Investigations On The Effect Of Heat Treatment On Mechanical Properties Of Nacre And Crossed-lamellar Structures In Shells

High strength and high toughness are usually mutually exclusive in most engineering materials,that is,the higher the strength,the lower the toughness is.However,as a kind of natural ceramic material,mollusk shells possess both high strength and toughness.The fantastic mechanical properties of the shell materials rely on the complexly hierarchical structure consisting of calcium carbonate and a little organic substance.Despite the fact that the content of organic substance is very low,it can still play an extremely important role in the fracture resistance of shell materials.As the organic substance is sensitive to temperature,the role of organic substance on the nacre and crossed-lamellar structures in shell materials were explored by heat treatment in the present work,and Cristaria plicata and Cymbiola nobilis shells were selected as the experimental materials.The effects of organic substance on the mechanical properties of two kinds of structures in mollusk shells were investigated by a series of structural characterizations and mechanical experiments on unheat-treated fresh shells and heat-treated shells.The microstructures of fresh samples and heat-treated samples(at 100?,200?,300?,400?)of C.plicata shell were investigated by scanning electron microscopy(SEM),and it is found that the C.plicata shell presents a sheet nacre structure.At room temperature(RT),the brick-and-mortar structure of nacre is composed of tiles of polycrystalline aragonites and organic substance with a uniform structure and regular arrangement.After a heat treatment at 100?,a small number of tiny holes appear on the aragonite tiles,and with the increase of heat-treatment temperature,the number and size of tiny holes increase.After a heat treatment at 400?,cracking occurs along the boundary of the aragonite tiles.The mechanical properties of fresh and heat-treated samples(at 200?,300?)in C.plicata shell were investigated by three-point bending and shear tests.The results show that the resistances to bending and shear of the samples decrease after heat treatments.Compared with those of the unheat-treated sample,the bending strength,shear strength and fracture work of samples heat-treated at 200? for 15 min all decline slightly,which is due to the loss of water.As for samples heat-treated at 300? for 15 min,the loss of organic substance results in a significant decrease in the bending strength,shear strength and fracture work of samples.The results show that organic substance is an important factor affecting the mechanical properties of nacre structure.The microstructures of fresh samples and heat-treated samples(at 180?,300?,400?,500?)in C.nobilis shell were characterized by SEM.It is observed that the C.nobilis shell shows a crossed-lamellar structure,and its structural elements are aragonite fibers.After a heat treatment at 180?,there is no obvious change in the microstructure.After a heat treatment at 300?,the organic substance is uniformly attached to the surface of aragonite fibers.After a heat treatment at 400?,the aragonite fibers are broken.In contrast,aragonite fibers are melt,and the crossed-lamellar structure is transformed into a homogeneous block structure after a heat treatment at 500?.The mechanical properties of fresh and heat-treated samples at different temperature with different heat preservation time in C.nobilis shell were investigated by three-point bending tests.It is found that both the heat-treatment temperature and time will affect the mechanical properties of the crossed-lamellar structure.The bending strength and fracture work of samples heat-treated at 180? for 15 min decrease due to the water loss in samples.However the bending strength and fracture work of samples heat-treated at 300? for 15 min are improved,as the organic substance is melt and bonds the aragonite fibers together.For different heat-treatment time(5 min,10 min,15 min,20 min,25 min)at 300?,with increasing heat-treatment time from 5 min to 15 min,the bending strength and fracture work of shell increase,since the organic substance in shells becomes gradually melted.However,as the heat-treatment time is over 15 min,the bending strength and fracture work of shell begin to decrease,since the molten organic substance gradually tends to be decomposed.By comparing the difference in mechanical properties of nacre and crossed-lamellar structure in fresh and after heat treatments at different temperatures,it can be found that the arrangement of "brick-mortar-brick" of nacre makes its mechanical properties more dependent on the organic substance.The complex three-dimensional crossed arrangement of the crossed-lamellar structure can weaken such a dependence of the mechanical properties on the change of composition form,and even to a certain extent,the change of the organic substance form can be appropriately utilized to improve the mechanical properties of the structure.Therefore,this research finding can provide a useful inspiration for strengthening and toughening of layered composites,e.g.,adjusting the soft-hard phase combination of layered composite materials to improve their mechanical properties.