What Is Flame Spray Coating? The Process and Where It Fits

What Is Flame Spray Coating?

Flame spray coating is the original thermal spray process: a wire or powder feedstock is melted in an oxy-fuel combustion flame — typically oxygen and acetylene or propane — and a compressed-air jet atomizes and propels the molten material onto a prepared surface at roughly 80–150 m/s. Each droplet flattens on impact and solidifies within microseconds, building up a coating layer by layer, the same mechanical-splat principle behind every thermal spray process. What sets flame spray apart is its combustion temperature and particle velocity, both well below HVOF or D-Gun, which makes it the simplest, most portable, and lowest-cost thermal spray option — and also the one with the widest gap between as-sprayed and premium performance.

Because the equipment is comparatively simple — a torch, a gas supply, and a wire or powder feeder — flame spray is often the first process a shop installs and remains in service alongside HVOF and D-Gun for the jobs that don't need their bond strength or density. For where flame spray sits against the rest of the family, see our guide on what thermal spray coating is.

How the Flame Spray Process Works

There are two feedstock formats. Wire flame spray feeds a continuous metal wire into the flame, where it melts at the tip; a compressed-air jet atomizes the molten tip into droplets and carries them to the substrate, the same wire-feed principle arc spray uses, but heated by a combustion flame rather than an electric arc. Powder flame spray instead carries powder feedstock into the flame through a carrier gas, useful for materials that aren't practical to draw into wire — ceramics, carbides, and certain self-fluxing alloys.

In both formats the flame temperature (roughly 3,000 °C for oxy-acetylene) is enough to melt common metals and alloys but well short of a plasma arc's 8,000 °C+, so flame spray's material range stops short of the refractory ceramics plasma spray handles. Particle velocity out of the nozzle is also modest — 80–150 m/s against 600–900 m/s for HVOF — which is the single biggest reason flame spray coatings behave differently in service than HVOF or D-Gun coatings of the same nominal material.

Flame Spray vs HVOF vs D-Gun: How They Compare

Particle velocity on impact drives everything downstream — bond strength, porosity, and hardness — so the honest way to compare processes is side by side on the numbers that actually matter for a specification decision.

| Property | Flame Spray | HVOF | D-Gun (Detonation Spray) | |---|---|---|---| | Heat source | Oxy-fuel combustion flame | Supersonic fuel/O₂ combustion | Detonation shock wave | | Particle velocity | 80–150 m/s | 600–900 m/s | 700–1,000+ m/s | | Bond strength | 10–30 MPa | 60–80 MPa | > 80 MPa | | Porosity | 10–15% | < 2% | < 1% | | Typical hardness (carbide grades) | Lower — as-sprayed metals/alloys | 1,100–1,400 HV (WC-CoCr) | Up to ~1,400 HV (WC-CoCr) | | Relative cost | Lowest | Moderate | Highest | | Best fit | Build-up, repair, non-critical wear, large-area work | General-purpose hard-chrome replacement | Critical, high-value, highest-duty-cycle wear surfaces |

The gap in bond strength and porosity is not a defect to be tolerated everywhere — for build-up and repair work, higher porosity is often irrelevant, and for large-area corrosion coatings on structural steel, arc-sprayed metalizing (a related low-velocity process) actually uses porosity to its advantage. But for a wear surface exposed to erosion, abrasion, or corrosive fluid ingress, flame spray's numbers explain why HVOF or D-Gun is the default recommendation. See HVOF vs hard chrome and what is D-Gun coating for the high-performance end of the family in more depth.

Materials Sprayed by Flame Spray

Flame spray's moderate temperature and velocity suit it to a narrower, softer materials list than HVOF or D-Gun:

• Mild and stainless steel wire — for dimensional build-up and repair on worn shafts, housings, and bearing seats
• Bronze and babbitt alloys — for plain-bearing surfaces and low-friction wear applications
• Zinc and aluminium wire — for corrosion protection on structural steel, though arc spray metalizing is more commonly specified for large-area work; see our metalizing guide for that process
• Self-fluxing nickel alloys (NiCrBSi) — sprayed and then fused with a secondary torch or furnace pass, which melts the coating into a metallurgically bonded, near-fully-dense layer with bond strength and porosity that can approach HVOF territory — the one route by which flame spray closes most of the performance gap
• Molybdenum and ceramic powders — used selectively for specific wear or dielectric requirements where flame spray's lower cost outweighs the property gap versus plasma spray

Where Flame Spray Coating Fits

Flame spray earns its place in a coating shop's process list on jobs where its cost and portability matter more than maximum bond strength or minimum porosity:

• Dimensional build-up of worn shaft diameters, keyways, and housing bores before precision machining back to print size — often cheaper than sourcing a replacement part
• Non-critical wear surfaces where moderate abrasion resistance is enough and duty cycle doesn't demand HVOF- or D-Gun-level density
• Bond coats under a plasma-sprayed ceramic top coat, where a thin NiAl or similar layer is flame- or arc-sprayed first to manage thermal-expansion mismatch
• Field and on-site repair, since flame spray equipment is portable and doesn't require the compressed-gas infrastructure HVOF or D-Gun systems need
• Fused self-fluxing alloy coatings on pump sleeves and valve seats, where the post-spray fusing step delivers near-metallurgical bond at a lower cost than HVOF

When Flame Spray Is the Wrong Choice

Being direct about flame spray's limits is as important as knowing where it fits — specifying it in the wrong place is a common and avoidable error:

• Specifying flame spray on a seal face, plunger, or any surface with sustained sliding contact under load — 10–15% as-sprayed porosity and 10–30 MPa bond strength will not hold up where HVOF or D-Gun is the metallurgically correct call
• Using flame spray for hard-chrome replacement — the process that actually replaces hard chrome on wear-critical components is HVOF or D-Gun tungsten carbide, not flame spray; see hard chrome replacement
• Leaving an as-sprayed flame spray coating unsealed in corrosive service — the interconnected porosity that's harmless on a build-up repair becomes a direct path for corrosive fluid to reach and undercut the substrate
• Assuming all flame spray coatings perform the same — an as-sprayed wire coating and a fused self-fluxing NiCrBSi coating are not comparable; the fusing step is what changes the bond and porosity numbers
• Skipping grit blasting before spraying — flame spray's bond is purely mechanical, so surface preparation matters even more here than in higher-velocity processes where some particles achieve partial metallurgical bonding

Choosing Between Flame Spray, HVOF, and D-Gun

A short checklist covers most process-selection decisions:

• Is the surface load-bearing, sealing, or under sustained wear? → HVOF or D-Gun, not flame spray
• Is this a dimensional build-up or repair before machining, with no direct wear duty on the sprayed layer itself? → Flame spray is usually sufficient and cheaper
• Does the part need corrosion protection over a large structural area rather than a precision wear diameter? → Arc spray metalizing, not flame spray, is the standard route
• Can the coating be fused after spraying (self-fluxing alloy, appropriate substrate)? → Fused flame spray can approach HVOF-level density at a lower process cost
• Is the component high-value or is unplanned downtime expensive? → Specify D-Gun or HVOF; flame spray's lower bond strength is not the place to save cost on a critical part

Get a Quote for Flame Spray Coating

Lotus Surface Tech is an ISO 9001:2015-certified thermal spray facility in Chennai, Tamil Nadu, established in 2015, operating flame spray alongside HVOF, Super-D-Gun, plasma spray, and arc spray metalizing, with capacity for components up to Ø800 mm × 7 m. We apply flame spray for dimensional build-up, repair, and fused self-fluxing alloy coatings, and will tell you directly when your part actually needs HVOF or D-Gun instead.

Full process capability is on our technologies page. When you are ready to move forward, contact us to get a quote — share your part drawing, the wear or corrosion problem, and the service environment, and we will recommend the right process and provide a firm price.