Metco 443 Microctrusture Trouble
07-29-2006, 08:47 AM, (This post was last modified: 01-19-2007, 02:52 PM by Gordon.)
#1
Metco 443 Microctrusture Trouble
Dear all,
We sprayed bond coating of Metco 443 powder by Metco 6P Gun. The fine unmelted particles were observed in the coating Microctrustures. Is there any method to reduce its emergence?
Thanks,
Best regards,

William

[Image: 443ns-2.jpg]
Regards, William
Reply
07-29-2006, 06:15 PM,
#2
RE: Metco 443 Microctrusture
Hi William

Welcome to the Surface Engineering Forum.

I think most of the small round shaped particles are not strictly unmelted particles, they are produced when a larger rather fluid particle stikes the surface and on "splatting" ejects or splashes smaller particles that then become included in the coating. True unmelted particles are usually much larger, being large particles that have not seen enough heat to soften or melt sufficiently to splat on impact.

Is the attached image a typical view of the microstructure or the worst case to ilustrate the problem? To some extent these particles and unmelts are a natural, though unwanted part of these coatings. I have noticed that sometimes when true unmelts are very low in numbers, the higher the occurance of these smaller rounded particles and also when unmelts are high in number, the fewer small rounded particles are seen.

Out of interest is the top coat a nickel graphite abradable coating or similar?
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07-31-2006, 11:06 AM,
#3
RE: Metco 443 Microctrusture
Dear Gordon,
I want to thank you very much for responding to my request so promptly. The attached image is worst test piece to ilustrate my problem. I didn't know how to reduce the small round shaped particles, Can you help me.
You are right, the Top coating is the nickel graphite abradable coating.

Thanks,
Regards,

William
Regards, William
Reply
07-31-2006, 04:41 PM,
#4
RE: Trouble with Metco 443 Microctrusture
Hi William

Do you know why Metco 443 was chosen as a bond coat? The nickel/graphite top coat would normally be rated for use up to about 480 C maximum. Metco 443 through the 6P gun does not form "self bonding" coatings and may not be the best bond coat for this application. I think Metco 450 or 480 NiAl would provide superior bond coats.

I think if the microstructure image is showing the worst case, then I think most aerospace specification would regard this as acceptable. If all the recommended parameter settings are being used correctly, there should not really be problem, unless you have a bad batch of powder. Altering the powder feed rate only or altering the fuel/oxygen flows only as you would for adjusting parameters for achieving the right hardness for your nickel/graphite coating may give some scope to improving the coating, but really this should be the last resort.

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08-01-2006, 12:24 AM,
#5
RE: Metco 443 Microctrusture
Dear Gordon,
You've been very helpful.

thanks,
best regards,

William
Regards, William
Reply
10-15-2006, 11:10 AM,
#6
RE: Metco 443 Microctrusture
Hi William,

I am curious about the microstructure of the Metco 443 you sprayed.
I see there are also excessive oxide (linear oxide) exist in the mircostructure, correct me if I am wrong. Is the oxide level considered acceptable or rejectable?

What system do u use to spray this metco 443? a Plasma or Flame spray? As I spray this powder quite often and our process has been stable. Perhaps I could help you on this, but I am using a 9MB gun.

Thanks

Regards,
Alex



William Wrote:Dear all,
We sprayed bond coating of Metco 443 powder by Metco 6P Gun. The fine unmelted particles were observed in the coating Microctrustures. Is there any method to reduce its emergence???
Thanks,
Best regards,

William

[Image: 443ns-2.jpg]
Reply
10-15-2006, 01:35 PM,
#7
RE: Metco 443 Microctrusture
Quote:Hi William,

I am curious about the microstructure of the Metco 443 you sprayed.
I see there are also excessive oxide (linear oxide) exist in the mircostructure, correct me if I am wrong. Is the oxide level considered acceptable or rejectable?

What system do u use to spray this metco 443? a Plasma or Flame spray? As I spray this powder quite often and our process has been stable. Perhaps I could help you on this, but I am using a 9MB gun.

Thanks

Regards,
Alex

Hi Alex

If the Metco 443 coating had been plasma sprayed, I would certainly understand you being curious and question acceptability. I believe the coating in question was flame sprayed using a Metco 6P gun, which is also required to apply the abradable top coat. A flame sprayed coating microstructure will appear coarser, with higher oxide concentration and larger oxide inclusions than the equivalent plasma sprayed coating.
This link illustrates the effect.
Same at lower magnification

[Image: t43c250.jpg][Image: t43f250.jpg]

[Image: p43c250.jpg][Image: p43f250.jpg]

These photos are NiCr coatings similar to Metco 443 (except they have no additions of Al) Try to guess which are plasma sprayed. Follow links above for more info.
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05-08-2007, 12:58 PM,
#8
RE: Metco 443 Microctrusture
Hi Gordon,
I have one question with this coating.
Why this kind of coating produce suspected unmelted particles, when we use plasme or flame to spray it.

best regards,

william
Regards, William
Reply
05-08-2007, 03:24 PM,
#9
RE: Metco 443 Microctrusture
Hi William

First we need to be clear on what we mean by unmelted particle:
  • particle included in coating that has not melted (proper definition)
The generic term probably covers these as well
  • particles that have melted but solidified before contact
  • small rounded particles produced from impact splashes
  • Any round shaped particle

We also need to be aware that not all unmelted particles are necessarily harmful. Many good high velocity sprayed coatings in fact do not melt the powder particles and thus will contain very high levels of unmelted particles. I prefer the term "unreacted? or non-reacted particle for any particle that has not inter-reacted with the coating in the desired way.

Unmelted particles, unmelts or unreacted particles are a fact of life with thermal spray coatings and it would be difficult to find a coating total devoid them. Lets take Metco 443 powder; typical size range 45 to 120 micrometres, spraying this material with a well balanced set of parameters will tend to over heat the smaller particles and under heat the larger, while hopefully majority in the middle being heated correctly. Also, other process factors such as powder injection, flame turbulence, coating/substrate temperature etc. may contribute.

Metco 443 is composited with fine aluminium particles that allow for exothermic heating during spraying. Chances are that not all particles are coated with aluminium evenly. This is probably another factor towards uneven heat input.

Reduction of unmelted particles can be achieved by choosing a powder with tighter particle size distribution, but this tends to push costs up dramatically. Making changes to spraying parameters to put more heat into the powder will reduce unmelted particles, but at the expense of increased levels of oxidation and fume output. At the end of the day it is a compromise between all the factors.
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