Influence of water on the machinability of calcium fluoride crystal

Calcium fluoride (CaF2) optical components are produced by ultra-precision machining, but the often-neglected ambient water may potentially affect its processing. This work provides a systematic study on the role of water in the machinability of single-crystal CaF2. Micro-scratching experiments revealed poor machinability and reduced deformability under water adsorption. Atomic scratching simulations reproduced the mechanical deformation and identified the enhancement in dislocation nucleation with water, which led to the reduction in deformability due to dense cross-linking of dislocations. Atomic scale cutting simulations also identified easier formation of subsurface cracks under water adsorption. With the aid of mechanical schematic models based on the simplification of the scratching process, this work revealed that while water enhances the plastic flow of the material during machining, the accumulation of dislocations creates stress conditions sufficient to induce material fracture, which is a unique influence of water adsorption on the reduction in machinability of CaF2.