Study On Lapping And Polishing Experiments Of Sapphire Substrate And Surface Roughness Model Of CMRF

As a widely used monocrystal material,Sapphire has strong chemical stability,high hardness,good resistance to abrasion,high strength,good electrical insulation,and high dielectric property.It has applied in this military field such as the infrared window,optical sensor and the fairing material of high-speed aircraft.Additionally,it is broadly applied to the civilian fields,such as substrate materials of the thin film of high-temperature superconductors and the large-scale IC's SOI and SOS.What's more,it is also used in substrate materials of light-emitting diode(LED),the screen and terminal touch panel of the mobile phone,the LED(light-emitting diode)illumination and so on.Along with the rapid development of LED and 5G communication technology,the demand for high-efficiency,high-precision,low-cost and no subsurface damaged sapphire substrate is urgently needed by using ultra-precision machining technology.However,sapphire has the characteristics of high hardness,easy brittleness,and strong chemical stability.The requirement of the sapphire substrate is high and no subsurface damage in industrial application,which makes the ultra-precision processing of sapphire substrate more challenging.Aiming to obtain the best surface quality,the sapphire substrate was polished by the cluster magnetorheological finishing with dynamic magnetic fields.The composition of magnetorheological fluid and processing parameters were optimized through a single-factor test and orthogonal test.Then,the Ra model of cluster magnetorheological finishing with dynamic magnetic fields was established and the pre-order lapping process was systematically optimized.The particular research contents and results are as follows:Firstly,the single-sided lapping experiments for 2-inch c-direction sapphire was carried out on the KD15 BX Precision Lapping Machine.The influences of abrasive species,sizes of abrasive particle,rotational speeds of lapping plate,lapping pressure and mass fraction of abrasive on the material removal rate and surface roughness were researched by single factor experiment.Then,the processing parameters were optimized by orthogonal experiment.And the processing flow of single-sided lapping was obtained: to realize the thinning and flatness of substrate,the W40?W14 diamond abrasive were selected to be rough and semi-lapping onthe cast iron disk under the conditions of rotational speed of lapping disk is 60 rpm,mass fraction of abrasive is 3 wt%,lapping pressure is 5.238 psi or 4.408 psi respectively.And then the W3 diamond abrasive was selected to be precision lapping on the synthetic copper disk under the conditions of the rotational speed of lapping disk is 60 rpm,mass fraction of abrasive is 1 wt%,lapping pressure is 4.408 psi.Finally,the uniform surface of scratch-free with Ra 7 to 9 nm was obtained.Secondly,the sapphire substrate after single-sided lapping was polished by cluster magnetorheological finishing with dynamic magnetic fields formed by synchronous rotational of magnetic poles.The composition of polishing slurries and processing parameters were optimized through experiments.And the results showed that the maximum material removal rate and the lowest surface roughness could be obtained in the magnetorheological fluid mixed with silica sol solution.The surface roughness of Ra was reduced to 0.78 nm when the mass fraction of abrasive and carbonyl iron powder is 16 wt%,particle sizes of abrasive and carbonyl iron powder are 120 nm and W3 and pH is 10.Based on the optimized magnetorheological polishing fluid components,the orthogonal experiment was carried out to obtain the optimum processing parameters.When the machining gap is1 mm,the rotational speeds of the polishing disk,magnetic poles,and workpiece are 40r/min,45 r/min and 350 r/min,the lowest surface roughness can be obtained.Finally,the sapphire was polished by cluster magnetorheological finishing with optimized magnetorheological polishing fluid and processing parameters.The average surface roughness decreased from Ra 7~9 nm to 0.58 nm after polishing 8 hours and the super smooth surface was obtained.Finally,the material removal behavior of cluster magnetorheological finishing for the sapphire substrate was deeply analyzed.The number of active particles of abrasive in contact with the workpiece in cluster magnetorheological finishing and the polishing force acting on the single abrasive particle was derived theoretically.And the surface roughness Ra model of cluster magnetorheological finishing was established by considering the trajectory length of the single abrasive particle moving on the workpiece.The effects of mass fraction of abrasive and carbonyl iron powder on surface roughness Ra was theoretically analyzed and compared with the polishing experiment.The results have a good consistency.