Importance of accurate air jet set up in plasma spray

Plasma spray coatings are being specified nowadays in more and more industrial components from aircraft engine parts to pump components and other commercial industrial parts. The primary reason for the specification of plasma spray coatings is the enhancement of surface engineering properties such as wear resistance, anti-galling properties and such. Plasma spray coatings such as tungsten carbide, chromium carbide and such are extremely popular where high surface wear resistance is required. However, in order for plasma spray coatings to perform at their peak levels, attention must be paid to the metallurgical quality of these coatings since a coating with a poor metallurgical quality will perform worse than a coating with a superior metallurgical quality.

While several variables affect the metallurgical micro structural quality of plasma spray coatings, in this article we will limit ourselves to the variable of proper set up of the plasma spray job to be coated. More specifically, we will address the importance of accurate air jet set up mounted on the gun holder. Air jets in plasma spray coating systems perform two basic functions. The first function is to cool the part during the spray operation and the second function is to blow away the unmelted particles from the coating plasma plume before they impinge on to the surface being coated. It is this second aspect that is sometimes overlooked by operators, especially those that are not well trained. The general rule of thumb is that the air jets must be set up so that the intersection point of the air coming out is located at one half of an inch beyond the stand off distance. For example if the stand off distance specified in the plasma spray parameter for the particular coating system at hand is four inches, then the air jet intersection must be kept at four and a half inches. By keeping it longer than four inches, the unmelted particles will not be removed from the coating. By keeping it shorter than four inches, there will be too much of turbulence introduced in the plasma plume leading to very bad coating quality.

In order to accurately set up the air jet intersection, it is recommended that the air jet intersect be set up first accurately before placing the part at the proper stand off distance from the gun. Many operators have a tendency to set the part at the correct stand off distance and then set up the air jets. This latter method can lead to erroneous air jet intersection and almost always leads to poor coating quality. Hence, as described in this article proper air jet set up is an imperative to producing high quality plasma sprayed coatings.