Study On The Tribological Behavior Of Typical Mammalian Tooth Enamel

After a long period of evolution,mammalian teeth have formed unique geometric shapes and anatomical structures.Compared with human beings,other mammals have rougher diets and harsher chewing conditions,but the service time of their teeth is still almost equal to their life span.Therefore,it is necessary to reveal the tribological mechanisms of typical mammalian teeth.The findings will enrich and develop the theoretical system of bio-tribology,and provide valuable insights into the bionic designs of medical devices and engineering equipment parts.By using the nano-indentation/scratch techniques,reciprocating friction and wear tests,and many microscopic examinations,the mechanical properties and tribological behavior of typical mammalian tooth enamel including bovine(herbivore),porcine(omnivore),and canine(carnivore)enamel were studied in vitro.The main conclusions are summarized as follows:(1)There exists an adaptation in the microstructure and tribological performance of mammalian enamel with mammalian feeding habits.The enamel rods and interrod in bovine enamel are aligned in a lamellar pattern,and the hydroxyapatite(HAP)nanofibers in the rod and interrod are highly ordered and in decussation.This microstructure endues bovine enamel occlusal surface with high hardness and good resistance against crack propagation,which effectively reduces the risk of excessive wear and fatigue fracture of bovine molars which are subjected to prolonged chewing(about 10 hours per day).Porcine/canine enamel has a microstructure consisting of hard keyhole-like rods embedded in soft interrod enamel,and the fibers are approximately perpendicular to the occlusal surface in the enamel rods and tend to be a little angled and slightly unorderly in the interrod enamel.As a result,the enamel of porcine and canine has a high load tolerance,which is suitable for the high biting force of omnivores/carnivores(up to 2 kg).(2)Different enamel rod sizes lead to different fracture behavior in porcine and canine enamel.The microstructure of hard rods embedded in soft interrod enamel endows porcine/canine enamel with the toughening mechanisms of crack deflection and bridging.The rods have smaller size for porcine enamel than for canine enamel;thus,porcine enamel has more rod/interrod interfaces and interrod enamel.As a result,the cracks in porcine enamel are more prone to initiate,deflect,and converge,as compared to canine enamel,which causes the material removal by delamination occurring more easily on the worn surface of porcine enamel.Hence,porcine enamel is inferior to canine enamel in wear resistance.(3)The crack growth behavior is related to the micro-nano structure of enamel.Cracks on bovine enamel occlusal surface tend to initiate in the rod/interrod boundaries.The rod/interrod HAP nanofibers in decussation is responsible for the preferential propagation of the cracks along the rod/interrod interface by inducing crack deflection,bifurcation,and bridging.And the staggered HAP nanofibers in the interrod enamel play an important role in hindering cracks from traversing the interrod through a toughening mechanism of crack zigzag.The unique alignment manner of HAP nanofibers functionally guides the cracks on bovine enamel occlusal surface to grow along the rod/interrod interface rather than across the interrod enamel.The anisotropic crack growth behavior helps prevent the enamel from fractue and chipping.(4)The corrosion/tribocorrosion behavior of mammalian enamel is significantly correlated with its surface microstructure.The acid agents tend to penetrate into the enamel along the straight interfaces between the longitudinally arranged HAP nanofibers,when enamel surface encounters acid corrosion,while the transversally arranged HAP nanofibers with zigzagged interfaces inhibit the penetration of acids inward,causing the occurrence of corrosive damage only in the surface.Clearly,apart from the chemical composition,the microstructure of enamel also involves in its surface corrosion.HAP nanofiber orientation changes the acid permeability into the enamel surface.Transversal nanocrystallites have the potential to enhance the corrosion resistance of mammalian teeth and then prevent excessive tooth wear.