SUMMARY
The use of incorrect finishing procedures or poor quality abrasives carries the risk of obtaining defective and poor quality surfaces, even if, perhaps, until the finishing phase the standard was high. The last phase of the production process represents a step of fundamental importance. In the event that this is not successful, the work done up to that point is totally invalidated, since very often these are non-recoverable errors.
PROBLEM
The excellence of the surfaces is very often a fundamental requirement in the production of precious objects. Sometimes, however, the surface quality is compromised by halos, opaque areas, scratches, smears that appeared only in the finishing phase (see Photo 1).
A more in-depth observation carried out with the aid of microscopes allows us to highlight the fact that these types of defects are made up of the presence of scratches, even of microscopic size, with particles planted on the surfaces located at the apex of these smears. This phenomenon is commonly referred to as the comet effect. As you can see from the following photos (see photos 2, 3, 4, 5) the streak was caused by the dragging of particles (of a more or less pointed shape) which then planted themselves in the surface.
Morphology in itself is already very indicative of the true nature of the problem, but the definitive diagnosis is provided by the microanalyses conducted on the inclusions (see microanalysis 1).
In the case under examination, the particles were therefore composed of Si oxides which, together with Fe and Al oxides, make up the main constituents of abrasives commonly used in surface finishing operations. In the event that such operations are not carried out correctly, using an inappropriate abrasive/metal pairing or employing low-quality products, there may be a detachment of abrasive particles which, carried by the brushes, causes the defect mentioned above.
SOLUTION
The solution to the problem under consideration consists in paying attention to all the details of the finishing operations:
- Use of high-quality abrasives (brushes, pastes, papers)
- Use of specific products depending on the type of alloy to be polished