Thermal spray coatings variables

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What differentiates a high quality thermal spray coatings shop from a so-so quality thermal spray coatings shop, I was pondering the other day. Well, there are a bag full of variables that one can list that goes into producing high quality thermal spray coatings. The high quality thermal spray coatings facility most likely tries to keep these variables under tight control whereas the sloppy thermal spray coatings facility has poor controls on these variables. So if you want to convert your thermal spray shop from a low quality house to a high quality house, all you have to do is to simply put in more controls, right? Now that is simple. Yet why do so many thermal spray companies continue to produce bad thermal spray coatings? This question is almost akin to someone saying that you make money in the stock market by buying low and selling high; now that is simple and yet everyday even the most savvy of investors loses money off and on ( some more often than others ) in stocks.

Well, at least the thermal spray coatings business is not nearly as hazy as Wall Street. So there is some hope for quantifying and reasoning out the solutions to this burning issue of poor thermal spray coatings quality and the controls on variables. That is essentially going to be the discussion in this article. To start off, one needs to list all of the variables that exist in the first place before you can go around trying to put in controls on them. You might be surprised, but many of the variables that are important actually go unnoticed. Just as an example, next time you walk by the cubicle of a fellow plasma spray engineer or walk by a plasma spray operator, catch him off guard and ask him to list all of the variables that he thinks are important. See how many one can list quickly when caught by surprise. You will be amazed to say the least.

Now granted some parameters are more important than others and need stricter controls than others. But that is not the point. In order to accurately reproduce high quality on thermal sprayed coatings, one needs to identify all of the variables that affect the process, list the importance of each variable and then institute controls on them. New variables that were hitherto not considered may pop up from time to time and in that case they need to be included in the list to be controlled. Now this may seem like a task to be delegated to the R and D department only or to the engineering manager only. That is simply not the case. Everyone in the team from the engineering guy to the operator to the metallurgical lab person needs to work in unison to exert these controls so repeatable coating quality is ensured.

Talk about variables. You will be surprised but years ago when a company I was working in was audited by a quality auditor from a large “independent” entity, he actually made an “observation” ( an observation for those not in quality circles is not a finding and therefore does not require a corrective action report ), that the coatings stripping operation called for rinsing the parts in water. His observation said that there was no specification of the temperature of the rinse water. Room temperature water was not sufficient. His point was that we should have stated that the parts must be rinsed in water that is between fifty degrees to ninety degrees F. Be specific and accurate he said. He was an old man and I was a young engineer and needless to say I took offense to his observation. But now that I am getting older, I tend to think he was trying to teach me the value of specificity in producing repeatable results.


Everything that is a variable that is to be controlled must be specified with upper and lower control limits. What is the high end acceptable point and what is the low end acceptable point, be it temperature, rpm, feed rate, gun voltage or whatever. This is also applicable to set up data such as stand off distance, air jet intersect distance, gun override past part, etc, etc. The operator who does the set up of the plasma jobs needs to be a conscientious person who cares about strict adherence to the controls that are set up and provide feed back to the parameter and tech sheet writer if some controls are too difficult to achieve or some controls need to be tightened up. This level of interaction between people is needed. That is how the great coating houses get to be what they are. Are you one of them?