Influence of the Polishing Conditions on the Final Quality of a Single Crystal
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uence of the Polishing Conditions on the Final Quality of a Single Crystal K. R. Muratova, *, E. A. Gasheva, and T. R. Ablyaza a
Perm National Research Polytechnic University, Perm, Russia *e-mail: [email protected]
Received February 2, 2020; revised February 2, 2020; accepted February 2, 2020
Abstract—The polishing of a lithium-niobate single crystal is considered. The influence of the pressure and relative speed in polishing on the productivity and the final quality of the crystal’s end surfaces is investigated. Formulas are derived for the total depth of the layer destroyed in finishing and the minimal time required for its removal. Keywords: polishing, single crystals, pressure, speed, polishing time, surface roughness, productivity DOI: 10.3103/S1068798X20100172
The polishing of brittle optical materials involves removing the layer disrupted by finishing; and establishing surface texture with defects considerably smaller than optical wavelengths (between λ/4 and λ/8). As a result, the surface is transparent and appears ideally smooth to the eye. At present, three main hypotheses exist regarding polishing: microabrasive action at the surface; plastic flow; and physicochemical interaction. Theoretical and experimental findings indicate that the best explanation is microabrasive action at the surface, accompanied by other physicochemical processes [1, 2]. The working surfaces of optical-glass components and crystals are polished with elastoplastic tools made of natural and synthetic materials, as well as resins of the pitch–rosin class. Various micropowders mixed in different proportions with purified water or other liquids are also employed. Friction at the tool, to which the powder grains are completely or partially affixed, polishes the layer damaged by finishing. The polishing rate depends on many factors: the pressure; the relative velocity of the tool and the workpiece; the tool properties; the characteristics of the micropowder; and the mechanical and physicochemical properties of the workpiece. The structure of the polished surface in the damaged layer resembles that after fine finishing by means of abrasive set in elastoplastic organic binder. This indicates that the great differences in the properties of laps and polishing tools and also in those of grinding powder and polishing powder are not associated with qualitative differences in the two processes. Their main difference is in the scale of the damaged layer.
After polishing, the disrupted layer of the crystal is inhomogeneous and has complex structure [3, 4]. Even after chemical–mechanical polishing by means of ultradisperse diamond powder and nanopowder, nanoscratches are seen on the surface: points or short (or even relatively long) lines, curved or straight. The scratches do not occupy the whole area; some areas have no visible scratches. Below such relief, we observe an elastoplastic zone that may include structural defects near the surface. The sum of such zones determines the depth of the disrupted layer formed in polishing. Polishing of
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