Ultrafine Polishing of Electroless Nickel–Phosphorus-Plated Mold with Magnetic Compound Fluid Slurry

Abstract

Electroless nickel–phosphorus (Ni–P)-plated mold for hot press of optical lens must be manufactured to nanolevel surface roughness. The single-crystal diamond turning (SCDT) process is generally used to machine the Ni–P plating layer to attain the required form and ultrafine surface finish. However, this process frequently causes concentrically arranged tool marks to appear on the work surface. In this study, the Ni–P plating layer was polished with magnetic compound fluid (MCF) slurry to remove the tool marks. Two MCF slurries were used, one containing high-quality carbonyl iron particles (CIPs) and alumina (Al2O3) abrasive particles (1 µm), and the other containing CIPs coated with zirconia (ZrO2) abrasive particles (50–100 nm). First, spot polishing without relative motion between the center of the MCF carrier and the workpiece was carried out using both slurries. Then, scanning polishing with the MCF carrier moving along a motion path was carried out using only the MCF slurry containing ZrO2-coated CIPs. Although both slurries removed the SCDT-induced tool marks on the work surface, the MCF slurry containing relatively large Al2O3 abrasive particles left scratches and frequently caused CIPs to be embedded in the Ni–P plating surface, which inversely worsened the work surface roughness. In contrast, the MCF slurry containing relatively small ZrO2 abrasive particles resulted in an improved work surface roughness, without scratches or the embedding of particles. The cross-sectional profile of the polishing spot manifests a characteristic symmetrical W-shape. The dot-shaped scratches were induced at the spot center, and the lengths of these scratches increase with the increase in the distance from the spot center. In scanning polishing, the W-shaped polishing spot moves along the designed scanning path and instantaneous overlapping occurs, which significantly improves the flatness of the Ni–P plating layer from 0.2 to 0.1 µm. In addition, almost all the particles can move relative to the work surface, and the surface finish of the Ni–P plating layer was improved, without scratches or the embedding of particles. The preliminary results show that the MCF polishing is applicable to the nanolevel finishing of soft magnetic materials.