Research On The Ultrasonic-assisted Magnet-orheological Polishing Technology Of Hemispherical Sapphire

Sapphire has broad application prospects due to its excellent physical and chemical properties,such as stable chemical properties,heat and wear resistance,good dielectric properties and thermal shock resistance,especially used as LED substrate and fairing material,in which the hemispherical fairing is the most familiar.However,the high hardness,brittleness and chemical inertness of sapphire material also make it very difficult to achieve ideal machining results by traditional polishing technologies,resulting in a low efficiency and a high cost.Therefore,carrying out research on the precision machining technology of sapphire spherical surface and improving its processing level,will not only help to promote the application of sapphire spherical domes,but also is of great significance to the processing of brittle material domes and curved parts.In this paper,the ultrasonic-assisted magneto-rheological polishing（UAMP）technology,which combined chemical mechanical polishing（CMP）,magneto-rheological polishing（MRP）and ultrasonic vibration,for hemispherical sapphire was systematically studied from the aspects of computer simulation and polishing experiment,and the optimal composite polishing process for hemispherical sapphire was obtained,and the material removal mechanism of sapphire in this composite polishing process was also studied.Firstly,Fe₃O₄/SiO₂ core-shell abrasives with different core-diameters and shell-thicknesses were prepared by sol-gel method to solve the problem that SiO₂abrasives are easy to aggregate or absent in the working area during traditional sapphire MRP process,and then characterized on the aspects of surface morphology,internal structures,phase components and magnetic properties.The prepared Fe₃O₄/SiO₂ core–shell abrasives exhibited better polishing performance for sapphire material,resulting in smooth and detect-free surfaces with a high material removal rate（MRR）compared to conventional mixed abrasives,especially the abrasives with 100 nm in core-diameter and 15 nm in shell-thickness can achieve the best polishing results for sapphire.The application of ultrasonic vibration could improve the MRR by more than three times,but the surface quality of polished sapphire wafer was slightly deteriorated.The mathematical models of normal force and shear force generated on sapphire wafer surface in UAMP process were established,and then the traditional MRR model based on Preston equation was modified.Experiments showed that the experimental results are in good agreement with the calculated results of the normal force,shear force and modified MRR models.The chemical reactions on sapphire surface during the core-shell abrasives magneto-rheological polishing were characterized,and the Al OOH was determined to be the main chemical product.Then,the material removal mechanism of sapphire surface in MRP process was investigated through the combination of scribing simulation and experiment of Fe₃O₄/SiO₂ core-shell abrasives against the chemical reaction layer on sapphire surface.In the early stage of scribing,the substrate material only undergoes plastic deformation,while with the indentation depth of abrasive-tool increases,the material located at the contact position between substrate and abrasive is subjected to greater contact stress and then damaged.The materials on both sides of scratch flow outward plastically cause ridges under the continuous extrusion process.The material at the bottom of scratch is damaged and removed during the continuous and more severe extrusion and shearing process,resulting in an uneven"jag-like"morphology,which is far rougher than the unscribed substrate surface.Therefore,the material removal mechanism of sapphire in MRP process with Fe₃O₄/SiO₂ core-shell abrasives is the combination of chemical reaction of surface material and the synthetic action of plastic deformation and shearing removal of the chemical products layer.Then,the mathematical models of MRR and surface roughness Ra,which consider the variations of the peak height of each point on rough spherical surface with polishing time,for hemispherical sapphire in UAMP process were established by the method of combining modified Preston equation and polishing motion trajectory.The effects of quantity and pole-arrangement of magnets on the magnetic field intensity and distribution on hemispherical polishing area as well as the polishing results of sapphire hemisphere were investigated by computer simulation,and the optimal quantity and pole-arrangement of magnets were screened out based on the calculated MRR and Ra.Through the experimental verification of hemispherical sapphire MRP process,the simulation results are in good agreement with the experimental results.And it was determined that the magnetic field generator with 25 magnets arranged in cross-involute arrangement would be employed for the following UAMP experiments of hemispherical sapphire.Finally,four key machining parameters in UAMP process of hemispherical sapphire were designed by orthogonal experiment,including rotation velocity X,swing period Y and swing angle Z of polishing cover as well as abrasives volume fraction W.The range analysis,variance analysis and grey correlation analysis were carried out on the experimental data,and the priority and the significance were determined for each parameter against evaluation indicators,and the optimal polishing processes were obtained.Different evaluation indicators correspond to different optimal polishing processes.Take the MRR as evaluation indicator,the optimal level combination of factors is X₃Y₁Z₁W₃,the largest MRR of 2.239μm/h was achieved in this condition,while the surface roughness Ra was 5.488 nm and the surface figure PV was 1.337λ.Take the Ra as evaluation indicator,the optimal level combination of factors is X₁Y₃Z₃W₁,the smallest Ra value of 0.933 nm and the lowest surface figure PV of0.324λwere achieved in this condition,while the MRR was 0.411μm/h.Take the combined effect of MRR and Ra with identical weight as evaluation indicator,the optimal level combination of factors is X₁Y₃Z₃W₁,which is consistent with that of Ra indicator.The good agreement between the experiment results and the simulation results in the UAMP of sapphire hemisphere verified the correctness of MRR and Ra models established by the method of combining polishing motion trajectory and modified Preston equation in this paper.