Chemical-Mechanical Polishing of Optical Glasses
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1157-E03-02
Chemical- Mechanical Polishing of Optical Glasses Elisabeth Becker, Andreas Prange, Reinhart Conradt Institute of Mineral Engineering, Department of Glass and Ceramic Composites, Mauerstrasse 5, D-52074 Aachen, Germany ABSTRACT This paper presents the Chemical-Mechanical Polishing of optical and fine-optical glasses, which is employed to fulfill the optical requirements for the surfaces of optical lenses. We present the effect of chemical interactions in the polishing process of optical lenses and show how these interactions can be influenced by the additions of certain chemicals. Furthermore, we present a thermodynamic simulation tool, by which these interactions can be modeled. Thus it is possible to understand the chemistry in the suspension and to simulate recent polishing processes in advance, with new additives or with various intrinsic glass ions from different glasses. INTRODUCTION The final step in the production of optical lenses is the polishing process, which is employed to bring about the desired surface quality (minimum roughness, absence of surface and sub-surface flaws) as well as the final adjustment to the macroscopic shape for optical applications. For both objectives, a certain removal of glass material is necessary. This production step is very critical because it is time and cost consuming. For our experiments we used the so called Synchro Speed Process: A rotating lens is polished by an also rotating polishing pad with a polishing suspension of water and polishing grain (CeO2). After the optimization of the mechanics of the polishing machine [1], the importance of chemical parameters became obvious. Therefore we focused on the polishing grain substrate [2], [3] and its behavior as a colloidal system [4] and on the interaction between the glasses, a multiplicity of oxides and non-oxides, and the aqueous system [5]. As an important factor, the polishing suspension is reused for polishing either the same glass or different glasses. In the first case, the suspension will be concentrated with intrinsic ions; in the second case, a mixture of ions from different glasses will be found in the suspension. The kind and amount of different ions influences the colloidal chemistry of the polishing suspension, its stability and the agglomeration state of the polishing grains. These interactions between the glass specific ions can cause instabilities in the polishing process yielding a decrease of the glass removal or scratches at the glass surface. EXPERIMENTS In several experiments, it was found that a mixture of glass ions can lead to instabilities in the polishing process. For example, the equilibria between Si, Ca and Zn ions in aqueous solutions are very sensitive and a shift of these equilibria can cause big differences in the removal rates. This is demonstrated in Figure 1, which shows the concentration of the diluted species (left ordinate) and the resulting removal rate (right ordinate) as a function of the polishing time for three different suspensions; suspension 1 and 2 are randomly
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