Dear Radek and Chirag,
Thanks for posting and start commenting on this very important topic.
Based on what I have seen and heard, I would say that image analysis (IA) of polished coating cross-sections is typically the "preferred" method of porosity evaluation as a quality control (QC) method; mainly in production environments of our TS community. It is because you need to find a compromise between having the porosity data and in a short processing time. I hypothesize that for many people, the mercury intrusion porosimetry (MIP) would be their "favourite" method, but it is not practical as QC.
About 20 years ago I participated in a round-robin initiative of TS coating porosity measured via IA (polished coating cross-sections). Several institutions participated in this initiative, but some porosity results were significantly different depending on the institution that evaluated the porosity.
That said, right now, 20 years later, it is amazing how these IA technologies and supporting equipment have evolved in the positive way, for example, (i) digital cameras, (ii) automated polishing machines and (iii) mainly the quality of the IA software packages available today.
As Radek stated in his message, a standardized polishing procedure is paramount. Today, with automated polishing machines, you can add the different polishing recipes to polish different coatings into the computer of the machine (via its touch screen). It includes the polishing paper type, load, RPM, time and cooling. Therefore, even if the polishing machine is operated by different users, as long as they follow all the polishing steps to the letter (which are uploaded into the machine), the human error/input will be drastically reduced. Of course, we cannot forget the initial steps of sample prepping, which includes the impregnation and cutting; which are equally important.
In my opinion, the most important supporting lab of a TS institution is the metallography lab. What I mean is; you may use a state-of-the-art computer-controlled console, a modern TS torch, measure Tp & Vp with great accuracy/precision and use an infra-red camera to measure the temperature of the substrate during coating deposition. This is fantastic. But if you employ a "non-optimized" metallography procedure to evaluate your coating, you will not have a representative microstructure and you will be led to wrong conclusions. Consequently, you will lose the effort you put in trying to make your coating as best as possible in the first place.
For those who are beginning to standardize polishing procedures of TS coatings, the ASM-TSS book Thermal Spray Technology: Accepted Practices; which is open access via the web, can provide very useful sets of info to our TS community.
The link is here:
Thermal Spray Technology: Accepted Practices | Handbooks | ASM Digital Library (asminternational.org)Of course, there are still important challenges remaining. For example, some people use SEM. Other people use OM. Moreover, there are issues about the magnification of the pic. Some people use 50X, others 100X, 200X or higher. All these issues will influence the measurements. When reporting the data in a report, I suggest (if possible) adding the following supporting info:
- For SEM: (i) magnification, (ii) working distance and (iii) kV
- For OM: (i) magnification and (ii) the exposure time
- For all: (i) the threshold value, (ii) the original picture and (iii) the threshold-marked pic (in this way, if the coating porosity value is 11%, people will see what it is meant by that)
Therefore, although there are still important challenges to be overcome, I guess that IA is becoming more and more acceptable as a QC method to measure the porosity of TS coatings in production environments and even labs, or any time you need to measure coating porosity on a regular basis.
However, for sure it is not a panacea. For a more fundamental understanding of the porosity nature of TS coatings, Archimedes and MIP are paramount. As an example, I am attaching a very interesting paper from Dr. Robert Vassen's group. Free-standing YSZ TBCs were removed from substrates using different methods, and their porosity values were measured via MIP. Depending on the way the YSZ TBC was removed from the substrate, the porosity values could be distinct. A very-very good paper.
I hope it helps!
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Rogerio Lima, PhD
Senior Research Officer
National Research Council of Canada
ASM Thermal Spray Society (TSS) Vice President
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Original Message:
Sent: 06-09-2022 06:51
From: Chirag Raval
Subject: Porosity evaluation
Hi Radek
Generally we measure porosity of coating with the help of optical microscope in Industry for coated sample. We used cross-sections images to check at 50X magnification in microscopic evaluation with std. software measurement analysis with the availability reference of coating porosity and structure.
I hope this information will be helpful to your question.
SEM is very much useful for research and development organisation to study in depth of coating structure, matrix, oxide and phase analysis.
Thanks and Regards
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Chirag Raval - M.Engg. in Met. & Mate. Engg.
Thermal Spray Applications & Know How Technology Transfer
chirag.ase@gmail.com
Original Message:
Sent: 06-08-2022 07:10
From: Radek Musalek
Subject: Porosity evaluation
Hallo everybody,
Can you briefly share what is your favourite coating porosity evaluation method and why? Please state your field/typical application if relevant. I am also interested in "how people from the industry see it".
There are generally many ways how to evaluate porosity in TS coatings - Archimedean, gas pycnometry, Hg intrusion, image analysis, X-Ray CT, SAXS, .... Each has some significant drawbacks.
In our academic lab, we mostly rely on image analysis (IA) of cross-sections observed by SEM.
Pros
- we do routine microstructural observations anyway :)
- SEM provides in back-scattered imaging mode nice materials/porosity contrast (we do not use light microscopy as it is not reliable for semi-transparent ceramics)
- SEM is able to provide even high-mag images -> porosity distribution may be acquired even for suspension plasma-sprayed materials with ultrafine porosity
- works for metals/ceramics/cermets sprayed from powders or liquids by different TS methods
- repeatible once the polishing method is standardized
- easy to provide statistics from various spots within one cross-section
- I understand the data (unlike other fancy but "black-box" methods)
Cons
- need to be sure that there are no pull-outs/sample preparion artifacts
- preparation method & image processing may be user-dependent
- relatively slow as vacuum embedding with low viscosity epoxy takes couple of hours -> usually not suitable for quick on-site porosity screening.
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Radek Musalek
Institute of Plasma Physics CAS, Prague, Czechia
Prague
+420266053077
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