Spraytime

As many in the industry know, thermal spray is a finicky game. With dozens of variables and incredibly refined physics, much 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. His name is Daryl Crawmer.

Rule #2

Crawmer had two laws of thermal spray. I am going to list his laws out of order for reasons that will soon become apparent. Crawmer’s second law is “There are no small carrier gas leaks.” In other words, “All carrier gas leaks are bad.” With so many variables to consider, this one can be easily 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 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. A consistent feed rate is key if you want to produce a consistent coating. Inconsistent feed rates lead 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, berries can start forming on the torch opposite the side of the powder injection port. If the flow is too low, the powder will not penetrate the boundary layer, and the powder port will get plugged or berries will form 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 gases. This can lead to rapid hardware failure and 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 periodic 15-minute leak checks on all powder feeders, hoses, and tubes used in the process. A 15-minute test could save you hours of troubleshooting and rework.

Rule #1

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 look at Crawmer’s second law again. 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 feedstock: 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, changing even one variable — no matter how small it may seem — can cause a lot to go wrong.

The optimistic side of this law is that thermal spray, like many processes, has its limits. But these limits are meant to be pushed. New things are meant to be tried, failed, and retried.

Conclusion

For the past century, thermal spray has been the hidden backbone of many influential industries like gas and oil, military, aerospace, and automotive. Today, new industries, such as electric vehicle, sustainable energy, commercial space travel, and artificial intelligence and quantum computing, will need that hidden backbone to help prop them up as they get legs and start to run. Thermal spray now faces more opportunities to push the boundaries of physics and materials engineering than ever before.

BENJAMIN GROW (benjamingrow@thermach.com) is operations manager, Thermach, Appleton, Wis.