying their mechanical competence using a three-dimensional failure criterion. Interface composition determines the mechanical hierarchy, spanning nearly an order of magnitude in strength. Dry twin boundaries provide the highest strength, reaching up to 6.5 GPa, through direct crystalline bonding. Conversely, protein-mediated interfaces exhibit the lowest strength (0.5-0.7 GPa) but demonstrate high damage tolerance. Water-mediated hydrogen-bonding networks enable this progressive failure, dissipating energy through sequential bond rupture and reformation rather than catastrophic separation. Water interfaces show thickness-dependent compliance: thin layers (<5[A] ) retain partial electrostatic coupling (>3 GPa), while thick layers allow controlled sliding (~0.8 GPa). These quantitative structure-property relationships provide transferable parameters for multiscale coral modelling, enabling researchers to bridge atomistic mechanisms with mesoscale mechanical response. The findings reveal how skeletal hierarchies integrate strength, stiffness, and energy dissipation, offering potential design princi-ples for biomimetic composites that reproduce the tunable mechanical properties of biomineralised materials. Statement of SignificanceCold-water coral skeletons are built from strong aragonite crystals but fail at much lower stresses than the crystals themselves. We used computer simulations at atomistic scale to understand why. Our study reveals that the interfaces between crystals, rather than the crystals control skeleton strength. We identified three interface types with different behaviours: crystalline twins are strongest and accommodate deformation through boundary migration, proteins with water layers are weaker but absorb energy through progressive failure, and water layers provide tunable properties depending on thickness. These findings explain how natural materials combine strong building blocks with compliant interfaces to achieve toughness. Our quantitative measurements provide design parameters for both predicting coral resilience to environmental stress and creating synthetic materials inspired by this hierarchical architecture.