Study On Tribological Mechanism Of Dental Enamel At Micro And Nano Scale

As the hardest mineral tissue in human body,enamel covers the surface of the tooth and has excellent performance to resist wear and fracture.Even after millions of chewing cycles,enamel disperses high chewing contact pressure with relatively limited damage rather than failure.The excellent mechanical properties of enamel have a close relationship with its orderly structure and complex chemical composition.With the rapid development of microscope technique and nanoindentation,the multi level structure and the mechanical properties of dental enamel have received a great deal of attention in recent years.However,lots of the previous studies on the mechanical properties and tribological behaviors of dental enamel mainly referred to the existing abrasion theory of metal materials.Few of them were based on the unique microstructure and inner bonded manner of enamel.In addition,the fundamental building blocks of enamel,hydroxyapatite crystallites,are at nanoscale.There have been considerable researches on the mechanical properties and tribological behaviors of tooth enamel at macro and microscales,but less attention has been paid to damage mechanism of enamel at nanoscales.So we need deep studies to understand the microtribological behaviors and mechanism of enamel at micro and nano scales based on its microstructure and chemical composition,and further establish the unique abrasion theory of enamel accordingly.The relevant research will not only enrich the basic tribology theory of enamel,but also lead new insight into how nature selects for tooth structure and provide new ideas to bioinspired designs.In this doctoral thesis,the microtribological behaviors of human tooth enamel at micro and nano scales are systematical studied.Firstly,the abrasive wear behavior of enamel by soft abrasives was investigated with a servo-hydraulic reciprocating sliding apparatus.The wear condition of enamel was investigated by an atomic force microscopy(AFM).Secondly,an understanding of how human teeth respond to the stresses associated with chewing at nanoscale was revealed by AFM and nanoindentor.The damage mechanism of a single HAP fibre was analyzed and a new model of nanoscale responses of enamel to chewing was built.Finally,with a high resolution scanning electron microscope(SEM),two orientations of enamel hydroxyapatite(HAP)fibres were clearly observed.The effect of HAP fibre orientations on the mechanical properties and microtribological behavior was studied by a nanoindentor.Based on these systematical investigations,the main conclusions can be summarized as following:(1)The effect of soft particles on the abrasive wear mechanism of dental enamel was revealed.The aluminum and brass spheres,which have hardness much lower than enamel,were choosen to conduct the wear tests.The effect of soft particles on the abrasive wear mechanism of dental enamel under one cycle was studied and the wear condition of enamel was revealed.The inner mechanism on the wear of enamel by soft particles was explained.The results indicated that enamel can be abraded by soft particles with tissue removal under one cycle.This demonstrated the wear of enamel by soft particles under one cycle mainly showed as abrasive wear and adhesive wear.Hydroxyapatite crystallites are “glued” together by proteins,and tissue removal requires only that contact pressure is sufficient to break the bonds holding enamel together.The research of microwear on dental enamel provides a reference for the following research at nanoscale.This also has important implications for understanding the relationship between micro wear and diet and would make it possible for the diet reconstruction of paleontology.(2)Three damage behaviors of enamel responses to chewing loads at nanoscale were researched and a new model of nanoscale responses of enamel to chewing was built.It was discovered that HAP crystallite responses are complex and multifaceted,and depend on the magnitude of applied force and load method: wear with “plucking”,deformation with compression and refinement of HAP particles.Critical contact pressure to initiate wear was the lowest,followed by deformation,and then refinement.Firstly,when the applied load exceeded the strength of the bonding protein “glue”,the HAP particle would be plucked from the surface,formed wear.Then,the applied load exceeded the strength of the bonding protein “glue” in the vertical direction,for the sufficient space within the enamel matrix,the HAP fibres would be compressed into the underside,formed deformation.Finally,when the applied load exceeded the strength of H-bonds that bind smaller nanoparticles into HAP particle,the particle would be fragmented to smaller ones,formed refinement.Further study showed that,lower contact pressures are required for a response with nanoscratching tests than nanoindentation tests.This study built the new nanowear theory for enamel.(3)The effect of HAP fibre orientation on mechanical properties and microtribological behaviors of dental enamel after slight erosion was demonstrated.After 1 min slight erosion by 0.001 M citric acid,the arrangements of HAP fibres on enamel surface was observed and the effect of HAP fibre orientation on mechanical properties and microtribological behaviors of dental enamel was demonstrated.As a result,at microscale,the orientations of HAP fibre in enamel rods and inter-rods enamel were different in occlusal surface and axial section.The arrangements of HAP fibres significantly influenced the mechanical and microtribological behavior of human tooth enamel.HAP fibres with long axes parallel to the surface had higher hardness and elastic modulus and showed better microtribological behavior.The force analysis of the different aligned HAP fibre based on the micro and nano wear mechanism of enamel was proposed to explain the experimental mechanism.This study futher enriches the theory of relationship between the structure and propertiey and the nano wear mechanism of enamel and provides the theoretical basis for the clinical dental prevention.In summary,as a natural complex material,enamel has its unique wear mechanism.The research of tribological behaviors of dental enamel at different scales would not only enriches the basic tribology theory for dental materials,but also provides the theoretical basis for the bioinspired design and clinical dental prevention.