Research On Fixed Abrasive Polishing Of Soft And Brittle CaF₂ Crystal

Owing to the unique properties of wide range of transmission, excellent achromatic and apochromatic etc., calcium fluoride(CaF2) crystal is widely used in high energy detection, infrared tracking and optical components. Especially in the ultraviolet optical system, CaF2 crystal has became the first lens material choice of lithography system and high energy laser system. However, due to its material characteristics, such as softness, brittleness, large thermal expansion coefficient and low thermal conductivity, it's difficult to realize the ultra-precision machining of Ca F2 crystal. In this paper, parameters of lapping, fixed abrasive pad(FAP), abrasive-free slurries and parameters of CaF2 crystal polishing were optimized based on fixed abrasive polishing technology. And based on chemical action, a material removal model of fixed abrasive polishing was set up. The main research work and results are as follows:(1) The parameters of fixed abrasive lapping of CaF2 crystal and FAP were optimized. The effects of pressure and pad speed on surface quality and material removal rate(MRR) of CaF2 crystal in lapping were studied. The optimal parameters are lapping pressure 10 kPa and pad speed 40 rpm,which the surface roughness Ra of CaF2 crystal lapped is 86.6 nm, and MRR is 975 nm/min. The effects of diamond abrasive and cerium oxide abrasive FAP, different diamond abrasives and different types of matrix on polishing of CaF2 crystal were investigated and analyzed. The 3-5 ?m diamond abrasive and soft type ? matrix were selected for optimized FAP.(2) Abrasive-free slurries were optimized. The effects of acid, neutral and alkaline slurries on fixed abrasive polishing of CaF2 crystal were studied. And the alkaline slurry was suitable for polishing of CaF2 crystal. Then the effects of five alkaline slurries on fixed abrasive polishing of CaF2 crystal were studied. Na3PO4 slurry was suitable for polishing of CaF2 crystal, which the surface roughness Sa of CaF2 crystal polished is 4.13 nm.(3) The parameters of fixed abrasive polishing of CaF2 crystal were optimized. Orthogonal experiment was used to study the effects of polishing pressure, slurry pH, pad speed and slurry flow on polishing of CaF2 crystal. And the optimal parameters are polishing pressure 6.7 kPa, slurry p H= 9,pad speed 40 rpm and slurry flow 60 ml/min, which the surface roughness Sa of Ca F2 crystal polished is 3.02 nm and MRR is 206 nm/min.(4) A material removal model of fixed abrasive polishing was set up based on chemical action. A metamorphic layer formation and removal model was set up by taking chemical action intoconsideration. And the relation between the metamorphic layer formation/removal and material characteristics of workpiece, concentration of chemical additives in slurry are made sure. There is a linear relation between polishing pressure and the cutting depth of abrasive without effects of chemical action. Finally, a material removal model fixed abrasive polishing was modified based on chemical action, and the reliability of the modified model was experimentally verified.