Investigations On The Size-Dependent Mechanical Behavior Of Nacre

During billions of years of evolution,the biological materials have successfully integrated two types of materials,inorganic minerals with high strength but low ductility and organic matrix with good ductility but bad strength,into complicated hierarchical structures by self-assembly,thus showing excellent comprehensive performances.Nacre,as one of the most important example in mollusk shells,is composed of?95 wt.%brittle inorganic CaCO3 and?5 wt.%organic materials,and it exhibits an attractive combination of high strength,toughness and stiffness,which cannot be matched by the artificial materials.Therefore,it is vitally necessary to explore the relationship between the microstructures and mechanical properties of nacre,to provide additional guidelines for designing and synthesizing high-performance biomimetic materials.In the present work,the microstructures and related mechanical properties of nacre in two kinds of shells in different water depth are investigated by structural characterizations and mechanical experiments.It is expected to reveal whether the water depth has influences on the microstructures and phase composition of nacre,and how the specimen dimension and platelet dimension affect the mechanical properties of nacre,thus offering beneficial references for further improving the performance of nacre-like materials.The microstructures and phase composition of nacre in the deep-sea Nautilus pompilius shell and shallow-water Cristaria plicata shell were investigated.It is found that the water depth in which the mollusk shells live has no effect on the basic process of biomineralization,but show an obvious influence on the microstructures of nacre.The results show that the inorganic composition of nacre in both N.pompilius and C.plicata shells are aragonite CaCO3.However,there are obvious differences in the platelet dimension of nacre between the two kinds of shells.Therefore,the harsh environment in the deep sea results in a slower rate of platelet deposition,thus causing thinner and smaller platelets of the deep-sea N.pompilius shell,but have no influence on the products of biomineralization.Three-point bending tests on a series of specimens with different thicknesses in C.plicata shell were performed,and the results show that the mechanical behavior of nacre exhibits an obvious macroscopic size effect.The value of bending strengths increases and then becomes roughly stable with increasing specimen thickness.Moreover,the mean value of work per volume rises continually with the increase of specimen thickness.Furthermore,as the specimen thickness increases,the crack traveling mode changes from penetration into the platelets into deflection along the interfaces,causing that the main failure mode of specimens is transferred from platelet fracturing into platelet pulling-out.The three-point bending tests on the nacre specimens with the same thickness in the two kinds of shells were conducted,and it is found that the mechanical behavior of nacre shows a noteworthy microscopic size effect.The value of bending strength of deep-sea shell is?42%larger than that of shallow-water shell,and the mean value of the work per volume of specimens of N.pompilius shell is about four times as that of C.plicata shell.Moreover,it is found that the cracks in N.pompilius shell deflect along the interfaces,but penetrate the platelets directly in C.plicata shell.According to the microcracking model proposed in this thesis,both the macroscopic and microscopic size effects of the mechanical behavior of nacre are closely related to the number of inter-layer interfaces.The more the number of inter-layer interfaces is,the higher the strength and the better the toughness of nacre under bending tests will be.However,as the number of inter-layer interfaces reaches a certain value,the value of strength tends to be stable,while the toughness is still getting better.Therefore,the present results can pave a way for designing strong and tough nacre-like materials with an appropriate dimension.The impact of the nano-asperities distribution density and dimension on the friction between adjacent platelets of nacre was analyzed quantitatively.It is indicated that the friction between platelets of nacre has an obviously positive correlation with the distribution density and dimension of nano-asperities on the surface of platelets.Compared with those in the shallow-water C.plicata shell,the distribution density and the dimension of nano-asperities on the platelet surfaces in the deep-sea N.pompilius shell are higher.Consequently,the friction between the platelets of nacre caused by the nano-asperities in the N.pompilius shell is far higher than that in the C.plicata shell,hence giving rise to the better bending strength and toughness of nacre in the N.pompilius shell.The mechanical properties of deep-sea N.pompilius shell and shallow-water C.plicata shell under bending tests were compared.It is suggested that the mollusk shells have evolved the most adaptive microstructures and mechanical properties to survive in their living environment through hundreds of million years.For example,the slightly lower density and higher open porosity of nacre in the deep-sea N.pompilius shell are beneficial for the shell to swim up and down in the sea more freely.Besides,compared with that of the shallow-sea shell,the variation of the bending strength is lower,which will help the shell inhabit in the deep sea with particularly high pressure,since there is less weak points.In addition,the thinner and smaller platelets with numerous nano-asperities on the surface are conductive for the shell to exhibit better mechanical performances,thus contribute to living in the deeper sea with a harsh environment.Therefore,the systematic study on the nacre can provide new designing ideas for synthetic high-performance nacre-inspired materials.