Thermal shock test

[derek]

Hi all coating experts, have you ever done thermal shock on a real part before? Can anyone share the experience with me? We have several time failures and try to find the solution. My coating system is NiCrAl (HVOF) + NiCrAl (APS) + TBC (0.020"-0.030" inch). What make the TBC fail? Thanks you all in advance.

[MVagusta]

(11-08-2013 04:52 PM)derek Wrote:  Hi all coating experts, have you ever done thermal shock on a real part before? Can anyone share the experience with me? We have several time failures and try to find the solution. My coating system is NiCrAl (HVOF) + NiCrAl (APS) + TBC (0.020"-0.030" inch). What make the TBC fail? Thanks you all in advance.

What do you mean with "TBC fail"?
Does the TBC spall off the piece? Does it crack as it shouldn't?
Give me some more detail.

[derek]

    The TBC spall off the piece? Please see attached picture.

Thanks for the replay.

[MVagusta]

(11-08-2013 10:45 PM)derek Wrote:  The TBC spall off the piece? Please see attached picture.

Thanks for the replay.

Umm... very interesting pic!
The way the picture was taken look familiar to me... Was the Thermal shock test done in a German laboratory?

Back to the issue...
The bonding to the substrate is very good, so not a bonding issue in my opinion. You should then look into TBC microstructure: Too low porosity might be the cause of the failure. What is the porosity range of the sample? How do you measure it? Do you take the average?

[derek]

yes, you are right, in a German laboratory.

The porosity is 13-16%. Not too low. Any other throught?

[MVagusta]

(11-13-2013 02:56 PM)derek Wrote:  yes, you are right, in a German laboratory.

The porosity is 13-16%. Not too low. Any other throught?

13-16% means nothing if you don't tell us also the evaluation method, are the values you are reporting the max found on a test coupon sprayed with the part? Are the values an average? Which is the absolute minimum porosity value found?

The part sample you are showing failed at the very beginning of the test as the base material is still sound!!
There must be a very big error (therefore easy to solve) when spraying that TBC if it fails so early!
As bonding appears to be good i 'd look into TBC structure and its porosity level. Yes...it's that easy!

If you can upload an image of the structure.

[derek]

Thanks for your reply, please see attached ficture, tell me the porosity you think. one is the best one is the bad spot.

[MVagusta]

(11-13-2013 10:09 PM)derek Wrote:  Thanks for your reply, please see attached ficture, tell me the porosity you think. one is the best one is the bad spot.

Thanks for the pics.
Honestly i think that those mounts have not been polished correctly and therefore porosity is not evaluable.
To me it seems that they were prepared for the metallic coating evaluation. Do you use the same polishing cycle for both evaluation? (metallic and TBC)

Are you expert in doing and evaluating TB Coating? Are you aware that, according to customer spec, there are special mounts preparation cycle necessary for a correct TBC porosity evaluation? (which differs a lot from the metallic preparation)

If my thought is correct i suggest you to double check with your customer.

Is there any other opinion out there?

[derek]

Gordon, what is your opinion?

[Gordon]

Hi Derek

I'm thinking along same lines as MVagusta. Check out your metallography, it is possible that your coatings are much denser then they appear. Once had a company complain that their TBC was too porous at 35% porosity (as tested by independent test house laboratory), when I re-prepared samples I found less than 4% They had spent weeks developing parameters to give them denser coatings, when in fact the original starting parameters were correct in the first place.

[derek]

Thanks a lot to MVagusta and Gordon. Our customer adjudge the TBC good or bad by thermal shock test, for me is really doesn't make sense because the IGT engine run is much smooth than thermal shock, some engine runs 4 years only restart 2-3 times and the cooling air is warm air.

We adjudge TBC by thickness, porosity, tensile test and engine run, what test do you do to make sure TBC is functional?

I will follow up the reason we find.