Research of Subsurface Damage Depth of Lithium Niobate Crystal by Fixed-Abrasive Lapping

Abstract

The subsurface damage depth of optical material introduced by lapping determines the efficiency of lapping and polishing process. In order to explore the subsurface damage (SSD) of fixed-abrasive lapping process, angle polishing method was employed to evaluate the SSD depth and damage form of soft-brittle material lithium niobate (LN) crystal. The influences of the grit size, the applied load and the hardness of pad matrix on SSD were investigated experimentally. The results show that: with the grit size decreasing form W28 to W14, the damage depth decreases by 70%. With the applied load decreasing form 15 kPa to 7 kPa, the SSD decreases by 40%. The dense and deep subsurface micro-cracks are caused by the hard matrix pad during lapping processing, while the matrix with low hardness causes plastic scratches on the surface/subsurface of the workpiece without obvious micro-cracks around the scratches. This study affords effective method for SSD evaluation and control of soft brittle materials such as LN crystal.