On The Anti-wear Mechanism Of Human Tooth Enamel And Its Inspiration To The Design Of Dental Ceramics

Enamel,the outermost layer of teeth,is the hardest tissue in the human body and is primarily intended to bear wear in everyday mastication.The acellular enamel of mature human teeth has limited capacity to self-repair after damage and can partially or completely lose its physiological functions under the action of pathological factors.Ceramics are commonly used for dental restoration,but are much stiffer and harder than natural enamel,thus often inducing severe abrasion and even damage of antagonist natural teeth.Therefore,an urgent clinical need is to reduce the abrasive potential of dental ceramics to natural teeth,without sacrificing its wear resistance,and a promising way to achieve this goal is to design and optimize dental ceramics based on the anti-wear mechanism of enamel.The research on the anti-wear mechanism of enamel and the bionic design of dental ceramics would help to not only explore natural mysteries and develop biotribology,but also promote the development of advanced dental ceramics,which has great theoretical and practical significance.By using mechanical tests,nano-scratch tests,reciprocating sliding wear tests and relevant analysis and characterization,the roles of minor protein component and multi-level micro-nano interface in providing the wear resistance of enamel were studied.Then a novel ceramic matrix composite with inverse enamel-like microstructure was developed,and its mechanical and tribological properties were systematically characterized.Main conclusions are drawn as follows:（1）A crucial function of minor protein component in enamel is involved in the orderly assembly of hydroxyapatite（HAP）crystals into nano-fibrils via interfacial protein bonding.During friction process,the occurrence of enamel wear requires to break the interfacial protein bonding between HAP crystals in individual nano-fibrils and the break dissipates considerable energy,which benefits the enamel to resist wear.Without the presence of protein,the aggregation of HAP crystals spontaneously occurs and the wear of enamel is dominated by the integral removal of aggregations,which results in significant decrease in the wear resistance.By mediating the assembly of rigid HAP crystals,minor protein component plays a crucial role in providing the excellent wear-resistence of enamel.（2）The tribological response of enamel is strongly dependent on its multi-level micro-nano interface.After heating at 300℃,the rod/inter-rod micro-interface of enamel remained intact,the nanometers thick protein interface disappeared,and microcracks initiated easily on the worn surface under high sliding cycles/high loads conditions.After heating at 1200℃,the micro-nano interface of enamel were absent,straight microcracks and flaky delamination appeared on the worn surface under high loads.Apparently,the multi-level nano-micro interface is of extreme importance to protect enamel against brittle wear.The nano-interface in enamel plays a role in suppressing the initiation of microcracks during friction process by dissipating friction energy,and the micro-interface contributes to inhibiting crack propagation and delamination through deflecting microcracks.（3）Inspired by the anti-wear mechanism of enamel,an inverse enamel-like microstructure was designed.Compared with enamel-like microstructure,the inverse enamel-like microstructure has a higher load-bearing capacity,which helps to avoid the dramatic decreases in material stiffness due to the introduction of flexible substances.Then a ceramic composite with inverse enamel-like microstructure was prepared by utilizing the diffusion-controlled gelation of an alginate sol,ceramic sintering,and resin infiltration.The bio-inspired ceramic composite has excellent load-bearing capacity and crack deflection and crack tip blunting mechanisms.When the ceramic volume fraction is between 40.42 vol.%and 71.48vol.%,the elastic modulus of the bio-inspired ceramic composite is 35.11 GPa～140.56 GPa,the hardness is 1.93 GPa～5.09 GPa,the flexural strength is 110 MPa～220 MPa,and the fracture toughness is 2 MPa·m^1/2～4 MPa·m^1/2,which are comparable to those of natural enamel.（4）Inverse enamel-like microstructure has good load-bearing capacity to protect bio-inspired ceramic composites from resin–ceramic separation during friction process.Under the normal chewing load,the coefficient of friction of the bio-inspired ceramic composites is slightly lower than that of enamel,and the wear is dominated by slight plastic deformation,which is similar to that of enamel.Under the high chewing load,the bio-inspired ceramic composites have a higher coefficient of friction and comparable wear volume,as compared to enamel.Microcracks initiated in hard and brittle ceramic matrix of the bio-inspired ceramic composites.Under the unusually high chewing load,the coefficient of friction and wear volume of the bio-inspired ceramic composites are higher than those of enamel.Cracking of ceramic matrix and crushing of resin fillers appear on the worn surface of the bio-inspired ceramic composites.The bio-inspired ceramic composite with a ceramic volume fraction of71.48%has the best match to enamel in terms of coefficient of friction,wear volume,wear rate,and has low abrasive potential to enamel.