Flame Spray
Flame spray is a thermal spray coating process in which wire or powder feedstock is melted by an oxy-fuel flame and propelled onto a prepared substrate to form a protective layer. It is used to build up wear-resistant, corrosion-resistant, or thermally functional coatings on shafts, plungers, valves, and cylinder liners, offering low cost and portability but higher porosity than higher-energy thermal spray methods.
In a CNC or machine shop, flame spray is typically scheduled as a repair or build-up operation. The part is first cleaned, grit blasted, and masked to create a roughened, contamination-free surface. A wire or powder feedstock is then melted in an oxy-fuel flame and sprayed onto the worn journal, bore, or face. Because the part stays relatively cool, distortion and metallurgical changes are minimized compared with welding. After spraying, the coating is inspected and then machined back to size on a lathe or mill, with grinding or honing used where tight tolerance and surface finish are required. Shops choose flame spray when cost, portability, and fast deposition matter more than maximum density. Typical deposits run from 0.005 to 0.250 inch, and deposition rates reach 5–20 kg/h. However, the coating often contains 10–20% porosity, so it is frequently sealed or paired with post-machining before service.
What is the bond mechanism of flame-sprayed coatings?
Flame spray coatings bond primarily by mechanical interlocking with a roughened substrate rather than full metallurgical fusion. The molten particles strike the prepared surface, flatten, and freeze, keying into the peaks and valleys created by grit blasting.
Why is flame spray considered a cold process?
Although the oxy-fuel flame is hot—around 2,500–3,000°C—the substrate remains relatively low in temperature during spraying. That low heat input prevents distortion and metallurgical changes in the base part, making the process useful for finished or heat-sensitive components.
When is flame spray preferred over plasma spray or HVOF?
Flame spray is preferred when cost, portability, fast build-up, and field repair matter more than maximum density or lowest oxide content. Plasma and HVOF produce denser coatings but require more expensive equipment and higher energy, making flame spray the economical choice for many repair jobs.