A new process chain for ultra-precision machining potassium dihydrogen phosphate (KDP) crystal parts

摘要

Potassium dihydrogen phosphate (KDP) crystal parts used in high power laser systems require high figure accuracy and high laser induced damage threshold (LIDT). However KDP crystal is extremely soft, hygroscopic, brittle and thermally sensitive, which make it difficult to meet the requirements via conventional processing methods. This paper puts forward a new process chain for ultra-precision machining KDP crystals, including single point diamond turning (SPDT), magnetorheological finishing (MRF) polishing and ion beam figuring (IBF) polishing processes. A compensation SPDT process is developed as the first step of the process chain to reduce machining errors (due to vacuum suction force, spindle unbalance, etc.). As a result high shape accuracy and fine surface roughness is obtained with high machining efficiency Non-aqueous and abrasive-free MRF polishing process is then employed to remove the diamond turning marks and further improve shape accuracy and increase the LIDT. Ion beam figuring (IBF) polishing is introduced as the final step of the process chain to remove the impurity layer. Experiments are carried out to evaluate the effectiveness of the proposed ultra-precision process chain.