Simulation and Experimental Study on Thermal Field Control Rules for Surface Morphology of Metal Tin Polishing Pads

Large-aperture planar optical components are key parts of the optical systems in high-end equipment such as photolithography machines and laser fusion experimental devices. Full-aperture polishing， also known as continuous polishing， is an important processing method for large-aperture planar optical components. The surface conditioning of polishing pads has been a research focus for scholars both domestically and internationally. Existing research results indicate that effective conditioning is one of the primary factor to enhance the processing accuracy of optical components. For polishing pads with a meter-scale diameter， such as 1 200 mm， turning or grinding processes are typically employed to achieve surface conditioning. However， these methods are not only time-consuming but also struggle to achieve high-precision finishing on the pad surface. Efficient and high-precision conditioning of meter-scale polishing pad surfaces has long been a challenge in the optical component polishing process.This paper proposes a method for surface topography conditioning of tin polishing pads through temperature field control. A composite polishing pad consisting of tin-aluminum-cast iron is constructed with three kinds of thin plates. A thermal field control system for the composite polishing pad is designed to investigate the internal temperature field distribution patterns of the pad under the influence of a controlled thermal field， and a gradient model of the internal temperature field within the composite polishing pad is established. Based on the bimetallic thermal deformation effect， five thermal stress distribution equations are established on the neutral surface of the three-layer composite polishing pad. By setting boundary conditions， a differential equation for thermal stress elastic deformation of the composite pad is established， and a mathematical model for surface thermal deformation of the tin polishing pad with parameter tcont as the variable is obtained through solving the equation.Using finite element simulation software， a simulation analysis was conducted on the thermal deformation of the composite material polishing pad's surface. The ambient temperature field was set to 20 ℃， with constraints and a controlled thermal field applied to the bottom surface of the composite polishing pad. When the parameter tcont of the controlled thermal field was set between 16 and 19 ℃， the thermal deformation of the polishing pad's surface was simulated， and the contour， peak value， and average value of the thermal deformation were analyzed. The simulation results indicated that when the controlled thermal field parameter tcont was set to 20 ℃， there was no temperature gradient change within the composite polishing pad， and no thermal deformation occurred on its surface. However， when tcont was less than 20 ℃， the composite material polishing pad exhibited an overall concave-shaped thermal deformation， and the magnitude of the thermal deformation became more pronounced as tcont decreased. Experiments were conducted to validate the regulation law of the thermal field on the surface of a tin polishing pad. Ensuring an ambient temperature of 20 ℃ in a constant-temperature workshop， measurement tools such as laser displacement sensors were used to measure the surface of the polishing pad. The deformation law of the polishing pad's surface was examined at parameter tcont values ranging from 16 to 19 ℃. Finally， through nine sets of processing experiments on optical components， the impact of the parameter tcont on the processing results was verified. The experimental and simulation results showed consistency in the trend of thermal deformation of the polishing pad's surface. Moreover， by adjusting the thermal field control parameter tcont， the surface accuracy of the processed optical components was optimized.Through theoretical analysis， simulation， and experimental verification， the paper draws the following conclusions： 1） A controllable thermal deformation prediction mathematical model for the surface topography of the polishing pad was established based on the multi-metallic thermal deformation effect； 2） A finite element model for thermal field regulation of the polishing pad was developed and experimentally validated. Within the temperature range tcont of 16 ℃ to 19 ℃， the finite element analysis results of thermal deformation of the polishing pad were consistent with the experimental results in terms of their changing trends； 3） The thermal deformation generated by the polishing pad thermal field regulation system is the greatest at tcont=16 ℃， with a maximum deformation of 16.9 μm and an average deformation of 12.5 μm； it is the smallest at t=19 ℃， with a maximum deformation of 2.2 μm and an average deformation of 2 μm； 4） Adjusting the parameter tcont can enhance the processing accuracy of the component surface. When tcont=18 ℃， the polishing accuracy of planar optical components is the highest. Compared to when tcont=20 ℃ and tcont=16 ℃， the surface accuracy PV value improves by an average of 20%， and the RMS value improves by an average of 15%； 5） According to the comparative analysis， the mathematical model of thermal deformation on the polishing pad surface and the simulation analysis cannot accurately predict specific values of the thermal deformation. Precise model analysis will be conducted in subsequent research.