Development of High Purity Alumina

[Albeezy]

Hello all!

I've lurked these forums for a few years now, it's great to have a discussion resource like this available

Anyways, we're working on developing coating parameters for plasma-sprayed high-purity alumina (99%+) on an anodized aluminum substrate. We are running into issues with coating adhesion. We've run through a couple parameters that have been known to work, (~500A, 40-50 slpm Ar, 8-12 slpm H2) but these parameters had a NiAl bond coat applied before them.

We've tried grit blasting the surface to increase mechanical bond, but that had a very limited effect. Additionally, the customer would like to avoid having to damage the anodization too much. It seems that the anodization is too hard to get a good adhesion at low temperature, but at higher temperature the thermal mismatch between substrate and coating gets too great, and coating pops off as the two cool. Currently, this is only an R&D project, so only spraying 1"dia. anodized Al buttons.

Any ideas, thoughts, or suggestions for things to try to get this coating to stick? Thanks y'all!

[Vadim Verlotski]

Hello all!

I've lurked these forums for a few years now, it's great to have a discussion resource like this available

Anyways, we're working on developing coating parameters for plasma-sprayed high-purity alumina (99%+) on an anodized aluminum substrate. We are running into issues with coating adhesion. We've run through a couple parameters that have been known to work, (~500A, 40-50 slpm Ar, 8-12 slpm H2) but these parameters had a NiAl bond coat applied before them.

We've tried grit blasting the surface to increase mechanical bond, but that had a very limited effect. Additionally, the customer would like to avoid having to damage the anodization too much. It seems that the anodization is too hard to get a good adhesion at low temperature, but at higher temperature the thermal mismatch between substrate and coating gets too great, and coating pops off as the two cool. Currently, this is only an R&D project, so only spraying 1"dia. anodized Al buttons.

Any ideas, thoughts, or suggestions for things to try to get this coating to stick? Thanks y'all!

Hi Albeezy,
Why do you coat an anodized aluminum surface? Doubled Al2O3 coating brings nothing but disadvantages: additional costs for anodizing and miserable adhesion of plasma layer to anodized surface. This is just as pointless as applying a carbide layer on nitrided steel surface.

If the customer decides to use thermal spraying, he must understand that his previous surface treatments (chrome plating, nitriding, anodising, shot peening and so on) must not be used any longer.

Regards
Vadim

[Albeezy]

Hi Albeezy,
Why do you coat an anodized aluminum surface? Doubled Al2O3 coating brings nothing but disadvantages: additional costs for anodizing and miserable adhesion of plasma layer to anodized surface. This is just as pointless as applying a carbide layer on nitrided steel surface.

If the customer decides to use thermal spraying, he must understand that his previous surface treatments (chrome plating, nitriding, anodising, shot peening and so on) must not be used any longer.

Regards
Vadim

This is an old spec that the customer submitted a while ago, before I got started at this company. I believe it never got out of the development phase, maybe because of the challenges faced?

Either way, our customer is not the end customer, so the requirement comes from higher up then I can affect. That being said, I agree with you. It's not an ideal situation, but sometimes the people making these decisions aren't the most technically aware of the process.

We'd like to give it our best shot, as far as achieving a suitable coating, perhaps in the meantime we can convince them to rethink what they're asking. This undertaking is a fun challenge, if nothing else!

I appreciate your input Vadim!