measuring strain in coating
09-26-2007, 11:51 PM,
measuring strain in coating
I'm not sure if this forum is a place for this question.

I would like to measure the ultimate strain in a coating.

Using finite elements, its been determined that the material the coating rests upon undergoes a strain of 18%. I'd like to determine how much strain my coating can withstand.

The catch is, the scale is very small. My coating thickness is 10 um, the diameter of the material the coating rests upon is 90 um.

What would be a good way to accurately measure the ultimate strain of my coating.

09-27-2007, 05:50 PM,
RE: measuring strain in coating
Hi Erich

Sign0016 to the Surface Engineering Forum.

Interesting, but not really my area of expertise, but here's my twopence worth of thought anyway:

Assuming your coating will have a lower strain than your substrate. One problem will be observing the first signs of failure in the coating, which probably will occur in a multitude of places along the filament length, rather than one place as you would expect for the substrate. So, I think you will probably require a microscope to view the coating surface to detect the onset of failure. Without going too "high tech" I can only think of making up a small jig so that you can strain the filament ( I assume it will be a bit like trying to test a human hair, scale wise Happy0193) in small increments, while microscopically observing the coating surface for failure at each small step in strain. This should give you a good idea of the relative strain between substrate and coating in combination. Remember, you are trying to test two materials joined together, one will effect the results of the other, so be careful not to interpret results in isolation. To do that you would need to test the coating alone without the substrate.

Hope that helps.
09-27-2007, 07:58 PM,
RE: measuring strain in coating
Thanks for the welcome Gordon.

This is how I tried to do it.
I coated a pebax tube with coating and subjected it to tensile testing.
I measured the strain on it, and marked when the force had a sharp dropoff. This is assuming at a certain force, the only thing holding the pebax together is the coating. So the sharp force drop off would be when the coating fails.

However one potential problem was, I didn't know if the coating was delaminating off the pebax. If it was the strain measured wouldn't accurately reflect the strain of the coating.

The method you described seemed more qualitative. I'm looking more for an actual number or %.
10-05-2007, 01:14 PM,
RE: measuring strain in coating
Hi Erich

Sympathise with your problem, wish I could help more. If you could accurately determine the strain properties of your coating, I doubt whether this information would enable you to predict how it would perform in practice on a substrate. Chances are that coating/substrate bond would fail long before any appreciable strain is reach in either coating or substrate. To determine if your coating is fit for purpose in your specific application, I can only suggest testing the actual part. If its purely an academic exercise to establish fundamental strain properties, then this approach won't work.

Good luck Big Grin

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