Ultrafine polishing of optical polymer with zirconia-coated carbonyl-iron-particle-based magnetic compound fluid slurry

Optoelectronic devices made using precision polymers must be polished to achieve nanolevel surface roughness for proper device operation. One promising novel finishing technique is magnetic compound fluid (MCF) polishing. An MCF is a mixture of magnetic fluid (MF) and magnetorheological (MR) fluid, both of which use the same solvent. Because conventional carbonyl iron particles (CIPs) used in common water-based MCFs lack the ability to prevent aqueous corrosion, zirconia (ZrO2)-coated CIPs are used in MCFs. For ultrafine polishing of polymethylmethacrylate (PMMA), surface roughness values achieved using ZrO2-coated CIP-based MCF slurries of various compositions were investigated. For comparison, the surface roughness values achieved with conventional non-coated CIP (i.e. HQ CIPs by BASF Japan Ltd.)-based MCF slurry and ZrO2-coated CIP-based MR fluid slurry were examined as well. The results are explained in terms of the status of the used MCF slurries and the generated normal forces during polishing. The use of relatively large abrasive particles (alumina (Al2O3)) in the ZrO2-coated CIP-based MCF slurries did not achieve better surface roughness and higher improvement rates than the conventional HQ CIP-based MCF slurry. Better surface roughness and a higher improvement are attained with MCF slurry than MR fluid slurry, because the magnetite particles within the MCF slurry help distribute the abrasive particles uniformly and stiffen magnetic clusters. In the absence of both Al2O3 abrasives and α-cellulose, better surface roughness is achieved using ZrO2-coated CIP-based MCF slurry; the best surface roughness of Ra 0.913 nm is achieved with a CIP concentration of 70 wt.% at a considerable improvement rate. The ZrO2-coated CIP concentration should be less than a certain value (in the current work, 70 wt.%); otherwise, the MCF slurry will show bad particle dispersion ability and be prone to dryness, which will result in the loss of polishing ability.