Ultra-precision lapping of H₂O(g) plasma-treated CaF₂ by porous diamond grits

The calcium fluoride (CaF₂) component has excellent light transmission and a high laser damage threshold in the UV band, and thus is a potentially ideal material for high-power laser facility. However, the low-damage surface is difficult to attain via the existing processing technologies due to its brittleness, especially the dissociation property of the (111) surface. We innovatively propose H₂O(g) plasma-assisted porous diamond abrasive lapping technology to achieve low-damage and high-precision surface processing of CaF₂ component, which means that the CaF₂ component is modified first and then the softened modified layer is removed by the porous diamond abrasive lapping. The H₂O(g) plasma modification mechanism was elucidated through a combination of molecular dynamics simulation and experiment and comparison with H₂O(g) plasma etching. The modification layer including an oxide layer and a phase change layer was produced by adjusting the plasma processing process (power of 25 W and H₂O(g) content of 10 ml). Then the modified CaF₂ component was lapped by the porous diamond abrasive tool on the ductile domain layer which is more than the modified layer and less than the sum of the modified layer and plastic domain. When the lapping process parameters were 200 rpm platen rotation speed and 20 N lapping pressure, a surface accuracy of 15.34 nm PV, 2.29 nm RMS (φ1 mm) was obtained. This study provides technical support for the low-damage processing of CaF₂ component.