Hierarchical Structure And Mechanical Behavior Of The Two Natural Bioceramics-the Wild Tusk's Enamel

In this paper,the adult wild boar's and wild wolfs tusk with the attack as the main function were used as the research objects.The hierarchical structure of the two tusk's enamel and the effect of the hydration condition and temperature on the mechanical behavior of the enamel were comparatively investigated.Besides,the fracture mechanism and toughening mechanism of them were analyzed.The research results indicated:The enamel of the wild boar and wolfs tusk had the same four-level hierarchical structure.They were the hydroxyapatite(HAP)crystal fiber(first-level constituent);enamel rod(second-level constituent);enamel types(third-level constituent);enamel patterns(forth-level constituent).The maximum find in this paper was that the high-density nano-twins and high-order hierarchical nanotwins were found in the HAP fiber of the wild boar tusk's enamel,but numerous grains with a diameter of 5-10 nm in the HAP fiber of the wild wolf tusk's enamel.This results provided the new idea for the development of the artificial materials.The enamel of the two tusks showed the strong anisotropy.The wild boar tusk's enamel presented an excellent nanohardness,elastic modulus and fracture toughness along the parallel to the axis direction of the enamel rod.However,the wild wolf tusk's enamel showed a better nanohardness and elastic modulus,only the fracture toughness was slightly lower along the perpendicular to the axis direction of the enamel rod.Besides,the mechanical performances of the enamel were related to the hydration condition and protein content.In the enamel of the wild boar's tusk,the micro-hardness was characterized by the highest outer layer and the lower inner layer,the nano-hardness increased at first and then decreased,the elastic modulus increased at first and then kept plateau,while the fracture toughness showed an opposite tendency from the outer enamel surface to the dentine-enamel junction,which made that the enamel patterns presented a structure characteristic of the external strength and internal toughness.However,the nano-hardness and elastic modulus of the enamel of the wild boar's tusk decreased at different degree,but the fracture toughness and the creep ability increased no matter along the parallel or perpendicular to the axis directions of the enamel rod under hydration condition.The heat-treatment of the enam el of the wild wolfs tusk at different temperatures showed that the nano-hardness,elastic modulus and fracture toughness were allreduced with the inactivation of the protein,particularly along the perpendicular to the axis direction of the enamel rod.The research found that the crack deflection and bridging,crack propagation forward along the weak interrod and cutting the enamel rod off were the mian reasons with the high strength and toughness.Based on the results above,the enamel with the attack as the main function had excellent toughness for two reasons:one was that the protein existed in the adjacent HAP fibers and enamel rod reduced the force intensity factor at the tip of the crack;another was that the water molecule in the enamel acted as an effective lubricant to promote the flexibility of the macromolecule chain or segments.