Accurate measurement of cutting-edge radius on a single-crystal diamond tool

Abstract

Single-crystal diamond (SCD) tools are hard and can maintain a uniform sharp cutting edge and hence used in ultraprecision machining of optics. The tools typically have edge radii in the range of 10-100 nm. Direct measurement of the cutting-edge radius on the SCD tool accurately in the order of tens of nanometers using a scanning electron microscope (SEM) is not possible and arduous using an atomic force microscope (AFM). In this study, we have demonstrated a more accurate edge reversal technique to measure the SCD tool’s cutting-edge radius. A nano indent is made using the SCD tool on two polished pure copper blocks held together in a precise vise. After the nanoindentation, the copper blocks are separated, and the cross-section profile of the indentation mark, a replica of cutting-edge geometry, is observed in an SEM. A fitted circle on the indentation profile is used to obtain the edge radius. Due to the elastic recovery in the material, the indentation profile geometry changes slightly, and the radius measured from the SEM image is not the same as the actual tool radius of the SCD tool. We performed finite element method (FEM) simulations to quantify the amount of elastic recovery and calculate the elastic recovery factor for a given SCD tool geometry as a function of indentation depth and edge radius. From the simulation results, we found the error due to elastic recovery effect in edge radius measurement is only 10%. Due to the stick-slip condition and high stress concentration factor at the cutting edge of the tool, material pull-out is observed at the middle of the cutting mark on the substrate but at the ends of cutting mark on the substrate material was negligible. In this method, compared to the previous literature work the three primary sources of errors, elastic recovery effect, convolution effect of AFM tip, and material pull-out are eliminated. Hence, this method provides a more accurate measurement of the edge radius and can be adopted by research labs, universities, and industries having ultra-precision machining facilities without needing a special purpose SCD tool edge radius characterization facility.