Cracks in abradable coatings

[fhyuan]

Hi, everyone:

We spray abradable coatings (similar to Al-BN) for aeroplane engines. However, the abradable coatings were cracked in the as-sprayed condition. Sometimes the coatings crack during a machining process or an engine test. The cracks are generally locally circumferential in form for the inside coating of an ring piece.
The workpiece is cooled on the outside during its spraying inside. I wonder if it is better to direct the compressed cooling air towards the spray area instead to decrease the thermal residual stress in the coating and avoid cracking of the coatings?

Thanks for your advice.

[Gordon]

Hi Fhyuan

I think you have answered your own question to some extent Take the heat out of the coating, rather than out of the substrate.

[djewell]

When diagnosing cracks, it is helpful to think about the cooling and contraction rates of both the substrate and the coating. Ideally, they should contract the same amount, leaving the coating in a state of neutral stress. When you get tension cracks in the coating, either during spraying or during machining, try to keep the coating cooler or allow the substrate to get hotter. If the substrate gets hotter, it has to cool more, and it will shrink more, canceling some of the tensile stress in the coating.

Personally, I try to avoid cooling a coating because of the thermal shock of the "cold" air flow on the "hot" coating, which can also cause cracks.

[fhyuan]

Thank you, Gordon and djewell!
I am now testing with Almen specimens. It is evident that the blasting process produces compressive residual stress in the specimens and the sprayed abradable coating is in a tensile state.

I'll test next with the Almen specimens as a function of spray parameters, heat treatment, machining, heating and cooling ways for the coating and the substrate.