| Silicon carbide(SiC)ceramic has an excellent material property such as high hardness,low coefficient of thermal expansion,satisfactory abrasion resistance and chemical corrosion resistance,which has been used widely in the fields of satellite,aerospace,weapon and space telescope.It is still a primary method to machine the SiC ceramic with the aid of grinding technique.However,a removal mechanism and crack behavior are still unclear in the process of grinding of the SiC ceramic.It has an important help to obtain a good grinding surface quality and lower subsurface damage by studying deeply a basic behavior of damage evolution and material removal during grinding.Grinding is a complex process,so it is difficult to analyze directly the material removal process and the formation and extension mechanisms of surface and subsurface damages.A scratching experiment is one of the most common and effective methods to study the behavior in the process of materials machining.Therefore,the scratching experiment is used widely to study the grinding removal process and crack forms of hard and brittle materials.The SiC ceramic is sintered with SiC particles and its structure is complex,which result in a difficulty to study directly the above issues.Hence this paper selects a research way from the single SiC to SiC ceramic,from sub-nanometer to micrometer,from plastic removal to brittle removal.In this paper,a grinding removal behavior and propagation mechanism of surface/subsurface cracks of a reaction bonded silicon carbide(RBSC)ceramic were researched in depth by the method of molecular dynamics(MD)simulation,finite element method(FEM)and scratching test with different modes.To analyze the deformation and removal mechanism of SiC material at sub-nanometer scale during grinding,the MD simulation method was utilized to study a micro-dynamics removal process of 6H-SiC and 3C-SiC.The effect of sharp and different shape abrasives on grinding removal process of SiC materials wa s studied.And characters of crystal structure,distribution of bond length and angle and high pressure phase transform at a machining/machined area were analyzed.These results provide an atomic-scale theory foundation to investigate a plastic deformation mechanism and removal behavior during the SiC materials grinding.The plastic deformation and removal mechanism of 6H-SiC and 3C-SiC at nano-scale were studied deeply with the aid of nano-scratching,focused ion beam(FIB)and transmission electron microscope(TEM)experiment and detection means.And the existence of amorphous phase of SiC material on the machined surface layer was firstly identified by experiment directly.The removal mechanism of SiC materials was analyzed systematically according to the results of MD simulation.RBSC ceramic is a typically hard and brittle material.So fractures and collapses produce easily during grinding the material.To put forward corresponding damage control measures,it is helpful to understand the distribution cha racter of cracks in the machining area by researching in depth the material fracture property and crack growth mechanism.Because the material is a multi-phase material with a complicated geometry structure.So the analysis of fracture property is very har d no matter from the aspect of theory or test.In this paper,an extended finite element method basing a virtual crack closure technique was employed to establish a three dimensional model of looping and embedding silicon that considering RBSC material structure.By analyzing the influence of material structure on the fracture mechanics parameters distribution of the crack tip and the dynamic growth behavior of cracks.In this paper,the damage distribution mode and crack growth behavior during the RBSC ceramic grinding were studied deeply by using the simulation and nano-scratching experiments with a variable load.It was found that the scratching crack in homogeneous material can be divided into three crack forms: the median crack,transverse crack and radial crack.It is easy to distinguish the crack forms attributed to the ruled crack morphology.However,it is difficult to discern the subsurface scratching crack of RBSC material.Because the material structure and crack form are complex;and a secondary crack and an irregular micro-crack often occur.The distribution forms of scratching cracks can be explained reasonably by simulation results.In this paper,simulations and experiments with multi-shape abrasives were used in research of material removal mechanism and friction and wear behavior of the SiC material from su-nanometer scale to micro-scale.The results provide strong support for the research of precision / ultra precision SiC ceramic material grinding and polishing technology.The results lay a theoretical foundation for the research on reducing the surface damage of RBSC grinding and the depth of subsurface crack. |
