Research On The Mechanisms And Processing Of Ductile Machining Of Cubic Silicon Carbide In Diamond Cutting

Cubic silicon carbide(3C-SiC)is a typical hard and brittle ceramic material with high electron mobility,high band gap,good corrosion resistance,oxidation resistance and wear resistance,and has been widely used in the fabrication of nanoelectromechanical systems optoelectronic devices operating in extreme environments.Surface integrity has an important impact on the optical,electrical and mechanical properties of 3C-SiC optoelectronic devices,for which the machined surface roughness less than tens of nanometers is desired.Currently,ultra-precision single-point diamond turning technology has been proven to be an effective machining method for preparing ultra-smooth surfaces of various hard and brittle materials,due to its advantages of high machining freedom,high precision and low subsurface damage.In particular,the ductile regime cutting of hard and brittle materials is one key processing route to obtain its high-quality surface.Below the critical depth of cut(DOC)for the brittle-to-ductile(BTD)transition,the ultrasmooth surface of hard and brittle materials such as single crystal silicon and germanium can be realized by diamond turning.However,there is currently a lack of fundamental understanding of the BTD transition behavior of 3C-SiC.During diamond cutting of 3CSiC,brittle defects such as crack propagation and grain pull-out are easily formed on the machined surface,which introduces serious subsurface damage and restricts the preparation of ultra-smooth surface,thus limiting the service performance of 3C-SiC devices.It is urgent to study its BTD transition behavior and mechanisms of ductile regime cutting.Therefore,this thesis conducts in-depth research on the mechanisms and processing of ductile regime machining of 3C-SiC carbide in ultra-precision diamond cutting.Firstly,molecular dynamics(MD)simulations are carried out to study the microscopic deformation mechanisms of 3C-SiC,and the mechanical properties and plastic deformation characteristics of 3C-SiC are obtained.MD simulations of bulk compression,uniaxial compression,and shear deformation of 3C-SiC are carried out.By comparing the simulation results with the experimental data,the empirical potential with high precision in describing the atom interaction in 3C-SiC is obtained.Subsequently,MD simulations of nanoindentation of 3C-SiC are carried out,and the atomic-scale material deformation mechanisms of 3C-SiC under mechanical loading are revealed.The influence mechanisms of crystal orientation and grain boundary properties on the deformation behavior of 3C-SiC are further studied.Secondly,a finite element(FE)simulation model of mechanical deformation of 3CSiC,which combines the crystal plasticity constitutive law with the cohesive zone model,is established to investigate plastic deformation dominated by dislocation slip and brittle fracture induced by crack initiation and propagation.The nanoindentation experiments of3C-SiC under different indentation depths are carried out,and the load-depth curves,mechanical properties and indentation surface morphologies of 3C-SiC at different indentation depths are obtained.Based on the experimental and FE simulation results of nanoindentation,the material deformation behavior of 3C-SiC under mechanical loading is quantitatively analyzed from microscopic scale.The mechanisms of the coupled effect of the crystallographic orientation of indentation surface and the indenter geometry on the anisotropy of surface pile-up are revealed.The crack evolution process in the surface and subsurface of 3C-SiC at large indentation depth is analyzed.Thirdly,the experimental and simulation investigation of ultra-precision diamond cutting of 3C-SiC are carried out to reveal the BTD transition behavior and material removal mechanisms.The machining behavior of 3C-SiC at different cutting depths,including chip formation,crack initiation and propagation,cutting force evolution,grain boundary deformation,dislocation activity and structural phase transformation,is investigated.The correlation mechanisms of internal microstructure of material with deformation modes,machined surface quality and subsurface damage are elucidated.The results demonstrate that the crack initiation and propagation at grain boundaries dominate the brittle regime cutting of 3C-SiC.Based on the developed FE simulation model combining plastic deformation and brittle fracture of 3C-SiC,FE simulations of diamond cutting of 3C-SiC are carried out,which effectively capture the ductile cutting behavior at small cutting depths and the brittle cutting behavior at large cutting depths.The influence of processing parameters on the ductile machinability of 3C-SiC is systematically investigated,with an emphasis on the grain boundary-related deformation behavior such as intergranular fracture.Finally,the experimental and simulation investigation of ultrasonic elliptical vibration-assisted diamond cutting of 3C-SiC are carried out,and an effective processing method for improving the ductile machinability of 3C-SiC is obtained.By combining focused ion beam sample preparation,high-resolution transmission electron microscopy observation and Raman characterization,the influence mechanisms of high-frequency elliptical vibration of diamond tool on the microscopic deformation behavior of 3C-SiC are studied.Combined with MD simulation,the large plastic deformation mechanisms of3C-SiC based on the formation of high-density oriented stacking faults is revealed.Experiments and FE simulations of ultrasonic elliptical vibration-assisted diamond cutting of 3C-SiC under different tool amplitude are carried out,and the influence of tool amplitude on its critical DOC associated with BTD transition is obtained.Based on the optimal amplitude parameter,the critical DOC associated with BTD transition of 3C-SiC has been increased by 6 times,the wear degree of diamond tool has been significantly improved,and the ultra-smooth surface of 3C-SiC with a surface roughness lower than10 nm is obtained,which provides a processing foundation for the preparation of ultrasmooth surface of 3C-SiC by ultra-precision diamond cutting.