Supersonic flame spraying, also referred to as high velocity air-fuel (HVAF) or high velocity flame spraying, is a modern thermal spray process that represents an advancement over conventional flame spraying. The technique makes use of a fuel gas (commonly propane, propylene, or kerosene) mixed with oxygen or compressed air, ignited to produce a combustion jet that reaches supersonic velocity. Powdered coating material is injected into this jet and accelerated toward the substrate, where it forms a dense, tightly bonded coating layer.

Compared with traditional flame spraying and even some other thermal spray techniques, supersonic flame spraying delivers distinct performance improvements. Below are the detailed advantages:

1. High Particle Velocity and Kinetic Energy

One of the most significant advantages of supersonic flame spraying is the extremely high particle velocity it achieves. Coating particles are propelled at speeds exceeding 1000 meters per second, significantly higher than those in conventional flame spraying. The high kinetic energy of these particles results in strong deformation upon impact, which directly contributes to:

Better adhesion strength, both mechanical and metallurgical.
Improved cohesion between deposited layers, leading to dense and uniform coatings.
Enhanced wear resistance, as the tightly packed structure is more durable under stress.

This high particle velocity is the key factor that distinguishes supersonic flame spraying from older methods.

2. Dense, Low-Porosity Coatings

Porosity is a critical factor in evaluating coating quality. Traditional flame spraying often produces coatings with relatively high porosity levels, making them less effective against corrosion or fluid penetration. Supersonic flame spraying, however, produces coatings with porosity levels often below 1%.

This density translates to:

Superior corrosion resistance, since corrosive agents cannot easily penetrate.
Better barrier properties against gases and liquids.
Increased coating life span, as there are fewer weak points for damage initiation.

3. Reduced Oxidation and Material Degradation

Because the process relies more on kinetic energy than on excessively high flame temperature, the exposure time of particles to combustion heat is shorter. This means less thermal damage and oxidation of the sprayed material.

For heat-sensitive coating powders, such as tungsten carbide (WC-Co) or chromium carbide, this is a major advantage. Carbides maintain their hardness and phase composition better when sprayed by supersonic flame processes, resulting in coatings that retain their original material properties.

4. Strong Adhesion Strength

The impact of particles traveling at supersonic speeds leads to excellent bonding with the substrate surface. Unlike traditional flame spraying, where coatings may sometimes peel or spall under stress, supersonic flame spraying achieves:

High bond strength, often exceeding 80 MPa.
Improved resistance to delamination under mechanical loads.
Reliable performance even in demanding industrial environments, such as turbines, pumps, or cutting tools.

5. High Deposition Efficiency

Another important advantage is the efficient use of material. A greater proportion of sprayed powder adheres to the surface rather than being lost in overspray. Deposition efficiency in supersonic flame spraying often exceeds 70%, which is significantly higher than conventional methods.

This translates to:

Lower operating costs, as less powder is wasted.
Faster build-up rates, enabling quicker coating of large surfaces.
Reduced overall project time, improving productivity in both manufacturing and repair operations.

6. Superior Surface Properties

The microstructure of coatings applied with supersonic flame spraying is markedly different from those created by slower particle deposition. These coatings typically exhibit:

High hardness due to dense packing and minimal oxidation.
Superior wear resistance against abrasion, sliding contact, and particle erosion.
Good impact resistance, since the strong adhesion prevents chipping or flaking.

For industries that require parts to endure friction, impact, or erosive particles—such as mining, oil and gas, and aerospace—these qualities are particularly valuable.

7. Wide Range of Applicable Materials

Supersonic flame spraying is highly versatile in terms of the coating materials it can apply. Options include:

Metals: stainless steel, nickel alloys, aluminum, copper.
Carbides: tungsten carbide-cobalt, chromium carbide-nickel chrome.
Ceramics and composites: for applications requiring extreme wear resistance.

The ability to apply such a broad range of materials allows engineers to customize coatings for very specific performance requirements, from wear protection to thermal insulation.

8. Better Surface Finish and Less Post-Processing

Coatings produced by supersonic flame spraying are smoother than those from conventional flame spray techniques. This improved finish often reduces or even eliminates the need for extensive grinding, machining, or polishing after coating.

Benefits include:

Time savings during finishing operations.
Lower overall cost in producing usable parts.
Improved dimensional accuracy, especially important in aerospace or precision-engineered components.

9. On-Site Application and Repair Flexibility

Supersonic flame spraying, like other thermal spray methods, is a cold process relative to welding. It does not involve melting the substrate, which means there is minimal risk of thermal distortion. Additionally, it can be applied on-site without removing the component.

This flexibility is highly valuable for:

Large equipment and structures that cannot easily be transported to a workshop.
Maintenance and repair operations, reducing downtime.
Quick restoration of worn surfaces, such as pump shafts, turbine blades, or rollers.

10. Environmental and Safety Advantages

Compared to plasma spraying or traditional flame spraying, supersonic flame spraying often uses air as the oxidizer instead of pure oxygen, and fuel gases that are safer and less costly. The process also has:

Lower flame temperatures, reducing harmful thermal effects.
Reduced fume and dust production, improving workplace safety.
Environmentally friendly operation, with lower gas consumption and higher efficiency.

Summary

Supersonic flame spraying is a significant improvement over conventional flame spray processes, combining efficiency, performance, and flexibility. The primary advantages include:

Extremely high particle velocity leading to dense, durable coatings.
Low porosity and reduced oxidation, improving coating longevity.
Superior adhesion strength and surface finish.
High deposition efficiency, reducing material and labor costs.
The ability to coat a wide range of materials for diverse applications.
Suitability for on-site repair and restoration without major disassembly.

Because of these qualities, supersonic flame spraying is now widely adopted in industries such as aerospace, power generation, oil and gas, automotive, and heavy machinery, where the durability of coatings directly impacts operational efficiency and cost savings.