Delamination in the bond coat sprayed on a Ti specimen

[Justin]

Dear all

I use 3MB gun with Ar/He to spray a bond coat of about 0.004" on a Ti specimen, and then spray a top coat of 0.06".

by a microscope, I see a delaminaiton as a gap within the bond coat on the overall blasted surface of Ti specimen.

Bond coat: METCO 450NS
Top coat: METCO 52CNS

How should I overcome this problem? Please advise.

[loriolo]

Hi Justin
Bond failures can occur on titanium substrates becouse this material should be indirectly preheated (it oxidise very fast, like alluminum), and you must take care during the surface preparation, the surface will be well blasted and as fast as possible it will be coated, take in mind that bond strenght for Ni/Al bond coat is about 4000 PSI, if you want something more bond, you must try with arc spray, so you can reach about 5400 PSI with Ni/Al and 6800 PSI with Sprabronce AA.

[kschewe]

450 bond well to Ti. It is the oem bondcoat for most aerospace oems. They recommend coarse grit mesh, 16 or 20 AlO. Try and spray asap if you can after blast. Clean the surface with Acetone or MEK. Most Oems allow 2-4h max after blast or reprocess. I have seen delamination in that combo. If 52c is applied with to much deposit per pass it will delaminate. I would suspect surface contamination though.
You can try using 480 as an alternate. It is a better bond coat then 450 if you are allowed to use it. It is the same 95Ni 5Al but an alloy instead of mix of powders

[Bler]

Hi Justin
Bond failures can occur on titanium substrates becouse this material should be indirectly preheated (it oxidise very fast, like alluminum), and you must take care during the surface preparation, the surface will be well blasted and as fast as possible it will be coated, take in mind that bond strenght for Ni/Al bond coat is about 4000 PSI, if you want something more bond, you must try with arc spray, so you can reach about 5400 PSI with Ni/Al and 6800 PSI with Sprabronce AA.

I spray Ni/Al on steel and Ti with F4 gun,roughness is about 3.3um on steel and about 2.3um on Ti .The TBT is about 40MPa on steel and 32Mpa on Ti. I consider the preheating and cooling may be the reason why there is delamination.

[Justin]

Thanks for your responses.

In order to show you what I am talking about, please refer to the following microscopic photo.

Delamination appears on a thin layer of bond coat.

Is it possible to result from the internal force when spraying?
If yes, how can I eliminate the force?

Thanks for your advice.

[derek]

First , when you spray Ti, re-heat is not recommend because the oxidation will lead the delamination.
Second, please dubble check the lab preperation is correct, some times the lab preperation will lead the delamination.

[Woderwick]

Justin,

We have seen this issue here as well.
After investigation it was found that the delamination was caused by the prep cycle in our lab.
Your micro shows the same as ours in that a thin layer of coating is still bonded well to the substrate and the delam is just above that (So the bonding was good). We removed this effect by reducing the amount of titanium base material on the sample by carefully cutting as much as possible away before mounting. This avoids extra stress on the coating due to the polishing of more base material than required.

Also, it can depend on the mounting technique used (We hot mount Titanium). We only view our samples for two hours after the completion of prep as the mounting material continues to work for some time after polishing (Effectivly pulling the coating off the subtrate) and the results get worst over time.

Hope this helps.

Woderwick

[Justin]

Hi Woderwick

Thanks for your reply.

We removed this effect by reducing the amount of titanium base material on the sample by carefully cutting as much as possible away before mounting. This avoids extra stress on the coating due to the polishing of more base material than required.

I don't understand the above clear.
Do you just remain a very small piece of as-sprayed coupon after cutting?

Could you please describe it further about the above?

Also, it can depend on the mounting technique used (We hot mount Titanium). We only view our samples for two hours after the completion of prep as the mounting material continues to work for some time after polishing (Effectivly pulling the coating off the subtrate) and the results get worst over time.

So you view the sample within two hours once you finish polishing hot mount, and the result will continue becoming worst and worst with time. Right?

[sreenuvundela]

I totally agree with woderwik comments.

From the micro structure, it is very clear that delamination is in coating itslef.

If delamination is inbetween substrate and coating, then it may be becasue of poor shot blasting and substrate oxidation.

But, in this case it is because of poor metallography sample preparation. It generally occurs because of poor cutting of the sample.


Regards,

SREENIVAS

[KevinS]

Looking at the photo provided it appears like too much energy was applied during the grinding and or polishing of the sample as this is what that condition typically looks like. If your coating is not too thick and you grind and polish at 150 rpm and 20-25N the problem should go away. If your coating is thick >0.040 inches then your problem is that the coating is thick and the grinding/polishing procedure will have to be adjusted to cover the thick coating.

Dear all

I use 3MB gun with Ar/He to spray a bond coat of about 0.004" on a Ti specimen, and then spray a top coat of 0.06".

by a microscope, I see a delaminaiton as a gap within the bond coat on the overall blasted surface of Ti specimen.

Bond coat: METCO 450NS
Top coat: METCO 52CNS

How should I overcome this problem? Please advise.