Thermal spray is one of the most important families of industrial surface engineering technologies.

It is used to apply metallic, ceramic, carbide, and other functional coatings onto components that require improved wear resistance, corrosion resistance, thermal protection, dimensional restoration, electrical properties, or other surface functions.

Unlike laser cladding and other fusion-based deposition processes, thermal spray generally does not intentionally melt the substrate to create a metallurgical fusion zone.

Instead, the feedstock material is heated, accelerated, and projected toward a prepared surface, where successive particles flatten, solidify, and build up into a coating.

This fundamental difference gives thermal spray several important advantages:

• Relatively low heat input to the substrate
• Minimal dilution
• Broad coating material selection
• High deposition efficiency
• Large-area processing capability
• Good suitability for heat-sensitive components
• Ability to apply metals, carbides, ceramics, and specialized functional materials

However, “thermal spray” is not a single process.

It includes several different technologies, each with its own particle temperature, particle velocity, feedstock, coating structure, cost, and application range.

The most important industrial processes include:

• HVOF – High Velocity Oxy-Fuel
• HVAF – High Velocity Air-Fuel
• APS / Plasma Spray – Atmospheric Plasma Spray
• Arc Spray
• Flame Spray

Understanding the differences between these processes is essential when selecting the correct coating technology for a specific component.

1. What Is Thermal Spray?

Thermal spray is a group of coating processes in which a feedstock material is heated and accelerated toward a prepared substrate.

The feedstock may be supplied as:

• Powder
• Wire
• Rod
• Cord
• Other process-specific forms

Depending on the thermal spray technology, the material can be partially melted, fully melted, or heated to a softened condition before impact.

When particles strike the substrate at high velocity, they deform into flattened structures commonly referred to as splats.

Large numbers of these particles accumulate to form a coating.

The resulting coating properties depend on:

• Particle velocity
• Particle temperature
• Feedstock material
• Particle size
• Substrate preparation
• Spray distance
• Gas chemistry
• Torch design
• Coating thickness
• Residual stress
• Porosity
• Oxidation level

For this reason, thermal spray performance should always be evaluated as a complete process rather than simply by coating material.

2. How Thermal Spray Differs from Fusion-Based Coating

One of the most important characteristics of thermal spray is that the substrate is generally not intentionally melted.

This is fundamentally different from:

• Laser cladding
• PTA hardfacing
• Conventional weld overlay

In those processes, part of the substrate melts and becomes part of a metallurgical fusion zone.

In thermal spray, the objective is typically to deposit material onto the surface while keeping the substrate below its melting point.

This can reduce:

• Thermal distortion
• Heat-affected zones
• Dilution
• Changes to substrate metallurgy

This is why thermal spray is attractive for:

• Thin components
• Heat-treated components
• Large surfaces
• Components requiring carbide or ceramic coatings
• Applications where substrate melting is undesirable

However, because a fusion zone is not intentionally created, thermal spray coatings rely on different bonding mechanisms.

These can include:

• Mechanical interlocking
• Localized metallurgical interaction
• Diffusion
• Physical and chemical adhesion
• High-velocity particle deformation

The dominant mechanism depends on the spray process and coating material.

3. Main Thermal Spray Technologies

The major thermal spray technologies differ primarily in how they generate heat and how they accelerate the feedstock.

A simplified comparison is:

Each process serves a different engineering need.

4. HVOF – High Velocity Oxy-Fuel

4.1 Working Principle

HVOF uses the combustion of fuel and oxygen to generate a high-temperature, high-pressure gas stream.

Fuel gases or liquid fuels may be used depending on the torch design.

The combustion gases accelerate through the torch and produce a high-velocity jet.

Powder is injected into this jet, where particles are heated and accelerated toward the substrate.

The combination of high velocity and controlled particle temperature produces dense coatings with strong adhesion and relatively low porosity.

4.2 Feedstock

HVOF primarily uses powder feedstock.

Common material families include:

• Tungsten carbide-based materials
• Chromium carbide-based materials
• Nickel-based alloys
• Cobalt-based alloys
• Stainless steels
• Metal alloys
• Cermets

Carbide coatings are among the best-known HVOF applications.

4.3 Coating Characteristics

HVOF coatings are known for:

• High density
• High hardness
• Strong wear resistance
• Good adhesion
• Low porosity
• Relatively low oxidation compared with many conventional thermal spray processes
• Good corrosion resistance depending on material

The high particle velocity helps particles impact the substrate with substantial kinetic energy, improving coating compactness.

4.4 Temperature Characteristics

The combustion temperature is high, but the substrate itself does not need to melt.

This allows HVOF to achieve a useful balance between:

Sufficient particle heating + high velocity + relatively low substrate thermal influence

This is particularly important for carbide systems, where excessive thermal exposure can damage or decompose hard phases.

4.5 Typical Applications

HVOF is widely used for:

• Hydraulic rods
• Pump components
• Valves
• Oil and gas equipment
• Industrial rolls
• Wear sleeves
• Paper industry rolls
• Steel industry components
• Erosion-resistant surfaces
• Corrosion-resistant coatings
• Heavy industrial wear components

It is especially attractive where a relatively thin, dense, highly wear-resistant coating is required.

5. HVAF – High Velocity Air-Fuel

5.1 Working Principle

HVAF is related to HVOF but uses compressed air rather than pure oxygen as the primary oxidizer.

Fuel is combusted in a high-pressure air environment, creating a high-velocity gas stream.

Powder particles are injected into the stream and accelerated toward the substrate.

Because combustion temperatures are generally lower than in HVOF, HVAF can reduce excessive thermal exposure of the particles.

5.2 Why Lower Particle Temperature Matters

Many coating materials benefit from high particle velocity but do not benefit from excessive heating.

This is especially relevant for carbide-metal composite materials.

If carbide-containing powder becomes too hot, undesirable effects can include:

• Decarburization
• Oxidation
• Phase transformation
• Degradation of hard phases

HVAF is therefore attractive because it emphasizes:

Very high particle velocity + relatively lower thermal load

5.3 Coating Characteristics

HVAF coatings can offer:

• Very high density
• Low oxide content
• Low porosity
• High wear resistance
• Good carbide retention
• Strong coating adhesion
• Smooth coating structure

The exact properties depend on equipment design, material, particle size, and process parameters.

5.4 Typical Applications

HVAF is well suited for:

• Carbide coatings
• Oil and gas components
• Pump parts
• Valve components
• Hydraulic components
• Industrial rolls
• Corrosion protection
• Erosion-resistant surfaces
• Severe wear applications

It is particularly useful when particle velocity is critical but minimizing overheating is also important.

6. HVOF vs HVAF

HVOF and HVAF are often compared because both use combustion to generate high-velocity particle streams.

The key difference is the oxidizer.

HVOF: oxygen
HVAF: compressed air

This changes combustion temperature and particle heating characteristics.

A simplified comparison is:

Neither process is universally superior.

The correct choice depends on the coating material and required properties.

7. APS / Plasma Spray

What Is APS?

APS stands for Atmospheric Plasma Spray.

It uses an electric arc to ionize a gas and generate a plasma jet with extremely high temperature.

Powder particles are injected into the plasma, where they are heated and accelerated toward the substrate.

Because plasma temperatures can be extremely high, APS can process materials with very high melting points.

This is one of its most important advantages.

8. Plasma Spray Working Principle

Inside the plasma torch, an electric arc forms between an electrode and nozzle.

A process gas such as argon, nitrogen, hydrogen, helium, or a controlled gas mixture flows through the arc region and becomes ionized.

The resulting plasma jet provides intense thermal energy.

Powder is injected into the jet, heated rapidly, and transported toward the workpiece.

After impact, particles solidify and accumulate to create the coating.

9. Plasma Spray Feedstock

APS mainly uses powders.

One of the major advantages of plasma spray is its ability to process materials that are difficult or impossible to deposit efficiently with conventional combustion-based systems.

Typical materials include:

• Alumina
• Zirconia
• Chromium oxide
• Titanium oxide
• Ceramic composites
• Metallic alloys
• Cermets
• Thermal barrier coating materials

This gives plasma spray a particularly important role in ceramic and thermal protection applications.

10. Plasma Spray Coating Characteristics

Depending on material and process parameters, APS coatings can provide:

• High-temperature protection
• Thermal insulation
• Electrical insulation
• Wear resistance
• Corrosion resistance
• Controlled porosity
• Ceramic functionality

Unlike HVOF, where very high particle velocity is often the primary advantage, plasma spray places greater emphasis on its ability to generate extremely high particle temperatures.

11. Typical Plasma Spray Applications

Common applications include:

• Thermal barrier coatings
• Ceramic wear coatings
• Electrical insulation coatings
• High-temperature components
• Industrial rollers
• Pump components
• Energy industry components
• Specialized functional surfaces

APS is particularly valuable when ceramic materials are required.

12. Arc Spray

Working Principle

Arc spray, also called twin-wire arc spray, uses two electrically conductive wires as feedstock.

The wires are continuously fed toward each other.

When they meet, an electrical arc melts the wire tips.

Compressed air or another gas then atomizes the molten metal and propels the droplets toward the substrate.

The particles solidify upon impact and build up into a metallic coating.

13. Arc Spray Feedstock

Unlike HVOF and APS, which primarily use powders, arc spray typically uses wire.

Common wire materials include:

• Zinc
• Aluminum
• Zinc-aluminum alloys
• Stainless steel
• Carbon steel
• Nickel alloys
• Other conductive metals

Because wire can be continuously fed, arc spray can achieve high deposition rates.

14. Arc Spray Coating Characteristics

Arc spray offers:

• High deposition productivity
• Relatively low operating cost
• Efficient large-area coverage
• Low substrate heat input
• Good corrosion protection capability

However, compared with HVOF or HVAF, arc spray coatings can have:

• Higher porosity
• Lower density
• Rougher surface structure
• Lower particle impact velocity

These characteristics are not necessarily disadvantages when the application is large-area corrosion protection.

15. Typical Arc Spray Applications

Arc spray is widely used for:

• Bridges
• Steel structures
• Offshore structures
• Marine components
• Tanks
• Pipes
• Large industrial structures
• Corrosion protection
• Dimensional restoration

For very large surfaces, arc spray can offer excellent economic efficiency.

16. Flame Spray

Working Principle

Flame spray is one of the oldest and simplest thermal spray technologies.

A fuel gas and oxygen generate a flame that heats the feedstock material.

The feedstock may be supplied as powder or wire.

Compressed gas then transports the heated particles toward the substrate.

Compared with HVOF and HVAF, particle velocities are significantly lower.

17. Flame Spray Characteristics

Flame spray is attractive because of:

• Simple equipment
• Low capital cost
• Easy operation
• Wide material availability
• Flexible field use

However, compared with more advanced thermal spray processes, coatings generally have:

• Higher porosity
• Lower density
• Lower bond strength
• Greater oxidation
• Lower wear performance

For demanding industrial applications, HVOF, HVAF, or plasma spray may therefore be preferred.

18. Typical Flame Spray Applications

Flame spray can still be useful for:

• General corrosion protection
• Dimensional restoration
• Low-to-medium duty wear applications
• Maintenance
• Field repair
• Large non-critical components

Its simplicity remains an advantage where the highest coating performance is not required.

19. Thermal Spray Bonding Mechanisms

One of the most important differences between thermal spray and laser cladding is bonding.

In thermal spray, the substrate generally remains solid.

Coating bonding can result from several mechanisms:

Mechanical Interlocking

The substrate is normally roughened before spraying.

Particles impact the rough surface and mechanically anchor to it.

Particle Deformation

High-speed particles flatten or plastically deform upon impact.

This is particularly important in high-velocity spray processes.

Localized Metallurgical Interaction

At certain contact points, localized diffusion or metallurgical interaction may occur.

Chemical and Physical Adhesion

Surface chemistry and interfacial forces can also contribute.

The exact bonding mechanism depends on the process.

This is why proper surface preparation is critical for thermal spray.

20. Surface Preparation Before Thermal Spray

Thermal spray performance depends heavily on substrate preparation.

Typical preparation may include:

• Degreasing
• Cleaning
• Machining
• Grit blasting
• Surface roughening
• Masking
• Preheating where appropriate

Grit blasting is particularly common because it:

• Removes surface contamination
• Increases surface roughness
• Improves mechanical anchoring

Poor surface preparation can lead to coating failure even if the spraying parameters themselves are correct.

21. Typical Thermal Spray Coating Thickness

Coating thickness varies significantly by process.

General ranges may include:

HVOF / HVAF

Often:

Tens of micrometers to several hundred micrometers

and potentially thicker depending on material and application.

Plasma Spray

Often:

Several tens of micrometers to more than 1 mm

depending on the coating system.

Arc Spray

Can achieve:

Hundreds of micrometers to several millimeters

for appropriate applications.

Flame Spray

Can also produce coatings from:

Hundreds of micrometers to millimeter-scale buildup

depending on material and process.

These ranges are indicative rather than fixed limits.

22. Typical Thermal Spray Materials

One of thermal spray’s strongest advantages is material flexibility.

Metallic Materials

Examples include:

• Nickel alloys
• Stainless steel
• Aluminum
• Zinc
• Copper
• Cobalt alloys

Carbides

Common examples include:

• WC-Co
• WC-CoCr
• Cr3C2-NiCr

These are especially important in HVOF and HVAF.

Ceramics

Common plasma spray materials include:

• Al2O3
• ZrO2
• Cr2O3
• TiO2

Composite Materials

Thermal spray can also deposit engineered metal-ceramic and cermet systems.

This broad material capability is one of the reasons thermal spray remains important in modern surface engineering.

23. Thermal Spray Applications

Thermal spray is used across many industries.

Common sectors include:

• Oil and gas
• Steel
• Power generation
• Mining
• Marine
• Paper
• Printing
• Pumps
• Valves
• Hydraulic equipment
• Heavy industry
• Automotive manufacturing
• General machinery

Typical objectives include:

• Wear resistance
• Corrosion resistance
• Erosion resistance
• Thermal insulation
• Oxidation resistance
• Dimensional restoration
• Electrical insulation
• Surface conductivity
• Friction control

24. How to Choose Between HVOF, HVAF, Plasma Spray, Arc Spray and Flame Spray

The correct process depends on what the coating needs to achieve.

Choose HVOF When:

• Dense hard coatings are required
• Carbide materials are used
• Wear resistance is critical
• Relatively low porosity is needed
• Strong coating adhesion is important

Choose HVAF When:

• Carbide retention is especially important
• Lower particle temperature is desirable
• Oxidation must be minimized
• High particle velocity is required

Choose Plasma Spray When:

• Ceramic coatings are required
• Extremely high-melting-point materials are involved
• Thermal barrier coatings are needed
• Electrical insulation is required

Choose Arc Spray When:

• Large surfaces must be coated economically
• Zinc or aluminum corrosion protection is required
• High deposition rate is important
• Wire feedstock is suitable

Choose Flame Spray When:

• Equipment simplicity is important
• Cost must be minimized
• Field operation is required
• Coating performance requirements are moderate

25. Thermal Spray vs Laser Cladding

Thermal spray and laser cladding are often discussed together because both can improve industrial component surfaces.

However, they solve different engineering problems.

The biggest difference is the bonding mechanism.

Thermal Spray

The substrate is generally not melted.

This provides:

• Low thermal influence
• Minimal dilution
• Broad material flexibility
• Excellent suitability for carbides and ceramics
• Large-area coating capability

Laser Cladding

The laser creates a controlled molten pool involving the deposited material and a thin layer of the substrate.

This produces:

• Metallurgical bonding
• Low dilution
• Dense metallic deposits
• Precise dimensional rebuilding
• Multi-layer capability
• Localized repair
• Strong integration with DED additive manufacturing

26. Thermal Spray vs Laser Cladding: Quick Comparison

This comparison should not be interpreted as one technology being better in every category.

27. When Thermal Spray Is Better Than Laser Cladding

Thermal spray can be the better choice when:

• The substrate must not melt
• Carbide coatings are required
• Ceramic coatings are required
• Very low heat input is important
• Large areas must be processed
• Coating thickness is relatively limited
• Dilution must be completely avoided
• Corrosion protection is the main objective

For example, an HVOF carbide coating on a hydraulic component may provide excellent wear performance without the need to create a fusion zone.

28. When Laser Cladding Is Better Than Thermal Spray

Laser cladding becomes especially attractive when:

• Metallurgical bonding is required
• Material loss must be rebuilt
• The component requires dimensional restoration
• Thick metallic buildup is required
• Localized repair is needed
• Complex geometry must be processed
• The component has high replacement value
• Low dilution is important
• DED additive manufacturing capability is valuable

For repair and remanufacturing, these advantages can become decisive.

29. Thermal Spray and Laser Cladding Are Complementary Technologies

Thermal spray and laser cladding should not always be viewed as competitors.

In many industrial environments, they are complementary.

A facility may use:

• HVOF for carbide wear coatings
• Plasma spray for ceramic thermal barriers
• Arc spray for large-area corrosion protection
• Laser cladding for precision repair and dimensional rebuilding

Each technology addresses a different part of the surface engineering spectrum.

This creates a more useful decision framework:

Choose the process according to the failure mechanism, material, geometry, thermal limitations, coating thickness, and lifecycle economics.

30. From Thermal Spray to Integrated Surface Engineering

At GREENSTONE, laser cladding and Directed Energy Deposition remain core technologies for industrial surface engineering, repair, remanufacturing, and metal additive manufacturing.

However, many industrial components do not require a fusion-based process.

Where carbide coatings, ceramic layers, low thermal input, or large-area surface protection are more important, technologies such as HVOF, HVAF, plasma spray, arc spray, or other thermal spray processes may offer technical and economic advantages.

For this reason, surface engineering should not begin with the assumption that every component requires the same process.

The correct approach is to evaluate:

• Substrate material
• Failure mechanism
• Coating material
• Required thickness
• Heat sensitivity
• Bonding requirement
• Surface area
• Component geometry
• Production volume
• Post-processing
• Total lifecycle cost

Where thermal spray provides the better solution, it should be considered.

Where metallurgical bonding, dimensional rebuilding, low dilution, precision repair, or DED capability becomes more important, laser cladding can provide significant advantages.

The objective is not to force a component into a particular technology.

It is to select the right surface engineering process for the right industrial application.

Frequently Asked Questions

What is the difference between HVOF and thermal spray?

HVOF is one specific thermal spray process. Thermal spray is the broader category that also includes HVAF, plasma spray, arc spray, flame spray, and other deposition technologies.

What is the difference between HVOF and HVAF?

HVOF uses oxygen as the primary oxidizer, while HVAF uses compressed air. HVAF generally operates at lower combustion temperatures while maintaining very high particle velocity, which can reduce oxidation and preserve carbide phases.

What is the difference between plasma spray and HVOF?

Plasma spray provides extremely high particle temperatures and is especially suitable for ceramics and high-melting-point materials. HVOF emphasizes high particle velocity and is especially effective for dense carbide and metallic wear-resistant coatings.

Which thermal spray process is best for tungsten carbide coatings?

HVOF and HVAF are widely used for tungsten carbide-based coatings. The best process depends on the specific carbide composition, required hardness, oxidation limits, coating density, and operating environment.

Can thermal spray repair worn components?

Yes. Thermal spray can be used for dimensional restoration in suitable applications. However, when substantial material buildup, metallurgical bonding, or complex precision repair is required, laser cladding may provide advantages.

Does thermal spray melt the base material?

Generally no. Thermal spray is designed to deposit heated particles onto the substrate without intentionally melting the base material.

Is thermal spray better than laser cladding?

Neither technology is universally better. Thermal spray is often preferred for carbide, ceramic, low-heat-input, and large-area coatings. Laser cladding is particularly strong for metallurgical bonding, dimensional rebuilding, precision repair, and DED-based manufacturing.