Effect Of Grain Boundary On Mechanical Properties Of Nano-Diamond Polycrystalline Materials

The material with a Vickers hardness greater than 40 GPa is defined as a superhard material.Diamond is the hardest material known in nature,with high Vickers hardness,good incompressibility and wear resistance,but it is easy to break along the（111）cleavage plane,weak compressive strength,low toughness,which limit its applications to some extent.The diamond synthesized by the addition of the metal component shows improvement in toughness,but greatly reduces its hardness.How to control the mechanical properties of diamond structured materials,synthesize materials with high hardness or fracture toughness,and reveal the relationship between microstructure and mechanical properties have always been important topics in the research field of superhard materials.In this paper,nano-diamond based superhard material with different microstructures are synthesized from the precursors of a mixture of nano-carbon（such as fullerene and graphene）and nano-diamond powder with varied molar ratio of each component via a high temperature and high pressure（HPHT）technology.The results on the microstructural changes and their effects on the mechanical properties of superhard materials can be summarized below:1.High temperature and high pressure technology has been used to sinter the mixture of fullerene and nanodiamond powder（average diameter 30nm）with different molar ratios of 0%,2%,5%and 20%,and the products are characterized by XRD,Raman,TEM,etc.It was found that diamond nanocrystals in the HPHT sintered pure diamond nano-powder showed obvious grain refinement and defects increase.In contrast,when fullerene was added,no significant grain refinement or growth was observed.The diamond nanocrystals can still maintain their original size.Further studies revealed that the addition of fullerenes can significantly adjust the hardness of the material.The hardness of the material can be up to about 120 GPa when proper amount of C₆₀0 was added,which exhibits a 60%or higher hardness enhancement compared with that of the sintered diamond without addition.The microstructure of the sample was studied by TEM.Stacked fault and twisted lattice structure appeared in all products.Combined with the selected area electron diffraction,it was found that refined diamond nanoparticles appeared between the diamond nnaocrystals in the HPHT sintered pure diamond product.In the products from diamond nanocrystals/fullerene mixture,amorphous carbon is formed between part of grain boundaries,which may act as a cement component.The EELS measurement on such boundary region indicates that the amorphous carbon has sp³hybridization characteristics.The hardness and toughness values of the samples were studied,and it was found that the sample made from the precursors containing fullerene had a higher hardness value,and the fracture toughness was comparable to that of the commercial polycrystalline diamond（metal additive）.According to the analysis,the optimized carbon boundary may have strong effect on the hardness enhancement.2.We found that when graphene is added（2%or 5%molar ratio）,the nanocrystalline diamond grains of the products grow up.The crystallinity of diamond is obviously improved,which may be due to the residual non-carbon groups in the reduced graphene.The groups may have the function of catalyzing grain growth under high temperature and high pressure conditions.In the diamond materials synthesized under these conditions,there are also stacking faults and lattice distortion in the grains,and amorphous carbon is also present between some grain boundaries,which acts as boundary adhesion to bond the diamond grains.The EELS study showed that the amorphous carbon is in sp²and sp³bonding states.The hardness and toughness values of the samples were examined.It was found that the hardness value of the products with graphene additive was similar to that of the HPHT sintered pure diamond nano-powder without additive but the fracture toughness was significantly improved,increasing 50-60%.The fracture toughness enhancement should be related to the unique hybrid amorphous carbon boundary with sp²and sp³bonding in the products.3.Through comparative studies,it is found that the addition of fullerenes at high temperature and high pressure can significantly reduce the refinement of diamond nanocrystals compared with diamond blocks without addition of synthetic materials.Fullerene has a certain stress and strain on diamond under high temperature and high pressure.Buffering,some fullerenes will transform into a small amount of amorphous carbon boundary under high temperature and high pressure conditions,thus increasing the hardness of the composite block.However,as the proportion of addition increases,the amorphous boundary becomes wider or becomes more,which causes less mechanical properties,thereby reducing the hardness.Based on the studies,it can be found that the additive of C₆₀0 into the precursor significantly reduce the refinement of diamond nanocrystals compared with the case of HPHT sintered pure diamond nanocrystals,and amorphous sp³boundary appears between some grains,and the hardness of the composite is improved.In contrast,the addition of graphene leads to grain growth in the products,the boundary density decreases,and amorphous carbon boundary with sp²and sp³bonding is formed at part of the grain boundary.The fracture toughness of such synthesized diamond polycrystalline material is improved.These results indicate that the additive of nano-carbons at high temperature and high pressure can tune the boundaries and grains in the nano-diamond polycrystalline material,thereby improving its mechanical properties.