What Is Thermal Spray and Its Different Forms

The phrase "thermal spray" refers to a variety of coating methods used to apply metallic or nonmetallic coatings. 

We classify these procedures into flame spray, plasma arc spray, and electric arc spray. These energy sources melt or semi-melt the coating material (in powder, rod, or wire form).

Process gases or atomization jets accelerate and drive the heated particles toward a prepared surface. We form a link with the surface upon impact, and other particles cause thickness growth and the formation of a lamellar structure.

What is Thermal Spray?

Thermal spraying is a low-cost way of applying coatings to improve surface characteristics. 

In its most basic form, it entails heating a feedstock material. Then, to deposit the feedstock, a stream of gas or compressed air is used to drive it, resulting in a surface structure on a particular substrate.

The coating feedstock substance may be a single metallic element. 

Still, it is more typically an alloy, ceramic, composite, carbide, or mix that produces a surface with physical qualities that we can only obtain by the thermal spray method.

Thermal spray is a process family, with each member offering distinct options for designed surfaces.

Because of nonuniform heating and unexpected drag forces that shear molten material from the parent wire or rod, the utilization of wire and rod feedstock materials results in particle size dispersion.

Consequently, these coating flaws' severity varies according to the thermal spray technique employed, the operating parameters used, and the substance sprayed.

Types of Thermal Spray and Corresponding Feedstock

There are the following types of thermal spray are used metallic and nonmetallic coatings.

Atmospheric Plasma Spray

Industries use this procedure to defend against wear and corrosion and for thermal insulation, repair, restoration, and reinforcing processes like surface plasma treatment

In addition, they may deposit coatings at high deposition rates onto diverse metallic substrates utilizing the widest variety of powder feedstock materials made of metals, alloys, carbides, ceramics, and others since it is the most versatile thermal spray technique.

thermal spray
Atmospheric Plasma Spray

As a result, atmospheric plasma spray becomes a staple process in depositing coatings for thermal protection, corrosion control, wear resistance,  dimensional control and restoration, and various other applications.

Between a positive electrode pole (anode) and an oppositely charged pole (cathode), a powerful electric arc is formed (cathode). 

It converts the flowing process gases into plasma. Next, the powder feedstock material is sent into the plasma jet, where it melts and propels the powder particles onto the workpiece surface.

Process Fundamentals:

  1. Arc as a heat source
  2. Feedstock: powder (ceramics, alloys, metals, blends, carbides, and others).
  3. Plasma temperature: about 16000°C (28 800°F).
  4. Particle velocity can reach 450 m/s (1500 ft/s).
  5. 4 to 8 kg/h (9 to 18 lb/h) is an approximate application rate.

High-Velocity Oxygen Fuel Spray

Surfaces resistant to wear and corrosion are required to protect important components in various industries. 

We provide this protection through the High-Velocity Oxygen Fuel (HVOF) process, which produces highly thick, hard coatings with tiny microstructures.

High-Velocity Oxygen Fuel (HVOF) spraying creates a flammable mixture by combining oxygen and fuel. 

The fuel for liquid-fuel HVOF (HVOF-LF) is typically liquid kerosene. HVOF-GF (gas-fuel) uses fuel gases such as propylene, hydrogen, propane, or natural gas (methane). 

Within the cannon, we thoroughly mix the fuel with oxygen before being combusted and discharged at supersonic speeds through a nozzle.

We feed the powdered feedstock material via the cannon, which uses nitrogen as a carrier gas. As the powder material exits the gun and we blast into the workpiece surface, the ignited gases envelop and uniformly heat it.

Process Fundamentals:

  1. Source of heat: combustion
  2. The powder is a feedstock (metals, alloys, carbides).
  3. Flame temperature: about 2 800 °C (5 000 °F).
  4. Particle velocity ranges from 400 to 800 m/s (1300 to 2600 ft/s).
  5. The application rate is 40 to 200 g/min (5 to 26 lb/h).

Spraying Using Electric Arc Wire

Electric arc wire spray is famous for on-site coating work because of its mobility and flexibility; nevertheless, it is also renowned for in-shop work. 

We frequently use it to cover welded tube seams, capacitors, piston rings, varistors, boiler walls, and massive infrastructures.

Melting metallic alloys and pure metal wires using electricity and compressed air allows for high spray and application rates.

Process Fundamentals:

  1. Arc as a heat source.
  2. Solid or composite metallic wires as feedstock.
  3. The temperature of the arc: is about 4000°C (7200°F).
  4. Particle velocity: approximately 150 m/s (500 ft/s).
  5. Application rate: 15 to 3,300 g/min (2 to 440 lb/h).

Spraying with Combustion Wire

Manufacturers frequently use combustion wire spray to provide anti-corrosion coatings and to restore surface dimensions. 

Metal-based wires are used in-shop or on-site to cover immense steel structures such as chemical/petrochemical tanks and pipelines and for marine applications. 

Because of the low equipment requirements, it is a popular choice for anything from on-site work to high-volume in-shop manufacturing.

The feedstock substance, in wire form, is continuously fed into a fuel gas-oxygen flame, where the heat of combustion melts it. Propylene, propane, and acetylene are the most common fuel gases. 

Next, operators surround the flame with compressed air, which atomizes the molten tip of the wire.

It causes the molten particles to move faster towards the prepared surface of the workpiece. 

Combustion wire spray is popular for machine element maintenance and general corrosion-resistant coatings.

Process Fundamentals:

  1. Source of heat: combustion.
  2. Wire as a feedstock (metal).
  3. Flame temperature: about 3100°C (5600°F).
  4. Particle velocity can reach 200 m/s (650 ft/s).
  5. The application rate is 15 to 1400 g/min (2 to 185 lb/h).

Combustion Powder Spray

Combustion powder spray is appropriate for machine component maintenance, repair, and restoration, and abradable coatings utilized in clearance control applications. It is a relatively adaptable method that can apply alloys,  metals, carbides, polymers, and even certain ceramic powders.

Workers continuously feed the powder feedstock material into a fuel gas-oxygen flame, where the heat of combustion melts it. In most combustion powder spray technologies, the powder is supplied through a feeder and transported to the spray cannon through a carrier gas. The mixed vapors accelerate the prepared surface of the workpiece by the melted particles, where they quickly solidify to produce a covering.

Process Fundamentals:

  1. Source of heat: combustion.
  2. Feedstock material (metals, metallic blends or ceramics, alloys).
  3. Maximum flame temperature: 3100°C (5600°F).
  4. Particle velocity can reach 50 m/s (165 ft/s).
  5. Application rate: 35 to 150 g/min (4.5 to 20 lb/h).

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