Critical condition of subsurface crack formation for optics

Based on indentation experiments and continuous stiffness method, the relationship curves between indentation depth and hardness/elastic modulus were obtained for fused silica optics. The process of ductile to brittle transition was systematically analyzed, and the critical loads and critical depths were determined in static/quasi-static indentation or dynamic scratch process. The results of gradient load scratch experiments show that the crack induced by scratch process has a strong dependence on the normal load, and the material removal is ductile removal with a small load. With the increase of normal load, the median cracks perpendicular to the specimen surface and lateral cracks parallel to specimen surface are produced, while no obvious features are observed on specimen surface. Lateral cracks are extended and the bright regions are formed, eventually the radial cracks are induced, whose propagation directions are perpendicular or approximately perpendicular to indenter movement direction, so brittle material removal is achieved.