Test Coupon Issue
03-19-2013, 01:08 PM,
Test Coupon Issue
Hello all,

I am new to the Thermal Spray world trying to get started. I have quite a bit figured out already and my coatings are coming out looking pretty good from what I can tell.

My question is in regards to surface cracking. Post spray my test coupons look perfect, good consistency and density, and no visible cracks, but my most recent set of approval coupons came back with what the testers claim to be surface cracks.

Two parts to my question.

Is there a way to check for surface cracks before sending out my samples, and without destroying them in the process?

What could be causing the surface cracking?

The sample bond lines are excellent, density and porosity is good.

I am spraying 410 SS substrate using Wc80/Co16/Cr4

Any incite would be much appreciated.
03-19-2013, 02:44 PM,
RE: Test Coupon Issue

Sign0016 to the Surface Engineering Forum.

First I would check that the cracks are not being produced by your testing lab. A common problem is inducing cracks during cutting-off of specimen for metallography.

If the cracks are genuinely present after spraying, make sure you are not overheating test coupons during spraying and keep thickness per pass of gun to the lowest possible.

Other than close-up visual inspection of the coating, dye penetrant flaw detection, but this can be a bit hit and miss.
03-19-2013, 03:34 PM,
RE: Test Coupon Issue
Thanks for the quick response.

Yes it was my theory that this cracking was being done in the sectioning process. However the test facility assures me that isn't the case, that they have "perfected the process".

I hadn't thought about Dye Pen testing however and will give that a try.

As far as part temp is concerned, the spray process is temperature controlled to a robot input which should the digital temp exceed 250F the spray process is stopped and will not continue until the temperature drops below 200.

Are these temperatures acceptable? Under the powder specs they are, but maybe lower is better in this instance.

BTW great website. Very helpful and easy to navigate!
03-20-2013, 11:07 AM,
RE: Test Coupon Issue

Can't see a problem with those temperatures. What deposit thickness per pass of the gun are you getting?
03-20-2013, 12:28 PM,
RE: Test Coupon Issue
Deposition rate is 0.00023" per pass (avg).

The coupons are coated to a thickness of 0.016" as per customer specifications.

I was reading some more last night and I'm wondering if my cooling system is quenching the part too quickly after I'm done coating.

Would I be wise to allow the coupons to air cool without assistance?
03-20-2013, 01:00 PM,
RE: Test Coupon Issue

Deposition rate sounds ideal.

I would not have thought the cooling would be a problem at the end of spraying, but a natural cool is easy to try just to rule it out.

During spraying though, I would not let the parts go cold.
03-23-2013, 09:56 AM,
RE: Test Coupon Issue
Hi .......
I can not tell you the solution yet, because in a thermal spray proces there are a lot of parameters that can alterate the coating resoult, and this depend of the kind of proces, powder, spray parameters, spray room, etc, etc, I can only reccomend you to do an Almen strip test, in order to know if the "residual coating stress", is a compressive or tension stress.To do this you must measure de deflection on a sample sprayed of steel (about 25mmX120MMX2mm), ones you know the stress in your coating, you can select the parameters to be modificate.
Best regards
11-07-2013, 09:31 PM,
RE: Test Coupon Issue
Without a picture of the cracking and the steps in process the sample it is imposible to even guess at the cause. Different process can crack the coating differently.
From the lab side suggest
Do not use a grinding stone
use cold mount
Do not exceed 150 rpm on grinding or polishing
make sure the sectioning speeds and feeds are proper
see if the surface cracking goes away both coarse and fine grinding disks are very likely to be a cause for surface cracking visible in a mounted sample.
11-08-2013, 03:13 PM,
RE: Test Coupon Issue
An easy way to check for surface cracks is to wet the COLD coating with acetone or alcohol. As it evaporates, you will see cracks (if there are any). Make sure you do not do this on hot coupons, as the thermal shock could crack your coating.

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