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  • 1.  Liquid leakage from pores during SEM examination

    Posted 08-25-2022 05:28
    Dear Thermal Spray Community,

    currently I`m facing a problem regarding the optical characterization of Al2O3 coatings. During SEM analysis, a liquid is visible on the specimen surface. It comes out of the pores. Probably the liquid is pulled out of the pores by the vacuum in the SEM. I have already tried to fix the problem by placing the samples in the vacuum oven at 80°C for 3 hours. With no success.

    The sample was only in contact with ethanol and a water-based suspension while grinding and polishing the sample.

    Has anyone had a similar problem in the past and was able to find a solution to it?



    Thank you in advance!

    ------------------------------
    Paul Junge

    Research Assistant
    Department Coating Technology
    Institute for Machine Tools and Factory Management IWF
    Berlin University of Technology

    ✉ Email: paul.junge@tu-berlin.de
    ------------------------------
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  • 2.  RE: Liquid leakage from pores during SEM examination

    Posted 08-25-2022 09:49
    Dear Paul,

    I was talking with our metallography technician about your issue. He has 20+ years of experience and he also saw what you reported. According to him, during the polishing procedure with ethanol and water-based suspensions, they became impregnated into the porosities of the sample.

    These are his recommendations to circumvent this problem:

    1) Use heated resin impregnation (e.g.; Struers Caldofix II@ 75°C) under vacuum before metallographic preparation.

    2) If unsuccessful, you can redo your vacuum drying (80°C) and keep the coating in contact with an absorbent fabric. So, you need to get a flat plate and lay an absorbing fabric on it. Then you put your metallographic mount on the fabric, with the polished coating cross-section facing the absorbent fabric. Add a little weight on the metallographic mount and perform your vacuum drying at 80oC. The fabric will help to absorb the liquid impregned into the porosities of your sample.

    I hope it helps!

    Cheers!



    ------------------------------
    Rogerio Lima, PhD
    Senior Research Officer
    National Research Council of Canada
    ASM Thermal Spray Society (TSS) Vice President
    ------------------------------

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  • 3.  RE: Liquid leakage from pores during SEM examination

    Posted 08-26-2022 03:42
    Dear Rogerio,

    thank you for giving me these valuable recommendations. I will firstly try method 2) and check if it will solve the problem. I will let you know if it has worked!

    All the best
    Paul

    ------------------------------
    Paul Junge

    Research Assistant
    Department Coating Technology
    Institute for Machine Tools and Factory Management IWF
    Berlin University of Technology

    ✉ Email: paul.junge@tu-berlin.de
    ------------------------------

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  • 4.  RE: Liquid leakage from pores during SEM examination

    Posted 08-26-2022 16:44
    Hi Paul,
    In addition to the previous answer - preventing ingress of polishing media into pores is best done by using vacuum impregnation.  The important part is to use a low viscosity/high wetting viscosity.  The recommendation given would work fine (Buehler equivalent is EpoHeat), both that and the one mentioned in the prior answer are oven curing epoxies. 

    Some critical parts to vacuum impregnation of pores with epoxy:
    - The sample is dry inside the pores as well as outside prior to mounting
    - Introducing vacuum to the sample prior to pouring resin is much better than pouring resin and then vacuuming, for two reasons - it helps remove residual liquid in the pores, and evacuates air - which allows the resin to wick into the very deep/small gaps.  A vacuum of 25inhg is typical.  
    - once impregnated with resin, cycle the vacuum between ambient and maximum vacuum at least 3 times - 5-7 can continue to generate improved impregation.  This is most easily done on equipment such as the Buehler SimpliVac (will automatically run through the cycles).  If this is not available, you can simulate it by manually releasing and re-introducing the vacuum
    - do no exceed the vacuum recommended, as evaporation of the epoxy system will change the resin performance

    There are a few tricks for drying samples.  Vacuum drying at temperature can help if you have the equipment.  A more accessible approach is to dry the sample after final polishing, by spraying with ethanol and air drying.  Then soak the sample in ethanol.  Ethanol is hygroscopic, and will absorb water.  Any subsequent drying method will then be more successful at removing the ethanol than it would a water filled sample.  I typically place in a standard oven to dry.  You could use this approach on mounted and prepared samples, as well as prior to mounting.

    I like Rogerio's recommendation to use a flat plate/absorbant cloth to help remove liquid!

    ------------------------------
    Michael Keeble
    BUEHLER,Buehler a Division of ITW
    Lake Bluff IL
    (847) 393-3645
    ------------------------------

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  • 5.  RE: Liquid leakage from pores during SEM examination

    Posted 08-29-2022 01:47
    Dear Paul.

    I addition to the other valuable answers I would like to add one other "trick" that I often find useful to remove fluids from polished cross sections of porous TS coatings:
    - submersion of the sample in a glass with ethanol (enough to cover the sample)
    - the glass with the submerged sample is placed in a small ultrasonic bath (the bath fluid to be adjusted so that the glass is not "drowning")
    - run the ultrasonic bath for 10-15 mins to force fluids out of the sample by vibration

    Maybe this could work on your sample as well. Good luck.



    ------------------------------
    Jesper V. Carstensen
    Senior Research Engineer
    MAN Energy Solutions
    Copenhagen SV
    +45 3385 1322
    ------------------------------

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  • 6.  RE: Liquid leakage from pores during SEM examination

    Posted 08-30-2022 03:42
    Dear Paul,

    thank you for starting this valuable thread.

    Just my comments to what was already said.

    We also face this issue in our lab quite often. The principal problem is the open interconnected porosity typical for many TS coatings.

    My additional hints would be:
    - first of all, use highly penetrating low viscosity resin. You may need to sacrify speed of the curing process and hardness of the epoxy.
    - clean and dry your samples thoroughly before embedding. In the voids, you may have leftovers of cutting liquid which often contains lots of "bad stuff". I would recommend rinsing in acetone or isopropanol and drying of sample for couple of minutes under hot air.
    - from the photo you seem to have a serious problem with resin adhesion to the sample. You should use resin with minimal/zero shrinkage, especially if you have a smooth coating surface. BTW, I have seen samples where wrongly chosen epoxy delaminated the whole coating (even after several weeks! - bad for samples archivation) although the coating was otherwise OK.
    - in our lab, we use EPOFIX resin from Struers combined with vacuum embedding with satisfactory results.
    - minimize polishing times to suppress polishing fluid/lubricant penetration. Also, do not leave samples on the polishing cloths longer than necessary.
    - you may try playing with lubricant if applicable as some are "more messy", especially if you see that the "sweat" is "oily".
    - I also use the trick with immersion of samples in ethanol or isopropanol.
    - I sometimes put the "polished but still sweating" sample in vacuum, so that the "sweat" is driven out (actually, like in SEM), and then use ethanol immersion/cleaning. I had samples where it needed to be repeated, but in the end, result was OK.
    - you may also use my favourite TS metallography trick. After the final polishing step, rinse your sample under running water, then put a small drop of dish detergent on the polished surface (we use JAR, which also smells nice :) and rub the it with clean latex glove under running water. This is especially useful for getting rid of OP-S residues and removing stains, but may also help in your case.

    - PS: Be very careful with ultrasound, especially with ceramic samples. If you really need to use it, I would recommend to use low intensity of sonication if your ultrasound bath allows it to be set.



    ------------------------------
    Radek Musalek
    Institute of Plasma Physics CAS, Prague, Czechia
    Prague
    ------------------------------

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  • 7.  RE: Liquid leakage from pores during SEM examination

    Posted 08-31-2022 00:53

    Dear Paul, I did read your post and want to help you finding the correct sealer. We have specialized on sealer production for thermal sprayed coatings and do offer a wide range of solutions.

    https://www.diamant-polymer.de/wp-content/uploads/2022/04/DIAMAND_Broschuere_Siegler_ENG_FINAL.pdf

     

    Mit freundlichen Grüßen,

    Kind regards,

     

    Dipl.-Ing. (FH) Carsten Kunde

    DIAMANT Metallplastic GmbH

     




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