Surface Preparation for Thermal Spray Coatings

An essential feature of any coating system is the bond between the coating and the substrate.  Thermal Spray operations are typically based on the materials being applied to the substrate in the plastic (non-molten) state.  Therefore, the bond is not due to fusion between the coating and the substrate.  In addition, there is usually little or no chemical reaction between the coating and the substrate, so the bond is not chemical in nature.  What is the bond mechanism? 

Coatings applied using thermal spray processes typically depend on a mechanical (interlocking) bond.  The nature of the substrate surface is therefore a key to quality Thermal Spray Coatings.  For successful coatings, the substrate surface needs to rough and pitted to provide a “foot-hold” (Splat-Hold) for each splat of powder that impacts the substrate.  In addition, the surface needs to be clean and free from contamination that would fill the pits and prevent locking of the splats.  How is this achieved? 

Grit blasting is popular for surface preparation, which is simply pressurizing an abrasive media with compressed air and aiming the stream of accelerated particles at the surface being prepared.  Many are familiar with grit blasting for cleaning surfaces prior to painting.  However, grit blasting for thermal spray is quite different since more than removal of oxides is needed; instead, pits and crevices need to be formed where the molten thermal spray particles “splat” into the rough surface and adhere.   

Grit blasting in preparation for thermal spray depends on dry abrasives. The grit blast material should be sharp and angular so that it will cut into the substrate on impact. It is also beneficial if it produces under-cut pits for a strong mechanical bond. The need for sharp, angular grit is the reason that grit for Thermal Spray operations needs to be changed-out more often than the grit used for surface cleaning operations. The most common materials used for grit blast is aluminum oxide and chilled iron. The typical; surface finish after grit blast is anywhere from 15 RA to 100 RA. The variables that will effect the final finish include media size, media morphology, media hardness, air pressure, distance from the work piece, angle of impingement, and anything that will effect the speed of the media hitting the work piece. 

The substrate is physically deformed during grit blast operations resulting in residual stresses being formed on the surface.  This is easily demonstrated by grit blasting a thin strip of test material, often called an almen strip, and observing how the metal “bows”.  This is due to the higher residual stress that is created on one side of the strip being grit blasted.  “Shot-peening” has the opposite effect of reducing residual stress on the surface. 

In summary, Grit Blast operations for Thermal Spray need to be properly used and controlled to provide a consistently strong bond by providing the proper splat pits for the coating material.