If there was a Godfather of thermal spray, who would it be?
By Ben Grow - Engineer, Thermach
Crawmer’s Laws of Thermal Spray
As many of you know, thermal spray is a finicky game. With dozens of variables and incredibly refined physics, there is a lot that can go wrong. Through decades of experience, one man was able to refine most of his knowledge into two laws to govern the entirety of thermal spray. He has contributed countless innovations and insights to the industry. He was even able to share some of his knowledge in the classroom as an Adjunct Professor at the University of Wisconsin-Madison. This man was also honored as a Fellow of ASM and was inducted into the Thermal Spray Hall of Fame in 2009. His name: Daryl Crawmer.
"For advancing thermal spray technology through numerous innovations in equipment and process design, and for being an invaluable source of information and advice to the thermal spray community."
I am going to list Daryl’s laws out of order for reasons that will soon become apparent. Crawmer’s second law is “There are no small carrier gas leaks.” Written another way: “All
carrier gas leaks are bad.”
With so many variables to consider, this is one variable that can easily be overlooked. A carrier gas leak anywhere from the point of control to the injection point into the plasma or flame can cause a multitude of problems, from inaccurate feed rates to powder injection problems, which in turn can cause downtime or produce a coating that is out of spec. This leads to rework and costs going through the roof.
Carrier gas leaks can also lead to inconsistent feed rates. If you would like to produce a consistent coating, a consistent feed rate is key. With an inconsistent feed rate, we are right back to rework and rising costs. Aside from feed rate issues, carrier gas leaks can result in plugs in the powder port or the production of “berries”. This is due to inconsistent powder penetration at the boundary layer of the plasma or flame. If the carrier gas flow is too high, you can start producing berries on the torch opposite the side of the powder injection port. If the flow is too low, powder will not penetrate the boundary layer, and you will plug the powder port or get berries on the same side as the powder port. Another issue with leaks is the aspiration of air or other gases that might not be process gasses. This can lead to rapid hardware failure, oxides/impurities in the coating, and it can alter the spray parameters. All three scenarios lead to excess material costs and, of course, more rework. For these reasons, we urge our customers to perform daily 15-minute leak checks on all powder feeders, hoses, and tubes used in the process. Not to sound like Geico®, but a 15-minute test could save you hours of troubleshooting and rework.
Crawmer’s First law is “The only absolute in thermal spray is that there are no absolutes.” You might be wondering why this law is here and out of order. Well, let’s take Crawmer’s second law as an example. Just because you have a carrier gas leak does not mean you cannot produce the desired coating. You can certainly produce a coating that is well within spec, it is just going to be more difficult and costly. Another example is with feed stock, the powder and wire used to spray. You can use the same powder from the same vendor in the same spec and it will work beautifully for your process. Then one batch comes in and it doesn’t work. The vendor insists that the powder is within spec, but it isn’t working as nicely. Maybe that specific cut has a slightly different shape to the particulates. Maybe the particulate size variation is off ever so slightly to interfere with the standard process. With so many variables in thermal spray, a lot can go wrong by only changing one variable no matter how small it may seem.
The optimistic side of this law is that thermal spray, like many industries, has its limits. But these limits are meant to be pushed. New things are meant to be tried, failed, and re-tried. For the past century, thermal spray has been the hidden backbone of many influential industries: gas and oil, military, aerospace, and automotive to name a few. Today, we are seeing new industries such as EVs, sustainable energy, commercial space travel, and AI and quantum computing which will need that hidden backbone to help prop them up as they get legs and start to run. Thermal spray may have just as many, if not more, opportunities today to push the boundaries of physics and materials engineering than ever before.
Remember, Thermach is not your competitor; your other options are. We don’t coat; we only supply thermal spray equipment and parts. You can proceed with confidence, knowing that our team is dedicated to creating a custom system that meets your exact specifications.
Manufacturer of Equipment & Parts for the Thermal Spray Industry. Backed by Reliable, Timely Service & Support.
Thermach manufactures thermal spray coating systems for the coating industry. We don't coat. We provide the equipment and parts necessary for you to run a successful coating operation. Our mission is to ensure your coating process is running smoothly with Thermach equipment.