Research On Grinding Damage Formation Mechanism And Suppression Technology Of Reaction Bonded Silicon Carbide Ceramics

Owing to its unique physical characterizes,such as low density,low thermal expansion coefficient,high specific stiffness and high chemical inertness,RB-SiC ceramics can be used in harsh environment.It is widely applied in MEMS,military and space exploration,especially as a favorite candidate substrate material of large-scale light-weight mirrors for space-related application.However,in order to obtain higher image resolution and longer lifetime,the space mirror must possess large diameter,super smooth surface and lower subsurface damage.Unfortunately,high hardness and brittleness of RB-SiC ceramics leading to surface/subsurface damages are inevitable in ultra-precision machining process,which will dramatically degrade the performance and mechanical strength of RB-SiC based devices.Therefore,in order to suppress fracture damage,ductile removal of RB-SiC ceramics must be achieved.Up to date,material removal mechanism and the critical condition from ductile to brittle transition of RB-SiC ceramics are still unclear,which limits the development of RB-SiC ceramics.Consequently,this thesis takes the RB-SiC ceramics as the research object.Advanced test methods,such as single grit scratch,nano-mechanical property and material microstructure characterization technologies,were used to investigate the material removal mechanism of RB-SiC ceramics,the formation mechanism of surface/subsurface damages and the effect of machining parameters on material removal characters in grinding process.After that,a new grinding force model that taken the grits distribution on grinding wheel,material properties into the consideration for micro-machining RB-SiC ceramics was developed.At last,laser assisted grinding technology was proposed to acquire high efficiency,high quality and low damage in the machining of RB-SiC ceramics.This thesis will provide theoretical guidance for optimizing grinding RB-SiC ceramics process.And it is also great significance to enrich the ductile grinding of hard and brittle materials.Firstly,a series of single grit scratch tests were performed to simplify the grinding process.And the morphology and microstructure of chips were analyzed to investigate the removal mechanism of RB-SiC ceramics.In this context,the critical depth of ductile-brittle transition model for RB-SiC ceramics was developed and the influence factors of ductile-brittle transition were studied.It was found that there are two removal modes containing plastic deformation and brittle fracture of RB-SiC ceramics in scratch testes.Amorphous phase transition of SiC induced by scratching shear stress is the main plastic removal mechanism of RB-SiC ceramics,while no amorphous phase of SiC was found in brittle fracture chips.Besides,the increase of scratch velocity can improve the critical depth of ductile-brittle transition.The brittle fracture prone to occur at the weaken zone such as grain boundary and phase boundary,which result in the fluctuation of friction coefficient.The fluctuation of friction coefficient increased with the increase proportion of brittle facture.The influence of grinding particle distribution characteristics on the formation law of grinding surface of RB-SiC ceramics was studied.The three-dimensional surface morphology of grinding wheel was characterized,the protrusion height of grits on the grinding wheel was statistically analyzed,a model of grits protrusion height on the grinding wheel was established in accordance with the normal distribution probability.Then,the elastic deformation force,plastic deformation force and brittle fracture force models in the grinding process were constructed by combining the abrasive particle distribution characteristics,material properties and the plasticity and brittleness removal mechanism of RB-SiC ceramics.The grinding force and machining quality of hard and brittle ceramics are predicted more accurately.The subsurface damage features and formation mechanisms underneath the grinding plastic zone and brittle fracture zone of RB-SiC ceramics were studied by analyzing the microstructure of SiC and Si phases underneath the grinding substrate.The results showed that the surface damage of RB-SiC ceramics includes micropits,transgranular cracks and intergranular cracks.Defects such as sintering aid,grain boundary and phase boundary in RB-SiC ceramics will lead to dislocations accumulation.When the stress concentration caused by dislocation accumulation exceeds the critical tensile stress of 33.9-36.5GPa,brittle fracture occurs in RB-SiC ceramics.In terms of subsurface damage,it was confirmed that the subsurface damage of SiC phase underneath the plastic region of RB-SiC ceramics includes basal plane<a>dislocation,stacking faults and twins,and that no amorphous transition of SiC phase was found.The subsurface damage of Si phase includes amorphous transition and dislocations.The anisotropic stress caused by dislocations is the main reason for the split of the Raman peak corresponding to Si-I phase.The subsurface damage of SiC phase underneath brittle fracture region includes microcracks and high density dislocations,and no amorphous transition of SiC phase was found.The microcracks are mainly medium and lateral cracks.The dislocations mainly include the total basal plane dislocations with Burgers vectors b=1/3<-2110>and the Shockley incomplete dislocations with Burgers vectors b=1/3[1-100]and b=1/3[-1010].The subsurface damage of Si phase mainly includes amorphous transition and{111}plane dislocation.Finally,the technology of laser assisted grinding of RB-SiC ceramics to suppress surface/subsurface damage was investigated.It was found that laser heating can soften and toughen RB-SiC ceramics,thus improving the removal ratio of plastic deformation in grinding process.Laser assisted grinding can effectively reduce the subsurface crack depth and surface roughness.The subsurface damage of traditional grinding includes medium and transverse cracks,while the subsurface damage of laser heating assisted grinding is mainly transverse cracks.At the same time,SiO₂amorphous phase was found on the laser radiation surface of RB-SiC ceramics,which result indicate that the oxidation reaction occurred during the laser radiation process with suitable temperature.