Study On Haedening Mechanism Of Superhard Diamond Surface And Its Nanohardness Evaluation Technology

Natural single crystal diamond is the hardest material known in the world.Various diamond tools are widely used in cutting,polishing and petrochemical industries.Many researchers have done plenty of researches on superhard materials,and several kinds of superhard materials have been obtained,but all of the superhard materials were compared with natural diamonds to show that the obtained materials have higher hardness than that of diamond,and there is no quantitative measurement of the hardness of the superhard material.The author's research group obtain a superhard diamond surface after the post-treatment of diamond tools.In the previous research work of the research group,the mechanical properties of the diamond surface after post-treatment were measured using nanoindentation technology,and the elastic modulus of the hardness of the superhard diamond surface reached a maximum at a depth of 6 nm,which was 650 GPa and 24000 GPa,about 6.5 times the real hardness and 12 times the real elastic modulus of the diamond.For the hardening mechanism of superhard diamond surface,no relevant research and analysis has been conducted.At present,all kinds of indenters with different shapes used on the nano indenter are made of diamond.For the tested samples whose hardness is much smaller than the diamond hardness,the existing Oliver-Pharr hardness calculation method can well obtain the hardness of the material.However,for the above-mentioned superhard materials,the diamond indenter may produce elastic deformation or even plastic deformation during the nanoindentation process,without the consideration of the indenter deformation,results may lead to a great error.This paper focuses on the mechanism of hardening of superhard diamond surface and the hardness evaluation technology of superhard materials.The specific research results cover the following content.Firstly,molecular dynamics simulation method was used to verify the effect of the Airebo potential function on the graphene.Secondly,the nanoindentation process of the superhard material of the graphene diamond system was simulated by molecular dynamics.The loading and unloading curves of superhard materials were compared to that of nanoindentation experiments of superhard materials.At the same time,the fracture process of graphene carbon-carbon bonds and the expansion process of diamond dislocations during nanoindentation of superhard materials were observed.It is concluded that the excellent mechanical properties of graphene retard the expansion of dislocations in diamond crystal,and the high-strength carbon-carbon bonds of graphene increase the surface free energy of the diamond,which is the main reason for the extremely high hardness of superhard diamond surfaces.Secondly,based on the elastic-plastic contact theory,the relationship between the load and indentation depth during the elastic deformation of the indenter is deduced,the applicable range of the method is determined,and the relationship between the contact area of the indenter and the maximum indentation displacement is obtained in the derivation process.The feasibility of the theory is verified by nanoindentation experiments with diamond indenter loaded diamond samples;the calibration methods of the two area functions are compared at the same time,especially in the case of small indentation depth,Two area functions calibration methods was compared,especially at the small indentation depth,the results showed that the area function obtained by the direct measurement method is more accurate.Thirdly,in the case of plastic deformation of the diamond indenter,a dimensional analysis method was used to analyze the relationship between indenter tip radius R and the load P,indentation depth h,the nominal elastic modulus Er,and the initial tip radius r0,and come to the corresponding formula;then through the nanoindentation experiment,the formula is fitted.Considering the plastic deformation of the indenter,the radius of the indenter edge also changes correspondingly.The relationship between the change law of the radius of the indenter edge and the indentation depth is established through experiments.The expression of the area function of the plastic deformation of the indenter is obtained,based on the comprehensive consideration of the plastic deformation of the tip radius and that of the indenter edge.Finally,atomic force microscopy was used to determine the diamond indenter was in the elastic deformation range during the superhard diamond surface nanoindentation.Based on above results,the hardness value of the superhard diamond surface at 5nm was measured to be 13.1GPa,which is 1.2 times the hardness of the diamond at this depth.