Biological armors such as mollusk shells have long been recognized and studied for their values in inspiring novel designs of engineering materials with higher toughness and strength. However, no material is invincible and biological armors also have their rivals. In this paper, our attention is focused on the teeth of black carp (Mylopharyngodon piceus) which is a predator of shelled mollusks like snails and mussels. Nanoscratching test on the enameloid, the outermost layer of the teeth, indicates that the natural occlusal surface (OS) has much higher wear resistance compared to the other sections. Subsequent X-ray diffraction analysis reveals that the hydroxyapatite (HAp) crystallites in the vicinity of OS possess c-axis preferential orientation. The superior wear resistance of black carp teeth is attributed to the c-axis preferential orientation of HAp near the OS since the (001) surface of HAp crystal, which is perpendicular to the c-axis, exhibits much better wear resistance compared to the other surfaces as demonstrated by the molecular dynamics simulation. Our results not only shed light on the origin of the good wear resistance exhibited by the black carp teeth but are of great value to the design of engineering materials with better abrasion resistance.
Supports for this work from the Early Career Scheme (ECS) of Hong Kong RGC (No. 533312) and the Opening Fund of State Key Laboratory of Nonlinear Mechanics from Institute of Mechanics, CAS are acknowledged. HW acknowledges the financial support from National Natural Science Foundation of China (No. 11321202). The authors thank Dr. Zhiping Xu, Dr. Kaimin Shih for helpful discussions on MD simulation and XRD analysis, respectively. We also acknowledge Dr. Vengatesen Thiyagarajan's group and Dr. Xinrui Niu's group for providing assistance in FESEM imaging and nanoscratching tests respectively.
[Fu, Jimin; He, Chong; Yin, Qifang; Yao, Haimin] Hong Kong Polytech Univ, Dept Mech Engn, Kowloon, Hong Kong, Peoples R China; [Xia, Biao; Li, Yan; Feng, Xue] Tsinghua Univ, Dept Engn Mech, AML, Beijing 100084, Peoples R China; [Xia, Biao; Li, Yan; Feng, Xue] Tsinghua Univ, Ctr Mech & Mat, Beijing 100084, Peoples R China; [Feng, Qiong; Wang, Hongtao] Zhejiang Univ, Inst Appl Mech, Hangzhou 310027, Zhejiang, Peoples R China; [Shi, Xinghua] Chinese Acad Sci, Inst Mech, State Key Lab Nonlinear Mech, Beijing 100190, Peoples R China