Hi Milad,
These are very important questions, but not easy to be answered. In fact, it may take few hours of discussion to address them; and a huge amount (years) of experience in testing to better understand coating failure. But I will try to do my best to summarize the situation in few paragraphs.
Regarding your 1st question... to the best of my knowledge there are no specific thermal cycle standards for thermal spray coatings. Thermal spray coatings are used in different applications where thermal cycle is present. However, specifically for thermal barrier coatings (TBCs), the furnace cycle testing (FCT) is highly used worldwide by OEMs, feedstock/equipment manufacturers, service centres and university/government laboratories to screen the performance of these coatings. That said, each institution has its own set of specs on how the test should be done; i.e., they create their own standards. In more simple terms, the temperatures, heating rates, cooling rates and dwell times at high temperature are not (necessarily) the same. In addition, some FCT equipment are bought, others are home-made (and this is another source to cause different performances) and the criteria of TBC failure may very from lab to lab. Moreover, although the majority of labs use puck-shaped samples, some use square ones and even rectangular ones. If you go to the literature and try to find the combination of the acronyms "FCT" and "TBC", for sure you will find many references to guide you. It is important to highlight that the FCT is not the only test used to thermal cycle TBCs, but most likely the most used due to the fact that after the set-up is done, it is low cost to operate, you can test for few to dozens of samples at the same time and it can run 24/7.
Regarding your 2nd question... you need to test the coatings you are developing against a benchmark, reference or control coating; which represents a coating already in use by the industry. There are different ways to get this "benchmark/control coating" produced. These are some of them:
1) You get it from industries that are manufacturing them and applying them for real use; i.e., they will make the "control coating" to you.
2) You can get the recommended spray parameters from an industry and make your "control coating".
3) You can get the recommended spray parameters from equipment/feedstock manufactures to make you "control coating".
Now having your "control coating", you need to test it in your thermal cycle testing system; e.g., FCT and define a criteria of failure; e.g., 20% spallation in area (from the total substrate coated-surface). So, supposing your "control coating" lasts on average 500 cycles, you are going to compare the coatings you are developing to this value. If they last more than 500 cycles, this is "good". If they last less than 500 cycles, it means that you are not there yet.
There is another important aspect. In the lab you need to accelerate the coating failure, in the sense that some coatings can operate in the real environment for years. But if you accelerate it "too much", the coating will not fail in the way it is supposed to be, and your results will tend to be biased and lead you to wrong conclusions.
Again... these are very important questions, but not easy to be answered.
I hope it helped!
Cheers!
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Rogerio Lima, PhD
Senior Research Officer
National Research Council of Canada
President of Thermal Spray Society (TSS)
e-mail:
rogerio.lima@nrc-cnrc.gc.ca------------------------------
Original Message:
Sent: 12-22-2022 13:49
From: Milad Rad
Subject: Thermal Cycling
Hello Everyone,
I have two questions regarding the failure of coatings due to thermal cycling. I would be grateful if you could share your experience and wisdom with me.
- Is there any well-known standard for conducting thermal cycling tests specifically for thermal-sprayed coatings?
- Prior to using coatings for industrial applications, it has to be proven that the coating will not fail in under the actual thermal loadings. I know that the working conditions of the coatings that are subjected to thermal cycling vary from one application to another. Is there an accepted practice regarding the number of cycles that must be considered in an accelerated test to conclude that the coating is reliable?
Thanks in advance for your attention.
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Milad Rad, Ph.D.
Assistant Professor
University of Southern Indiana
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