NiCrAlY interface separation

[edrazee]

Hi

New user here! This forum has been useful for my thermal spray journey!

I am facing an issue with interface separation when spraying NiCrAlY coating using F4 gun. I have tried taking extra precautions when grit blasting but it does not seem to improve the situation. My suspect is that there are oxides on the interface and they are being pulled out.

Anyone with experience can chime in on this? Am I guessing wrongly?

Thanks in advance!


Ed

[Lemster68]

You should let us know the substrate and all your spray parameters, including powder used.

[edrazee]

You should let us know the substrate and all your spray parameters, including powder used.

Substrate is Inconel 718+.

Spray parameters:
Metco F4 gun with 8mm nozzle and 2.0mm powder injector
9MPE-CL powder feeder
Current 600A
Argon 50 NLPM
Hydrogen 6 NLPM
Carrier gas flow 3 SLPM
Powder feedrate 40 g/min
Standoff distance 140mm

Do you see anything off with my parameters? Also attached some photos.

   

   

   

[Lemster68]

I don't have an answer for you yet, just more questions: 75, 90, 105 degree injection? What volts are you getting? Preheat? Yes/no? If yes, how hot? Surface speed and step distance? Still do not know what specific powder you are using.

[edrazee]

I don't have an answer for you yet, just more questions: 75, 90, 105 degree injection? What volts are you getting? Preheat? Yes/no? If yes, how hot? Surface speed and step distance? Still do not know what specific powder you are using.

1) 90 degree injection
2) ~75V
3) No preheat
4) 75 m/min and 4mm step (if i remember correctly)
5) I started out with Amdry 962 but recently bought PAC 9620

Additional info
1) Around 0.0015" per pass
2) Grit blasting with #60 brown aluminum oxide. Surface roughness of around 110 Ra.
3) Vacuum impregnated at room temperature.

[Lemster68]

Great, now that you specifically named one of your powders, I found this on the forum. Look to the end.
http://www.gordonengland.co.uk/sef/Threa...PS-process

[edrazee]

Thanks for the link! Really useful.

However, my main question here is about the "interface separation" on my samples. Any thoughts on that?


Regards
Ed

[Vadim Verlotski]

Thanks for the link! Really useful.

However, my main question here is about the "interface separation" on my samples. Any thoughts on that?


Regards
Ed

Hi Ed,

It is not for nothing that NiCrAlY layers are normally produced using VPS and not APS. With APS process it is much more difficult to solve the problems with the oxidation of the substrate (adhesion problem) and the problem with the tensile stresses in the layer (temperature problem).
In your case, I would have tried to drastically reduce the layer thickness per pass (at least 3 times faster relative burner movement than now: < 0.0005" per pass). This helps to reduce the tensile stresses in the coating.

Regards
Vadim

[edrazee]

Hi Ed,

It is not for nothing that NiCrAlY layers are normally produced using VPS and not APS. With APS process it is much more difficult to solve the problems with the oxidation of the substrate (adhesion problem) and the problem with the tensile stresses in the layer (temperature problem).
In your case, I would have tried to drastically reduce the layer thickness per pass (at least 3 times faster relative burner movement than now: < 0.0005" per pass). This helps to reduce the tensile stresses in the coating.

Regards
Vadim

Hi Vadim

Thank you for chiming in on this. I am relatively new to TBC coatings. I have a few questions if you don't mind.
1) Will reducing the thickness per pass reduces the formation of oxide due to less heat build up?
2) What is the relationship between tensile stresses in the coating and formation of oxides?
3) What does preheating the substrate do to the coating?

[Vadim Verlotski]

Hi Ed,

the problems with oxidation and dangerous tensile stresses in the layer have to be considered separately, although they often arise simultaneously.

Oxidation first:
Your substrate is always oxidized and you cannot avoid it with "normal" APS (there are a few methods to remove oxides from the oxidized substrate surface during APS, but it does not apply to your burner and powder). In a VPS process, the substrate is cleaned of the oxide layer using vacuum annealing at 900°C and this is one of two most important VPS advantages.

The second advantage of the VPS process over the APS is the very high substrate temperature (900°C) during spraying. Thanks to this temperature, the layer on the substrate can plastically deform and the high residual stresses cannot arise.
With an APS, you have to cool the substrate and do not exceed 150°C during coating. Preheating to 150-200°C is possible, but counterproductive. When spraying on a cold substrate, tensile stresses in the layer are inevitable.

The only thing you can do is keep these tensions as low as possible. For this, as thin as possible monolayers are helpful.

Regards
Vadim

[edrazee]

Hi Vadim

Thank you for your detailed reply. I have learnt so much. I will work towards reducing the thickness per pass to see if i can resolve this problem.

Thanks again!