KH₂PO₄, or KDP, is an important electro-optic material. It is currently used for frequency conversion of the OMEGA laser at LLE, and it will be used for the same purpose in the National Ignition Facility (NIF) currently under construction at LLNL. It is commonly used in electro-optic devices such as Pockels cells. KDP is representative of a class of optical crystals which are difficult to polish because they are soft and water-soluble.

Single point diamond turning (SPDT) is the current state-of-the-art process for finishing KDP. Surfaces with an rms roughness of 1 to 3 nm are achieved for flat plates up to 30 cm in diameter. SPDT is done by showering mineral oil over the KDP plate and cutting the surface with a stiff, multi-million dollar CNC machine. The oil, a flammability hazard, is removed with toluene or xylene.

The ability to readily change the MRF carrier fluid from water to a non-aqueous liquid opens up the possibility for processing soft, water-sensitive parts. A search for a carrier fluid evaluated candidates for compatibility with KDP and the MRF process. A number of water-miscible fluids tested were found to leave a "fog" on the surface of KDP. Even just a few minutes contact with 200-proof ethanol transferred enough moisture from the air to leave visible defects. A dicarboxylic acid ester was eventually identified as a successful carrier fluid. It has a very low vapor pressure, a low viscosity, it does not evaporate, and it is easily cleaned out of an MRF machine. Single crystal plates of potassium dihydrogen phosphate (KDP) have been successfully polished, demonstrating that MRF can smooth to the nm level. It also removes the tool marks left from SPDT.

Removal of Tool Marks Left from Single Point Diamond Turning of a KDP Crystal
Diamond-Turned	polished with MRF

S. D. Jacobs, “Manipulating mechanics and chemistry in precision optics finishing,” Science and Technology of Advanced Materials   8 153-157 (2007).

S. R. Arrasmith, I. A. Kozhinova, L. L. Gregg, H. J. Romanofsky, A. B. Shorey, S. D. Jacobs, D. Golini, W. I. Kordonski, P. Dumas, and S. Hogan, "Details of the Polishing Spot in Magnetorheological Finishing (MRF)," in Optical Manufacturing and Testing III, edited by H. Stahl (SPIE, Bellingham, WA, 1999), Vol. 3782, pp. 92-100.