Preparation of coating application areas is as important as the actual spraying of the coating. If not completed correctly the coating could delaminate or not perform as required in service.
Inspect the Part First
Make a visual inspection of the part for cracks, corrosion, etc. Also, check the part’s dimensions and ensure that it is within the required tolerance. Parts that are heavily pitted or are slightly undersize may require some machining or build-up to achieve the required dimensions and thereby save time in the long run and to avoid having to use excess material.
Degrease Thoroughly
Clean all surfaces of any oil, coolant or machining residue as these will prevent the coating from adhering to the surface. Solvent wiping or ultrasonic cleaning is suitable for most components, depending on the component geometry.
Mask What Must Stay Uncoated
Masking of threads, holes and bearing surfaces such as bearing tracks and bearing seats prior to grit blasting is a critical factor to ensure a successful coating application. In general, the masking must be able to withstand the grit blasting as well as the subsequent thermal spray coating application. Therefore, hard wearing plugs, tapes or caps made from silicone are generally used to mask off areas that are not intended to be coated.
Grit Blast to the Right Profile
The grit blasting creates the mechanical key with which the coating will bond to the surface of the part. For most types of thermal spray, the surface should be of a roughness profile of Ra 6 to 10 microns and best achieved using angular grit at the correct pressure. This can be read about in more detail in the TWI guide to how surface cleanliness affects thermal spray bond strength.
Spray Without Delay
As soon as the blasted surface of steel is left to the air, it begins to oxidise within an hour or so, the thin layer of oxide that forms on the surface of the steel can reduce the adhesion of the thermal spray coating by a significant amount. For Thermal Sprays, visit poeton.co.uk/surface-treatments/thermal-metal-sprays/.
Proper preparation is key to a thermal spray coating having a working life of work.