| With the development of aerospace industry,the surface accuracy and surface quality requirements of space telescope mirrors with complex profiles are getting higher and higher,and the demand is also increasing.Reaction-sintered silicon carbide RB-SiC is one of the main materials for large-diameter mirrors due to its excellent physical,chemical and mechanical properties.Due to the difference in hardness between the silicon phase arnd the SiC phase in RB-SiC,the surface must be modified with amorphous silicon to obtain an ultra-smooth surface by polishing.The new ultra-smooth surface polishing method is studied to solve the problems of low polishing efficiency,high cost and surface abnormal corrosion in the existing ultra-smooth surface polishing method,which is of great significance for the ultra-precision processing of large-diameter RB-SiC mirrors.In this study,an internal ultra-smooth surface polishing method using a novel water-based viscoelastic polishing fluid was proposed.Based on the rheological properties of the polishing fluid and the wall slip characteristics,a fluid constitutive equation was established,and the fluid formulation and polishing process parameters were optimized.The ultra-smooth surface polishing of single crystal silicon and amorphous silicon is realized.An aqueous viscoelastic polishing fluid with water-absorbent resin and thickener compounded hydrogel as carrier was developed.It was proved that the polishing fluid has a climbing rod effect and an extrusion swell phenomenon.The abrasive particles in the polishing fluid do not settle and stabilize.The viscosity increases first and then decreases with the shear rate.By observing the microscopic morphology of the polishing fluid,it is found that a large amount of abrasive particles are distributed on the surface of the fluid,and the interior is a three-dimensional network structure.The rheological experiment of the polishing fluid reveals the dynamics of the fluid.The viscoelastic property increases as the concentration of the water absorbing resin increases,and increases as the mass fraction of the abrasive grains increases,and decreases as the particle size of the abrasive grains increases.The Mooney method proves that the polishing fluid has wall sliding characteristics,and the wall sliding velocity of the fluid increases with the increase of shear stress.The PTT non-Newtonian fluid constitutive model is used,and the fluid simulation software Polymat is used to fit the the coefficients of the model,a constitutive equation for the aqueous viscoelastic polishing fluid is established.The numerical simulation of the flow state of the polishing fluid in the polishing plate flow channel was carried out by using the fluid dynamics simulation software Polyflow.The pressure,wall slip speed and shear of the polishing fluid were analyzed under different groove depths,different inlet flow rates and different wall slip conditions.The influence provides a basis for the selection of the polishing process parameters and the construction of the experimental system.An internal circulation polishing system was developed.Firstly,the polishing experiment was carried out on the surface of grind single crystal silicon.The effects of groove depth,abrasive type,abrasive particle size and mass fraction on the surface roughness and material removal rate of the silicon wafer were obtained,it was found the polishing method can obtain a smooth surface by that abrasive grains removes the plastic domain material of silicon;the single crystal silicon wafers with different initial surface qualities were polished,the silicon wafer with initial roughness Ra of about 10 nm,the roughness was reduced to 6.18 nm,and the silicon wafer with initial roughness of about 1 nm kept the surface roughness substantially unchanged,and the material was also removed.Finally,the amorphous silicon modified layer was polished using a viscoelastic polishing fluid containing a 3 ?m alumina abrasive,and the roughness Ra was reduced from 3.12 nm to 1.64 nm. |
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