Gordon,

Any idea if chrome carbide & T800 coating applied by plasma or HVOF after heat treatment or stress relief at ~2200 Deg F would undergo any property change that negate the coating quality? Work piece with these coating still intact and looks good, thinking of not to replace them if there is no concern on the coating integrity after stress relief.

Pls advice.

Regards,
Alexangel1226

Hi Alex

Yes, at 2200 F (~1200 C) the coating properties will change. If done in vacuum or protective atmosphere for short time at temperature these changes could well be beneficial assuming the coating remains well attached to the substrate. T800 coatings are certainly claimed to be improved using 2150 F (1175 C) for ~ 5 mins, becoming denser and wear resistance is claimed to increase by a factor of 4.

Chromium carbide/nickel chromium I'm not quite so sure of, as there is little information on heat treatments. But, I once did metallography on one of these coatings that had accidentally been vacuum heat treated after spraying. Not sure now on exact heat treatment parameters, but it was 1100 C + and well above its recommended max service temperature. Anyway, the coating microstructure certainly did not appear to be harmed and if anything was greatly improved as far as lowering porosity and some melding (not sure on that word, but a little solid state diffusion going on, not proper fusion or welding) together of particle boundaries. My feeling was that the coating had been improved. I don't think the components were put into service though, because of the hassle of getting round aerospace approvals etc. A problem you may have to face

Good luck

Gordon,

Further to this issue, do you think the YZ powder would undergo thickness change as it may get more compact during heat treat cycle? I have a case that we spray a liner to 0.015", however, after engine run, torn down and investigate and find that the thickness is only at 0.010" to 0.012".

Well potentially, this is the work diligence issue where technician did a lousy job but oversighting the measurement, dint check, the measurement instrument fault,..etc, take this potential causes away, is there a possibility that during engine run, which is at extremely high temp, the thickness of the coating is change? as TBC is porous coating.

Is there anyway we could simulate that investigation in the furnace? what is your recommendation to the test?

Pls advice.

Hi Alex

I doubt very much that yttria stabilised zirconia coating would undergo a 20 - 30% volume reduction. Complete porosity densification (up to 15% ?), low density phase transforming to high density phase, extreme temperature - no I don't think so I think you need to look elsewhere for the difference in thickness readings. Yes, I think placing test pieces in a furnace simulating similar conditions would be useful.

Gordon- Txs. I also think the same. well in this case, root cause is very much in the area of human error and measurement. Gauge R&r has to be extablish to verify the different between a thickness measure from metallurgical evaluation which is on a average reading, vs the tools or instrument to check the thickness, an isoscope which uses eddy current application, flat anvil micrometer, a point micrometer used. and also the NC program used to spray on the part.