Surface–texture evolution of different chemical-vapor-deposited zinc sulfide flats polished with various magnetorheological fluids

Abstract

The macro-structure of chemical-vapor-deposited (CVD) zinc sulfide (ZnS) substrates is characterized by cone-like structures that start growing at the early stages of deposition. As deposition progresses, these cones grow larger and reach centimeter size in height and millimeter size in width. It is challenging to polish out these features from the top layer, particularly for the magnetorheological finishing (MRF) process. A conventional MR fluid tends to leave submillimeter surface artifacts on the finished surface, which is a direct result of the cone-like structure.Here we describe the MRF process of polishing four CVD ZnS substrates, manufactured by four different vendors, with conventional MR fluid at pH 10 and zirconia-coated-CI (carbonyl iron) MR fluids at pH 4, 5, and 6. We report on the surface–texture evolution of the substrates as they were MRF polished with the different fluids. We show that performances of the zirconia-coated-CI MR fluid at pH 4 are significantly higher than that of the same fluid at pH levels of 5 and 6 and moderately higher than that of a conventional MR fluid at pH 10. An improvement in surface–texture variability from part to part was also observed with the pH 4 MR fluid.