Thermal Spray Coatings | Coating Quality

By: Raj Krishnaswamy

Thermal spray coating processes inherently have several variables that need to be controlled in order to ensure consistency in coating thickness and metallurgical characteristics. Unlike a machining process where a single point or multi-point tool shaves off material from a piece of bar stock or plate stock, where the critical parameters of feeds, speeds and coolant flow determine part quality based upon the machining program, thermal spray coating processes have too many variables prior to the part even reaching the thermal spray coating booth, let alone what happens in the actual thermal spray process itself. In this article, we will deal primarily with the often overlooked variable of gun over travel and its effects on thermal spray coating quality.
Gun over travel is defined for the purposes of this discussion as the additional linear travel beyond the part, also known as overshoot. This gun over travel parameter is applicable BOTH to the grit blasting surface preparation step as well in the thermal spray step.

In the interests of saving time, especially in high volume automated grit blasting operations, when the grit blasting gun overshoot is kept to a bare minimum and the grit blasting media continues to impinge upon the part as it reverses its traverse direction, the ends of the part will not develop the same grit blast surface profile as the bulk of the part being prepared. This will change the profilometric readings towards the ends of the parts. Because thermal spray coatings are mechanically bonded to the surface, this will result in inconsistent bonding towards the edges of the parts being processed. Hence it is imperative that the gun over travel in the grit blasting processes be spelled out in grit blasting process operation sheets and be strictly adhered to.
A similar situation exists in robotic thermal spray coatings operations, where in the interests of keeping thermal spray target efficiencies high and saving on production times, gun over travel values are reduced. A key point to note is that as the robot arm reaches the end of its traverse, there is a certain amount of deceleration and similar acceleration as it ramps up speed on its reverse stroke. This additional time factor will cause higher coating thicknesses in the ends of the piece being coated. Sometimes, when coating small parts in high volume, where several pieces may be racked on to a thermal spray fixture, entire parts on the ends of the fixture may depict higher coating thicknesses than parts that are in the middle, where the robot arm maintains a consistent velocity. Hence as in grit blasting, the gun over travel amount must be spelled out and documented in the actual thermal spray coating process also to ensure consistent part to part and within the part coating thickness.

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