Study On Grain Growth Of Nano-Micro Composite Ceramic Tool Materials In Sintering Process

Because of its high hardness,high temperature resistance and good wear resistance,ceramic cutting tool materials has an incomparable advantage over traditional cemented carbide tool materials in cutting hard machined superhard materials.However,due to its low fracture toughness,it has limited large-scale promotion and application.In order to solve this problem,a variety of toughening and reinforcing mechanisms have been developed one after another.The appearance of the nano composite ceramic tool material greatly improves the toughness of the tools.The macroscopic mechanical properties of ceramic tool materials depend on microstructure,study the microstructure evolution process and predict the final structure,which has important theoretical significance for the development of nanocomposite ceramic tool materials.In this paper,the theory of grain growth in the ideal sintering process of nano micron composite ceramic tool materials and the mechanism of material transfer between grain and grain are studied.Based on Surface Evolver software,The force equal diameter spherical particle model was established,and the energy change and balance configuration problem of particle system was discussed in sintering process.The energy of the granular system decreases first and then increases with the sintering time.The densification of linearly arranged spherical particle system is discussed during sintering.The results show that the surface diffusion mechanism material is transported to the neck with the surface tension driven by the sintering process.The sintered neck was gradually formed while the original shape of the particles remained unchanged.Volume diffusion or grain boundary diffusion reduces the center distance between particles,resulting in densification.However,it is impossible to retain the vacancy from the particle interior or through viscous flow to the grain boundary.At the same time,it must be diffused through the lattice to the outside of the particle.When the matter flows to the neck,the center of mass of the particle will still move close to each other.In addition,the mass transfer mechanism of surface diffusion and evaporation condensation does not cause the crystal lattice to move relative to the grain boundary,but the center of mass of the particles also moves toward the neck because of the transfer of matter.Two unconstrained particles may also be in equilibrium form after a long period of centroid movement under the non-densification mass transfer mechanism.The evaporative condensation and surface diffusion mass transfer mechanisms only promote the growth of the sintered neck at the beginning of sintering but do not cause the shrinkage of the pair of particles.Simulation of the growth of the neck of the three spherical particles modelled initially in an L-shaped arrangement in the sintering process.The simulation results show that even if the size and shape of the particles are completely the same,the specific arrangement of the particles will lead to the emergence of unstable neck growth.The change of topological structure in the microstructure evolution of materials is discussed.During the sintering process,the evolution of particle surface and grain boundary changes the topological structure between particles.The topology evolution of the particle surface network and the grain boundary network follows a certain rule.Grain growth occurs at the same time as the topological structure of the grain boundary changes.The smaller crystal grains in the system shrink through grain boundary motion and eventually disappear,and the average size of the remaining grains becomes larger.The sintering process of spherical particles with different particle size ratios was simulated.The simulation results show that the closer the size of the two particles,the greater the effect of the small particles on the evolution of the whole system.The diffusion coefficient ratio D mainly affects the rate of the sintering process without affecting its evolutionary mode.The contact dihedral angles between particles have a significant effect on the morphological evolution of the granular system,but the smaller dihedral angle is more favorable for the small particles to maintain spherical shape throughout the simulation sintering process.The neck length curve between the particle pairs with different volume ratio shows that the smaller the particle volume ratio is,the higher the growth rate of the neck is at the beginning of sintering.Finally,the effect of particle diameter ratio on the rate of pore annihilation was simulated.The results show that in the non-equal spherical particle model,the neck between the nanoscale particles first begins to grow and then the neck between the micro-nanoparticles begins to grow,and eventually the interparticle pores disappear.Micron-sized particles grow,nanoscale particles shrink.Sintering experiments were conducted on Al₂O₃ matrix nano and micro composite ceramic tool materials.The influence of different sintering temperature and holding time on microstructure evolution between microparticles and nano micron particles was investigated.The microstructure of the experimentally obtained material agrees well with the simulation results,which verify the correctness of previous simulations.