Magnetic-Assisted Polishing for High-Precision Fused Silica Glass: A Process Optimization Study

1. Introduction 'Fused silica glass' is a typical wide-bandgap dielectric material, widely used in the production of devices such as gratings, vacuum windows, shielding plates, and lenses. To achieve mass production of high-precision 'fused silica glass', the process of 'grinding shaping → precision grinding → ultra-precision polishing' is commonly used domestically and internationally. Polishing, the final step in precision/ultra-precision manufacturing of optical materials, significantly impacts component processing quality and surface integrity.

Traditional chemical-mechanical polishing achieves smooth and flat surfaces through the combined action of chemical and mechanical energy. However, the normal cutting action of abrasive particles during workpiece processing can lead to mechanical defects like scratches, micro-cracks, or pits below the surface. These defects are susceptible to inducing material melting and explosive damage under strong laser irradiation.

In recent years, magnetic-assisted polishing, which utilizes fluid dynamic pressure shearing for material removal, has gained considerable attention from scholars worldwide. This technique has evolved into a method for achieving 'near-zero' defect surfaces on high-power optics. Based on the different magnetic particles (micron-sized iron powder or nano-sized iron oxide) in the polishing fluid, magnetic-assisted polishing techniques are primarily classified into three types: magnetorheological fluid (MRF), magnetic fluid (MF), and magnetic composite fluid (MCF) polishing.

Magnetic-assisted polishing employs magnetic particles, non-magnetic abrasive particles, cellulose, and deionized water to create a viscous semi-solid flexible polishing head. The abrasive particles beneath the polishing head come into contact with the workpiece, engaging in relative motion and micro-cutting, resulting in low damage and high-precision polishing. Shi et al. analyzed the feasibility of elastic MRF polishing based on the theory of elastic-plastic deformation. They achieved chemical-dominant elastic MRF polishing of large-diameter 'fused silica glass' by adjusting the composition of the magnetorheological fluid and polishing parameters, ultimately attaining a super-smooth surface with a roughness Ra of 0.167nm.

Jiang et al. compared the differences in normal polishing force, tangential polishing force, material removal rate, and surface roughness of the workpiece during traditional MCF polishing and ultrasonic-assisted MCF polishing. Their findings indicated that ultrasonic-assisted polishing effectively enhances the material removal rate and surface smoothness of the workpiece. Guo et al. demonstrated that the material removal rate of BK7 glass during end-face MCF polishing is directly proportional to the speed of the carrier disk and inversely proportional to the polishing gap. They also established a material removal rate model related to the workpiece's tangential force and normal force.

These studies, both theoretical and experimental, have investigated the material removal mechanism and surface quality formation mechanism of magnetic-assisted polishing technology, driving the adoption of this technique in optical manufacturing.

This research examines the influence of varying polishing gaps and iron powder volume ratios on the material removal rate, surface roughness, and transmittance of 'fused silica' elements during magnetic-assisted polishing. Through analyzing these parameters and spatial magnetic induction intensity simulation, the study clarifies the impact of spatial magnetic induction intensity and iron powder volume ratio on material removal efficiency and surface quality. The research proposes a novel polishing process: 'small polishing gap + high iron powder ratio polishing fluid → large polishing gap + low iron powder ratio polishing fluid'. This process provides a theoretical foundation and technical support for achieving efficient and low-defect processing of high-power laser elements.

2. Experiment

2.1 Sample Preparation In this experiment, magnetic-assisted polishing was performed on 'fused silica glass' elements using a self-built circular polishing machine. The polishing equipment, depicted in Figure 1, consists of a horizontal main spindle that drives a ring-shaped neodymium-iron-boron (Nd-Fe-B) magnet with a magnetic flux density of 0.4T to rotate at a speed of nt, creating a spatial dynamic magnetic field. A ring-shaped polylactic acid (PLA) baffle is installed on each end face of the magnet. Both the magnet and the baffle have an outer diameter of 40mm and an inner diameter of 25mm, with thicknesses of 8mm and 4mm, respectively. The ring-shaped baffle and ring-shaped magnet collectively form a polishing wheel, with a polishing gap δ between the wheel and the workpiece below it.

A 40mm × 40mm × 5mm 'fused silica glass' specimen was used as the processing object. Before polishing, the workpiece underwent double-sided grinding with W5-W10 silicon carbide, achieving an initial surface roughness Ra of 0.2-0.25μm and a sub-surface crack depth within 4.5μm.