Mechanisms of force magnetic shear combined with chemical rheological polishing (FMS-CRP): A case study in sapphire processing

Sapphire crystals are extensively used in laser high-energy systems due to their exceptional optical properties. However, achieving high surface quality and minimal damage in sapphire crystals is extremely challenging. This paper presents a novel method, force magnetic shear combined with chemical rheological polishing (FMS-CRP) based on shear-induced thickening and magnetically-induced thickening combined with chemical interaction, designed to enhance the quality of sapphire. A model of polishing pressure (P_d) in the FMS-CRP zone was developed based on Reynolds and magnetisation equation. The material removal rate (MRR) was derived from active abrasive theory. According to FMS-CRP experiments, the maximum variance between theoretical and experimental values was 8.6%, confirming the validity of the MRR theoretical model. The risk of subsurface damage (SSD) was mitigated using maximum depth of cut and crack depth theories. Material Studio (MS) software simulations, along with X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR), were used to analyse the complexation reaction process of sapphire and to identify the composition of the chemically softened layer. Under optimal polishing conditions (D_a = 4 μm, pH = 10, h₀ = 1.0 mm, T = 25 °C, w_a = 30 wt%, v_f = 2.5 m/s, and B = 300 mT), the accuracy of sapphire faceting significantly improved, achieving a surface roughness of R_a = 0.2 nm and a peak-to-valley (PV) value of 10 nm. SSD was controlled within 0.5 μm, ensuring excellent surface quality. Thus, the FMS-CRP processing method is shown to produce high-precision sapphire crystals with substantially improved surface quality and controlled subsurface damage.