Origin of material removal mechanism in shear thickening-chemical polishing

Abstract

A shear thickening-chemical polishing (ST-CP) approach exploiting the recombination mechanism of shear-thickening and chemical-physical friction is proposed for ultraprecision machining of optical materials. The ST-CP slurries with dynamic rheological behaviour are characterized, and the optimal preparation process is explored for high-efficiency polishing of workpieces. A critical shear rate (CSR) prediction model in the flow field of slurries is systematically investigated and experimentally verified in detail. A mathematical control of the material removal rate (MRR) is modelled and developed for ST-CP. The shear-thickening-induced micro-cutting and chemical-physical friction contribute to the material removal mechanism in the ST-CP process. A special chemical reaction layer consisting of Li2O and Nb2O7 evoked on the workpiece, which can soften the surface layer of lithium niobate (LiNbO3), increases the chemical-physical friction and material removal through micro-cutting and shearing. The material removal process in ST-CP is a dynamic equilibrium process in which atoms of the workpiece surface are continuously involved to form new substances or oxides to achieve a soft chemical reaction layer, accompanied by the shear-thickening-induced micro-cutting action. A series of ST-CP experiments validate that the maximal error between theoretical and experimental data is less than 11.5%, which shows the high degree of accuracy of the MRR prediction model. Measurements and calculations are performed to explore the effects of shearing velocities, Al2O3 content, pH value, and oxidant content on surface roughness and MRR. When the shear-thickening induced micro-cutting and chemical reaction reach a dynamic equilibrium, a maximum MRR of up to 65.8 mg/h is achieved, and surface roughness is significantly reduced within 120 min from Ra 36.04 nm–1.46 nm with low subsurface damage (<5 nm). This investigation reveals that ST-CP is a progressive ultra-precision manufacturing approach for optical polishing and finishing of crystal materials.