Hi Steve
Some great answers already. The "best practices" link posted by James Quinn is a great reference and well worth reading.
There is too much detail to put in a post like this, but I will cover many of the key areas to consider. Apologies in advance for the length of the post!
Your challenge will be retaining brittle areas, and keeping the sample flat. All preparation steps are important, and we can help more specifically as you start in the work - but key considerations are:
- Protect the sample during any sectioning step. This can include orientation, correct selection of consumable, conservative parameter selection and sometimes (especially with test coupons) mounting
before sectioning
- Make sure specimen is cleaned, and completely dried prior to mounting. Consider
- ethanol soak (to pick up water - easier to get ethanol out of the sample than water)
- oven drying/vacuum drying to get deeply absorbed moisture out of porosity
- use a low viscosity epoxy
- mount under vacuum, cycle the vacuum 3-5 times
- cure under ambient conditions (usually OK) or in a pressure pot (@ 2 bar, sometimes help ensure no shrinkage gaps on some samples)
- Supporting the epoxy with a filler (such as FlatEdgeFiller) or second sample can help protect edges - but can also slow preparation steps a bit.
- 1st grinding step should match the level of damage potentially introduced from sectioning. Grinding can do more damage than sectioning!
- You have options with grinding/polishing steps - pay attention that each step is long enough to remove damage from the prior step.
- The grinding/polishing step -most often
not done correctly is the first diamond slurry step after fixed abrasive grinding. This must be long enough to recover grinding damage.
- Ideally use very flat polishing surfaces such as UltraPol at 9micron, Trident at 3micron/1micron
- Aim to keep polishing steps to a minimum - <2mins. If scratches remain after that much time, correct the penultimate step to leave finer scratches. This helps to avoid rounding in edges. A manufactured foam polishing pad (ChemoMet) may give better results than a traditional napped surface.
Some more general recommendations:
- Inspect the sample at each step.
- Avoid abrasive that is similar in size to the coating/layer dimension (or brittle feature)
- If using Diamond grinding discs, follow with a short (<1min) step on SiC paper - fresh SiC is much less damaging that diamond grinding discs at the same grit size
- keep polishing surfaces well wetted, but not flooded
- Use complementary rotation (head and base in same direction) for all steps
- Use central force preparation if possible (easier to maintain flatness of samples). If using individual/single force, be careful to grind more conservatively/finer grit sizes
We can give best advice once you've started - especially when we see how your specimens react to various stages, and can advise on "tweaks". Email
techsupport@buehler.com at any time (or you can contact me!)
Vibratory polishing
Recommendations likely depend on the manufacturer, but these are really easy to use. Best use is to vibratory polish for 20-60 minutes
after a full mechanical preparation. This can enhance polishing results and help with flatness. Typically we would use a napped cloth (Microcloth or TexMet) and the same polishing abrasive that is recommended for final polishing - most often colloidal silica. There are various options regarding appropriate load and movement speed settings that depend on sample size and circumstance, which we can assist with. Vibratory polishing can be used for other steps, but is not usually so advantageous for steps other than final polishing, for this kind of sample.
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Michael Keeble
BUEHLER,Buehler a Division of ITW
Lake Bluff IL
(847) 393-3645
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Original Message:
Sent: 01-12-2023 20:38
From: Steven Matthews
Subject: Cross section preparation of oxides on coatings & Vibratory polishing
Hi,
I have a grad student working on thermally sprayed bondcoats for thermal barrier coatings. We are going to be looking a lot at the oxide formed on these coatings, both as stand alone coatings, as well as in composite layers within a thermal barrier system (ceramic top-coat, on top of a metallic bond coat, with a thermally grown (TGO) oxide in between).
In the past I have found it very challenging to prepare nice cross sectional images of the oxide layer, particularly at the surface of coatings, but also within the TBC layered structure. Mostly this has been due to fracture and pullout of the oxides using my "conventional" prep method for thermal spray coatings. I realise that there is a lot of proprietary knowledge/experience on TBC sample prep....but I would sincerely appreciate any advice on preparing cross sections in a manner that retains the surface or TGO layers for analysis.
Related to this......our technician recently "rediscovered" an old vibratory polisher when we relocated labs. I have struggled to find much information on recommended procedures for vibratory polishing or information regarding the sort of materials that it is best suited for. Has this type of polishing been superseded by more recent polishing consumables....or does it still have value in particular cases, particularly for oxidised cross sectional samples?
Thanks in advance
Steve Matthews
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Steven Matthews
The University of Auckland
Auckland
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