Thermal spray with poor grit blast -- a guaranteed disaster

The mechanism of bonding of plasma spray coating processes is essentially mechanical in nature. Once the coating particles reach a plastic state and are propelled to the surface that is to receive the coating, the bonding between the coating and the underlying substrate is essentially mechanical. Hence the surface characteristics of the substrate prior to coating is extremely important. Any oils and contaminants on the surface should be thoroughly removed by the use of proper degreasing fluids. After ensuring that the surface is contaminant free, grit blasting can proceed. However, if the grit blasting operation is improper, then subsequent plasma spray will not be worth pursuing since the bonding of the coating to the substrate will get affected.

The choice of the proper grit blasting materials such as aluminum oxide is essential. It is preferable that virgin materials be used all of the time. The grit size of the grit blasting media is important. Some operators prefer a mix of two different grit sizes. If such a blend is chosen, then ensuring the proper ratio every time is critical for reproducibility. The grit blasting pressure is another significant parameter and must be tightly controlled as well as the grit blast nozzle angle since the angle of impingement of the grit particles makes a huge difference to the grit blast profile. Stand off distance of the grit blasting gun from the surface being worked on is equally important. Grit blast quality can be checked using a surface profilometer and should be frequently checked and the data logged.

Excessive grit blasting pressure or using aggressive grit blast media can cause grit particles to be embedded into the surface getting coated. Several thermal spray coatings specifications call out for limits in the amount of such contaminations that are allowed and you want to ensure that these are not exceeded. Poor grit blasting will result in lower numbers for the tensile bond test data. This is why many aerospace thermal spray coatings suppliers will specify grit blasting as a significant operation in their thermal spray coatings line.