| The thesis initiates a systematic study of the microindentation method for the measurement of near-surface mechanical properties of optical materials. These material properties can be used to predict the material's response in optical manufacturing operations involving elastic, plastic, and fracture processes, mainly the optical fabrication phenomena in fixed and loose abrasive microgrinding. Experimental observations regarding the indentation size effect, indenter geometry effect, hardness anisotropy of single crystals, and coolant effect, are reported. Applications in deterministic microgrinding, the Twyman effect, and the lapping hardness are investigated. Surface roughness under deterministic microgrinding conditions (fixed infeed rate) at the Center for Optics Manufacturing with bound abrasive diamond ring tools with various degrees of bond hardness is correlated to a material length scale, identified as a ductility index, involving the hardness and fracture toughness of glasses. This result is in contrast to loose abrasive grinding (fixed nominal pressure), in which surface microroughness is determined by the elastic stiffness and the hardness of the glass. In the Twyman effect, the results indicate that in brittle grinding conditions the grinding force and the depth of the compressive layer correlate well with glass mechanical properties describing the fracture process, such as indentation crack size. The maximum surface residual compressive stress decreases and the depth of the compressive layer increases with increasing abrasive size. In lapping conditions the depth of the abrasive grain penetration into the glass surface scales with the surface roughness, and both are determined primarily by glass hardness and secondarily by Young's modulus for various abrasive sizes and coolants. In the limit of small abrasive size (ductile-mode grinding), the maximum surface compressive stress achieved is near the yield stress of the glass. An interpretation of the lapping hardness of commercially available optical glasses in terms of a micromechanics model of material removal by subsurface lateral cracking is also presented. The results show that lapping hardness is a function of the glass mechanical properties involving the hardness, fracture toughness, and Young's modulus of glasses. |
