The mechanisms which ultimately effect material removal take place on the boundary surfaces between the polishing grains and the workpiece surface. The abrasives in table 8-2 have proved effective for the polishing of metallic and hard — brittle materials.
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Table 8-2. Abrasives used for the processing of metals, hard and brittle materials and glass [WASC93]
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Applicability for smoothing and removal depends in the case of polishing clay not only on grain shape and grain size distribution, but also on the amount of the two crystal modifications y — and a-Al2O3 and on the tendency to agglomeration. The quality is strongly influenced by the starting materials and the finishing technology.
The use of natural diamond as abrasive material has experienced broader application since the development of finer grain sizes. Properties such as its blocky shape with sharp cutting edges and its high hardness are of particular interest, for which reason natural diamond is used for processing both metals and extremely hard substrate materials like sapphire or silicon. The advantages of diamond also include high removal rates with small grain sizes in conjunction with good surface qualities [WASC93]. Because of its chipping effect, diamond is used preeminently for pre-polishing and intermediate polishing and less often for final polishing. Both natural and synthetic diamonds can be used. Whereas monocrystalline natural diamond is blocky and has relatively few cutting edges, synthetic diamond is polycrystalline and has many cutting edges. Its grain size values range from 0.1 to approximately 50 qm.
As a result of recent developments in the semiconductor industry, colloidal silicon dioxide has come to be regarded as an effective abrasive for final polishing [OPIL83, GUES91]. Colloidal slurries are dispersions with small particles with sizes of 1 — 1000 nm. The dissolved material is not present as ions or individual particles, but rather in the form of agglomerations of atoms or molecules. Colloidal SiO2 slurrys usually have a particle size of < 100 nm, the solid content lying in the region 40 — 50 %. Most important is the high pH value of 9 to 11, which causes a chemical-mechanical removal. This is usually increased by adding surface-active materials.
A typical parameter used to describe an abrasive is its “grip”. The faster the glass is removed by the abrasive, the thinner the to be removed layer and the shorter the required process time, the higher the grip.