There are several stages that must be carried out before an adequate coating can be applied to a part. All of the stages have to be carried out properly to avoid the coating failing due to lack of adhesion or the part being worn down to incorrect dimensions whilst being surface finished.
Cleaning and Degreasing
Solvents such as acetone, MEK or heptane are used for cleaning parts prior to coating in order to remove oil, grease, etc. from the surface. This can be done by wiping down the part with a solvent soaked rag or by using an alkaline wash such as a detergent solution.
Grit Blasting
Surface cleaning is a first step in preparation to remove dirt, oil, grease, etc. from the surface. This is typically done with solvent wiping or an alkaline wash depending on the part surface. After cleaning the surface, the surface is then grit blasted to create a mechanical key for the molten particles to key to. A typical abrasive used is aluminium oxide or steel grit with a surface profile of around 50 to 100 microns. Without surface cleaning and grit blasting, the coating will not adhere properly and fail early.
Masking
Masking of components is completed using a variety of items, such as tape, plugs or even custom-made tooling. This prevents the unwanted areas of the part receiving a coating, which could result in rework if discovered after the coating process. Masking also enables strict control of coating thickness to that required by the part’s drawing.
The Spray Process
This is what people are referring to when they are talking about Thermal Spray Coating. In this process, the feedstock material (in the form of powder or wire) is fed into a heat source, typically in the form of a plasma torch or a combustion flame. The resulting melted or softened particles are then propelled through a nozzle and, upon hitting the prepared surface, they flatten rapidly and build up layer upon layer of a dense deposit. The thermal spray processes are defined in detail by ASM International in terms of how they are classified, including plasma, HVOF and arc wire spray processes.
Thermal Spray Coating is covered in more detail at https://www.poeton.co.uk/surface-treatments/thermal-metal-sprays.
Post-Spray Finishing
Most coatings are ground or machined back to the specified dimensions after application, as the deposit is typically supplied slightly proud. Many coatings are then sealed using a variety of resin and/or polymer type sealers to close surface porosity and to enhance corrosion resistance.
Final Inspection
The coating thickness is then verified with a thickness gauge and adhesion testing is conducted on test pieces that have been processed at the same time as the part. Should the results of the above testing be outside of the specified limits the item is returned to the assessment team for re-evaluation.
The final outcome of the coating application, and the ultimate protection of the part, depends on each stage of the process being done correctly, reinforcing each other and resulting in a coating that has the properties intended by the design.
