On the improvement of subsurface quality of CaF2 single crystal machined by boron-doped nano-polycrystalline diamond tools

The subsurface quality of single-crystal calcium fluoride (CaF2) is examined by ultra-precision cutting experiments using a boron-doped nano-polycrystalline diamond (B-NPD) tool and compared with that of CaF2 machined with a single-crystalline diamond tool (SCD). The critical depth of cut, surface morphologies, and process force did not change significantly in the plunge-cut tests. In the ultra-precision cylindrical turning tests, the surface roughness was almost the same, whereas the subsurface damage depth (SSD) changed remarkably, from ten to several hundred nanometers. The SSD depth was reduced by the use of B-NPD. Additionally, shear deformation and rotational deformation of the subsurface layer were observed. The lowering of friction coefficient at the interface between the cutting tool and the machined surface is assumed to affect the subsurface layer.