SAMPLE PREPARATION: Polishing
Once you have cast your samples and standards in an epoxy mount, it is time to polish the surface. Again, the objective here is to obtain an analytical surface without more than a few microns of relief -- and negative relief is much better than positive relief. For samples that are much harder/softer than the epoxy, you will need to strike a balance between eliminating small scratches and creating surface relief by overpolishing (resulting in the softer material being removed faster).
There are a number of ways to polish an epoxy round and we can't offer detailed advice on each. We outline the general method used at UW below, which uses a progression of diamond grits on a spinning polishing wheel. No matter which method you choose to use, we strongly urge you to practice your approach before polishing the mount(s) you will analyze at WiscSIMS. Polishing is a destructive process and it is possible to polish right though your sample and/or standard grains!
There are some third-party companies that will polish your sample for a fee, but we do not have extensive experience with any of them. You will want to carefully describe and document the degree of polishing you desire, and be sure to communicate the scientific value of each mount. If samples show polishing relief, it may be possible to fix with a diamond lapping film.
Third-party polishing services:
- Spectrum Petrographics Inc.; Vancouver, WA
UW polishing method:
- Grind analytical surface with 6 micron diamond mesh polishing pad. Lubricate spinning pad with constant flow of water.
- Using a different low-nap pad for each grit, polish the analytical surface with a progression of 6,3,1 micron diamond paste.
- For some samples, a final polish is achieved with a 0.05 micron colloidal alumina solution. The alumina solution is placed on a vibrating pad and the mounts are placed (face-down) on the vibrating pad under ~100 g weights for 30-200 seconds depending on the relative softness of sample/epoxy. Rinse mount with warm tap water and scrub lightly with a soft toothbrush to remove any colloidal alumina residue.
- If you introduce too much surface relief at any step, you'll need to start back over at the grinding step (diamond mesh).
Materials used for polishing at UW:
- Diamond mesh grinding wheel
- Low-nap pads
[for polishing grits 15-6 μm: Allied Gold Label adhesive-back polishing cloth, 8" diameter (part no. 90-150-210)]
[for polishing grits 3, 1, 0.25 μm: Allied TECH-cloth adhesive-back polishing cloth, 8" diameter (part no. 85-150-360)]
- Diamond paste / polycrystalline diamond suspensions (e.g. Buehler MetaDi Supreme series)
- Vibrating pad
- Colloidal alumina