Colloidal silica is an exceptional final polishing solution for a wide variety of different materials. It uses a mixture of chemical and mechanical polishing to improve removal rate whilst also reducing surface deformation, making it the polishing compound of choice for high quality preparation. When used correctly it is an invaluable tool in a metallographer’s utility belt. However, in the wrong hands it will only bring misery.
If you find large scratches suddenly appearing during finish polishing, it may be due to contamination from crystallized colloidal silica. If colloidal silica is allowed to dry onto a cloth the amorphous, sub-micron silica will crystallize to form larger particles that don’t break down. These silica crystals can create large scratches – especially in soft samples – ruining someone’s hard work, as the only way to fix it is to go back a couple of stages and do it again with a fresh cloth.
To prevent problems, follow these simple steps:
- Wet the cloth with water prior to use – this prevents the colloidal silica drying out when its applied
- Keep the cloth damp by regular addition of polishing media – never let it get dry
- Rinse the cloth, samples and holder with water during the last 20-30 seconds of the polishing step. Most of the work in keeping the cloth in optimum condition is achieved by doing this.
- At the end of each period of use, run water onto the cloth and set to low rpm. Use a hard plastic edge to squeegee the cloth, removing any remaining colloidal silica.
One can also use Colloidal Silica with additives that minimize crystallization, likes Buehler’s MasterMet 2 Colloidal Silica. These reduce the amount of cleaning necessary, but usually have an inferior polishing rate.