Optimal Magnetorheological Fluid for Finishing of Chemical-Vapor-Deposited Zinc Sulfide

Sivan Salzman

Hopeman 224

Magnetorheological finishing (MRF) of polycrystalline, chemical-vapor-deposited (CVD) zinc sulfide (ZnS) optics tends to leave visible surface artifacts known as “pebbles” (image below). These artifacts are a direct result of the material’s inner structure that consists of cone-like features that grow larger (up to a few millimeters in size) as deposition takes place. This work explores ways to improve the surface finish of CVD ZnS processed with MRF through modification of the magnetorheological (MR) fluid’s properties. A materials science approach is presented to define the anisotropy of CVD ZnS through a combination of chemical and mechanical tests with single crystal samples of ZnS, whose cuts and orientations represent most of the facets known to occur in the polycrystalline CVD ZnS. Magnetorheological finishing experiments were performed to explore the influence of material anisotropy on the material removal rate during MRF. By adjusting fluid’s viscosity, fluid’s pH, and abrasive type and concentration to find the chemo-mechanical conditions that equalize removal rates among all single crystal facets during MRF, an optimized and novel MR formulation was developed to finish CVD ZnS with minimal surface artifact and roughness.