In principle, one could use metallography to distinguish between (A) gas-atomized, spherical, metallic feedstock particles not remelted in thermal spraying and (B) thermal sprayed particles of the same powder lot that melted and resolidified in flight.
The as-etched microstructure of a particle can be greatly influenced by its solidification rate. So (A) and (B) particles of the same true size might be distinguishable if the solidification rates of (A) in manufacturing and (B) in spraying differed.
Note, though, that a metallographic cross-section you see may not be through the "equator" of the particle, which makes comparisons tricky---big particles can look smaller in cross-section than they really are, while solidification rates vary with
true size.
Also note that, if the thermal spray standoff distance is too low, some particles might have too little time to melt. If the standoff is too high, some molten droplets might solidify before impact.
About plasma fluctuations: As you indicated, these can affect the heating rate of sprayed particles. They're not all exposed to the same melting conditions. Some recent equipment designs give exceptionally stable plasma jets. Here's one new example:
https://www.arzell.com/plasma-spray-system/------------------------------
Robert Miller
Materials Engineering Consultant
R. A. Miller Materials Engineering
Indianapolis IN
(317) 698-5479
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Original Message:
Sent: 07-18-2022 17:38
From: Milad Rad
Subject: Proper Method for Analyzing Nonmolten Particles
Hello Everyone,
I have two questions regarding the presence of nonmolten particles in thermal-sprayed coatings. I would be grateful if you could share with me your thoughts and experience.
- It is well-known that non-molten particles do not receive enough energy from the energy source (i.e. flame, plasma, etc.). It is also possible that they receive sufficient energy, but they lose it prior to reaching the substrate. Is there any method that can be used to understand clearly whether the particle was not melted at all, or it was melted and then resolidified in flight?
- One of the key reasons for the presence of non-molten particles is believed to be the fluctuation in plasma or flame during the spraying process that causes variance in the in-flight particle temperature and velocity. I am interested in knowing more about the quantitative impact of these unwelcome fluctuations. Could you please guide me about your own experience or any published research article that has explained the impact of these fluctuations from a statistical viewpoint?
Thank you in advance for your attention.
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Milad Rad, Ph.D.
Assistant Professor
University of Southern Indiana
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