Microstructural Characterizations And Mechanical Behavior Of Scapharca Broughtonli Shells

Through natural selection over hundreds of millions of years,biological organisms have developed fantastic materials with outstanding mechanical behavior,e.g.,shells,bones,etc.Despite the fact that these materials consist of inorganic mineral and biological macromolecules,their mechanical properties are far superior to those for the traditional synthetic materials.Shell,as a kind of species over billions of years,are renewed for their complex microstructures and excellent mechanical properties.In the present work,a kind of natural biological material called Scapharca broughtonli shell was selected as the target material,its microstructures,crystal compositions,vickers hardness,three-point bending mechanical behavior were studied and discussed by using a variety of analysis and testing techniques,in order for providing some valuable reference data for the in-depth investigation on the structure and its relation to mechanical behavior of natural biological materials.The microstructures of the Scapharca broughtonli shell on the dorsal,body and marginal parts were observed,and it was found that the shell exhibits different structures on the three parts.Specifically,the inner layer of the dorsal side shows a complex crossed-lamellar structure on the dorsal side,a homogeneous structure on the body part,while the outer layer exhibits a crossed-lamellar structure on both three parts.On marginal side,the crossed-lamellar structure occupies all areas.The crossed-lamellar structure is composed of the first-order lamellae,which can be divided into the second-order lamellar.Furthermore,the third-order lamellae construct the second-order lamellae.XRD analysis of the various layers of the different parts of Scapharca broughtonli shells demonstrates that the components of different part are different.The ingredients of different parts of this shell are made of crystalline CaCO3(aragonite)except for the inner layer on the body part,which is formed by CaCO3(vaterite).Thus,the aragonite-crystalline CaCO3 is the major component of the Scapharca broughtonli shells.Samples produced from the marginal side were subjected to thermal treatments at 200,300,400,500,600,700,800,and 900 ℃,respectively,in order to evaluate the crystalline transformations of the calcium phosphate structures in the process of heating.As the powder samples were annealed at temperatures higher than 300℃,an irreversible phase transformation from aragonite to calcite occurred.As the temperature increases to as high as 700℃,the calcite CaCO3 starts to be decomposed into CaO.Microhardness has been measured on different parts with different orientations of shells,and it was found that the microhardness of the marginal side is the highest,the next is on the body part,and the lowest on the dorsal side.The microhardness decreases with increasing temperature,and the area of indent tends to become larger.The lost of the water and the organic matrix have greatly detrimental influences on the mechanical properties of the shell.The results of three-point bending tests showed that the mechanical properties of this shell shows an obvious anisotropy,i.e.,the fracture stress and the elastic modulus of specimens along the perpendicular direction are higher than those along the parallel direction,and the fracture stress and the elastic modulus of the marginal side are higher than those on the body part.Along the perpendicular direction,the major fracture mechanism is the fracturing of the first-order lamellae,which can absorb more energy.However,along the parallel direction,the major fracture mechanism is the sliding of the first-order lamellae.The ability of organic to arrest the cracks is obviously lower than that of the inorganic lamellae,resulting in a poor mechanical behavior.