Hierarchy structure and fracture mechanisms of the wild wolf tusk's enamel

The self-growth and self-strengthening of natural biomaterials provided us strategies for new materials design. In this paper, the microstructure and fracture mechanisms of the wild wolf tusk's enamel were studied. The enamel included four-order hierarchies, which were the hydroxyapatite (HAP) fiber (first-order, nano-scale, ploy-crystals), enamel rod (second-order, micro-scale, rope-like), enamel type (third-order, meso-scale, mat-like) and the enamel patterns (forth-order, macro-scale), respectively. It was interesting to find that the numerous nano-grains distributed disorderly in a single HAP fiber. The thousands HAP fibers bundled together to form the rope-like enamel rod. The protein ligaments were discovered between adjacent enamel rods. The out enamel, inner enamel and P&D-zones showed a criss-cross type and ran through whole enamel pattern in three-dimensional space. The enamel of the wild wolf tusk exhibited an excellent fracture toughness based on the nanoindentation tests. The fracture morphology in transverse direction indicated that the cracks preferred to propagate along the weak interface (protein or interrod) and cut those enamel rods perpendicular to the propagation direction. However, the cracks extended obviously forward along the step-like paths from the outmost surface of the enamel to the enamel-dentin junction in the longitudinal direction. It was considered that the protein ligament was the main reason for the good fracture toughness of the bulk enamel. Our studies reveal that the design strategies of the natural material can be applied to guide the development of high-performance artificial materials.