Thermal Spray Welding: Processes, Materials & Uses

A power plant turbine shaft worn down by erosion costs a fortune to replace, yet the same shaft can often be restored to service with a sprayed coating measured in millimeters. That gap between ‘scrap it’ and ‘save it’ is exactly where thermal spray welding lives, and it is why aerospace, oil field and heavy equipment shops keep a spray gun next to the welding machine.

What Is Thermal Spray Welding? A Plain-English Definition

For many shops, the goal isn’t to melt an entire part back together, it’s to put brand new material exactly where it’s needed. That’s the simplest way to understand thermal spray welding, more commonly known as thermal spraying or a thermal spray coating. At its core, as defined by ISO and ASTM, thermal spraying is a group of coating processes in which finely divided feedstock is heated by a combustion flame or electrical arc and propelled against a prepared surface. Rather than being one single method, it’s an entire family of processes that all share this same idea: build a coating, don’t necessarily melt the part.

The key distinction here is fusion. Unlike traditional fusion welding, thermal spray generally does not melt the base metal, or substrate. Thanks to this low heat input, substrate temperatures often stay below about 150-200 °C, meaning distortion is minimal and heat-sensitive components can often be coated with little to no warping. These coatings are typically applied in layers from roughly 0.1 mm to several millimeters thick, allowing for precise restoration without the bulk of a weld build-up.

Not a Weld, But Often Spoken About With One

With that being said, it’s easy to see why the two worlds get lumped together. The term “thermal spray welding” is used heavily throughout industry, despite most jobs not creating a true, fused weld at all. This comes down to one massive difference: the bond type. In thermal spray, bonding is primarily mechanical, relying on the particles splashing, flattening, and locking into the microscopic peaks of a prepared surface. Only when a coating is post-treated by fusing or heat treatment can it create more of a metallurgical bond.

This is very different from weld overlay or hardfacing. Weld overlay fully melts filler metal into the surface of the substrate to create a metallurgical bond, and in the process, causes dilution, mixing the filler with the base material. Thermal spray takes the opposite route. By keeping the heat away from the substrate, it avoids creating a large heat-affected zone (HAZ), keeping the original properties of the base metal intact. For parts where changing the metallurgy of the parent material is not an option, this low-heat approach can be the deciding factor when comparing it to options like those covered under alloy steel welding electrodes.

The Vocabulary You’ll Need Going Forward

Before diving into the processes themselves, it’s important to know a few terms that will be referenced throughout the rest of this article. In many repairs, a bond coat will be sprayed first as a thin, adhesion-focused layer between the substrate and the final material. From there, a top coat is applied to provide the service properties the part needs, such as wear or corrosion resistance. With that build-up comes porosity, which are tiny, microscopic voids between the flattened particles. While some porosity can be expected, excessive porosity can become a leak path if left unsealed. Finally, deposition efficiency is the measure of how much of the feedstock actually ends up on the part versus how much is lost to overspray. These small details tend to matter far more than people realize when it comes to a coating’s success.

The Goal: The Right Coating For The Right Part

With all of this in mind, thermal spray welding is best defined as a coating family, not a shortcut to fusion. Its strength lies in its selectivity. It allows a shop to add specific materials to a specific surface, at specific thicknesses, with minimal heat and minimal distortion. For a reader looking to know if it suits their part, the answer is often yes when the repair is about surface restoration or protection, and not when full fusion of the base metal is a code-mandated requirement. With the vocabulary established, the next step is seeing exactly how these tiny, heated particles go from feedstock to a finished coating.

How the Process Works: From Feedstock to Finished Coating

Thermal spray welding sounds exotic until you watch it once. A gun throws heated particles at a part, the particles flatten, and a coating grows pass by pass. Underneath that simple picture is a controlled sequence where almost every step influences whether the coating stays on for ten years or flakes off in ten minutes. Here is the full workflow, from a raw casting to a finished, dimensionally restored surface.

The Physics in One Paragraph

Whatever the feedstock form, the gun does two things: it heats the material to a molten or semi-molten state, and it accelerates it toward the substrate at high velocity. On impact, each particle flattens into a tiny disc called a splat. Splats stack, lock into the rough profile of the surface and to each other, and build the coating layer by layer. That is why thermal spray is usually described as a mechanical bond rather than a fused, metallurgical one: the coating grips the anchor tooth of the blasted surface instead of melting into the base metal. The heat reaching the part stays low compared with welding, so distortion is minimal.

Step-by-Step: The Job Sequence

1. Inspection and specification. Confirm the part is the right candidate, define the failure mode being fixed, and write down coating material, thickness, porosity, bond strength, finish and sealing requirements.
2. Abrasive blast cleaning. The substrate is blasted with an angular media to a clean, rough anchor profile. This is the single biggest determinant of bond strength, because adhesion is mostly mechanical interlock. A smooth, oily or freshly machined surface will not hold.
3. Masking. Threads, sealing faces, fit-ups and areas that must stay bare are taped, plugged or shielded.
4. Preheat (if needed).驱 Moisture removal and a warmer surface improve wetting, especially on large or cold parts.
5. Gun setup. Set gas flows, current, powder feed rate, standoff distance and traverse speed. Most guns run with a standoff of roughly 75 to 250 mm depending on process, and spray rates range from a few kilograms per hour for wire flame or twin-wire arc up to 10+ kg/h for HVOF and large plasma systems.
6. Pass pattern. The operator (or robot) lays overlapping passes at a consistent angle and speed, watching interpass temperature so one zone does not overheat while another stays cold.
7. In-process monitoring. Watch for discoloration, dry spray, uneven build and gun spitting. Small corrections mid-job prevent grind-to-death fixes later.
8. Post-coating steps. Sealing to close porosity, then machining, grinding or honing to dimension; some self-fluxing alloys get a fused heat treatment instead.
9. Cleaning and final inspection. Remove blast residue, check thickness, verify finish, log the results.

Why Surface Prep Decides Everything

Ask any experienced sprayer where a coating failed and the answer is nearly always the surface, not the gun. Contamination, oil film, a delayed gap between blasting and spraying (flash rust sets in fast), or the wrong bond coat all break the mechanical lock. This applies equally to steel, stainless and aluminum work, which is why preparation guides like Aluminum Cleaner for Welding: How to Prep Aluminum Right stress cleanliness as the first real step. Blast media must be sharp and angular, dry, and correctly graded for the profile you need, and the blasted surface should be sprayed as soon as practical.

Job Setup Checklist

Item	Confirm Before Spraying
Specification	Material, thickness, porosity, bond strength, finish defined
Substrate	Clean, dry, blast profile verified (6–12 µm Ra)
Masking	Threads, seals and fit surfaces protected
Gun parameters	Gas flows, current, standoff 75–250 mm, feed rate set
Environment	Ventilation on, humidity controlled, blast residue gone
Post-processing	Sealer, machining method and inspection gauge ready

Get these six lines right and most of the coating problems people blame on the equipment simply never happen.

Major Thermal Spray Processes Compared: Flame, Arc, Plasma, HVOF, Cold Spray

Thermal spray welding is not one process but a family, and the differences matter more than most buyers expect. What separates a flame gun from an HVOF gun or a plasma torch comes down to two numbers: how hot the particles get and how fast they hit the part. Temperature decides which materials can actually melt. Velocity decides how densely the particles pack, how strongly they stick, and whether the finished surface machines like solid metal.

Combustion flame spray and twin-wire arc: the affordable workhorses

Powder flame spraying burns oxy-fuel (typically acetylene or propane with oxygen) to melt powder fed through the flame; rod and wire variants feed solid metal into the same heat source. Twin-wire arc melts two continuously fed electrodes in an electric arc and blows the droplets away with compressed air.

All of these run at low particle velocities, tens to low hundreds of meters per second, giving moderate bond strength and porosity you can usually see in a cross-section, typically several percent and higher if parameters drift. The payoff is cost: simple equipment, high deposition rates and thick build over large areas. This is the everyday route for zinc and aluminum corrosion protection, stainless and carbon steel build-up, molybdenum friction surfaces and routine alloy steel dimensional restoration. Bridge and water-tank metallizing lives here, under the terminology of ISO thermal spraying standards.

Atmospheric and vacuum plasma spraying

Plasma spraying replaces combustion with an electric arc struck inside a gas-fed nozzle, producing a plasma jet far hotter than any flame. That heat is the point: atmospheric plasma (APS) melts refractory ceramics such as yttria-stabilized zirconia, alumina and chromia that combustion processes simply cannot reach. Velocities are moderate to high, and coatings land fine-grained with low porosity when parameters hold.

Vacuum or low-pressure plasma spraying (LPPS/VPS) runs the same torch in a partial vacuum, which slows particle oxidation, raises velocity and produces extremely dense, bond-strong coatings for aerospace turbine sections. Plasma suits thermal barrier coatings, bond coats, oxide ceramics and precision wear surfaces, and it is the most parameter-sensitive process on this list. Related work sits alongside our aerospace welding resource.

HVOF and high-velocity air-fuel

High-velocity oxy-fuel burns in a cannon-style or continuous combustion chamber, accelerating particles to several hundred meters per second and beyond, with jet velocities well into the supersonic range. The dominant effect is kinetic energy, not raw heat. Particles arrive semi-molten and hammer flat, giving coatings commonly in the low single-digit porosity range, with bond strengths strong enough that they can be ground, machined and finished like metal. That is why WC-Co, WC-Co-Cr and Cr3C2-NiCr cermet wear coatings migrated from plasma to HVOF over the past two decades. HVAF variants burn below the flame’s dissociation temperature, which further reduces carbide decarbonization. Operating cost sits between plasma and wire arc: more gas and more sophisticated control, but far less downstream trouble.

Detonation guns, warm spray and cold spray

Detonation (d-gun) spraying pulses explosive combustion to deliver very dense, very smooth coatings at low deposition rates, mostly for premium aerospace work and bores. Warm spray meters oxygen to keep particles just below full melting. Cold spray goes further and deposits material without melting it at all: gas accelerated through a de Laval nozzle drives powder at hypersonic speed, and bonding happens through plastic deformation on impact. Because the substrate never gets hot, cold spray avoids oxidation and heat-affected-zone concerns in the base metal, which makes it attractive for copper, aluminum and other oxidation-prone metals, and for in-field repairs where welding heat is off the table. The surface-prep discipline is the same as any other coating work; our aluminum cleaning guide explains how finicky that gets.

Plasma-transferred arc: the boundary case

PTA sits on the border with weld overlay. It melts powder and the substrate surface together in a constricted arc, creating genuine fusion and a metallurgical bond at moderate dilution with much less heat than a stick or MIG overlay. Section 5 handles that comparison in full; for now, treat PTA as the answer when you need a sprayed powder feedstock but fusion-level adhesion and thickness.

Process comparison at a glance

Process	Heat source	Particle velocity	Coating character	Typical duty
Wire/rod flame & twin-wire arc	Oxy-fuel flame or electric arc	Tens to low hundreds m/s	Visible porosity, moderate bond	Corrosion protection, build-up
Atmospheric plasma (APS)	Electric plasma jet	Moderate to high	Fine, dense, melts ceramics	Ceramic wear, thermal barriers
Vacuum plasma (LPPS/VPS)	Plasma in partial vacuum	High	Very dense, low oxide	Aerospace hot-section parts
HVOF / HVAF	Combustion in cannon chamber	Several hundred m/s and beyond	Low porosity, machineable	Cermet wear, erosion, cavitation
Detonation gun	Pulsed detonation	High	Very dense, smooth	Premium aerospace, bores
Cold spray	Heated gas, no melting	Supersonic	Void-free, no HAZ	Conductive, reactive-metal repairs
PTA	Constricted transferred arc	Low (melt pool)	Fused, metallurgical bond	Thick heavy-duty fused layers

Choosing by service duty

Match the process to the failure mode, not to the catalogue:

• Erosion and abrasion: HVOF or HVAF for cermets; cold spray where the substrate must stay cool.
• Corrosion: sealed HVOF, or a well-bonded arc-sprayed aluminum/zinc layer over a properly anchored blast profile.
• Thermal cycling: APS ceramic top coat over a metallic bond coat, sized for the expansion mismatch.
• Low friction and anti-galling: flame or arc-sprayed molybdenum, or HVOF chrome carbide where wear is harsher.
• Refractory ceramics: plasma only; combustion processes cannot melt them.
• Thick, code-bound fusion work: step to PTA or weld overlay instead.

Feedstock Materials: Wires, Powders, Ceramics, Cermets and Polymers

A thermal spray coating can only be as good as the material you feed the gun. Feedstock is the single biggest lever you have for tuning hardness, corrosion resistance, thermal insulation, friction behavior and cost. The same gun that sprays soft zinc over bridge steel can put a wear-resistant carbide layer on a pump shaft with nothing more than a different wire. This section covers what can actually be sprayed, what form it arrives in, and how to match it to the failure you are trying to stop.

Metallic Wires

Wire is the workhorse feedstock for arc and flame spraying. It is cheap, dense, dust-free, stores indefinitely, and feeds through a drive motor with very little fuss. Common types include:

• Zinc and aluminum for atmospheric corrosion protection of structural steel. Both work like galvanizing: they sacrifice themselves cathodically so the steel underneath does not rust. Zinc-aluminum blend wires (typically 85Zn/15Al) pair zinc’s cathodic protection with aluminum’s barrier durability.
• Carbon and stainless steels for dimensional restoration of shafts, rolls and pump housings, and for mild abrasion duty.
• Nickel-chromium alloys for high-temperature oxidation and corrosion resistance, often used as a bond coat or build layer under ceramic.
• Molybdenum for dry lubricity and scuff resistance on gears, synchronizer rings and brake components.
• Copper and copper alloys for electrical contact rebuilds and thermal-conductivity surfaces.

Aluminum work is worth pausing on. Sprayed aluminum behaves differently from welding filler because it never becomes a weld pool, but the oxide discipline is identical: a freshly cleaned, dry surface is non-negotiable. If you have ever fought aluminum cleaner prep on a welding job, you already understand why contamination kills adhesion here too.

Powders: Atomized, Agglomerated and Sintered

Powder opens the door to materials that cannot be drawn into wire: ceramics, cermets and exotic alloys. Morphology matters as much as chemistry. Gas-atomized or plasma-densified powders are spherical, free-flowing and feed consistently through a gravimetric or volumetric feeder. Irregular crushed powders pour poorly, bridge in the hopper and give erratic spray density. Particle size distribution typically sits in the 5 to 45 micrometer band for plasma and HVOF work, with coarser fractions used for high-build arc and flame spraying. Feedstock must suit the gun: coarse crushed powder will not melt properly in a plasma jet, while fine ceramic powder blown through a twin-wire arc gun is simply wasted. Vocabulary and classification are covered in ISO 14917, the international reference for thermal spraying terminology and feedstock descriptions.

Cermets: The Erosion and Abrasion Workhorses

Cermets, hard ceramic particles held in a metallic binder, dominate erosion and abrasion service. The main families are:

• WC-Co (tungsten carbide-cobalt, usually 83/17 or 88/12) for abrasive wear on shafts, sleeves and valve seats.
• WC-Co-Cr where corrosion resistance must accompany abrasion resistance, such as pump and marine duty.
• Cr3C2-NiCr (chrome carbide-nickel chromium, typically 75/25) for high-temperature erosion, where WC oxidizes and softens above roughly 500 °C.

HVOF is the preferred route: high particle velocity and moderate temperature produce dense coatings above 95% theoretical density with bond strengths typically measured in tens of megapascals. Note that chrome carbide feedstock is the source of hexavalent chromium exposure concerns covered in OSHA’s thermal spraying guidance.

Ceramics and Thermal Barrier Systems

Ceramics are sprayed by plasma, whose extreme jet temperature melts alumina, titania, chromia, zirconia and yttria without decomposing them. Alumina is the general-purpose engineering ceramic for wear and electrical insulation; titania offers strong abrasion and corrosion resistance at lower cost; chromia suits severe corrosion and slurry erosion; zirconia and yttria-stabilized zirconia (YSZ) are almost exclusively thermal barrier duty.

Thermal barrier coating systems are two-layer by design. A metallic bond coat, usually an MCrAlY alloy (M = nickel, cobalt or a blend), protects the substrate from oxidation and gives the ceramic something to grip. A YSZ top coat then provides the insulating barrier, letting gas turbine components run at gas temperatures above the metal’s melting point. Ceramics are brittle, expand at a different rate than steel, and need careful thickness control.

Polymers, Composites and Advanced Feedstocks

Polymers are normally flame-sprayed from powder or rods, often as polymer-encapsulated composites where ceramic or PTFE particles are locked inside a nylon or polyester matrix. These give low friction, non-stick and mild wear resistance for food, packaging and textile machinery. At the sophisticated end sit amorphous (metallic glass) and nanostructured powders, which produce denser, finer-grained coatings with improved toughness, though they cost more and demand tighter process control.

Matching Feedstock to the Failure Mode

The fastest way to choose a material is to name the failure first, then pick the family that resists it:

Wear or Service Mechanism	Typical Feedstock	Usual Process
Abrasion, erosion	WC-Co, WC-Co-Cr, Cr3C2-NiCr	HVOF, plasma
Atmospheric corrosion of steel	Zinc, aluminum, Zn-Al	Wire arc, flame
Chemical or slurry corrosion	Ni-Cr-Mo alloys, stainless, chromia	Plasma, HVOF
High-temperature oxidation	Cr3C2-NiCr, NiCrAlY	Plasma, HVOF
Thermal barrier duty	MCrAlY bond coat + YSZ top coat	Air plasma
Low friction, dry running	Molybdenum, polymers, PTFE composites	Flame, arc
Dimensional restoration	Carbon steel, stainless, Ni-Cr wire	Wire arc, flame

Get the feedstock family right and most downstream process problems solve themselves. Get it wrong, and no amount of gun tuning will save the job.

Thermal Spray vs Weld Overlay, Hardfacing and Cladding

“Can you just spray it, or does it need to be welded on?” That question gets asked in almost every repair shop, and the honest answer is that neither route is universally better. They rely on different physics, they put different amounts of heat into the part, and they leave you with a coating that behaves differently under load. Understanding where the line sits is the difference between a repair that lasts a decade and one that spalls off in six months.

Weld Overlay: A Metallurgical Bond With a Dilution Problem

Weld overlay, hardfacing and cladding are fusion processes. An arc melts both the filler and a thin layer of the base metal, so the finished surface is metallurgically continuous with the substrate. TWI describes hardfacing essentially as weld-deposited wear-resistant material, and that fusion is the point: the bond strength is the base metal’s strength, and the layer carries impact transmitted from underneath.

The price you pay is dilution. Iron from the substrate mixes into the overlay and shifts its chemistry, which is why procedures control travel speed, bead shape and sometimes a “buttering” layer to keep the final pass composition on spec. Stainless and Alloy 20 corrosion overlays are the classic examples: get the dilution wrong and the corrosion resistance you paid for disappears. Heat input is the other cost. A multi-pass overlay on a 40 mm shaft means real thermal cycling, residual stress, and possible distortion of a part you already finished machining.

Thermal Spray: Low Heat, No Dilution, Mechanical Bond

Thermal spray never melts the substrate. Molten or semi-molten particles hit an abrasive-blasted surface, flatten into splats and lock onto the anchor profile. The coating is chemically what you fed the gun, essentially undiluted, and the substrate stays cool enough that a finished, heat-treated part can be coated and reground to size. Per ISO 14917 terminology, these are deposit processes, not fusion processes, and the bond is mechanical rather than metallurgical. Under shear or impact loading that distinction matters.

Characteristic	Thermal Spray	Weld Overlay / Hardfacing	Clad Plate (Roll or Explosion)
Bond type	Mechanical (splats on anchor profile)	Metallurgical	Metallurgical across full interface
Dilution	None to negligible	Measurable and controlled	Minimal, controlled at plate mill
Heat input	Very low; substrate stays cool	High; weld pool and HAZ	Localized at bond line only
Distortion risk	Low	Moderate to high	Low once plate is stress-relieved
Typical thickness	0.1–3 mm (some processes to 10 mm)	1–10+ mm	3–50+ mm corrosion or structural layer
Deposition rate	High for arc and HVOF	Moderate; slower with low dilution	Not a shop process; it is stock material
Re-coatability	Excellent; strip, blast, re-spray	Requires gouging and re-welding	Not repairable; replace the plate
Code acceptance	Rarely accepted for pressure boundary work	Widely accepted under ASME/NBIC	Standard for pressure vessels

Where Each Route Wins

Thermal spray is the better choice when you have large areas, thin and precise layers, or heat-sensitive parts:

• Dimensional restoration of worn shafts, rolls and bores where the finished size must be held to a few hundredths.
• Coatings on fully machined or heat-treated components that must not distort.
• Thermal barrier ceramics, low-friction coatings and seal faces where a fusion process is simply not applicable.
• Field work on large structures, such as zinc or aluminium corrosion protection on bridge steelwork, where welding is impractical.

Fusion overlay wins when the layer has to survive punishment or satisfy a code:

• Thick, heavy-duty hardfacing on crusher teeth, bucket lips and mixer blades seeing subsurface impact.
• Services where porosity cannot be tolerated, because a sprayed coating is inherently splat-laminated.
• Code work: ASME pressure equipment cladding and NBIC-regulated repairs generally require fusion processes with a qualified WPS and PQR. Thermal spray lives in repair and surface engineering, not in the pressure boundary.

Plasma-transferred arc (PTA) sits astride the fence: it is a fusion process, so you get a metallurgical bond, but with low dilution and powder feed it behaves operationally more like spraying than like stick overlay.

Removal matters too: an old hardfacing layer usually has to come off by carbon arc gouging, while a sprayed coating is stripped by grinding or blasting and simply re-applied. For most wear and corrosion duty on rotating equipment, that re-coatability, plus the negligible heat input, is why thermal spray has quietly taken over large areas of the repair market while fusion overlay keeps the thick, high-impact and code-regulated work.

Where Thermal Spray Welding Is Used: Industries and Real Applications

Most parts do not fail because the whole component is worn out. They fail because the outer few millimeters are gone. That single fact is why thermal spray welding shows up in almost every heavy industry: it puts fresh, purpose-built material back on the surface that is doing the work, without heating or distorting the part underneath.

Aerospace and Power Generation: Hot-Section Coatings

The highest-profile use is the thermal barrier coating (TBC) system on gas turbine hot-section components. A ceramic top coat, typically yttria-stabilized zirconia, sprayed over a metallic bond coat lets the metal run at temperatures that would otherwise soften it. ASM International documents these coating systems as a major enabling technology for higher firing temperatures and efficiency in both land-based turbines and aeroengines, which is why blades, vanes, seals and combustion hardware are routinely coated in aeroengine and utility maintenance shops.

On the colder, dirtier side of the plant, the same processes protect against wear rather than heat: boiler tubes, induced-draft fan blades, pulverizer rolls and erosion protection on blade tips and last-stage components. Repair shops handling pressure parts and boiler work, such as those profiled in Alamo Welding & Boiler Works, see this wear-metal buildup regularly. Aerospace work carries tighter paperwork still, with procedures qualified under programs like those described in Aerospace Welding.

Oil and Gas, Pulp and Paper, and Rotating Equipment

In oil and gas, the workhorse application is the spray-and-fuse coating on pump sleeves, shaft journals and valve stems in corrosive or abrasive service. Self-fluxing nickel-chromium-boron-silicon alloys are sprayed, then fused in place, giving a dense, metallurgically bonded, very hard surface. Downhole tools, progressive cavity rotor rods, seat faces and choke components get the same treatment. Pulp and paper is a close cousin: calender rolls, dryer cans, suction rolls and Yankee cylinders rely on sprayed coatings for smoothness, release and wear resistance, with chrome oxide and alumina ceramics common where paper cleanliness matters.

Automotive, Marine and Infrastructure

Automakers and Tier 1 suppliers use thermal spray for brake rotors, valve seats, cylinder bores, piston rings and engine bearings, and the trend toward aluminum and casting-intensive designs keeps surface engineering in demand. That is separate from repair work on aluminum parts, a different skill set covered in resources like Alloy Wheel Welding.

The largest single application by tonnage is corrosion protection of steel. Sprayed zinc, aluminum and zinc-aluminum coatings are specified on bridges, transmission towers, offshore structures and ship components precisely because a hot-dip galvanizing kettle cannot accommodate a 30-meter bridge girder. Spray metallizing of iron and steel architectural work is documented for long-term corrosion protection of monuments and bridges, and structural steel shops working on marine steelwork increasingly encounter it; see AJ Marine Welding for how coatings and welding procedure interact on marine projects.

Industry	Typical parts	Common process and material
Aerospace / power	Turbine blades, vanes, seals, boiler tubes	APS/LPPS, zirconia TBC over MCrAlY
Oil and gas	Pump sleeves, valve stems, rotor rods	HVOF or spray-and-fuse, WC-Co-Cr, Ni alloy
Pulp and paper	Calender rolls, dryer cans	Wire arc and HVOF, chrome oxide, stainless
Marine / infrastructure	Bridges, towers, ship steel	Arc or flame spray, zinc and aluminum
Mining and cement	Fans, chutes, mixer blades	HVOF and arc spray, carbides, white iron

Restoration, Mining, Food and Packaging

Mining and cement plants spray fan rotors, chutes, hopper liners, mixer blades and separator vanes, where erosion from abrasive bulk material is the enemy. Food, beverage and packaging machinery gets stainless and chrome oxide coatings on sealing faces, sealing bars and wear plates, chosen for cleanability and low friction. Architectural and historic restoration is quieter but steady: bronze statuary, cast-iron ornament and steel monuments are cleaned and re-protected by thermal spray rather than by hot-dip or paint, because the original geometry cannot be dipped or blasted aggressively.

A Fan Rotor: Repair or Replace?

Consider an erosion-worn fan rotor in a power station ID fan. The airfoil blades are scoured thin on the leading edges and the shaft journals are scored. Replacement means new forgings, weeks of lead time, alignment work and a planned outage. A repair route means blast-cleaning the affected zones, spraying the journals with a spray-and-fuse nickel alloy, machining them back to size and rebuilding blade edges with a hard-facing spray pass, all in a fraction of the downtime at a fraction of replacement cost. The same economics explain why restoring a worn shaft beats buying a new one: the coating’s material and labor are tiny compared with the forging, machining and scheduling cost of the replacement part.

Across all of these industries the pattern repeats: thermal spraying is chosen when a surface problem needs a surface solution, and the part is worth saving.

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“Ask a shop owner why they bought a thermal spray gun and the answer is rarely a prettier bead. It is that the process places a chosen alloy or ceramic onto a part without approaching the part’s melting point. That one property drives the advantages below, and it also explains the limits and safety obligations that come with the process.”

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Low heat input, minimal distortion. Substrate stays far below melting, so thin walls, precision bores and close-tolerance shafts come out where they went in. Warping and residual stress are fractions of a fusion overlay’s.

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Fast coverage. Several kilograms per hour is routine, making a boiler tube run, calender roll or ship hull section practical in a way weld overlay is not.

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Thickness control and re-coatability. Builds go on in measured passes, then grind or machine to size. Worn coatings can be stripped and re-sprayed instead of scrapping the component.

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Restoration at a fraction of replacement cost. A worn pump shaft or fan rotor often costs more to source and refit than to blast, spray, machine and return to service. That economics is why aerospace and power plants treat spray booths as capital equipment. For aluminum parts passing through the same shops, the surface prep in [link] is a useful parallel.

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Benefits, Limitations and Safety Concerns

Ask a shop owner why they bought a thermal spray gun and the answer is rarely a prettier bead. It is that the process places a chosen alloy or ceramic onto a part without approaching the part’s melting point. That one property drives the advantages below, and it also explains the limits and safety obligations that come with the process.

The Case for Thermal Spray

• Low heat input, minimal distortion. The substrate stays far below melting, so thin walls, precision bores and close-tolerance shafts come out of the booth dimensionally where they went in. Warping and residual stress are fractions of what a fusion overlay produces.
• Fast coverage of large areas. Several kilograms per hour is routine, making a boiler tube run, calender roll or ship hull section practical in a way weld overlay is not.
• Tailor-made surface chemistry. WC-Co on a steel shaft, a thermal barrier ceramic on a nickel superalloy blade, zinc on structural steel: the base metal never has to accept any of it metallurgically. Dissimilar combinations are normal.
• Thickness control and re-coatability. Builds go on in measured passes, then get ground or machined to a number. Worn coatings can be stripped and re-sprayed instead of scrapping the component.
• Restoration at a fraction of replacement cost. A worn pump shaft or fan rotor often costs more to source and refit than to blast, spray, machine and return to service. That economics is why aerospace and power plants treat spray booths as capital equipment. For aluminum parts passing through the same shops, the surface prep in Aluminum Cleaner for Welding: How to Prep Aluminum Right is a useful parallel on how much surface condition decides the result.

The Honest Limitations

• Porosity. As-sprayed coatings contain interconnected pores. Unsealed, they become leak paths and corrosion highways, especially in immersion service. Sealing, sealing plus a topcoat, or a fusion step is usually non-negotiable.
• Lower bond strength than fusion. Sprayed bonds are largely mechanical interlocks, and most processes deliver adhesion below the parent metal’s strength. Coatings are poor choices for heavily loaded impact or fatigue surfaces without a bond coat and correct preparation.
• Thickness ceilings. Building beyond a few millimetres in one job invites residual stress, cracking and spalling, particularly with ceramics whose thermal expansion differs sharply from steel’s.
• Edge and sharp-corner coverage. Particles travel in straight lines, so sharp edges shadow themselves and finish thin. Knife-edge designs need rethinking or masking.
• Post-machining and finish work are usually required, along with lot and procedure qualification for regulated work.

Safety: What the Gun Puts in the Air

Thermal spraying belongs with welding, cutting and brazing as a fume-generating operation. OSHA’s thermal spraying guidance sits inside the same welding, cutting and brazing standard family. Engineering controls and ventilation come first; respiratory protection is required wherever fume or dust exposure exceeds the applicable limits.

• Metal fumes and fine inhalable dust from the feedstock. Spraying stainless steel or chromium carbide generates hexavalent chromium, a documented exposure risk that calls for periodic monitoring under NIOSH criteria. Nickel and manganese follow from most stainless and alloy wire. Some specialty alloys carry beryllium, and tungsten carbide/cobalt dust is its own housekeeping problem, with cobalt linked to respiratory sensitization.
• Fire and explosion. Spraying aluminum, zinc, titanium, zirconium or antimony produces combustible dust; titanium and zirconium dust is pyrophoric. Guidance recommends against spraying reactive metals unless the specific hazard has been assessed, and many shops ban Ti and Zr without dedicated inerting and housekeeping.
• Oxygen-enriched atmosphere. HVOF and oxy-fuel powder guns run fuel gas and oxygen at pressure. Leaks around hoses, regulators and the gun head can enrich a confined space enough to make ordinary materials burn readily.
• Optical radiation. Wire arc and plasma guns emit UV and IR intense enough to demand the same eye and skin protection as arc welding.
• Noise. High-velocity and plasma equipment routinely exceeds 85 dBA at the operator’s position, so hearing protection and acoustically treated enclosures are standard design practice, not optional extras.

Handled properly, the balance still favors thermal spray. The benefits are economic and metallurgical, the limitations can be designed around, and the safety costs are known, budgetable and documented by AWS and ASM International. Ignored, the limitations show up as failed coatings and the safety gaps show up as exposure records. The difference is almost always planning, not equipment.

Quality Control, Inspection and Governing Standards

A coating you cannot measure is a coating you cannot promise. The difference between a repair that lasts and one that flakes off in a month comes down to specification, qualification and inspection: qualify the procedure, test the coupon, inspect the part, and cite the standard behind every acceptance decision.

Qualify the Procedure Before You Touch the Part

Production spraying starts on a coupon, not on the customer’s shaft. It is sprayed on a representative substrate using the exact production recipe: same blast media and anchor profile, same gun, gas flows and current, same standoff, same powder lot. The coupon is then pulled, sectioned, hardness checked and measured, proving the process works before a pass lands on real hardware.

Everything goes into a spray log: media lot, profile readings, preheat and interpass temperature, spray rate, powder consumed versus deposited, and finishing steps. When a coating fails, that log shows whether the problem was the process or the day it was sprayed.

Adhesion, Porosity and Thickness: The Three Numbers Everyone Specs

Porosity, thickness and bond strength are the most frequently specified criteria, typically joined by surface preparation profile and finish:

• Bond strength is measured by tensile pull-off under ASTM C633: a dolly is glued to the coating, the assembly is cut free, and a machine pulls until something breaks, showing whether the interface, coating or adhesive gave way.
• Porosity is read on a polished cross-section by point counting or image analysis; corrosion-critical specs add permeability or electrolytic leak testing to catch connected pores.
• Thickness is read with magnetic induction or eddy current gauges on the finished part and with micrometers on coupon edges; rough surfaces need a surface-compensating probe or a masked spot.
• Hardness is confirmed by Rockwell, Vickers or microhardness traverses, which also expose overheated carbides or decarbonization left by a bad oxygen-fuel ratio in HVOF work.

Checks tied to service conditions are handled separately:

Check	Method / reference	What it proves
Bend & fatigue	Coupon bending, rotating-bend rigs	Survives strain and cyclic loading
Corrosion	ASTM B117 salt fog, immersion	Barrier holds over time
Thermal cycling	Furnace/quench rigs	No spallation from CTE mismatch

Performance Testing and In-Service Inspection

Coupon tests prove the procedure; performance tests prove it will survive the service. Salt-spray exposure under ASTM B117 is the standard corrosion check, while thermal cycling exposes ceramics sprayed too thick or with too much residual stress. Bend and fatigue coupons matter whenever the part flexes.

Once the part is in service, inspection shifts to nondestructive testing. Ultrasonic methods find disbondment and measure thickness, dye penetrant reveals surface-breaking porosity and cracks on ground finishes, and sprayed corrosion barriers are holiday-tested with a spark probe, since one connected pore is a corrosion starter. Established NDT guidance covers technique qualification and operator certification.

Who Writes the Rules: Standards Bodies and Document Families

Several organizations share the job of defining thermal spray welding practice:

• AWS publishes the C-series thermal spray documents on process definitions, safety and practice, and runs a certified thermal spray operator program that verifies operator competency rather than company capability. Training paths such as an aluminium welding course build the same discipline.
• ASTM committees C21 and C28 own the test methods: C633 for adhesion, B117 for salt fog, plus hardness and porosity standards.
• ISO 14917 covers thermal spraying terminology and classification, giving buyers and shops a common vocabulary, while ISO 2063 covers metal coatings for corrosion protection by spraying, including preparation and thickness expectations for metallizing.
• NACE/SSPC (now AMPP) set the surface preparation standards that decide bond quality before the gun is lit, from near-white metal blast cleaning to profile verification; dedicated aluminum cleaning and prep steps get their own procedures too.
• ASME and NBIC govern repair context, because a sprayed or overlaid repair on pressure-retaining hardware must fit the jurisdiction’s repair code, as in boiler-shop practice like Alamo Welding & Boiler Works boiler services. Aerospace adds Nadcap-style approval, familiar from aerospace welding standards and careers.

The quality system boils down to this: prove the procedure on a coupon, measure the numbers that matter, expose a sample to the service environment, and inspect in service before small defects become big ones. Shops that do this stop guessing, and their coatings behave like engineered materials instead of applied paint.

Common Mistakes, Troubleshooting and How to Fix Them

Every spray booth has a story about a coating that looked fine coming off the gun and failed a week later. In most of those stories the gun was never the problem. Thermal spray is unforgiving about preparation and parameters, and the failures show up in predictable patterns: coating that lifts, coating that pores through, coating that cracks, coating that comes out the wrong hardness. The good news is that almost every one of these has a recognizable signature, and once you can read the signature you can fix it without guessing.

Adhesion Failures: The Coating Simply Lets Go

Poor adhesion is the most common and most avoidable defect. The bond in thermal spray is mechanical, so anything that fills or contaminates the anchor profile kills it. The classic mistake is waiting too long after blasting, or worse, handling the prepared surface with bare hands. Flash rust can form on bare steel within hours in humid shops, and a fingerprint is enough to create a release layer. Oil left from machining, coolant residue, and moisture all do the same job. If coating lifts or delaminates shortly after service, contamination or a delayed spray is almost always the root cause. Surface cleanliness is the same discipline you would apply when you prep aluminum with a dedicated cleaner before fusion welding: clean, protect, and spray promptly.

Fixes are procedural rather than exotic: re-blast, spray within the specified window (many procedures require coating within four hours or before visible flash rust appears), use clean gloves and dedicated fixtures, and verify the dew point. On stainless, zinc, or very smooth substrates, a thin bond coat of nickel chrome or molybdenum often bridges what the top coat cannot key into on its own.

Porosity and Dry Spray

A dull, friable, sponge-like coating that machines away to nothing is dry spray: particles arrived cold, at the wrong angle, or from too far away, so they stacked without flattening into fully bonded splats. Excessive standoff distance, an off-normal spray angle, low flame or arc energy, and too-fast traverse speeds are the usual culprits. Material is rarely to blame by itself.

Shorten the standoff to the specified range, bring the gun perpendicular to the surface, slow the traverse enough to get proper overlap, and confirm particle temperature by checking gas ratios and powder feed rate. Where porosity remains open in corrosion or hydraulic service, seal it with a penetrating sealer after spraying. Unsealed porosity is a leak path, plain and simple.

Cracking, Spalling and Edge Lift-Off

Thick ceramic coatings sprayed in one heavy pass trap residual stress, and steel and alumina expand at very different rates when the part heats and cools. The result is cracking or spalling, especially on thin sections and sharp edges. The fix is architectural: build thickness in multiple lighter passes, control interpass temperature, break sharp corners with a generous radius, and use a compliant bond coat between steel and ceramic. Multi-layer bond-coat/top-coat systems exist precisely to absorb that expansion mismatch.

Symptom	Likely Cause	Fix
Coating lifts or flakes off	Delayed spray after blasting, flash rust, oily or handled surface, wrong bond coat	Re-blast, spray inside the time window, glove handling, add bond coat
Dull, porous, friable deposit	Standoff too far, angle off-normal, cold particles, traverse too fast	Reset distance and angle, verify gas flows and feed rate, seal afterward
Cracks or spalling in service	Single heavy pass, high residual stress, CTE mismatch ceramic on steel	Multi-pass build, interpass control, bond coat, radius sharp edges
Wrong hardness or brittle carbide	Overheated carbide, decarbonization, wrong oxygen-to-fuel ratio	Tune O2-to-fuel ratio and particle temperature, re-qualify parameters
Uneven thickness, thin at edges	Masking and fixture errors, gun speed inconsistent, no path plan	Fix fixtures, use travel guides, map pass pattern, gauge between passes
Contamination between passes	Interpass temperature ignored, dust or grinder residue on surface	Monitor interpass temp, keep adjacent areas covered, clean between passes

Decarbonization deserves special attention in HVOF carbide coatings: too much oxygen or too hot a particle burns carbon out of the tungsten carbide, leaving soft, brittle chrome- or cobalt-rich phases. Tuning the oxygen-to-fuel ratio and particle temperature restores hardness. Symptom, cause, and fix are described well in TWI’s technical resources on thermal spray defect analysis.

Spec-Writing Mistakes

Some troubleshooting failures begin before anyone touches the gun. A specification that demands 10,000 psi tensile bond strength from a flame-sprayed zinc coating is asking the process to do something it physically cannot, and the shop will either fail qualification or quietly re-test until something passes. Other common spec errors include omitting surface preparation standards, not defining porosity limits, ignoring sealing requirements in corrosion duty, or specifying a finish that no process in the family can hold without machining allowance.

Troubleshooting in thermal spray is mostly listening to the coating. Read the fracture surface, check the blast profile, pull the parameter sheet, and the defect usually tells you exactly which step went wrong.

How to Choose a Process, Getting Started and What’s Next

Faced with a worn or corroded part, most people’s first question is which spray gun to buy. That is the wrong question. The right sequence is to understand what failed, what the part must survive next, and what acceptance criteria will prove the fix worked. Get those right and the process choice becomes obvious. Here is a flow that works on any job.

A Seven-Step Selection Flow

1. Define the failure mode. Erosion, abrasion, corrosion, cavitation, oxidation, thermal cycling, and low-friction service each point to different coatings. A cavitation-eaten pump sleeve and an oxidized heat-exchanger tube have little in common except the gun that coats them.
2. Define the substrate. Carbon steel, stainless, cast iron, aluminum, brass, ceramic, and polymer-lined surfaces all behave differently under blast cleaning and heat. Aluminum brings its own surface chemistry problem, as this guide to cleaning aluminum before welding explains.
3. Define thickness and finish. A 100 µm corrosion barrier that runs as-machined is a different job than a 2 mm build-up turned to size afterward. Some processes hold tight thickness control; others are build-rate tools.
4. Define service temperature and environment. Chromia and alumina ceramics hold up in hot, corrosive gas streams; WC-Co cermets shine at room-temperature abrasion but oxidize above roughly 500°C; yttria-stabilized zirconia is the classic thermal barrier.
5. Set acceptance criteria before quoting. Bond strength, porosity ceiling, hardness band, finish, and sealing requirements belong in the spec up front, not in an argument after delivery. ISO 14917 gives you the vocabulary for that spec.
6. Budget the whole job, not just the coating. Surface preparation, masking, powder or wire, fixtures, inspection, and finishing usually cost more than the spray time itself.
7. Decide in-house or outsourced. This is a business decision as much as a technical one.

Factor	In-house gun	Job-shop contract
Capital outlay	Gun, control unit, blast booth, compressors, dust collection	Minimal; pay per part
Operator skill	You must hire or train and retain it	Borrowed, usually certified
Throughput	High volume, repeat work, large structures	Occasional or oversized parts
Qualification	You own procedure and lot documentation	Shop’s procedure qualifies the job
Best fit	Continuous production, field crews, structural coating	Repairs, exotic materials, one-off R&D parts

Budgeting in-house also means budgeting gases and consumables. Our gas and equipment buying guide shows how cylinders, wire, and powder actually stack up.

Getting Started: The First Steps That Actually Matter

Shops that succeed with thermal spray do the same unglamorous things first:

• Run a coupon before the part. Spray a test plate of the same substrate, blast it identically, and pull-test it. Ten minutes of coupon work prevents a scrapped component.
• Qualify a written procedure. Record media, profile, distance, gases, current, powder feed rate, pass pattern, and finishing steps so the job repeats with a different operator.
• Buy the right blast media and meter your powder. Feedstock flow and size distribution drift more than most people expect; a cheap feed-rate controller pays for itself.
• Keep a spray log. Every part, date, lot number, parameters, operator, and inspection result. If a coating fails in service, that log is your only defense.
• Train and certify operators. Certification programs run through bodies such as AWS, and structured training, including a formal welding course path, shortens the learning curve.

What’s Next for Thermal Spray

Several trends are reshaping the field. Cold spray keeps expanding beyond its defense and aerospace roots into electronics, additive repair, and corrosion-resistant cladding, largely because it never melts the feedstock, so heat-sensitive substrates stay intact. Automated and robotic spray cells with closed-loop parameter monitoring are cutting porosity scatter and dependence on any one operator’s hand. Feedstock research follows the same path, with nanostructured and water-strength powder formulations reaching production use, a shift ASM International tracks closely.

Spray cells now log their own data and flag drift in real time. There is also a sustainability angle that rarely gets credit: recoating a worn component generally consumes far less energy than making a replacement from raw material, which makes thermal spray a genuine repair-over-replace story. Meanwhile, the line between additive manufacturing and thermal spray is blurring, with high-rate processes used to build geometry, not just cover it.

In short, thermal spray welding is a family of coating processes that heat feedstock to a molten or semi-molten state and propel it onto a prepared substrate, building a layered surface without melting the base metal. It sits beside fusion welding rather than inside it: choose it when you need low heat, precise thickness, large coated areas, or dissimilar materials joined by mechanical bonding; choose weld overlay for a fused, dilution-controlled metallurgical layer or code-qualified heavy build-up. Define the failure, set acceptance criteria, prepare the surface, qualify the procedure, and the right process reveals itself.

Frequently Asked Questions

Is thermal spray welding the same as welding?

No. Thermal spray melts or heats feedstock (wire or powder) and propels it onto a surface, but it normally does not melt the base metal. Fusion welding melts both the filler and part of the substrate to create a welded joint, while thermal spray builds a coating that bonds mostly by mechanical interlock.

What is the difference between thermal spray and HVOF?

HVOF (high-velocity oxy-fuel) is one specific thermal spray process, not a separate technology family. It burns fuel and oxygen in a chamber and accelerates particles at very high velocity, producing denser, harder, better-bonded coatings than flame or wire arc spray.

How thick can a thermal spray coating be?

Most coatings fall between about 0.1 mm and 3 mm per job, with individual passes kept thin to control stress. Heavier build-ups are achieved with multiple passes, and very thick layers are usually better served by weld overlay or a machined and shrunk-on sleeve.

Does thermal spraying damage the base metal?

Substrate temperature is usually kept low enough that there is no meaningful heat-affected zone, which is one of the main advantages of the process. The part still needs to be clean, dry, and sometimes preheated, and the surface must be roughened by blast cleaning.

Which thermal spray process is best for wear resistance?

For abrasion and erosion, HVOF-sprayed tungsten carbide-cobalt or chromium carbide-nickel chromium coatings are the usual first choice because they are dense and hard. For high-temperature oxidation or sliding wear, NiCr-based alloys, molybdenum, or ceramic coatings may be more suitable.

Are thermal spray coatings waterproof or leak-tight?

As-sprayed coatings contain some porosity and are generally not leak-tight. For corrosion or barrier duty they are commonly sealed with penetrating epoxies, waxes, or polymers, or applied as a multi-layer system with a bond coat and dense top coat.

Is thermal spray coating safe for food or drinking-water service?

It can be, but only with a coating system and process qualified for that service, using approved materials and seals. The porosity in as-sprayed coatings must be addressed, and the applicator should be able to document material compatibility and cleaning requirements.

What safety gear is needed for thermal spray work?

At minimum: respiratory protection or supplied air where fume and dust controls are not fully effective, eye and face protection against arc/UV, hearing protection, gloves and flame-resistant clothing, and good ventilation or a spray enclosure. Reactive metal powders and HVOF oxygen lines add fire and ignition risks that need a specific hazard assessment.

Can a thermal sprayed part be repaired or recoated?

Yes, and that is one of the biggest advantages: worn coatings can be stripped by grinding or blasting and re-sprayed. Each recoat cycle requires requalification of the surface profile and removal of any sealers or oils before the new bond coat goes on.

How long do thermal spray coatings last?

Service life varies enormously with the failure mechanism, coating material, thickness, and sealing, ranging from a few years in severe erosion to decades for sacrificial zinc or aluminum corrosion protection on steel. A properly specified and inspected coating usually lasts far longer than the same part bare.