Mechanism and experimental study of negative pressure rotating jet-assisted light-induced colloidal polishing of calcium fluoride

Addressing the challenges encountered in machining calcium fluoride (CaF ₂ ) crystals on ultra-smooth surfaces, this paper proposes a negative-pressure rotating jet-assisted polishing method to enhance the processing efficiency and surface quality of photo-induced colloidal jet polishing. A nozzle structure incorporating this functionality was designed. First, the material removal mechanism synergizing photo-induced chemical adsorption with jet impact shear is elucidated. Second, computational fluid dynamics (CFD) analysis compares the flow field characteristics (including velocity, pressure, and turbulence intensity distribution) between negative-pressure rotating jet and conventional rotating jet nozzles. Finally, polishing experiments evaluated the actual improvement effects of both nozzles on the surface morphology and roughness of CaF ₂ crystals using atomic force microscopy (AFM). Simulation results indicate that the negative-pressure rotating jet nozzle effectively suppresses turbulent diffusion, with an exit turbulence intensity (15.5) significantly lower than that of the conventional nozzle (20.9). It also exhibits greater jet vorticity and a more stable flow field. Experimental results demonstrate that after polishing with this nozzle, the surface roughness Sa of CaF ₂ workpieces decreased from an initial 1.92 nm to 1.36 nm, while the maximum peak-to-valley height Sz was substantially reduced from 457 nm to 33 nm. Surface flatness was significantly improved, with overall performance surpassing that of conventional rotating jet nozzles. Both simulation and experimental results confirm that this technology significantly enhances polishing uniformity through stable flow fields, providing reliable technical support for achieving ultra-smooth surface processing of calcium fluoride crystals.