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

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.