Plasma Jet Colour/Temperature
02-09-2009, 12:45 PM, (This post was last modified: 02-09-2009, 01:16 PM by Rich.)
#1
Plasma Jet Colour/Temperature
Hi, I'm new to this board but have been reading it for a while. I hope you may be able to help me!

I'm an LPPS process engineer with a problem...

My LPPS systems run on argon primary gas and helium secondary. The powder carrier is also argon.

I am investigating the effect of varying the primary gas wrt coating deposition area, i.e. the spot size of MCrAlY coating that would be created if sprayed onto a flat plate.

By increasing the argon from 85psi to 120psi over a 5-10 second duration, the plasma jet visibly changes. It appears to change from a blue to a purple colour. It is spraying XPT 852 powder at 1500A gun current throughout this process. The only variable is the primary (argon) gas pressure over the orifice. I can't remember the orifice size off hand, and don't know the actual flow rate of the gases, just that these changes are noted with an increasing pressure and hence flow rate.

Doe the colour change from blue to purple indicate a temperature change of the plasma jet? The EM spectrum would indicate a shorter wavelength of light emitted (i.e. higher temp), but I don't know how this relates to plasma jets. A typical flame colour is determined by the combusted material. As for plasma jets, I just don't know!

Also, the plasma jet appears to become narrower and more focused as the primary gas pressure is increased. In trials this appears to give a greater weight gain, or rather a more focused sprayed spot size.

The spraying distance is approx. 12-16 inches.

Does anyone have any views on the plasma jet temperature related to colour, or the effects the primary gas have on it?

Does anyone have any views of the spray area ("spot size") variation wrt to primary gas pressure?

Or any links to papers that have investigated this previously. I have drawn a blank after many hours of searching, except for this:

https://en.wikipedia.org/wiki/File:Plasma_scaling.svg

From this paper, although I don't know where they got the colours from on the previous link:
(link dead)

I'm in at the deep end with this. New job and all that!

Many thanks,
Rich
Reply
02-09-2009, 01:45 PM,
#2
RE: Plasma Jet Colour/Temperature
Hello

Do you have 2 pictures?

If you have more Argon, plasma jet temperature will be lower and gas speed faster, so particles will be less heating and a speed higher (so particles stay less time in plasma jet and go less in plasma jet center).
If temperature is high (if you have less argon temperature is higher and particles stay longer in plasma jet) more evaporation from particles you have, this evaporation can change plasma color jet.

regards


(02-09-2009, 12:45 PM)Rich Wrote: Hi, I'm new to this board but have been reading it for a while. I hope you may be able to help me!

I'm an LPPS process engineer with a problem...

My LPPS systems run on argon primary gas and helium secondary. The powder carrier is also argon.

I am investigating the effect of varying the primary gas wrt coating deposition area, i.e. the spot size of MCrAlY coating that would be created if sprayed onto a flat plate.

By increasing the argon from 85psi to 120psi over a 5-10 second duration, the plasma jet visibly changes. It appears to change from a blue to a purple colour. It is spraying XPT 852 powder at 1500A gun current throughout this process. The only variable is the primary (argon) gas pressure over the orifice. I can't remember the orifice size off hand, and don't know the actual flow rate of the gases, just that these changes are noted with an increasing pressure and hence flow rate.

Doe the colour change from blue to purple indicate a temperature change of the plasma jet? The EM spectrum would indicate a shorter wavelength of light emitted (i.e. higher temp), but I don't know how this relates to plasma jets. A typical flame colour is determined by the combusted material. As for plasma jets, I just don't know!

Also, the plasma jet appears to become narrower and more focused as the primary gas pressure is increased. In trials this appears to give a greater weight gain, or rather a more focused sprayed spot size.

The spraying distance is approx. 12-16 inches.

Does anyone have any views on the plasma jet temperature related to colour, or the effects the primary gas have on it?

Does anyone have any views of the spray area ("spot size") variation wrt to primary gas pressure?

Or any links to papers that have investigated this previously. I have drawn a blank after many hours of searching, except for this:

https://en.wikipedia.org/wiki/File:Plasma_scaling.svg

From this paper, although I don't know where they got the colours from on the previous link:
(link dead)

I'm in at the deep end with this. New job and all that!

Many thanks,
Rich
Reply
02-11-2009, 10:33 PM,
#3
RE: Plasma Jet Colour/Temperature
Hi Rich

Sign0016 to the Surface Engineering Forum.

Firstly, I have not had a lot of experience with LPPS (Low Pressure Plasma Spray) and I am not a plasma physicist Sad Interesting questions and I will give some of my thoughts, but bear in mind the above caveat Happy0193.

Quote:Does anyone have any views on the plasma jet temperature related to colour, or the effects the primary gas have on it?
Is this colour condition with or without powder feed? If we are talking just about the plasma, then I'm sure the emitted light or electromagnetic radiation spectrum as well as the absorption spectrum would yield information on temperature, composition and ionic state changes etc., but colour is the perceptual quality of light and is a subjective response by the brain to light stimulating the eye Cool The observed colour glow around the plasma may well be indicative of its temperature, but I think it is far more complex than estimating temperature from the colour of steel at say 800C (which is complex enough Happy0193)

I once did a little bit of development work for plasma spraying titanium alloys in an argon chamber, though my part was all conducted in APS conditions. The equipment I had to work with would not produce suitable coatings using pure argon plasma, just not enough energy. Helium secondary gas was needed. Anyway, out of interest I ran a trial using plasmas from pure argon, through various mixes of argon/helium to pure helium all roughly at the same total gas flow. I do remember being surprised by a sudden transition in plasma conditions as plasma went into helium only mode. The surrounding colour glow distinctly changed towards red/violet rather than normal blue/violet. Now, as perceived colours go, one might expect shifting towards red from blue would indicate lower temperatures, but I'm pretty sure helium plasma was running very much hotter than argon and argon/helium plasma.

To a large extent I think plasma temperature is a bit academic by itself. The important aspect is the particle heating and acceleration properties of the plasma of which temperature only plays a part.

As far as primary gas flows are concerned, I'm not sure I totally agree with landemarre in that the plasma temperature will be lower with higher gas flows, but tend to agree with resultant effects on particle heating and velocity. Higher gas flows should make the plasma more restrictive - increase resistance needing more energy (higher voltage), increasing arc length and thermal pinch effect which should result in higher peak temperature and velocity. Complex for sure even without considering the particular equipment, nozzle geometry, or whether plasma gas is rotating, lamina, turbulent flow etc..

Quote:Does anyone have any views of the spray area ("spot size") variation wrt to primary gas pressure?
I'm sure it does, but it will also be dependent on many other factors.

Really you will need to consider all parameter conditions together to try and make any sense of this. For instance you are making one change (primary gas pressure), this in itself may change plasma voltage (energy), will change plasma composition (argon/helium mix) again effecting plasma properties. Also the changing properties of your plasma may now make your powder injection parameters less than ideal etc... The more you look into it, the more complex it seems to get Happy0193 but I suppose that makes life more interesting.

Sorry for rambling, but I think I've only scratch the surface of this topic. Good luck with your endeavours.
Reply
02-13-2009, 10:37 AM, (This post was last modified: 04-28-2009, 10:02 AM by Rich.)
#4
RE: Plasma Jet Colour/Temperature
Gordon, you are right it is a complex subject. "The more you know, the more you realise you don't know" seems to be a fitting phrase.

Plasma Temperature:
Before we go any further I would like to emphasise that I am not looking to quantify the temperature wrt the plasma gases - I am simply looking (at the moment!) to examine if the temperature increases or decreases as the plasma gas mixture varies.
I initially observed these changes whilst running powder as the colour changes are most noticable under these conditions. I have since done it with just the plasma jet.

You seem to contradict yourself partly:

If we are talking just about the plasma, then I'm sure the emitted light or electromagnetic radiation spectrum as well as the absorption spectrum would yield information on temperature, ...

And,

Now, as perceived colours go, one might expect shifting towards red from blue would indicate lower temperatures, but I'm pretty sure helium plasma was running very much hotter than argon and argon/helium plasma.

Could you clarify that please? By common sense I agree with your first statement, but by your practice it appears that it might not be correct after all.


Spot Size:
What I've observed is that by increasing the primary gas (argon) the plasma jet narrows down. This would be the pinch effect you mention due to nozzle geometry. It also focuses the powder better and appears to have less powder wastage, i.e. more powder is successfully melted in the jet and less just bounces off into space.


Another question I've been thinking about for a while is monitoring the coating thickness of production parts without sectioning parts. The process at the moment consists of development work to obtain the correct thickness and coating distribution - verified by sectioning and microscopy. This thickness is correlated with a weight gain of the whole component after LPPS spraying. When production parts are sprayed their weight gains due to coating are recorded, and trends can be observed. As long as the weight gain on all parts is within our specification then the coating thickness is assumed correct.
As it's MCrAlY coating we can't use methods such as we'd use for TBC coatings etc. Are there other methods people use that I could adopt?

Many thanks,

Rich



(02-11-2009, 10:33 PM)Gordon Wrote: Hi Rich

Sign0016 to the Surface Engineering Forum.

Firstly, I have not had a lot of experience with LPPS (Low Pressure Plasma Spray) and I am not a plasma physicist Sad Interesting questions and I will give some of my thoughts, but bear in mind the above caveat Happy0193.

Quote:Does anyone have any views on the plasma jet temperature related to colour, or the effects the primary gas have on it?
Is this colour condition with or without powder feed? If we are talking just about the plasma, then I'm sure the emitted light or electromagnetic radiation spectrum as well as the absorption spectrum would yield information on temperature, composition and ionic state changes etc., but colour is the perceptual quality of light and is a subjective response by the brain to light stimulating the eye Cool The observed colour glow around the plasma may well be indicative of its temperature, but I think it is far more complex than estimating temperature from the colour of steel at say 800C (which is complex enough Happy0193)

I once did a little bit of development work for plasma spraying titanium alloys in an argon chamber, though my part was all conducted in APS conditions. The equipment I had to work with would not produce suitable coatings using pure argon plasma, just not enough energy. Helium secondary gas was needed. Anyway, out of interest I ran a trial using plasmas from pure argon, through various mixes of argon/helium to pure helium all roughly at the same total gas flow. I do remember being surprised by a sudden transition in plasma conditions as plasma went into helium only mode. The surrounding colour glow distinctly changed towards red/violet rather than normal blue/violet. Now, as perceived colours go, one might expect shifting towards red from blue would indicate lower temperatures, but I'm pretty sure helium plasma was running very much hotter than argon and argon/helium plasma.

To a large extent I think plasma temperature is a bit academic by itself. The important aspect is the particle heating and acceleration properties of the plasma of which temperature only plays a part.

As far as primary gas flows are concerned, I'm not sure I totally agree with landemarre in that the plasma temperature will be lower with higher gas flows, but tend to agree with resultant effects on particle heating and velocity. Higher gas flows should make the plasma more restrictive - increase resistance needing more energy (higher voltage), increasing arc length and thermal pinch effect which should result in higher peak temperature and velocity. Complex for sure even without considering the particular equipment, nozzle geometry, or whether plasma gas is rotating, lamina, turbulent flow etc..

Quote:Does anyone have any views of the spray area ("spot size") variation wrt to primary gas pressure?
I'm sure it does, but it will also be dependent on many other factors.

Really you will need to consider all parameter conditions together to try and make any sense of this. For instance you are making one change (primary gas pressure), this in itself may change plasma voltage (energy), will change plasma composition (argon/helium mix) again effecting plasma properties. Also the changing properties of your plasma may now make your powder injection parameters less than ideal etc... The more you look into it, the more complex it seems to get Happy0193 but I suppose that makes life more interesting.

Sorry for rambling, but I think I've only scratch the surface of this topic. Good luck with your endeavours.
Reply
02-26-2009, 12:38 AM,
#5
RE: Plasma Jet Colour/Temperature
Hi Rich

Sorry for delay in reply.

Quote:You seem to contradict yourself partly:

If we are talking just about the plasma, then I'm sure the emitted light or electromagnetic radiation spectrum as well as the absorption spectrum would yield information on temperature, ...

And,

Now, as perceived colours go, one might expect shifting towards red from blue would indicate lower temperatures, but I'm pretty sure helium plasma was running very much hotter than argon and argon/helium plasma.

Could you clarify that please? By common sense I agree with your first statement, but by your practice it appears that it might not be correct after all.

Really, all I was trying to say is that the overall colour we perceive by eye is probably a poor way to assess temperature as opposed to detailed analysis of the spectrum.

Quote:Another question I've been thinking about for a while is monitoring the coating thickness of production parts without sectioning parts. The process at the moment consists of development work to obtain the correct thickness and coating distribution - verified by sectioning and microscopy. This thickness is correlated with a weight gain of the whole component after LPPS spraying. When production parts are sprayed their weight gains due to coating are recorded, and trends can be observed. As long as the weight gain on all parts is within our specification then the coating thickness is assumed correct.
As it's MCrAlY coating we can't use methods such as we'd use for TBC coatings etc. Are there other methods people use that I could adopt?

Measuring weight gain ie amount of coating is I think a good and relatively easy method to check whether everything is going to plan. Although, the correlation to thickness will be dependent on keeping coating distribution and density constant.

Where coating properties are similar to that of the substrate, the use of portable electronic thickness measuring devices will be difficult if not impossible. See coating-thickness-measurement-t-66.html
coating-thickness-measurement-t-202.html
I think you will have to rely on measurement of component before and after coating.

Good luck, it sounds like your making good progress Smile
Reply
02-27-2009, 02:36 PM, (This post was last modified: 04-28-2009, 10:03 AM by Rich.)
#6
RE: Plasma Jet Colour/Temperature
Hi Gordon, thanks again for the reply.

Agreed, perception by eye is not accurate by any means. I was simply hoping to know that, say, if it looks more blue or more red it's getting hotter or colder respectively in a purely qualitative way. I'll have a play when I can get some more time on the machines!

As for the coating thickness measurements it's as I thought. We've used weight gains for a long time with no problems, and as long as the gun/sting axes are not altered then the coating distribution varies very little.

Anyway, thanks again. I've now got more problems and questions that I've got to investigate when I get time on the machines. If I had a machine all to myself for development work... That would be ideal!

(02-26-2009, 12:38 AM)Gordon Wrote: Hi Rich

Sorry for delay in reply.

Quote:You seem to contradict yourself partly:

If we are talking just about the plasma, then I'm sure the emitted light or electromagnetic radiation spectrum as well as the absorption spectrum would yield information on temperature, ...

And,

Now, as perceived colours go, one might expect shifting towards red from blue would indicate lower temperatures, but I'm pretty sure helium plasma was running very much hotter than argon and argon/helium plasma.

Could you clarify that please? By common sense I agree with your first statement, but by your practice it appears that it might not be correct after all.

Really, all I was trying to say is that the overall colour we perceive by eye is probably a poor way to assess temperature as opposed to detailed analysis of the spectrum.

Quote:Another question I've been thinking about for a while is monitoring the coating thickness of production parts without sectioning parts. The process at the moment consists of development work to obtain the correct thickness and coating distribution - verified by sectioning and microscopy. This thickness is correlated with a weight gain of the whole component after LPPS spraying. When production parts are sprayed their weight gains due to coating are recorded, and trends can be observed. As long as the weight gain on all parts is within our specification then the coating thickness is assumed correct.
As it's MCrAlY coating we can't use methods such as we'd use for TBC coatings etc. Are there other methods people use that I could adopt?

Measuring weight gain ie amount of coating is I think a good and relatively easy method to check whether everything is going to plan. Although, the correlation to thickness will be dependent on keeping coating distribution and density constant.

Where coating properties are similar to that of the substrate, the use of portable electronic thickness measuring devices will be difficult if not impossible. See coating-thickness-measurement-t-66.html
coating-thickness-measurement-t-202.html
I think you will have to rely on measurement of component before and after coating.

Good luck, it sounds like your making good progress Smile
Reply
02-27-2009, 05:15 PM,
#7
RE: Plasma Jet Colour/Temperature
Hi Rich

Quote:Agreed, perception by eye is not accurate by any means. I was simply hoping to know that, say, if it looks more blue or more red it's getting hotter or colder respectively in a purely qualitative way. I'll have a play when I can get some more time on the machines!

Well I certainly use the relative intensity (brightness as well as colour) of light given off by the particles within the spray stream to judge degree of particle heating (which is not necessarily anything to do with actual plasma temperature).

Observing changes within the spray stream, whether that is colour, brightness, shape, noise etc. I think is extremely useful in development. Also, from an operator point off view, who has his senses tune to the process will/should notice when things are not right or going wrong. Though, increasing advances in equipment technology tend to make us more reliant/trusting on the machines "intelligence" or senses more than our own Rolleyes
Reply
10-13-2010, 08:06 AM,
#8
RE: Plasma Jet Colour/Temperature
love to see the pictures , plz share with us . thanks in advance .
Reply
10-19-2010, 06:02 PM,
#9
RE: Plasma Jet Colour/Temperature
Rich,
This is probably too late to help you out, but here goes:

Disclaimer: I am not a plasma physicist (rather an optical physicist) so this may not be entirely correct...but here is my simple way of understanding color/temperature in plasmas!

Measuring objects using color assumes they can be reasonably approximated by a black body. See: http://en.wikipedia.org/wiki/Black_body The wavelength of light is determined by the random thermal motion of the atoms in the material and tends to be fairly continuous.

Plasmas are entirely different. In the plasmas we are talking about here, most of the electrons are essentially completely free from their atoms. This is why the plasma becomes conductive and you can pass 500Amps across a gap with only 40 Volts (V=IR the resistance is very low). The temperature of the plasma is basically the average velocity of the constituents (free electrons, nuclei, and entrained gases). This is why when you add hydrogen or helium you increase the enthalpy of a plasma...their nuclei have a low mass, thus a high velocity. The wavelength of light emitted is determined by the very discrete energy levels of the constituents. The color of the light is determined by the emission/absorption spectra of the constituents and the temperature of the plasma...rather than just the temperature as in a black body.

In your case, by changing the concentrations of the various constituents, you will suppress and/or excite various emission lines. You could vary easily be lowering the temperature of the plasma while suppressing a lower energy level line. This would make the plasma appear more purple. In addition you are also changing the amount of entrainment of any atoms you have floating around in your vacuum chamber...maybe trace amounts of oxygen or nitrogen (I did some work a number of years ago where I measured the temperature and relative concentration of entrained molecular oxygen in an argon plasma using a complicated spectroscopy technique...I understood the optics end of things, but didn't do the plasma physics end of things). This would change the perceived color of your plasma by adding new emission lines. All this combined will probably have a much heavier weight on the perceived color of the plasma than temperature. In order to measure temperature, you would need to take the spectrum of the plasma and do some complicated math.

Unless you want to turn this into a science project, you would be better off optimizing particle temperature, velocity, and injection rather than plasma temperature.

Thanks,
Dan
Reply
10-20-2010, 04:40 PM,
#10
RE: Plasma Jet Colour/Temperature
Hi Dan

Thanks for your interesting comments. I think that explains better my comments on the perceived colour differences between argon and helium plasma Smile
Reply




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