Microstructural Characterizations And Mechanical Behavior Of Cristaria Plicata Shell

After thousands of years of evolution,various kinds of unique self-assembled structures have been constructed in shells;these structures are generally unreachable in engineering materials.Nacre(mother-of-pearl),which is found in the shinny interior of many mollusk shells,has fascinated materials scientists and engineers over decades due to its superior mechanical properties.In the present work,Cristaria plicata shell with a hierarchical structure was selected as the target material,and its microstructures,phase compositions,Vickers’hardness,compressive behavior and three-point bending behavior were tested by using a variety of analysis and testing techniques,focusing on the relationship between microstructures and mechanical properties.This study will pave the way for the development of high-performance biomimetic materials.The microstructures on the surface and cross section parts of the Cristaria plicata shell were observed by scanning electron microscopy(SEM).At room temperature,the Cristaria plicata shell is a kind of nacre structure with a“brick and mortar”morphology,and the platelets in the nacre structure are arranged parallel to the outer surface.There are some intrinsic structural defects observed in this shell,including sheet stacked structure,"dovetail"structure,uneven sheet thickness,natural holes and other defective structures.X-ray diffraction(XRD) and thermodynamic analyses of the powder samples of Cristaria plicata shell demonstrate that the whole shell is composed of aragonite(CaCO3).The diffraction peaks of the block samples reveal that the preferred orientation of aragonite calcium carbonate crystals is(002)crystallographic plane.Thermodynamic analysis results show that the water in the nacre layers is～0.8%by weight,and the organic is～3.2%,which begins to decompose at 200℃.The morphologies on the surfaces and cross sections of heat treated samples at different temperatures were observed by SEM.Some small holes were found on the cross sections of the platelets after a heat treatment at 100℃ for 30 min.The organic matrix loses its activity and begins to decompose at 200℃,since the smell of burnt feathers can be strongly smelled when the samples are heat treated at such a temperature.Some interfaces between the sheets tend to fracture,and the number of holes on the platelets with a loose feature increases.As the temperature increases,the number of pores continues to increase,and some macrocracks were observed on the surface of nacre.The microhardness on the plane and cross section samples of Cristaria plicata shells has been measured by Vickers’ hardness tests.It is found that the hardness on the surface of nacre is～8.3%higher than that on the cross section,indicating that the microhardness of the nacre structure exhibits an anisotropic feature.The indentation was surrounded by some microcracks,and some toughening mechanisms such as crack deflection and mineral bridge during crack propagation were observed.The hardness value of surface samples heated at 100℃ is 200 ± 9.7 HV,which is～16.5%lower than that at room temperature.Some platelets were broken into small pieces around the indentation.However,the indentation morphology shows a pit-like shape on the surface of the sample heated at 200℃,which indicates that the organic matrix plays the most important role in the mechanical properties of nacre.Compressive tests were performed on the samples of Cristaria plicata shell,and the results reveal that the compression behavior of nacre structure still shows an anisotropic characteristic.The stress-strain curve is jagged when the loading direction is parallel to the sheet direction(Orientation A),indicating that local cracking,or even multiple crackings have occurred before reaching the maximum stress.The value of the compressive strength is the largest with the loading direction perpendicular to the sheet direction(Orientation B),along which the samples show the characteristics of brittle fracture.The results of Weibull statistics show that the compressive strength along the orientation B is 96%higher than that along the orientation A.The three-point bending mechanical behaviors of Cristaria plicata shell samples heat treated at different temperatures were investigated.It is found that,with the increase of temperature,the bending strength decreases,and a sharp decrease occurs at 200℃.It is indicated that the major factor influencing on the bending properties of shells is the existence of organic matrix.The fracture surfaces are relatively flat at 100℃.Interface cracking between the platelets commence at 200℃,and there are also some microcracks on the platelets.