Enhancing Ductile-mode Cutting of Calcium Fluoride Single Crystals with Solidified Coating

Positive improvements have been observed during machining brittle materials under high hydrostatic pressure, but the techniques to achieve such desirable effects often utilize complex and expensive equipment or tools. This work presents a cost-efficient method to achieve ductile-mode machining of brittle materials at higher uncut chip thicknesses, by the application of a solidified coating on workpiece surface before a machining process. Orthogonal microcutting experiments were conducted on calcium fluoride single crystals oriented with the (111) plane and an increase in critical uncut chip thickness was observed with the solidified coating. The primary cause has been resolved to be mechanical-related and results in a stabilized microcutting process. Transmission electron microscopy provided evidence of slip deformation occurring in the machined subsurface regions and a layer thickness of subsurface damages reduced by ~ 45% under the influence of the solidified coating. In addition, erratic fluctuations in direction of the resultant machining force were subdued with the applied coating, which is proved to be caused by the compressive stresses induced from the sandwiching of the CaF2 material between the tool and the solidified coating. The proposed technique successfully reduces the cost and pollution in the fabrication process of optical components from the use of coolant in an ultraprecision machining process to the time consumed by eliminating the subsurface damage with abrasive slurries in post-machining polishing.