Thermal Spray Grit Blast

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Thermal spray engineers everywhere know of the myriads of variables that can affect the outcome of the process from the stand point of quality. This is because the thermal spray coating process itself is built up of various sub-processes, each of which has its own set of variables and therefore problems. It IS the job of thermal spray coatings process engineers to pay specific attention to EACH of these sub processes and institute proper controls on each of them in order to ensure final product quality.

It is my opinion that none of these sub processes can be taken for granted and none of them is either more important or less important than the other. I know, as must you, that in many thermal spray coatings houses, the plasma spray operator is generally considered the king ( or the queen to be politically right ) in comparison to the parts cleaning operator and this is not correct from a processing stand-point. Everyone is equally important and I do not mean this from a socialism stand-point but rather from a technical point of view.

Grit blasting prior to thermal spray is one such sub-process that is very important in itself. The variables dealing with proper grit blasting cannot be over emphasized. You do want the proper grit blast coverage and the appropriate level of grit blasting, but not over grit blasting by way of too high a grit blasting pressure or too aggressive a grit size. I know of thermal spray coatings companies that require their operators to have a grit blast release coupon at the beginning of every job that gets tested for proper surface roughness measured by way of a profilometer and that is the right way to ensure that the surface has been properly prepared.

Grit blasting is a key link in the overall thermal spray process because at the very heart of proper adhesion is a properly prepared surface. Grit blasting prior to thermal spray is considered a *significant process* by many OEM aerospace companies and rightfully so, because thermal spray is inherently a mechanically bonded phenomenon other than in cases where a diffusion heat treatment is followed up.

Personally, the variables in grit blasting are so many and the process itself can become problematic at times that I felt that it deserved a separate blog all by itself and you are welcome to visit my blog on abrasive grit blasting entitled http://abrasivegritblasting.blogspot.com wherein I plan to discuss not only grit blasting as a surface preparation process prior to follow on processes but also as a means of coating removal. ( Of course, some of us will claim never to have had a need to strip and recoat, right? wink, wink ) Anyway, that is another site that you may want to visit in your spare time.

Thermal spray companies that perform shot peening have another headache to deal with; I guess that is why they pay them the big bucks :) But shot peening becomes another process with variables that need to be controlled and monitored.

A case in point to be remembered if you are planning to institute grit blast cabinet release by way of coupon control, ( if you have not already done so far ) is to ensure that the coupon base material is the SAME as the part base material, otherwise the results wont mean much.