Nanofinishing of BK7 Glass Using Magnetorheological Finishing Technique with Solid Core Rotating Tool

Abstract

Surface finishing is a promising method to improve the optical characteristics of a crown glass. BK7 finds its applications in transmissive optics i.e. lens of binocular, lens of microscope, lens of telescope and light emitting diodes. The magnetorheological nanofinishing of optical glasses using solid core tool is found more advantageous than the other advance finishing processes in aspects like precision and accuracy. BK7 is a high quality crown glass and is used where additional benefits of fused silica glass such as in temperature sensitive applications are not required. It has low inclusion content and extremely low bubble, also finds application in lens manufacturing. The present work focuses on nanofinishing of the BK7 glass specimen for ratifying its utility in practical application. In this work, programmable logic controlled 3-axis motion are fed to the magnetorheological (MR) tool for finishing the glass specimen. On tool extremity, a hemispherical shape of MR polishing fluid is generated using magnetic control by an electromagnet. MR polishing fluid consists of cerium oxide abrasive powder product of Cepoll with grade 1663, magnetic iron powder and deionised H2O. Also, it acts as viscoelastic solid under the influence of magnetic field, which assists in reducing the surface roughness of glass upto nano-level range. The magnetorheological (MR) nanofinishing with solid rotating core tool is carried on the BK7 glass of size 10×10×3 mm. Optical properties such as transmittance, absorbance and reflectance of finished BK7 glass are analyzed and found suitable for lens manufacturing. Results of higher quality with excellent finishing are obtained by the present MR finishing process. After 90 minutes of finishing, the surface roughness values Ra and Rq are reduced to 17 nm and 27 nm from the initial values of 41 nm and 57 nm respectively. On considering the results of preliminary experiments and literature survey, the four parameters with three levels design were selected. Response surface methodology is conducted in order to find the optimum process parameters, where box-behnken design (BBD) is applied on those four process parameters and three levels. This research also presents the effects of each process parameters on the percentage change surface roughness such as the effect of composition of CeO2 abrasives in magnetorheological polishing fluid, effects of magnetising current, effect of tool’s (solid core) rotational speed and effects of working gaps. The best surface roughness Ra and Rq values are achieved as 22 nm and 32 nm from the initial of 41 nm and 57 nm at optimum parameters in 30 minutes of finishing time cycle. To study the surface morphology, scanning electron microscopy is performed on BK7 glass with sputter coating of gold on it.