SAMPLE PREPARATION: Polishing
Once you have cast your sample, standards, and labels in an epoxy mount, it is time to lap and polish the surface. Again, the objective here is to obtain an analytical surface with no 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. Below, courtesy of specialist Brian Hess, we outline the general method developed at UW, which uses a progression of diamond grits (both fixed and loose) 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!
Important Note: Our recipe does not gurantee success, and does not impart the requisite skills or decades of experience required to master this process.
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; please contact us for an up-to-date recommendation:
Materials used for lapping and polishing at UW:
- Fixed-diamond lapping pad (6-μm grit), 12" diameter (Buehler UltraPrep, part#: 15-6206)
- Magnetic backing, 12" diameter, for mounting fixed-diamond pad to rotating lapping wheel (Buehler MagnoMet, part #: 16-3072)
- Soft toothbrush and Liquinox soap [Critical cleaning liquid detergent, biodegradable, phosphate-free (part #: 1201)]
- Low-nap polishing pads with magnetic backs, 8" diameter, mounted to rotating polishing wheel (Allied High Tech)
- separate pads for 6- and 3-μm grits [Allied Gold Label polishing cloth (part #: 90-500-210)]
- for 1-μm grit [Allied White Label polishing cloth (part #: 90-500-500)]
- new pads are used after every ~5 mounts for SIMS-level polishing (would still be viable for polishing thin sections or for SEM/EPMA)
- Polycrystalline diamond suspensions (Buehler MetaDi Supreme Series)
- 6-μm (part #: 40-6632)
- 3-μm, (part #: 40-6632)
- 1-μm, (part #: 40-6632)
- Vibrating pad, slow-speed high-nap pad (Buehler Microcloth, 12" diameter with adhesive back, part #: 40-7222)
- Colloidal alumina 0.05-μm poslishing suspension (Buehler MasterPrep, part #: 40-6377-032)
- Grind the analytical surface with the 6-μm fixed-diamond lapping pad. At UW, we use a 12" pad on a wheel spinning at 1140 RPM and lubricated with a constant flow of water. The goal here is to prepare a flat surface for polishing, and to appropriately expose the samples and standards as needed. Note that after lapping, the polishing process will still remove some material (a few μm thickness).
- After each lapping/polishing step, examine the surface under magnification with reflected light to assess progress, exposure and surface roughness. First, you'll need to rinse the mount with warm tap water and scrub lightly with a soft toothbrush and concentrated Liquinox soap to remove any lapping/polishing residue. Do a final rinse swith distilled/R.O. water (to avoid deposits from tap water) and blow dry with clean air before examining under magnification.
- After lapping, and using a different low-nap pad for each grit, polish the analytical surface with a progression of 6,3,1-μm polycrystalline diamond suspensions. At UW, we use an 8" polishing wheel spinning at 550 RPM, and only use the (slower-moving) inner 2.5" radius for polishing. No more than 1 (one) minute is spent on each grit. Examining the sample on a reflected light microscope after each step will reveal:
- After the 6-μm grit, the sample will not appear to be polished
- After the 3-μm grit, the samples will be noticeably different and will visually "pop"
- After the 1-μm grit, surfaces will appear almost clear of scratches/imperfections
- For some samples, a final polish is achieved with a 0.05-μm colloidal alumina solution. The alumina solution is placed on a vibrating pad and the mounts are placed (face-down) on the vibrating pad under ~900 g in custom weighted holders for 1-5 min (10 min for thin sections) depending on the relative softness of sample/epoxy. It is critical to repeat the cleaning with soap immediately after this step to remove colloidal alumina residue and give a final rinse with distilled/RO water. Check for polishing residue under reflected light.
- It is easy to fixate on removing small scratches during the polishing sequence, leading to overpolishing that can quickly cause topographic relief. You want no more than 3-μm of relief, and ideally <1-μm. If you introduce too much surface relief at any step, you'll need to start over at the grinding step (fixed-diamond lapping pad). A few scratches in a polished sample are (usually) easy to avoid during analysis, and are certainly preferable to the results of overpolishing.
- Once polishing is complete, and flatness is confirmed (see next step), cut the back of the mount off so that it is 4-5 mm thick. Ensure that the cut is parallel to the polished face so the mount has an even thickness, and avoid scratching the polished surface! After cutting off the back, you can improve label legibility and remove saw scuffs from the cut surface by briefly lapping it with the 6-μm fixed-diamond lapping pad.