Effect of different grinding strategies on subsequent polishing processes of sapphire

The hard-to-process properties associated with high hardness and high brittleness of sapphire drastically limit the services of optics, low-damage or damage-free properties are essential to the performance of sapphire optics. To achieve efficient and damage-free sapphire optics, a series of experiments involving ultra-precision grinding, polishing, and ion beam polishing were conducted. Initially, the surface and subsurface characteristics of sapphire ultra-precision grinding were examined, and the evolution of damage during polishing was discussed. Chemical etching was performed to evaluate the subsurface damage caused by polishing, ion beam polishing was used to eliminate subsurface damage. Finally, focused ion beam (FIB) and transmission electron microscopy (TEM) were employed to analyze the atomic structure of the subsurface. The results demonstrated that ultraprecision grinding can produce a ductile grinding surface and minimize subsurface damage, the difference in grinding damage on the outside and center of the workpiece was suppressed. The evolution of the surface roughness and damage behavior during polishing varied depending on the specific grinding strategy, ultraprecision grinding decreased damage removal time by 1/4. Photothermal absorption was utilized to monitor the damage progression during the polishing process and detect the presence of subsurface damage beneath the Beilby layer. Etching revealed that the subsurface damage was primarily caused by scratches, which was effectively eliminated after ion beam polishing. High-magnification TEM images provided compelling evidence of damage at the close-to-atomic scale for sapphire optics. Only amorphous structures smaller than 2 nm were observed on the subsurface, most of the atomic structure remained intact, albeit with irregular arrangement. Additionally, the grinding marks were mitigated after polishing.