LPPS Coating Temperature and 'Non-Stick' Coatings
05-27-2009, 02:07 PM,
LPPS Coating Temperature and 'Non-Stick' Coatings
I'm after some information and haven't been able to find what I need in a literature search so far, so I thought I'd ask here.

I'm asking about the temperatures experienced in an LPPS machine, in the plasma jet itself and in the atmosphere surrounding it.

It's a Metco 03CP plasma gun with an EPI 264A gas flow panel running on He and Ar. Metco 60C powder feed unit.

We typically spray revolving parts at a distance of approx. 12" from the nozzle exit. What I'm trying to find out is the temperature that might be experienced in the plasma jet 12" from the nozzle. Tough question!

I'd also like information regarding the temperature of the atmosphere in the coating chamber and perhaps the temperature gradient between the plasma jet and the inner wall of the chamber, although I can estimate this fairly easily once the plasma jet temperature is known.

It is quite clear that the jet would melt various superalloys if they were left in there stationary and the rate of melting is a function of time. What I want to determine is, based on the temperature of the jet, how long various alloys can be exposed to the plasma jet during both the TA cleanup process and coating process. Any pointers to answer this question would be greatly appreciated!

Another related question is about non-stick coatings, such as Teflon variants, that we could apply to tooling to prevent the adhesion of MCrAlY and reduce the need for acid stripping the tooling. This is directly related to the temperature related in the plasma jet area, or rather the temperature that the tooling might reach after x amount of time. If anyone has any advice or experience that they could share regarding non-stick coatings in high temperature environments, ideally with MCrAlY involved, then I'd appreciate that too.

05-27-2009, 04:14 PM,
RE: LPPS Coating Temperature and 'Non-Stick' Coatings
Hi Rich

I don't have any real hands on experience with LPPS (Low pressure plasma spray) process, so I will not even try to answer any of your questions directly. I will make some comments/observations that I hope will be helpful.

I am not sure knowing plasma/atmospheric temperatures by themselves will be particularly helpful in calculating the possible temperature of parts within the chamber. Knowing the amount of energy going in versus the amount exiting the system may be more useful. But then we are talking about the system as a whole at equilibrium not real world non-equilibrium conditions.

I know that heat transfer from plasma gun to parts/tooling is greatly increased when a coating is being deposited over just heating with gas/plasma alone. This effect on its own suggest to me that knowing temperatures alone will not be helpful. The temperature of a unloaded plasma plume is probably going to be higher than one loaded with powder, yet heating potential will be significantly lower.

I think the only sure way will be to actually measure/monitor the temperature of the parts your interested in under operating conditions.

What masking materials (if any) do you use at the moment? Fluorocarbon and silicone type coatings may be good, if temperature is kept relatively low. For parts in direct line of fire, I have my doubts though.
06-02-2009, 10:42 AM,
RE: LPPS Coating Temperature and 'Non-Stick' Coatings
Hi, thanks for the comments.

After a bit more reading around I've managed to find a paper based on a similar setup to our machine which demonstrates a local heat flux at the same spraying distance that we use of 600 W/cm^2 and a temperature of 10,000K. Obviously any metal in the line of fire from that sort of temperature and heat transfer will melt very fast, but parts are typically rotating and a point on that part might only briefly pass under the spray plume before rotating away from the plume and coming around again.

However, without knowing the heat transfer rate out of the tooling it's hard to estimate anything with any certainty. I might be able to estimate a ratio between a two different materials, one of which is a material we often use and one of which is a new material.

Tooling materials at the moment are typically a Hastelloy alloy. I'll look into your suggestions of flurocarbon and silicone coatings.

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