Co28Mo17Cr3Si powder
09-08-2006, 12:01 PM,
#1
Co28Mo17Cr3Si powder
Dear Gordon,
We are studying the Co28Mo17Cr3Si powder by plasma spray. But the result was not good, the unmelts and oxides didn?t meet requirement. We used 9MB Gun and 728 nozzle.
Can you help us,
How to solve this problem?
How should we do?
Thanks,
Best regards,

William
Regards, William
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09-08-2006, 01:26 PM,
#2
RE: Co28Mo17Cr3Si powder
Hi William

First, which powder are you using? I will guess Metco 68F (TRIBALOY T800).

What parameters? Ar/H2 or Ar only.

Are your requirements for a standard type coating or something special like a very high oxide content coating?

Is the 728 nozzle correct? Information I have suggests other nozzle types (Metco 68F).
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09-09-2006, 06:03 AM,
#3
RE: Co28Mo17Cr3Si powder
Dear Gordon,
Thanks for replying to my question so quickly.
My research is ?how to get low Oxides and low unmelts?. So we used 728 nozzle to spray this powder but unmelts didn?t meet my requirement. I have used the 733 nozzle to spray it too, but the Oxides didn?t make me satisfied.
The powders that I used have Metco 68F, Praxair Co-111 and Stellite T-800.

Do you have choices to the nozzle, parameter or equipment of other suggestions?
Can the Axial III plasma Torch System receive better result?

I?m very grateful to you.

Regards,

William
Regards, William
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09-09-2006, 02:10 PM,
#4
RE: Co28Mo17Cr3Si powder
Hi William

Oxide and unmelted particles (unmelts) are a bit of a balancing act. A typically good coating will contain some of both. The tendency is that reduce one and you will increase the other and visa-versa. I'm sure you probably have already noticed this effect. A very complex topic, which is worthy of, detailed discussion. I'm short on time at the moment, so I will return to this thread in a few days.
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09-09-2006, 02:27 PM,
#5
RE: Co28Mo17Cr3Si powder
I came across this pdf document a while ago while searching for something totally different. I think it could be a useful reference on this topic with respect to coating interpretation and evaluation.

Sorry link dead
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09-11-2006, 04:10 AM,
#6
RE: Co28Mo17Cr3Si powder
Dear Gordon,
Thank your replying.
This is a very complex topic.
If you have time, I?d like to discuss with you again.

Regards,
William
Regards, William
Reply
09-11-2006, 04:24 PM,
#7
RE: Co28Mo17Cr3Si powder - Powder
Hi William

Powder properties, feed rate and injection are critical factors.

Powder particle size range and the size distribution within this range will have a major impact on unmelts/oxide ratio and concentrations with APS (atmospheric plasma spray) APS. The finer the particles size the larger the relative surface area, which will make it easier to over-heat and oxidise. The tighter the size range and distribution the better for process control. If you have a poorly classified powder with a wide range of particle sizes, it is difficult to avoid over-heating with excessive oxidation of the fines and not melting the larger particles producing unmelts.

Coarse powders will tend towards low oxide coatings, but possibly with higher unmelts and porosity. Fine powders will tend towards denser more oxidised coatings.

link 1.
link 2.

Putting the powder into the right area of the plasma is also critical. With sideways injection, there is also a particle classification effect. Larger particles tending to penetrate through the plasma better than the smaller particles. It is very important that when developing plasma spraying parameters, that you always consider tweaking the carrier gas flows/powder feed rate to suit any changes that you make elsewhere. All to often I see people making changes effecting plasma velocity/enthalpy without thinking how this will influence the powder injection.

Optimising the powder feed rate is also a factor, higher feed rates tend to increase unmelts.

Other areas effecting oxide/unmelts I will cover in separate posts maybe tomorrow.
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09-12-2006, 01:01 AM,
#8
RE: Co28Mo17Cr3Si powder
Dear Gordon,
Thank you for your help.Ashamed0002
I will try to study in this direction.

Regards,
William
Regards, William
Reply
09-12-2006, 09:21 PM,
#9
RE: Co28Mo17Cr3Si powder - plasma parameters
Hi William

Plasma Parameters

For Metco 68F there are two basic recommended parameter sets covering 3MB/7MB/9MB:

GH or 732 nozzle for Ar/H2
704 or 727 nozzle for Ar only

These parameters should give you good coatings and probably will make good baseline standards to work from, certainly when considering Metco 68F powder. Have you evaluated these particular coatings yet?

To make changes to the coating, try increasing the primary plasma gas flow by say 10% over your baseline parameters. This will very slightly raise the plasma voltage/energy but will also increase plasma velocity. This change will in fact put less heat into the powder and increase particle velocity. You should find the coating has slightly less oxide and possibly slightly more unmelts. Try reducing primary flow by 10% from baseline and you should have the reverse effect. You could also try changing secondary (hydrogen) plasma gas flow (if using secondary gas). This will have significant effect on particle heating; increasing flow will increase oxide, reducing flow will increase unmelts. You could change the plasma current (Amps), but I would be inclined to leave this setting as a constant, certainly initially. Remember when making these changes, consider the effects on other setting such as powder injection. Spray distance, powder feed rate and hardware (nozzle) changes are other variables that could be considered. As you said a very complex topic, so I will stop here as I think there is enough food for thought.

To be honest, if you want to significantly lower the oxide/unmelts from the standard coating, you will really need to change the process or environment. Try using an argon or nitrogen shroud attachment on your gun or one step further spraying in an inert gas chamber or VPS. HVOF, HVAF and Cold Spray (cold gas-dynamic spraying process) are other processes worth consideration.
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09-12-2006, 09:38 PM,
#10
RE: Co28Mo17Cr3Si powder
Hi William

Just wondering, what is ultimate aim of your project?

Also, Just thought I would mention that the T800 coatings respond very well to heat treatment (~1200 C), this is a bit like sintering, closes up porosity, should remove problem effects of unmelts and improves corrosion and wear resistance.
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09-13-2006, 02:17 AM,
#11
RE: Co28Mo17Cr3Si powder
Dear Gordon,
My friend has participated in the 2002 Coating Workshop that GEAE has held, he tells me that can do the research of this one. I have read the Coating Interpretation & Evaluation file and other relevant research. The ultimate aim of my project is conformed to with F50TF45 CL-A requirement. I think this is a difficult research.

Thank your suggestions.
Regard,

William
Regards, William
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09-13-2006, 12:40 PM,
#12
RE: Co28Mo17Cr3Si powder
William

Thank you for starting this topic of discussion. Interesting thread, which is also relevant to all coating development, evaluation and interpretation. I think this thread only scratches the surface of subject.

I would be interested to hear how you get on with your project, the problems you encounter and hopefully what you learn from the exercise.

I have linked a few message archives that may be of some interest and relevance.

Key process variables for APS

T800 coating thickness limit

Cluster oxide in Inco718

Good luck
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