obstruction during deposition of amorphous powders
07-15-2010, 06:12 PM,
#1
Question  obstruction during deposition of amorphous powders
Dear "surfacers",

I am new to amorphous materials and have little experience with hvof thermal spray. I wonder if you could help me with spraying problems.
I'm using a tafa jp 5000 to do an amorphous iron based coating with a nanosteel powder, but only the first deposition runs ok while following coatings are impossible to obtain even changing thermodynamics by parameters such as oxigen and fuel flow and ratio.
No problems with other powders but when I try again with this again I can do only one coating, as at the second deposition obstruction of injectors happens and need to stop.
What can I do?
I worked with fuel enriched flow to preserve particles from oxidation.
Thanks,
Moira


"Progress of science requires opportunity for share with no restriction all results and all point of views: this freedom of communication is indispensable for the development and extension of scientific knowledge". [A. Einstein]
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07-16-2010, 02:33 AM, (This post was last modified: 07-16-2010, 02:39 AM by Simon Kim.)
#2
RE: obstruction during deposition of amorphous powders
Hi, Moira

I have some experience of amorphous materials.
That needs to chill as super cooling effect during phase transformation.
Almost the cooling speed is 1X10^6℃/sec or more high speed.
Maybe iron base amorphous material needs above speed than Nickel or Cobalt alloy.
So your first deposition was satisfied, but next deposition did not reach the cooling speed.
Twin wire arc spray can reach to cooling speed almost nealy, so that has 40-50% amorphous phase in coating layer.
Because arc temperature is more high than HVOF flame, so can to get amorphous phase easily by high temperature gradient.
If you can have to cool by proper treatment after every deposition step, layers are transformed to amorphous phase same as first deposition.

Simon
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07-16-2010, 10:07 AM,
#3
RE: obstruction during deposition of amorphous powders
Hi Simon,
thank you very much for your answer.
I do have high amorphous fraction also in the second coatings, but I have to stop the process perhaps the temperature is higher (don't know why as I stop the equiment between depositions) or some other problems occur in the powder flow before combustion chamber: at the second deposition the flexibles burn on the injectors and also the barrel has some internal deposits!
Then I change barrel, injectors and flexibles but still I can't get a good process. The only way is to spray some others powders for a while (that you know is not so easy with only one distributor available) and then spray the amorphous one to have the first good coating.
Can't go on in this way. How can I do?
Thanks a lot,
Moira


"Progress of science requires opportunity for share with no restriction all results and all point of views: this freedom of communication is indispensable for the development and extension of scientific knowledge". [A. Einstein]
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07-20-2010, 08:26 AM,
#4
RE: obstruction during deposition of amorphous powders
Hi Moira,
I heard the phenomenon, internal doposition, from my colleague.
That is happened by small particle size in powders.
My colleague solved by changing the barrel length to shorter size and increasing pressure with kerosene and oxygen.
Fine particles are often deposited on internal surface of barrel.

And I wonder what kind of the powder, is gas atomised or water atomised?
Maybe the powder is made by gas atomised, the size is under 325 mesh.
The gas atomised powder has no fine size after seiving.
However, if your powder is water atomised, there are many fine grains.

I wish your success.

Simon
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07-20-2010, 04:34 PM,
#5
RE: obstruction during deposition of amorphous powders
Hi Simon,
I'm spraying a gas atomized powder -53μm/+15μm. The particles are near-spherical with satellites attached. That's why I got problems. I already worked with high pressure (up to 110psi) and with the 4" barrel that is the shorter one, with no results. Now I arrive to have two deposition done before internal deposition in the barrel occur with an enlargement of the barrel ending. Next trial will be at lower powder feed rate with increasing of the flow of the carrier gas.
Thanks again for your interest,
Moira


"Progress of science requires opportunity for share with no restriction all results and all point of views: this freedom of communication is indispensable for the development and extension of scientific knowledge". [A. Einstein]
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07-21-2010, 03:20 PM,
#6
RE: obstruction during deposition of amorphous powders
Hi desMoira
One of my customers, had the same problem with Hastelloy C 276 powder, the fines in powder produce this kind of problem, I solved the problem, just pouring off from a can to another in front of a vane of an exhaust fan, in order to eliminate fines, I tried with different powder productors but the result was the same, finally I decided to change to HVOF wire system, and finally I had a good result, with satisfation of my customer, for all the other answers Simon was right.
Regards
Luigi
Best regards
Luigi
Reply
07-22-2010, 01:18 PM,
#7
RE: obstruction during deposition of amorphous powders
Wire system... very interesting. I'll look for information and soon I'll keep you up to date.
Thanks a lot Loriolo.


"Progress of science requires opportunity for share with no restriction all results and all point of views: this freedom of communication is indispensable for the development and extension of scientific knowledge". [A. Einstein]
Reply




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