Nickel Aluminide on Platinum

[k09]

Hello Forum members,

I had a query regarding Nickel Aluminide Coating on Platinum.

We need a ceramic coating on the platinum component to reduce its material loss. As per standard procedures a ceramic coating like alumina will require Ni-Al or Ni-Cr bond coat.

however, the client from his previous experience belives that Nickel and Aluminium react with Platinum at higher temperatures causing it to melt before BP. He is not very sure about it though.

I wanted to know if

1. Ni-5Al would actually react with platinum at high temperatures (1000-1200C)?

2. Would the coating fail if we donot apply the bond coat? the component is not exposed to any abrasion or any other relative action but the operating temperature is around 1000C.

I appreciate your inputs..

Regards

K09

[Gordon]

Hi K09

I would try first without any metallic bond coat. NiAl or NiCr bond coats are well beyond their best at these temperatures anyway and I would suspect some diffusion or reaction with Pt.

[k09]

Hello Mr. Grodon,

Could the diffusion lead to coating failure eventually?

We sprayed Ni-Al but then we got it removed by carefully grit blasting it.. This was very difficult as the component was thin walled (~0.5mm)

However, if we had not done this. would a 20micron thick Ni-Al on 0.5mm thick Pt under it and 200microns Alumina top coat, face any problems in an operational temperature of 1200C?

Would the coating or....even worse..the Pt base material itself get affected in such conditions?

As the client narrated his previous experience...Can the Ni really "eat into" the Pt base material at elevated temperatures? I do have a feeling this was slightly exaggerated...but still, I need to know how would a Ni-Al Coat on Pt behave at high temperatures.

Regards

K09

[Gordon]

Hi K09

Could the diffusion lead to coating failure eventually?

Solid state diffusion between NiAl and Pt I doubt would directly lead to coating failure. However, degradation of NiAl at 1000-1200C might. NiAl coatings generally are only rated up to ~650C for continuous use and up to 930C for intermittent.

However, if we had not done this. would a 20micron thick Ni-Al on 0.5mm thick Pt under it and 200microns Alumina top coat, face any problems in an operational temperature of 1200C?

Somewhat dependent on operating environment, but yes for the reason stated above.

Would the coating or....even worse..the Pt base material itself get affected in such conditions?

Possibility of coating failure through degradation of NiAl bond coat. Possibility of diffusion of NiAl with Pt. Not sure if contamination of platinum substrate will have impact on service, but your customer seems concerned with this aspect, so it may important.

As the client narrated his previous experience...Can the Ni really "eat into" the Pt base material at elevated temperatures? I do have a feeling this was slightly exaggerated...but still, I need to know how would a Ni-Al Coat on Pt behave at high temperatures.

"eat into" may be a exaggeration, but I think diffusion will occur over time. Platinum slowly changing to PtNi alloy.

[loriolo]

I agree with Gordon reply, Ni+5% Al is not indicated for high temperature, because there could be diffusion of nickel and aluminum in the base materials, a more indicated coating for high temperature is a bond coat with a tribaloy intermetallic powder Co-Cr-Mo and a top coat with Ytria stabilized Zirconia powdwer , that is indicated for temperature > 980 C, as coating on turbine components
Regards, Luigi

[k09]

Thanks for all the inputs!

Being a developmental job, we went ahead with the bond coat (Ni 5Al) with a top coat of Alumina. We also made another sample with only Alumina.

The sample was kept in a furnace at 1050 DegC. After 30 mins, we got both samples out for observation.

The one with the bond coat had delaminated COMPLETELY without application of any external force. The layer below the Alumina had turned blue-green. On the other hand the sample with only Alumina did not get affected. Bend test on the 2nd sample yielded good results even after the baking experiment.

To ensure wheter the blue-green compound formed between Alumina and Pt was a Ni-Pt alloy, we conducted another test. We placed the Ni-Al powder on Pt foil and kept it the furnace for a few minutes. when we removed it, the powder had turned green and amorphous but it had not affected the Pt foil. Perhaps longer exposure would have affected the base material.

Gordon and Luigi..thanks for being a part of this and giving your valuable inputs..

Regards

K09

[k09]

Hello every1,

this is an extension to the query i had posted earlier.

Is there any way to dissolve the Ni-Al sprayed on Platinum without corroding the platinum substrate?

We could go for grit blasting but it would cause some substrate material loss even at very low pressures.

Thanks in advance..

Regards

K09

[Gordon]

Hi K09

A commercial nickel strip salt as used in plating industry or nitric acid solution ( ~50% water by volume at room temperature) should do the trick without attacking platinum.