Low Pressure Plasma Spray Oxides Control

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The low pressure plasma thermal spray process is amenable to producing the best clean low oxide coatings in the thermal spray coatings family. So much so that while high velocity thermal spray processes try to come very close to LPPS thermal spray process coating quality and imitation may be the best form of admiration, LPPS still is the best choice for producing a high quality oxide free thermal spray coatings. I know this statement is going to create some stir among the hvof thermal spray fans, but lets face it – hvof tries to imitate LPPS quality when it comes to low oxide clean thermal spray coatings; LPPS came first; then came hvof. Such low oxide clean coatings produced by the low pressure plasma spray process find fantastic applications in aerospace turbine shrouds, dental implant coatings and other high tech applications. The key to producing high quality LPPS coatings though is to have an excellent maintenance program in place. Unlike other thermal spray processes, LPPS is a high maintenance “animal”. It is in this regard that the balance of this post is addressed.

Several years ago, I came across a thermal spray operation, where a thick, high quality CoNiCrAlY coating had to be deposited on thousands of parts consistently meeting stringent specifications where very low levels almost negligible amounts of oxides were allowable in the coating and the interface between the coating and the substrate had to be very clean with minimal contamination. Such coatings are usually metallographically evaluated at 200X magnification as well as 500X magnification. The low-pressure plasma spray system used was successfully producing this coating with very high levels of repeatability. Stand off distance was fourteen inches and reverse transfer arc cleaning was used. However, suddenly the coating quality started deteriorating with the oxide levels slowly rising. Obviously, the coating process was stopped and upon maintenance evaluation, air (oxygen) leak up rate was well within limits indicating no leaks; parameters were very carefully monitored and everything looked okay; and yet poor quality coatings were the result. The common sense was that if you are getting too many oxides, oxygen is leaking in to your system. The frustration led to an inch by inch evaluation of every single hose and fitting and guess what was found – the leak was actually coming from a pin hole in an argon hose carrying gas into the powder feeder. When the leak up rates were determined, the powder feeder was a part of the system but the supply lines were not and so argon was leaking out; when the plasma spray system was running, air was getting in to the system through the pin hole in the supply system, causing excessive oxide levels in the coating. Replacement of the argon hose solved the problem of excess oxides in the CoNiCrAlY coating. Seems simple now, but as I always say everything is difficult to understand until it is understood.