Thermal Spray
Is thermal spray a coating process or a welding process?
It is a coating process; the deposited material forms a bonded layer on the surface rather than fusing the entire substrate like welding or overlay welding.
Why is grit blasting so important before spraying?
It creates the surface roughness needed for mechanical adhesion and helps remove oxides or contamination that would otherwise weaken bonding.
Why do shops usually machine after spraying instead of before only?
Because thermal spray is commonly used to restore oversize allowance or add a functional layer, and the sprayed surface generally needs finish turning, boring, grinding, or honing to achieve final size and surface finish.
Thermal spray is a family of coating processes where powder, wire, or rod feedstock is heated to a molten or semi-molten state and propelled onto a substrate, rapidly solidifying into a functional coating. In machining repair, it restores undersize dimensions and adds wear, corrosion, or heat resistance before finish machining to OEM specifications.
On the shop floor, thermal spray usually follows pre-machining and surface preparation. The part is cleaned and grit-blasted to create a rough anchor pattern, then the spray torch deposits multiple passes while the operator controls stand-off distance, traverse speed, and substrate temperature. This builds material back up on worn shafts, bores, seal journals, turbine components, rails, and machine ways. Plasma spray, HVOF, flame spray, and wire arc are common choices depending on the coating material and service environment. After deposition, the coated surface is finish turned, bored, ground, or honed to restore the original tolerance and surface finish. Complex geometries are possible only where line-of-sight access and masking are acceptable. The process is used to restore dimensions and apply wear-, corrosion-, or heat-resistant layers that cannot be produced by cutting alone, making it a practical repair and enhancement step before final machining.
Spalling and delamination: Inadequate grit blasting or residual contamination prevents mechanical interlocking, so the coating loses adhesion and flakes away once the part goes into service.
Distortion and residual stress: Excessive heat input or unstable torch dwell can warp thin sections and alter substrate properties, destroying the tight geometry that precision CNC parts need after finish machining.
Torn or ragged coating edges: Sprayed layers machine differently than base metal, so wrong inserts, feeds, or cutting strategies can pull the coating, leave poor microgeometry, and cause premature service failure.