Re-applying Metco 308

[gejohn]

Hello again,

I am experimenting with this abradable coating and trying to prevent any more strip and recoats. Whenever this coating falls out in the metlab or in the machine shop, we have to strip the whole diameter.

In your experience, have you ever sprayed on top of machined Metco 308? I know there are powders that allow you to do this, but have not seen this yet for 85:15.

Any ideas?

John

[Gordon]

Hi John

No knowledge of re coating Metco 308, but generally would recommend stripping and starting afresh with any thermal spray coating. Sometimes this practice can be successful, but it carries a high risk factor.

[Madila Awalini]

Dear John,

I've heard about a defect called "LAYERING" that will happen when there is an interuption during spray process.
This can only be detected during microstructure examination.
But we have not done that practice in our shop, so always strip the existing coating prior to apply a new one.

John / Gordon, can you kindly give me the type of powders that allow us to do spraying on top of an existing coating system?

Thank you very much for your share.

~MaDiLa~

[Gordon]

Hi Awalini and John

Metallography invariably will show a fault line at the interface between old and new coating. How much this weakens the coating integrity is anybodies guess without conducting thorough coating evaluation or from experience of these coating systems working in practise.

Ideally, the best coatings are applied in a continuous manner at a constant coating/substrate temperature. I have seen metallography of coatings where the coating process was paused overnight and restarted next day. Just to say a good metallographer will probably be able to pin point this pause in spraying in the coating microstructure, even though the effect may be a subtle one and may not significantly effect the purpose of the coating. Re-coating of old coatings that are likely to have been contaminated or have been machined finished and grit blasted may even fail duing the spraying process or later during finish machining which i must say is better scenario then failure in service. In conclusion I would say strip back to the original sound substrate and start again. If you have to re-coat on top of a coating, make sure you test/evaluate the coating or are very confident from past experience that the coating will be fit for service.

Two successful jobs that I can think of, that were sprayed on top of existing coatings:

An old massive yankee dryer roll which had seen many thermal spray repairs, had a very thick arc sprayed 13 Cr steel coating. This was cleaned and ground (dry) true, then very carefully inspected and any holes or imperfections in the coating skilfully patch repaired using arc spray Ni5Al. This was grit blasted and re-inspected. The surface was then bond-coated with arc spray Ni5Al and then HVOF sprayed with a chromium carbide/nickel chromium. The coating was then finish ground. (note yankee roll was under internal steam heating 100 C+ throughout long process). At the time I thought this was a very risky procedure, but these guys were very experienced and skilful in their art and confident of success (and so they were). The procedure probably cut time and cost to less than half. I still would not recommend the procedure unless you really know what you doing.

Another example being large gate valves which had fused stellite like coatings. Many failed attempts at HVOF coating these parts before they were given to me for a last try. Can't remember what the HVOF coating was, but trying to coat these using recommended techniques resulted in the coating being rejected half way through the build-up. Anyway, knowing the existing coating had good heat resisting qualities, I preheated well above the normal 100 C to around 350 ? 400 C and then sprayed the coating without problems. It was nice to see the faces of many people involved that claimed it to be impossible to coat

So yes it is possible, but you must take account that it will compromise the integrity of the coating system and you need to be sure that the coating will be fit for service.

[zhoudesign]

you can spray this coat by METCO 6P equipment
SPRAYING MATERIAL： Ni/GRAPHITE
MATERIAL CODE: METCO 308-NS
EQUIPMENT
GUN TYPE： METCO 6P-II
SPRAYING NOZZLE： 6P7A-D
GAS CAP： 6P-4
POWDER FEEDER： METCO 3MP
METER WHEEL： S
AIR NVIBRATOR： USE
GASES
OXYGEN PRESSURE： 0.16-0.18MPa
ACETYLENE PRESSURE： 0.11-0.13MPa
AIR PRESSURE： 0.35-0.70MPa
NITROGEN PRESSURE： 0.37-0.40MPa
OPERATION CONDITIONS
OXYGEN GAS FLOW： 47?3
ACETYLENE GAS FLOW： 50?3
PRESSURE AIR GAS FLOW： 10?3L/h
NITROGEN GAS FLOW： 37-40
POWDER FEED RATE： 45?3 g/min
SPRAYING DISTANCE： 200-250mm
SPRAYING ANGLE： 70-80?

[zhoudesign]

you can spray this coat by METCO 6P equipment
SPRAYING MATERIAL： Ni/GRAPHITE
MATERIAL CODE: METCO 308-NS
EQUIPMENT
GUN TYPE： METCO 6P-II
SPRAYING NOZZLE： 6P7A-D
GAS CAP： 6P-4
POWDER FEEDER： METCO 3MP
METER WHEEL： S
AIR NVIBRATOR： USE
GASES
OXYGEN PRESSURE： 0.16-0.18MPa
ACETYLENE PRESSURE： 0.11-0.13MPa
AIR PRESSURE： 0.35-0.70MPa
NITROGEN PRESSURE： 0.37-0.40MPa
OPERATION CONDITIONS
OXYGEN GAS FLOW： 47?3
ACETYLENE GAS FLOW： 50?3
PRESSURE AIR GAS FLOW： 10?3L/h
NITROGEN GAS FLOW： 37-40
POWDER FEED RATE： 45?3 g/min
SPRAYING DISTANCE： 200-250mm
SPRAYING ANGLE： 70-80?

[Alexangel1226]

Gordon,

Some comment on this issue. I think recoating or not is a question of compliance, tolerance and integrity. A lot of job shops simply overcoat on existing coating..or localized repair without stripping the coating, and that is understanbly due to the undesirable rework cycle incurred. So it is the process owner integrity to stand firm on his professional expertise to dispose for a rework. It could be painful for the reworking.. but that's the right thing to do.

Also, unless the shop that produces the coating is not controlled by 100% metallorgical evaluation requirement to meet certain standard. else recoating will definitely fail the metallorgical evaluation. Imagine how are we going to simulate the tensile specimen, or microstructure test with some existing coating on it? Representativeness is questioned and coating integrity is at stake. There is no technical justification to say that it is alright. And all requirement (for example avaition standard) for any coating type is examined in as-sprayed condition onto a fresh new parent material or specimen.

So, if full compliance is a must, there seem little chance of easy way out...If a process owner is comfortable to proceed, then his decision is always challenged. Taking consideration that the work piece application in the aircraft that carry lives, I would doubt so to take any risk on this. Yet, if the application is not so crucial or life threatening ones, and given no straight control or standards to follow, I guess there is still room for compromise and exploring the recoating on existing layers, provided it passes all the post machining or grinding processes.



Regards,

[Gordon]

Hi Alex

In agreement with your comments.

Going back to Gejohn's original question, I can't see Aerospace specifications/procedures allowing this practice anyway, even under a concession I doubt it would be accepted.

My advice would be to strip back and start again, unless you really know what you are doing.

don't re-coat on top of old or failed coatings covertly (aim at cowboys/rogue traders) you only need a few bad/failed coatings to tar all thermal spray coatings with a bad name.

[Donald]

Re-coating on top of existing coating can be done but it's risky at best -- unless you thoroughly grit blast prep the already coated surface (which usually results in removing existing coatings in the case of conicraly for example) and preheat atleast 1 to 2 strokes over the media, it will more than likely show excessive delamination and or non-uniformity in oxide content during metallographic evaluation at anything higher than 50x. I can also imagine the erosion values would be nonconforming as well (if you do such testing on metco 308...)

Unless there's absolutely no way to remove the coating, I'd remove the existing coating down to substrate and recoat entirely. If the coating on top of the previously machined coating fails in service you'll have a lot more on your hands than the moneys you'd lose reworking the part in house.

Hope that helps.[/align]