Chemical mechanical polishing for potassium dihydrogen phosphate using four kinds of green developed slurries

Due to the soft-brittle, deliquescent, and temperature-sensitive nature, potassium dihydrogen phosphate (KDP) has become one of the most difficult-to-process materials. Nonetheless, high-performance devices strictly require the surface roughness of KDP less than 1 nm, which is a great challenge for the ultraprecision machining field. To overcome this challenge, four kinds of green slurries were developed. For instance, ceria and graphene oxide were selected as abrasives, sodium carboxymethylcellulose was used as thickener, fatty alcohol-polyoxyethylene ether was employed as emulgator, sodium carbonate was proposed as a pH regulator, and tiny water was applied to dissolving agent. Four kinds of various base solutions, such as corn oil, glycol, polyethylene glycol and glycerol, form four varieties of green slurries for the chemical mechanical polishing (CMP) of KDP. After CMP, the surface roughness Sa is 0.562 nm with a scanning area of 50 × 50 μm2, using the glycol slurry. Until now, to the best of our knowledge, this is the lowest surface roughness at a such large measurement area. X-ray photoelectron spectroscopy and infrared Fourier transformation confirm that CP bonds were generated between graphene oxide and KDP, resulting in the breaking away of H2PO4− from KDP. A soft layer was produced by the tiny water. CO32− reacts with H2PO4−, forming CO2 gas and protecting the concave area on the surface of KDP. The soft layer was removed by the synergistic effect among abrasives, thickener and a polishing pad. The developed green slurries suggest new insights to achieve sub-nanometer surfaces on KDP with uniquely physical nature, for the potential use in laser systems.