Thermal Spray

Start here: ten words every newcomer needs

Overheard on the job

Real-sounding lines from the floor, translated into plain English.

• Blast it to white metal, get your tooth up near 4 mils, and have the TSA on within the flash rust window.

  TranslationGrit blast the steel completely clean, create a deep surface profile of about 0.1 mm, and apply the thermally sprayed aluminum before the fresh surface starts to rust.

• Gun's spitting again, voltage is wandering, pull the nozzle before it puts nodules all over the bond coat.

  TranslationThe plasma gun is shedding molten buildup into the coating and its voltage is unstable, so replace the worn nozzle before it ruins the first layer.

• Mount shows unmelts and oxide stringers both, so the injection's off, check the carrier gas before you touch the amps.

  TranslationThe cross section has unmelted particles and oxide lines at the same time, which points to powder entering the plasma at the wrong depth, so check the gas flow that delivers the powder before changing gun power.

• Bond pull was a glue failure at 11k, so the carbide's good, release the rods to grind.

  TranslationThe tensile adhesion test broke in the epoxy at about 11,000 psi, meaning the coating held stronger than the glue, so the coated rods can go on to finish grinding.

• Pull the tape while it's warm or it'll bridge and lift the edge, then sweep blast the overspray off the shoulder.

  TranslationRemove masking tape soon after spraying so the coating does not form a continuous skin across it and peel up at the edge, then lightly blast away loose overspray from the adjacent area.

• Leave 10 mils grind stock on the journal, DE on this cut is lousy so budget extra powder.

  TranslationSpray about 0.25 mm extra thickness for finish grinding, and expect to use more powder because this particle size range deposits inefficiently.

• Strip and respray the liners, the TGO's grown thick and the top coat is spalling at the dome.

  TranslationRemove the old coating from the combustor liners and recoat them, because the oxide layer under the ceramic has thickened and the thermal barrier is flaking off.

The spray processes, guns, gases and booth hardware that put coatings on parts.

• Barrelalso gun barrel, nozzle extension
  The long tube on an HVOF or detonation gun where particles are heated and accelerated before exiting.
• Boothalso spray booth, spray cell
  The soundproofed, ventilated enclosure where the gun, robot and part sit during spraying. Shop talk
• Carrier gasalso powder carrier gas, carrier
  The gas stream, usually argon, that carries powder from the feeder through the hose into the spray jet.
• Cold sprayalso cold gas dynamic spray, kinetic spray
  Process that fires solid metal powder at supersonic speed so it bonds by impact rather than melting.
• Critical velocityalso Vcr
  The minimum impact speed a cold spray particle needs to stick to the surface instead of bouncing off.
• Detonation gunalso D-gun, detonation spray
  Process that fires powder from a long barrel using repeated explosions of oxygen and acetylene.
• Flame sprayalso combustion spray, oxy-fuel spray
  The simplest thermal spray process: powder or wire melted in an oxy-fuel flame and blown onto the part.
• Gunalso torch, spray gun
  Booth word for the thermal spray torch itself, whether plasma, HVOF, arc or flame. Shop talk
• HVAFalso high-velocity air fuel
  High-velocity air fuel: an HVOF-style process that burns fuel with compressed air instead of oxygen.
• HVOFalso high-velocity oxygen fuel, high-velocity oxy-fuel
  High-velocity oxygen fuel: burning fuel and oxygen under pressure to fire powder at supersonic speed.
• Light offalso light the gun, light-up
  To ignite the spray gun, whether striking a plasma arc or lighting an HVOF or flame torch. Shop talk
• Metallizingalso metalizing, metallizer
  Shop and field name for thermal spraying, especially zinc and aluminum sprayed on steel for corrosion. Shop talk
• Nozzlealso anode, anode nozzle
  In a plasma gun, the water-cooled copper anode the arc attaches to; in HVOF, the passage that shapes the jet.
• Plasma gasalso primary gas, secondary gas
  The gas mix ionized in a plasma gun: a primary gas for flow plus a secondary gas for heat.
• Plasma sprayalso APS, air plasma spray
  Spraying powder through an electric arc plasma jet hot enough to melt ceramics and refractory metals.
• Plumealso jet, flame
  The visible jet of hot gas and glowing particles streaming from the spray gun toward the part.
• Powder feederalso feeder, hopper
  The hopper and metering unit that delivers a steady, measured flow of powder to the spray gun.
• Spray and fusealso spray-fuse, fused coating
  Spraying a self-fluxing alloy, then remelting it with a torch, induction or furnace so it fuses to the part.
• Thermal sprayalso thermal spraying, metallizing
  Family of coating processes that melt or soften material and propel it onto a surface to build a layer.
• Twin-wire arcalso arc spray, electric arc spray
  Process that melts two electrically charged wires in an arc and blows the molten metal onto the part with air.
• VPSalso vacuum plasma spray, LPPS
  Vacuum plasma spray: plasma spraying inside a low-pressure inert chamber to make oxide-free metal coatings.

Powders, wires and the coating chemistries sprayed for wear, corrosion and heat.

• Abradablealso abradable seal, abradable coating
  A deliberately soft coating that rotating blade tips cut into, setting a tight seal clearance.
• Agglomerated and sinteredalso A&S, agglomerated-sintered
  Describes powder made by spray drying fine particles into balls, then sintering them to give them strength.
• Alumina-titaniaalso Al2O3-TiO2, 87/13
  Aluminum oxide blended with titanium dioxide, a tough general-purpose ceramic wear coating.
• Carbidealso carbide coating, tungsten
  Shop shorthand for a carbide coating, usually tungsten carbide, as in putting carbide on a part. Shop talk
• Chrome oxidealso chromium oxide, Cr2O3
  Plasma-sprayed chromium oxide, a hard, dense ceramic for wear parts and polished sealing surfaces.
• Chromium carbidealso chrome carbide, CrC-NiCr
  Cr3C2 carbide held in a nickel-chromium binder, a wear coating that keeps working at high temperatures.
• Cored wirealso tubular wire, powder-cored wire
  A metal sheath filled with powder, used in arc spray to spray alloys and carbides not made as solid wire.
• Cutalso size cut, size range
  Shop word for a powder's particle size range, such as minus 45 plus 15 micrometers. Shop talk
• Feedstockalso spray material, consumable
  The raw coating material fed into a thermal spray gun, as powder, wire, rod, cord or liquid suspension.
• Finesalso undersize
  Powder particles smaller than the specified size range, which cause buildup, spitting and poor flow.
• Fused and crushedalso F&C, fused-crushed
  Describes powder made by melting material into an ingot, then crushing and screening it to size.
• MCrAlYalso MCrAlY bond coat, NiCoCrAlY
  Family of oxidation-resistant alloys of nickel or cobalt with chromium, aluminum and yttrium.
• Molyalso molybdenum, moly coat
  Shop name for sprayed molybdenum, a scuff-resistant coating for piston rings, synchro rings and bond coats. Shop talk
• Nickel aluminumalso NiAl, nickel aluminide
  Nickel-aluminum composite that reacts in the flame to give a strongly bonded first layer.
• Self-fluxing alloyalso self-fluxing powder, NiCrBSi
  Nickel or cobalt alloy with boron and silicon that fluxes its own oxides when fused after spraying.
• TSAalso thermally sprayed aluminum, thermal spray aluminum
  Thermally sprayed aluminum: arc- or flame-sprayed aluminum that protects steel from corrosion.
• WC-Coalso tungsten carbide cobalt, WC-CoCr
  Tungsten carbide in a cobalt binder, the most common wear-resistant thermal spray coating.
• YSZalso yttria-stabilized zirconia, 8YSZ
  Yttria-stabilized zirconia: the standard ceramic for thermal barrier top coats.

Cleaning, grit blasting and masking that decide whether a coating sticks and where it goes.

• Anchor profilealso surface profile, blast profile
  The peak-and-valley roughness grit blasting leaves on a surface, which the coating locks onto.
• Bake-outalso burn-off, oven degrease
  Heating oily or porous parts before blasting to drive out absorbed oil that would ruin the bond.
• Bridgingalso bridged, bridging over
  Coating that forms a continuous skin across the edge of a mask, so it tears or lifts when the mask comes off. Shop talk
• Embedded gritalso grit entrapment, grit inclusion
  Abrasive particles left stuck in the substrate after blasting, visible at the bond line in a cross section.
• Flash rustalso rust bloom, flash rusting
  A thin rust bloom that forms on freshly blasted steel when it sits in humid air before coating.
• Grit blastingalso grit blast, abrasive blasting
  Blasting a surface with angular abrasive to clean it and roughen it so the coating can grip.
• Maskingalso mask, maskant
  Covering areas of a part that must not be blasted or coated, using tapes, plugs, shields or maskants.
• Potalso blast pot, pressure pot
  Shop name for the pressure blast vessel that holds grit and feeds it into the blast hose. Shop talk
• Preheatalso preheating, preheat passes
  Warming the substrate before spraying to drive off moisture and reduce stress at the coating interface.
• Sweep blastalso brush blast, brush-off blast
  A quick, light blast pass to refresh a prepared surface or clean overspray without cutting deep. Shop talk
• Tapealso masking tape, tape up
  Masking tape used for blasting and spraying, or as a verb, masking a part off before coating. Shop talk
• Toothalso bite, profile
  Shop word for the bite or roughness left on a blasted surface, the grip a coating needs to hold on. Shop talk
• Undercutalso undercutting, recess
  A recess deliberately machined below finished size so the coating can fill it to full thickness.
• White metalalso white metal blast, SP 5
  The highest blast-cleaning grade for steel: every trace of rust, scale and old coating removed.

The settings, gun motion and booth habits that control how a coating builds up.

• Cookalso cooking, cooked
  Booth slang for overheating a part during spraying, usually from slow traverse, short standoff or no pauses. Shop talk
• Cooling airalso air jets, air knives
  Jets of compressed air or CO2 aimed at the part during spraying to keep its temperature in check.
• Deposition efficiencyalso DE, deposit efficiency
  The percentage of the feedstock sprayed that actually ends up as coating on the part.
• Dustingalso dust, overspray dust
  Fine, cooled overspray that settles on a surface as loose powder and weakens the next layer sprayed over it. Shop talk
• Footprintalso spray spot, spray pattern
  The shape of the deposit a gun leaves in a short stationary shot or single line, used to check spray alignment. Shop talk
• Gun voltagealso arc voltage, torch voltage
  The arc voltage of a plasma gun, watched as a live indicator of gas mix, power and hardware wear.
• Line of sightalso line-of-sight process, shadowing
  The limit that thermal spray coats only surfaces the gun can point at directly.
• Milalso mils, thou
  One thousandth of an inch (25.4 micrometers), the usual North American unit for coating thickness.
• Oversprayalso over-spray, spray loss
  Spray material that misses the part or fails to stick, ending up in the collector or on nearby surfaces.
• Particle diagnosticsalso in-flight particle diagnostics, particle sensor
  Sensors that measure the temperature, speed and size of particles in flight to monitor the spray process.
• Passalso spray pass, stroke
  One sweep of the gun across the area being coated; the coating is built up pass by pass.
• Pulsingalso surging, puffing
  Uneven powder delivery that makes the spray stream surge and flicker instead of flowing steadily. Shop talk
• Running hotalso running cold, hot
  Booth talk for parameters that are overheating the particles or part; running cold is the opposite. Shop talk
• Shootalso spray, shot
  Booth verb for spraying a coating onto a part or coupon. Shop talk
• SLPMalso standard liters per minute, NLPM
  Standard liters per minute: gas flow corrected to standard temperature and pressure.
• Spittingalso spits, spit
  A gun shedding blobs of molten buildup into the coating, leaving lumps known as spits. Shop talk
• Spray anglealso impingement angle, gun angle
  The angle between the spray jet and the surface, ideally 90 degrees for the densest coating.
• Spray ratealso feed rate, powder feed rate
  How much feedstock goes through the gun per unit time, usually in grams per minute or pounds per hour.
• Standoff distancealso spray distance, standoff
  The distance from the gun exit to the surface being sprayed, which sets how hot and fast particles arrive.
• Stripingalso tiger striping, banding
  Visible bands or ridges across a coating caused by uneven overlap between passes. Shop talk
• Substrate temperaturealso part temperature, substrate temp
  The temperature of the part being coated, usually held below about 150 °C during spraying.
• Surface speedalso gun speed, traverse speed
  How fast the part surface moves past the gun, set by part rotation and robot traverse.

What a sprayed coating looks like inside and the ways it goes wrong.

• Bond coatalso bond, bondcoat
  The first layer sprayed on a substrate to help the following coat stick and sometimes protect the metal.
• Bond linealso interface, bondline
  The interface between a sprayed coating and the substrate, or between two coating layers.
• Delaminationalso disbonding, debonding
  Separation of a coating from its substrate or of one coating layer from another.
• DVCalso dense vertically cracked, segmented TBC
  Dense vertically cracked: a thermal barrier sprayed hot so it forms deliberate through-thickness cracks.
• Liftingalso edge lifting, edge lift
  Shop word for coating peeling up at an edge, often after tape removal, grinding or machining. Shop talk
• Mud crackingalso mud-cracking, craze cracking
  A network of surface cracks that looks like dried mud, from tensile stress in a coating.
• Oxidesalso oxide content, oxide stringers
  Thin oxide films formed on metal particles in flight, showing as dark stringers between splats.
• Porosityalso pores, voids
  The fraction of a coating's volume made of voids between and within splats, usually reported in percent.
• Pulloutalso pull-out, grain pullout
  Particles torn out of a coating during polishing, leaving holes that look like porosity in the mount.
• Residual stressalso coating stress, internal stress
  Stress locked into a coating and substrate after spraying, from splat shrinkage, cooling and impact.
• Spallingalso spallation, spall
  Flakes or chunks of coating breaking away from the surface, typically under thermal cycling or impact.
• Splatalso lamella, lamellae
  A single flattened, frozen droplet: the basic building block of a thermal spray coating.
• TGOalso thermally grown oxide, alumina scale
  Thermally grown oxide: the alumina layer that grows between bond coat and ceramic in hot service.
• Thermal barrier coatingalso TBC, thermal barrier
  A ceramic coating system that insulates hot-section metal parts from combustion gas temperatures.
• Top coatalso topcoat, functional coat
  The outer, functional layer of a multilayer coating, sprayed over the bond coat.
• Unmeltsalso unmelted particles, unmelted
  Lab shorthand for particles that hit the surface without fully melting and kept their round shape. Shop talk

Lab checks, quality evidence and the machining, sealing and repair work that follows spraying.

• Adhesive failurealso interface failure, adhesion failure
  A bond test break at the coating-substrate interface, meaning the coating let go of the base metal.
• As-sprayedalso as sprayed, as-coated
  Describes a coating straight from the gun, before any grinding, machining, sealing or heat treatment.
• Bond strengthalso adhesion strength, tensile bond
  How hard you must pull to tear a coating off its substrate, measured by gluing and tensile testing.
• Cohesive failurealso cohesive break, intra-coating failure
  A bond test break inside the coating, with coating left on both halves of the test specimen.
• Couponalso test coupon, witness coupon
  A small test piece sprayed alongside production parts to stand in for them in destructive tests.
• Dimensional restorationalso buildup, salvage
  Rebuilding worn or mis-machined surfaces to original size with thermal spray, then machining to finish.
• Dry sand rubber wheel testalso ASTM G65, G65
  Abrasion test that feeds sand between a coated sample and a spinning rubber wheel, then measures volume loss.
• Glue failurealso epoxy failure
  Lab slang for a bond test where the epoxy broke first, so the coating is at least as strong as the glue. Shop talk
• Grind stockalso finish allowance, stock allowance
  Extra coating thickness sprayed beyond the final size so it can be ground or machined to finish.
• Hard chrome replacementalso chrome replacement, HCR
  Using HVOF carbide coatings in place of electroplated hard chrome on rods, pins and landing gear.
• Microhardnessalso HV0.3, Vickers
  Hardness of a coating measured with tiny diamond indents on a polished cross section.
• Mountalso met mount, cross section
  A polished cross section of a coated coupon set in resin, used to check structure under the microscope. Shop talk
• Sealeralso sealant, seal coat
  A thin liquid resin brushed or soaked into a sprayed coating to fill its connected porosity.
• Strip and resprayalso strip and recoat, stripping
  Removing a failed or worn coating completely and applying a new one, a standard repair and rework. Shop talk

Frequently asked questions

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