Water dissolution ultra-precision polishing of KDP crystal and its precision cleaning

KDP crystal is a kind of excellent electro-optic material used in many laser facilities, which is also widely acknowledged to be extremely hard to machine. In this work, we developed a process to finish KDP crystal to a precision and clean engineering specimen. A micro-nano water dissolution principle and its planarisation mechanism in the machining process are illustrated, which turns the disadvantaged deliquescence property of KDP crystal into the driving force for ultra-precision polishing of the crystal. Micro emulsion fluid with nano water nuclei can precisely control the material removal and realize the selective polishing, thus forming a super smooth and precise surface. Then a subsequent cleaning process developed specially for this ultra-precision polishing method helps to get rid of the polishing fluid, and finally reduces the residue off the crystal to a clean and tidy surface with 1.964 nm rms roughness for engineering application.