Instrument to check coating thickness of Thermal Barrier Coatings

[odski0317]

Hi can you pls help, we are spraying thermal barrier coatings by plasma, we want to check coating thk after every passes, what type of measuring instrument or gage can I use to verify? Is coating thk gauge instrument possible?
Pls advise.

[Gordon]

Hi odski0317

to the Surface Engineering Forum.

Thermal barrier coatings are normally non-magnetic and non-electrical conductors so they should be suitable for measurement with the commonly available portable electronic thickness measuring devices typically aimed at the paint and organic coating industry. Make sure the device you choose covers the thickness range you require. It will be important to know the magnetic/electrical property difference between coating and substrate as only certain combinations will work. So a non-magnetic/non-electrical conductive ceramic coating should work well on a ferromagnetic substrate or on a good electrical conductive metal substrate. The bond coat material that you use may be much more difficult to measure though and may cause some calibration problems later for the ceramic coating. The instrument will require calibration on bond coated substrate as the bond coat will alter properties depending on thickness. These instruments can be very sensitive and accurate if set up well, but they really do need to be cross-checked/compared to other measuring methods like micrometer/vernier or cross-section metallography.

There are a number of threads on this subject which may be worth searching out and I've copied one across below from coating-thickness-measurement

Hello,
I am interested in portable coating thickness measurement instrument for thermal spray coating: all type cermet coatings WC/Co, CrC/NiCr; ceramic coatings Al2O3, Al2O3/TiO2; carbon steel; stainless steel, Mo ; self fluxing NiCrBSi coating and ?. on carbon steel, stainless steel and cast iron substrates. Can anyone to help?
Thanks

Hi Aturas

There are many portable electronic thickness measuring devices out there. Ideally, the ones using both magnetic and eddycurrent methods would best suit your needs. Select the models that cover your particular thickness measuring range. These devices are good for:

• non-magnetic coatings on ferromagnetic substrates like aluminium on steel
  
• non-electrical conductive coatings on conductive substrates like ceramic on austenitic (non-magnetic) stainless steel
  

You will find that these instruments will not work with every coating/substrate combination though. Always calibrate and test on specific coating/substrate materials before relying on their readings.

The measurement of metals on non-ferrous metal substrates is not easily achieved using eddy-current method unless there is a 3:1 ratio in the conductivity between substrate and coating. The calibration of instruments must be carried out using samples of the materials with known thickness. Such instruments do not use the normal calibration characteristics and would have to be specially made for the specific coating and substrate.

[Alexangel1226]

Gordon,

The bond coat material that you use may be much more difficult to measure though and may cause some calibration problems later for the ceramic coating. The instrument will require calibration on bond coated substrate as the bond coat will alter properties depending on thickness. These instruments can be very sensitive and accurate if set up well, but they really do need to be cross-checked/compared to other measuring methods like micrometer/vernier or cross-section metallography.

Specific to TBC application, do you mean the daily check or calibration on the master test piece may not be giving you accurate top coat thickness even though we know after the master test piece verification, we are very sure that the probe or isoscope is working fine?

In your opinion, if the probe/isoscope shows very little error (<1%) when checked on master test films, and yet shows a great variation of 0.002" ~0.003" difference compared to metallurgraphy cut-up thickness check, this is due the the bond coat thickness which has alter the
properties? Can u elaborate further?

I am having this prob. I conducted a studies on my instrument which uses eddycurrent application to check coating thickness, to see the variation compared to a flat anvil micometer, point micrometer, and metallurgical cut-up.

It surprises me when the variation between isoscope and metallugical cut-up is as such (0.002" to 0.003")at Mean! whereas the shape point micrometer very close to the metallurgical evaluated result. And Flat Anvil is inbetween the rpobe and the shaped point mic & metallurgy cut up. I do not do use the same bond coat surface and do the cal or zeroing. should I do so

Pls advice..

Regards,
Alexangel1226

[Gordon]

Hi Alex

When measuring as-sprayed coatings (with rough texture) using metallography you have a choice of what to measure; from trough or peak or estimated average value of the substrate or bond coat interface to the coating surface troughs or peaks or average value. When using micrometer/vernier or eddy current/magnetic probe you will be measuring max peak to max peak (assuming you calibrate on the prepared surface before spraying ). I think this is where some of the discrepancies lie between methods.

If you are carefully calibrating the probes on the prepared (bond coated) surface to be sprayed then there should be no real problem in determining peak to peak coating thickness of the ceramic coating. Problem comes if you are trying to measure the bond coat. The bond coat probably not being too dissimilar to the substrate properties (electrical, magnetic) may make eddy current/magnetic methods not work at all or will require special calibration using actual bond coating material as a posed to the plastic or brass calibration test shims. So determining the total combined coating thickness can be problematic using only eddy current/magnetic probes.

I think cross referencing the various measuring methods as you are doing is good thing, even if they don't accurately agree, they should be consistent (hopefully) and useful for general production checking.

[William Alexander]

Hi can you pls help, we are spraying thermal barrier coatings by plasma, we want to check coating thk after every passes, what type of measuring instrument or gage can I use to verify? Is coating thk gauge instrument possible?
Pls advise.

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Hi could you help me, we require to spray a thermal barrier coating could you recommend a coating to me,
William Alexander

[dienngothe]

Hi everybody,
We are using an arc spray equipment to create various metallic coatings. We have a problem when we measure the thickness of a magnetic coating. Can you suggest me a device for this problem? I have asked some thickness gauge manufacturers and they recommended me some devices that use ultrasonic wave. What do you think about this kind of device? Can they be used to measure the thickness of magnetic coating produced by arc spay technology.
Please give me your answers.
Thanks and best regards