Study On Toughening Mechanism Of Nano-polycrystalline Diamond

Nano-polycrystalline diamond（NPD）is an extremely hard material with mechanical isotropy and better performance than single-crystal diamond,and its Vickers hardness can reach 100～150 GPa,which is extremely suitable for ultra-fine machining,aerospace,deep well drilling,national defense,other fields,and belongs to the country’s future strategic materials.Free-oriented nanocrystals greatly optimize the mechanical isotropy of NPD.At the same time,the strong lattice resistance（P-N force）caused by strong covalent bonds in diamond hinders dislocation slip and limits plastic deformation.Although NPD exhibits extremely high hardness,the fracture toughness of traditional NPD is insufficient(K_IC≈5～11 MPa·m^0.5),which restricts the scope of application of NPD.The conflict between hardness and toughness in diamond is a key scientific problem to be solved.To explore the toughening mechanism caused by non-intrinsic dislocation slip is the key to solve this problem.At present,grain boundary design can realize the strength distribution of the connection within the material,forming a non-uniform energy barrier.By effectively designing the distribution of energy barrier,the crack can be induced to produce deflection,branching and bridging toughening phenomena,realize the dissipation and absorption of fracture energy,hinder the crack growth,and increase its extrinsic toughness.However,the toughening mechanism of grain boundary on NPD is still unclear.Exploring the toughening mechanism of grain boundary in NPD,solving the contradiction between hardness and toughness in NPD,and realizing the optimization of hardness and toughness of NPD at the same time are the key scientific issues in this paper.In this paper,by selecting different precursors and adjusting the experimental conditions of high temperature and high pressure（HPHT）,the grain boundary design was optimized,and different grain boundary density,grain boundary strength and composite texture distribution were constructed.In other words,different energy barrier distributions which can induce crack propagation were constructed in NPD,and the hardness and toughening mechanism of NPD were studied to prepare NPD with high hardness and toughness.The following innovative results were obtained:1,Using nano spherical graphite（Nano C）as raw material,polycrystalline diamond（PCD）with different grain sizes was prepared by pressure control.With the grain size increasing from 10 nm to 2μm,the grain boundary density decreases and the grain boundary connection strength increases（Young’s modulus increases）.（1）When the grains are at the nanoscale,the energy barrier passing through the grain is higher than the energy barrier at the grain boundary because of the weak grain boundary connection,so that the fracture mode is grain boundary fracture（IF）;（2）With the increase of grain size and grain boundary connection strength,some strongly connected grain boundaries form a high energy barrier that hinders fracture propagation,and the fracture mode presents a mixture of IF and transgranular fracture（TF）.（3）At large grain size,most grain boundaries are strongly connected,and the energy barrier spreading along the grain boundary is higher than that passing through the grain interior,mainly in TF mode.In other words,the gradual enhancement of the energy barrier at the grain boundary junction changes the fracture mode"IF→IF+TF→TF",and the fracture toughness first increases and then decreases.（4）Finally,a"big sawtooth"crack deflection is formed when IF and TF are mixed,effectively disperse the energy of crack propagation,and the fracture toughness(K_IC)reaches 13.0±2.7 MPa·m^0.5 when the hardness of NPD（HV=114.6±6.9 GPa）is basically unchanged.This toughness is about three times that of single crystal diamond.2,Nano-diamond coated with a thin graphite layer（T≈2～4 nm）as a precursor,using the flexibility and interlayer slip ability of two-dimensional graphite thin layer,under mild pressure（7 GPa）,nano-diamond particles form a coherent grain boundary strong connection.At the grain boundary of NPD,an energy barrier is constructed which is higher than the grain interior and can inhibit crack propagation.For the first time,a new crack deflection mode in which the coherent grain boundary prevents the crack from breaking along the grain is found.It is proposed that the fracture mode of NPD with coherent grain boundaries is mixed mode of IF and TF,and TF has the following crack deflection fracture modes:（1）In-grain crack deflection（IGCD）;（2）Crack deflection at low angle grain boundary（L-c GCD）;（3）Crack deflection at high angle grain boundary（H-c GCD）.Under the strong grain boundary energy barrier,the grain-boundary NPD exhibits higher hardness and toughness than the conventional NPD,with HV=121.5±6.4 GPa and K_IC=13.2±1.5 MPa·m^0.5,respectively.3,With highly oriented pyrolytic graphite（HOPG）as raw material,martensitic transformation under high temperature and high pressure is made use of its regular arrangement characteristics of high orientation,and the layered interlocking texture containing nano-lamellar diamond（T<100 nm）and nano-lamellar graphite（T<10nm）is constructed,forming a layered energy barrier with alternating strength.The layered interlocking texture not only obstructs the fracture perpendicular to the layer direction,but also,along the layer direction,the cracks spontaneously select low energy barrier to form multiple crack branches,and deflect at the interlocked structure,effectively disperzing the fracture energy,so that the sample can maintain high hardness（HV=120.8±6.3 GPa）at the same time.Toughness(K_IC=15.9±2.3 MPa·m^0.5)up to 3 to 4 times that of single crystal diamond.The construction of high hardness and high toughness NPD is realized.In summary,the key to maintaining high hardness and enhancing the fracture toughness of NPD is to optimize the grain boundary design in the sample.Through the grain boundary construction,the energy barrier is designed to prevent the diffusion of cracks,so that cracks spontaneously deflect and branch,forming the dispersion of fracture energy,which can effectively increase the toughness of NPD.Therefore,the design of high energy barrier to prevent crack propagation and the construction of strong grain boundary connection is the key to realize the development of high hardness and high toughness NPD by extrinsic methods.The research in this paper not only helps to improve the toughness of NPD,but also provides a new perspective for the future design of ultra-tough and extremely hard materials.