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Статья. Опубликована в журнале "LLE Review". – 2004. – Vol. 101 – P. 35-42.
Название на русском языке: Минимизация образования артефактов при магнитореологическом полировании плоскостей при химическом осаждении ZnS.Авторы: I.A. Kozhinova, H. J. Romanofsky, A. Maltsev, S.D. Jacobs, W.I. Kordonski, S.R. Gorodkin.Аннотация к статье на английском языке: The finishing performance of magnetorheological (MR) fluids prepared with a variety of magnetic and nonmagnetic ingredients was studied on CVD ZnS flats from different manufacturers. Surfaces studied were used as received, after polishing on pitch, after single-point-diamond-turning (SPDT), and after deterministic microgrinding. MR polishing using hard CI and standard MR fluid chemistry yielded a surface with high roughness and pebble-like structures. The severity of pebble-like decoration was shown to be related to the processed side of the ZnS puck. Pebbles were more pronounced on the free surface farthest from the graphite growth mandrel. Experiments on pitch-polished surfaces showed nanoalumina abrasives to be the least detrimental of four nanoabrasives tested. These abrasives were then combined with soft CI and altered MR fluid chemistries to enable several microns of ZnS material to be removed with suppression of pebble-like decoration. Surfaces that were initially microground, diamond turned, or pitch polished were all processed with this altered MR fluid to ~20 nm p–v and 2-nm rms. Diamond-turning marks were eliminated.
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MINIMIZING ARTIFACT FORMATION IN MAGNETORHEOLOGICAL FINISHING OF CVD ZNS FLATS
Minimizing Artifact Formation in Magnetorheological Finishing of CVD ZnS Flats Introduction and Motivation The development of optics for new applications is based on the use of materials that have the appropriate mechanical and optical properties. For infrared (IR) window and dome materials that protect imaging systems against damaging conditions, three main considerations are taken into account: material strength, thermal-shock resistance, and optical characteristics, such as transmission, absorption, scattering, and emissivity. To obtain maximum transmittance, a good IR material should exhibit minimum optical scattering due to a rough surface finish. The synthetic materials that meet the requirements for IR windows could be either single crystals or polycrystalline with cubic crystallographic structure, the latter being usually tougher, which is desired.1 Polycrystalline ZnS is an excellent candidate for these applications. The chemical vapor deposition (CVD) process was developed in the 1970s in response to the need for increased volumes of polycrystalline ZnS with improved transparency.2 The CVD process takes place in a chamber similar to an autoclave at increased temperatures (600°C to 1000°C) and at pressures below 100 Torr. Deposits of solid grains are formed on the inside walls of a graphite mandrel. The size of the grains depends on the deposition temperature and has a dramatic impact on the mechanical properties, such as the erosion rate.3 Two types of CVD ZnS are available on the market from domestic sources: regular grade and multispectral grade. The first type has a grain size of about 2 to 8 µm, is yellow-orange in color, and is used in the 3- to 5-µm-wavelength and 8- to 12-µm-wavelength regions. The “multispectral (or ‘water clear’) grade” ZnS is colorless, exhibits high transmittance over a wider range of wavelengths, and can be used in the 0.5- to 12-µm region of the spectrum. This latter material undergoes a hot isostatic pressing (HIP) process to grow the grain size to 20 to 100 µm, eliminating microscopic voids and other internal defects; however, this refining process leads to a reduction in strength and erosion resistance of the clear material.
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