Plasma spraying microspheres, problems with clumping
06-11-2015, 07:23 AM,
#1
Plasma spraying microspheres, problems with clumping
Hi everyone,
this is my first post on this forum so I briefly want to introduce myself before going on to ask for advice!
I'm a graduate student in physics and I just started learning about thermal spraying a year ago. Thermal spraying has amazing potential in my subfield of research in physics so we've been exploring a variety of uses of thermal spraying for engineering purposes in our research. I find thermal spraying to be extremely interesting and I'm hoping to learn a lot more by talking to you guys!

Recently, we have been plasma spraying microspheres (made of alumina and silica) mixed with alumina powder onto an alumina surface. However, we have been experiencing "clumping" of the powder, meaning that we the flow rate of the powder is not easily controlled and instead of getting a consistent flow of these microspheres out of the plasma gun, we'll get bursts where clumps get shot out and splatter onto the substrate. We have tried playing with the flow rate, cleaning the nozzle of the gun very frequently, and feeding the powder into different ports of the gun. None of this has solved the problem. I am starting to think that it is somewhere along the feed line (between the hopper and the gun) or the adaptor (between the feed line and the hopper) where there is a very small pinhole through which the powder is fed.

I'd love some advice! Thanks in advance!
Reply
06-11-2015, 08:51 AM, (This post was last modified: 06-11-2015, 10:20 AM by Vadim Verlotski.)
#2
RE: Plasma spraying microspheres, problems with clumping
(06-11-2015, 07:23 AM)objeong Wrote: Hi everyone,
this is my first post on this forum so I briefly want to introduce myself before going on to ask for advice!
I'm a graduate student in physics and I just started learning about thermal spraying a year ago. Thermal spraying has amazing potential in my subfield of research in physics so we've been exploring a variety of uses of thermal spraying for engineering purposes in our research. I find thermal spraying to be extremely interesting and I'm hoping to learn a lot more by talking to you guys!

Recently, we have been plasma spraying microspheres (made of alumina and silica) mixed with alumina powder onto an alumina surface. However, we have been experiencing "clumping" of the powder, meaning that we the flow rate of the powder is not easily controlled and instead of getting a consistent flow of these microspheres out of the plasma gun, we'll get bursts where clumps get shot out and splatter onto the substrate. We have tried playing with the flow rate, cleaning the nozzle of the gun very frequently, and feeding the powder into different ports of the gun. None of this has solved the problem. I am starting to think that it is somewhere along the feed line (between the hopper and the gun) or the adaptor (between the feed line and the hopper) where there is a very small pinhole through which the powder is fed.

I'd love some advice! Thanks in advance!

Hi objeong,
If I understand correctly, you are mixing large particles (spheres) of Al2O3 and SiO2 with a fine-grained powder Al2O3. In this way, you can really make a fine non-flowable Al2O3-powder flowable (agglomerates of fine powder "smeared" on the surface of large particles). I itself did manner mixture Cr-SiO2, wherein a particle size of SiO2 was 50-100 microns, and the chromium particles < 5 microns. The powder mixture was flowable and powder feeder fed it without problems to the plasma torch.
However, one last problem remains. When radial powder feed, the fine particles melt immediately and stick to the powder nozzle. Molten particles accumulate more and more on the nozzle and then "spit" as a big lump.
I know only one way to solve of this problem: you have to replace the radial powder feed to axial powder feed. I work with the plasma torch Axial III of Mettech. Only this burner allows the spraying of mixtures which comprise particles < 5 microns.

Regards
Reply
06-11-2015, 11:54 PM,
#3
RE: Plasma spraying microspheres, problems with clumping
(06-11-2015, 08:51 AM)Vadim Verlotski Wrote: Hi objeong,
If I understand correctly, you are mixing large particles (spheres) of Al2O3 and SiO2 with a fine-grained powder Al2O3. In this way, you can really make a fine non-flowable Al2O3-powder flowable (agglomerates of fine powder "smeared" on the surface of large particles). I itself did manner mixture Cr-SiO2, wherein a particle size of SiO2 was 50-100 microns, and the chromium particles < 5 microns. The powder mixture was flowable and powder feeder fed it without problems to the plasma torch.
However, one last problem remains. When radial powder feed, the fine particles melt immediately and stick to the powder nozzle. Molten particles accumulate more and more on the nozzle and then "spit" as a big lump.
I know only one way to solve of this problem: you have to replace the radial powder feed to axial powder feed. I work with the plasma torch Axial III of Mettech. Only this burner allows the spraying of mixtures which comprise particles < 5 microns.

Regards

Hi Vadim,
thank you so much for your detailed reply!
I work with spraying mostly microspheres (few % alumina powder), by the way. Providing some numbers, here are the average particle sizes.
Microspheres: 22 micron
Alumina: 60 micron

So you can see that these powders are not extremely fine. Do you still think that these powders are too fine? Are fine powders more likely to agglomerate together?

Also, if you don't mind, I don't quite understand why radial feed causes more agglomeration than axial feed. Could you explain to me why?

I very much appreciate the help! You have already given me so many ideas!
Reply
06-12-2015, 08:18 AM, (This post was last modified: 06-12-2015, 08:19 AM by Vadim Verlotski.)
#4
RE: Plasma spraying microspheres, problems with clumping
Hi objeong,

First I want to explain why a radial powder feed is worse than the axial powder feed:
If the powder particles with the same sizes and densities, there is no problem with the radial powder feed: so you can find a gas pressure of the powder feeder in which the largest proportion of particles goes in the plasma flame. If you want to plasma spraying a mixture of small and large particles, so there is a problem: "squeeze" to small particles in the plasma flame need higher gas pressure than for large particles. Choose a medium gas pressure, so the smallest particles are repelled from plasma flame, forming a cloud of powder to the flame and stick to powder nozzle; the largest particles against shooting through the plasma flame.
When using axial powder feed coming all particles in the flames core. The smaller particles are hot and faster than the larger ones, but all of the particles then form the layer. The temperature differences of small and large particles can be well used.

In your case, SiO2-spheres are small particles that go not well in the plasma flame. These are also easily fusible and stick as Glass on powder nozzle. If you want to stay with radial powder feed, you need take significantly larger SiO2-spheres (> 100 microns).

Regards
Reply
06-15-2015, 11:33 PM,
#5
RE: Plasma spraying microspheres, problems with clumping
(06-12-2015, 08:18 AM)Vadim Verlotski Wrote: Hi objeong,

First I want to explain why a radial powder feed is worse than the axial powder feed:
If the powder particles with the same sizes and densities, there is no problem with the radial powder feed: so you can find a gas pressure of the powder feeder in which the largest proportion of particles goes in the plasma flame. If you want to plasma spraying a mixture of small and large particles, so there is a problem: "squeeze" to small particles in the plasma flame need higher gas pressure than for large particles. Choose a medium gas pressure, so the smallest particles are repelled from plasma flame, forming a cloud of powder to the flame and stick to powder nozzle; the largest particles against shooting through the plasma flame.
When using axial powder feed coming all particles in the flames core. The smaller particles are hot and faster than the larger ones, but all of the particles then form the layer. The temperature differences of small and large particles can be well used.

In your case, SiO2-spheres are small particles that go not well in the plasma flame. These are also easily fusible and stick as Glass on powder nozzle. If you want to stay with radial powder feed, you need take significantly larger SiO2-spheres (> 100 microns).

Regards

Hi Vadim,
thanks again for your reply! I'll try it out and let you know how it goes! It seems like a radial feed system (Metco systems) is much more popular than the axial feed! The companies that I have contacted don't carry axial powder feeds!
Thanks!
Reply
06-16-2015, 01:21 AM,
#6
RE: Plasma spraying microspheres, problems with clumping
(06-12-2015, 08:18 AM)Vadim Verlotski Wrote: Hi objeong,

First I want to explain why a radial powder feed is worse than the axial powder feed:
If the powder particles with the same sizes and densities, there is no problem with the radial powder feed: so you can find a gas pressure of the powder feeder in which the largest proportion of particles goes in the plasma flame. If you want to plasma spraying a mixture of small and large particles, so there is a problem: "squeeze" to small particles in the plasma flame need higher gas pressure than for large particles. Choose a medium gas pressure, so the smallest particles are repelled from plasma flame, forming a cloud of powder to the flame and stick to powder nozzle; the largest particles against shooting through the plasma flame.
When using axial powder feed coming all particles in the flames core. The smaller particles are hot and faster than the larger ones, but all of the particles then form the layer. The temperature differences of small and large particles can be well used.

In your case, SiO2-spheres are small particles that go not well in the plasma flame. These are also easily fusible and stick as Glass on powder nozzle. If you want to stay with radial powder feed, you need take significantly larger SiO2-spheres (> 100 microns).

Regards

Hi Vadim,
sorry to bother you with this again but is Mettech Axial III the only spray gun that takes an axial powder feed? It seems like the Metco 3MB, 7MB, and 9MB are the most popular plasma guns and all of them are radial powder feeds.
Thanks in advance!
Reply
06-16-2015, 07:32 AM,
#7
RE: Plasma spraying microspheres, problems with clumping
Hi objeong,

That is absolutely correct. Axial III of Mettech is unique plasma torch with axial powder feed. Since all other plasma torch have the radial powder supply, the work almost exclusively with narrow fractional coarse powders from interval 20-120 microns, mostly 45 ± 10 microns. Such mixtures, how do you want to spray, are to spray only with Axial III.
However, to try this, you need not have its own Axial III; You can spray your mixture in a any spray company with Axial III. I can also spray it for you.

Regards
Reply
06-16-2015, 11:26 PM,
#8
RE: Plasma spraying microspheres, problems with clumping
(06-16-2015, 07:32 AM)Vadim Verlotski Wrote: Hi objeong,

That is absolutely correct. Axial III of Mettech is unique plasma torch with axial powder feed. Since all other plasma torch have the radial powder supply, the work almost exclusively with narrow fractional coarse powders from interval 20-120 microns, mostly 45 ± 10 microns. Such mixtures, how do you want to spray, are to spray only with Axial III.
However, to try this, you need not have its own Axial III; You can spray your mixture in a any spray company with Axial III. I can also spray it for you.

Regards
I'll PM you! Thanks!
Reply
06-17-2015, 05:22 PM,
#9
RE: Plasma spraying microspheres, problems with clumping
Hi
you need to set parameters because you are making a mixture and to set-up a set of spray parameters for a new powder is very long, and in your case you need to have a good powder feeder, and different powder injectors and supports with different angles, and a powder feeder without fluidized bed.
Best regards
Luigi
Reply
06-19-2015, 12:05 AM,
#10
RE: Plasma spraying microspheres, problems with clumping
(06-17-2015, 05:22 PM)loriolo Wrote: Hi
you need to set parameters because you are making a mixture and to set-up a set of spray parameters for a new powder is very long, and in your case you need to have a good powder feeder, and different powder injectors and supports with different angles, and a powder feeder without fluidized bed.

Hi Ioriolo,
why is a fluidized bed powder feeder bad? Also, you are also suggesting an axial powder feed?
Thanks!
Reply
06-24-2015, 02:26 PM,
#11
RE: Plasma spraying microspheres, problems with clumping
Fluidized bed powder feeder are chipper for the OEM, but when you need to aline the powder plume inside the plasma flame, you need to increase or reduce the carrier gas, you can do this without any change in the powder feeding with a mechanical transport of the powder, but in the fluidized bed, changing carrier gas pressure you change all the other settings, is a very instable kind of powder feeder.
Best regards
Luigi
Reply




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