Characterization of fracture toughness and micro-grinding properties of monocrystal sapphire with a multi-layer toughening micro-structure (MTM)

The application of sapphire in micro-machining is often limited due to its high brittle feature. This study proposes and successfully fabricates a novel multi-layer toughening micro-structure (MTM) sapphire for improving its machinability in micro-grinding. A predicting model, which quantitatively predicts toughening effect Ujc, is built based on both toughening theory and regression method. The fabrication procedures of MTM sapphire with different micro-structures (LOCA& slots, only LOCA) are presented and LOCA by ultraviolet curing plays a vital role in this paper. Results from static tests of a fatigue testing machine show that MTM sapphires have particular fracture behaviors compared with normal monocrystal sapphire. The breaking force Fc is enhanced from 1.49 kN to 7.29 kN and 16.75 kN. A central circle fracture and an annular breakage are observed, and all crack propagations are perpendicular to the annular breakage. A group of experiments to investigate the fracture behavior and toughening effects in micro-grinding of MTM sapphire are carried out by this study. The toughening effects in micro-surface grinding of MTM sapphire with large processing parameters are significant and big brittle fractures are reduced compare with normal sapphire. A damping effect is found in micro-grinding force of MTM sapphire, the amplitude and average value of micro-grinding force are all reduced comparing with normal sapphire, Fy shows the largest damping effect, the analytical coefficients of toughening effect are also given by this study. Finally, a micro-scale thin-wall (150 μm width, 900 μm depth, 2000 μm length) is achieved by micro-grinding of MTM sapphire, it is demonstrated that the method which this study proposes successfully enhances the machinability in micro-machining of sapphire without harming its optical performance.