Dark grey particles in Ni-Al bond coat?

[fhyuan]

Hi, all

Now we are struggling with the Ni-Al bond coat as significant grey particles present in the bond coat.
Initially we thought the powder had been contaminated so We purged the feed line and the powder feeder before spraying, but the problem persists. I have suspected the particles consist of aluminum oxide formed during spraying, however some EDM analyses indicated they are nickel rich.

Does anyone else have any experience with this powder?

Thanks

Yuan

[Gordon]

Hi Yuan

Composite Ni/Al powders like Metco 450 will typically give 5-7% oxide inclusions (predominantly aluminium oxide) in plasma sprayed coatings. NiAl alloy powders will probably be less. Another source may be from grit blasting with alumina grits, but you should easy recognise the difference. A micro-photograph may help.

[fhyuan]

Thank you, Gordon. In general the oxide forms in the form of stringers or oxide clusters, but it is roughly globular shape about 20-30 microns or large in our case. The particles are in the coating not at the interface, so it is not the alumina grits.
The Ni-Al powder is AMDRY 956.

[Gordon]

Hi Yuan

AMDRY 956 is similar to Metco 450 in that they are composite powders, a nickel core with about 4.5% very fine aluminium powder glued to the outside. During spraying the aluminium burns with oxygen from the air. This gives the exothermic reaction these types of powders are known for and not (as false claim years ago) a nickel/aluminium reaction forming aluminides. (Metco 404 Ni/20Al is an exception, 20% aluminium core with solid nickel shell). Essentially, the resulting coating is nickel containing aluminium oxide. Normally, the presence of free aluminium is not liked, but inevitably some may be found along with small amounts of nickel oxides.

I suspect your coatings may well be typical, but without actual looking at the microstructure I can't really say

[fhyuan]

Hi, Gordon:

I tried and but failed to attatch the coating micrograph, would you please tell me how to post the micrograph?

[Gordon]

Hi Yuan

The simplest way would be to use the forums attachment up-load facility, that should be visible on the reply or new post page. Ideally for photographs the jpg file format will be best. Files larger than 1024 KB will be rejected, so try to make image web and forum friendly. If you have trouble using this method please let me know in detail so I can fix problem. In the meantime you could email the image to me here and I will place the image in this thread for you.

[fhyuan]

Hi Yuan

The simplest way would be to use the forums attachment up-load facility, that should be visible on the reply or new post page. Ideally for photographs the jpg file format will be best. Files larger than 1024 KB will be rejected, so try to make image web and forum friendly. If you have trouble using this method please let me know in detail so I can fix problem. In the meantime you could email the image to me here and I will place the image in this thread for you.

   

[Gordon]

Hi Yuan

Is this micro-photograph a typical/average view or worse case? I assume you are working to a specification that calls for something like "oxide: Acceptable if distributed and fine. Continuous oxide stringers and clusters diameter greater than 60 micrometers are unacceptable".

What equipment, parameters, set-up for spraying samples? Is test piece sprayed in isolation to job or under simulated conditions e.g. non-ideal conditions such as internal diameters? How many spray passes? Any powder feed problems - pulsing, powder port/nozzle face fouling etc.?

[fhyuan]

Hi, Gordon

You are right as to the specification you mentioned. The micro-photograph is a typical view.

The test pieces were sprayed in isolation for spray ability test with 9MB spray gun. I believe we do not have the powder feed problems, no pulsing, no fouling.

The spray parameters are as follows: 500A/69V/80 Ar/ 17H2/160 mm standoff and ~60g/min feed rate/GH nozzle.

[Gordon]

Hi Yuan

Try reducing secondary H2 flow from 17 to 15, standoff 160 to 130, increasing powder feed from 60g/min to 70 g/min (powder port #2, 37 Carrier gas flow). Spray coating in one layer/single pass if possible.

If you see no significant improvement in coating and you are sure all equipment is calibrated and working as it should, I would try AMDRY 956 from other powder batches to make sure this is not down to product variation/quality issue, or try alternative powder like Metco 450NS.