Skip to content
MecAgentMecAgent
Lesson 11.10CoatChapter 11 · Coatings and surface finishing

Thermal spraying (plasma, HVOF, arc)

Melted droplets fired at a part stack into a layer

Thermal spraying melts a coating material into tiny droplets and fires them at a part. They flatten on impact and stack up into a thick layer. Sprayed zinc protects steel bridges, sprayed ceramic shields jet-engine parts from heat, and sprayed metal rebuilds worn shafts.

Thermal spraying (plasma, HVOF, arc): a real photo
Photo: zircotec.com, Attribution, via Wikimedia Commons
Molten droplets fly at high speed and flatten into a coating on the partA plasma gun shown; HVOF and wire-arc guns melt the material in other ways (bottom)plasma gasargon, hydrogenpowder inmetal, ceramic, carbidecoating, 0.1–1 mmthe partstays fairly cool2electric arcturns the gas into plasma3molten dropletshundreds of m/sClose-up: droplets splat, freeze and stack into a coatingmolten droplet5coating, built layer on layer4splatsflatten and freeze1rough surfacegrit-blasted firstThree ways to melt the materialPlasmaan electric arc turns gasinto plasma, about 10,000 °CHVOFfuel and oxygen burn underpressure: a supersonic jetWire arctwo wires arc and melt;air blasts the melt outpart, powder and coatingspray gunarc and plasma jetplasma gas
Thermal spraying · plasma gun, droplet splats, three heat sourcesThe top shows a plasma gun at work; the close-up shows how the coating builds. Circled numbers match the steps below.

How it works

  1. Prepare the surface. The part is cleaned, areas that must stay bare are masked, and the surface is grit-blasted (blasted with hard grit) to roughen it. The rough surface gives the droplets something to grip.

  2. Melt the material. The gun melts powder or wire. A plasma gun uses an electric arc to turn gas into a plasma (an electrically charged gas) at about 10,000 °C. HVOF guns (high-velocity oxygen fuel) burn fuel with oxygen, and wire-arc guns melt two metal wires with an electric arc.

  3. Fire the droplets. The hot gas jet hurls the molten droplets at the part at hundreds of meters per second.

  4. Splat and freeze. Each droplet hits, flattens into a thin pancake called a splat, and freezes almost instantly. The part itself stays fairly cool, often below about 150 °C.

  5. Build up and finish. A robot sweeps the gun back and forth, so splats stack into a coating typically 0.1–1 mm thick. The surface comes out rough, so precise parts are ground or polished afterwards.

Key facts

Materials
Metals (zinc, aluminum, stainless steel, nickel alloys), carbides (tungsten carbide-cobalt), ceramics (alumina, zirconia), some plastics
Typical parts
Shafts, rolls, turbine blades, boiler tubes, pump parts, implants, bridges
Part size
From small implants to whole bridges and storage tanks; the gun only needs to reach the surface
Precision
Coatings roughly 0.05–3 mm thick, about ±0.05 mm as sprayed; tighter after grinding
Surface finish
Matte and rough as sprayed, like fine sandpaper; carbides can be ground to a mirror
Batch size it suits
One-off repairs to production runs of aerospace and car parts
Tooling and machine cost
Thousands of dollars for a flame or arc gun; often hundreds of thousands for a plasma or HVOF cell with robot and booth
Speed
Typically several kilograms of coating per hour; a part takes minutes to hours

Strengths and limits

Strengths

  • Thick coatings, up to several millimeters
  • Almost any material that melts: metals, ceramics, carbides
  • Part stays fairly cool, so it does not warp or lose its heat treatment
  • Rebuilds worn or mis-machined parts

Limits

  • Line of sight: the gun must see the surface, so deep holes are hard
  • Coatings hold small pores and grip mechanically rather than fusing
  • Rough as sprayed; precise parts need grinding
  • Loud, dusty and bright: needs a sealed booth or a robot

Where it's used

  • AerospaceHeat-barrier ceramic on turbine blades, hard carbide on landing gear
  • InfrastructureZinc and aluminum against rust on bridges, wind towers and tanks
  • EnergyCorrosion-resistant layers on boiler tubes and pump parts
  • Paper and steel millsHard carbide coatings on large rolls
  • MedicalPorous titanium or hydroxyapatite (a bone-like mineral) on hip implants, so bone grows in
  • RepairRebuilding worn shafts, bearing seats and pump housings

How to recognize it

  • A matte, slightly rough coating, often gray, that feels like fine sandpaper.
  • Coating edges that stop at a sharp masking line.
  • Under magnification, a layered, pancake-like structure with small pores.
  • A ground, mirror-bright carbide band on a shaft or roll, a different color from the steel around it.

Compare with

Cold spray11.11fires particles that stay solid and weld on impact; thermal spray melts them first.Hot-dip galvanizing11.4coats steel in zinc by dipping the whole part; sprayed zinc suits huge or fixed structures.Directed energy deposition8.7melts powder with a laser into a fully fused metal layer; thermal spray splats grip mechanically.

Watch it

Austchrome · How an HVOF gun makes dense, hard coatings

Curtiss-Wright Surface Technologies · The whole shop process: masking, grit blasting, robot spraying

MTU Aero Engines · Robots plasma-spraying jet-engine parts

Alec Steele · Flame, arc and powder spraying tried in a workshop

See real photos

Remember

  • Thermal spraying melts material, fires the droplets at a part, and lets them stack into a coating.
  • Coatings are thick (typically 0.1–1 mm) and rough, but the part stays fairly cool.
  • Plasma, HVOF and wire-arc guns differ mainly in how they make the heat.

More in Coatings and surface finishing

All 100 processes
11.1Spray paintingLiquid paint atomized into a fine mist that dries or curesOpen the lesson11.2Powder coatingCharged dry powder clings to the part, then melts in an ovenOpen the lesson11.3Screen printingInk pushed through a mesh stencilOpen the lesson11.4Hot-dip galvanizingSteel dipped in molten zinc forms a bonded zinc layerOpen the lesson11.5Electroplating (chrome, nickel, zinc)Electric current deposits metal from a bathOpen the lesson11.6AnodizingCurrent grows a hard oxide out of aluminum itselfOpen the lesson11.7ElectroformingPlating grown thick on a model, then lifted offOpen the lesson11.8PVD coating (thin films in vacuum)Atoms knocked off a target coat parts in vacuumOpen the lesson11.9CVD coatingGases react on a hot surface and leave a filmOpen the lesson11.11Cold spraySolid metal grains weld on by sheer speedOpen the lesson

Try it

Put AI to work in your CAD.

Model parts, generate drawings and automate the repetitive work inside your CAD software.

Download (Free)Book a demo