Micro water dissolution machining principle and its application in ultra-precision processing of KDP optical crystal

In this paper, a micro water dissolution machining （MWDM） principle is proposed for machining potassium dihydrogen phosphate （KDP） crystal using water-in-oil micro-emulsion as an abrasive-free polishing fluid. In addition, two instances of the application of this principle to ultra-precision machining of KDP crystals are presented. Computer-controlled optical surfacing （CCOS） and diamond wire cutting （DWC） process were carried out according to the MWDM principle. In the case of the CCOS technology, it is found that the micro-waviness was removed completely by following the MWDM principle. The surface undulation decreased from 40 nm to less than 10 nm, and the surface root-mean-square （rms） roughness obviously reduced from 8.147 to 2.660 nm. In the case of the DWC process, the surface rms roughness reduced from 8.012 to 2.391 gm, and the cutting efficiency was improved. These results indicate that the MWDM principle can efficiently improve the machining quality of KDP optical crystal and has a great potential to machine water-soluble materials.

Investigation on the cleaning of KDP ultra-precision surface polished with micro water dissolution machining principle

A potassium dihydrogen phosphate(KDP) optical crystal was machined to an ultra-precision surface with water-in-oil(W/O) micro emulsion polishing fluid. The micro water dissolution principle utilized in the machining process is discussed, its planarization mechanism is illustrated, and an ultra-precision polished surface with 2.205 nm RMS roughness is obtained. However, a substantial quantity of residual contamination remained on the polished surface after machining. This can seriously impact the optical performance of the crystal, and so it must be removed. Fourier transform infrared(FTIR) spectroscopy was used to conduct an investigation into the composition of the surface residue, and the results showed that the residue was comprised of organic chemicals with hydrocarbon chains and aromatic ether, i.e., mostly the polishing fluid. The cleaning method and the principle on which the KDP ultra precision surface investigation is based are discussed in detail, and the cleaning experiments with selected KDP-compatible organic solvents were then performed. FTIR transmittance spectra measurement and microscopic observations were employed to assess the effects of the cleaning process on the surface of the KDP crystal. The results showed that toluene cleaning achieved the most desirable results. This cleaning method produced a surface roughness of 1.826 nm RMS, which allows the KDP crystal to be applied to subsequent engineering applications.