Subsurface damage measurement of single crystal germanium and borosilicate glass BK-7

We present a new experimental method for measuring subsurface damage (SSD) on ground surfaces of single crystal germanium and borosilicate glass BK-7 based on the morphology and mechanical properties dependence on depth into the material. The material selection allows us to compare crystalline and non-crystalline materials. We use spots of different penetration depths on ground surfaces by Magnetorheological Finishing (MRF) spots to remove part or all of the damaged layer, and then evaluate the surface roughness, fracture toughness and material removal rate (MRR) of MRF at the deepest point penetration of MRF fluid into the spot. The dependence of these results on penetration depth into the material reveals the subsurface damage of the surfaces. It is shown that the subsurface damage depths revealed by each property (surface roughness, MRR, fracture toughness) match each other.