A novel water dissolution combined continuous polishing for soft-brittle crystals

Soft-brittle crystals such as KH₂PO₄ (KDP) pose significant challenges to traditional machining techniques due to their inherent brittleness and substantial sizes. To surmount these challenges, this paper presents the water dissolution combined continuous polishing (WDCCP) method based on the water dissolution principle for achieving high-efficiency and high-quality machining. The WDCCP process is a three-step methodology including initial precision polishing with an aqueous solution, subsequent ultra-precision polishing utilizing a water-in-oil microemulsion, and final cleaning with anhydrous ethanol. Comprehensive analyses of the slurry’s microstructure and viscosity are conducted to reveal the surface planarization mechanism. Multiple slurry dispensers ensure a more uniform distribution of various slurries across the interface between the KDP crystal and the pad, thereby enhancing surface quality. Experimental trials conducted on a large numerical control polishing machine demonstrate that precision polishing can eliminate sawn surface defects within a mere 5 min. The ultra-precision polishing significantly reduces the surface roughness root mean square (rms) from 474.974 to 2.779 nm and the peak-to-valley (PV) from 29.859 to 48.949 nm. After WDCCP, the integral morphology of a 180 mm × 180 mm KDP crystal exhibits a PV of 1.938 waves and an rms of 0.264 waves. The WDCCP method emerges as a potent and efficient solution for the machining of large-sized soft-brittle crystals.