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THERMAL SPRAY

All thermal spraying processes rely on the same principle of heating a feed stock, (Powder or Wire) and accelerating it to a high velocity and then allowing the particles to strike the substrate. The particles will then deform and freeze to the substrate. The coating is formed when millions of particles are deposited on top of each other. These particles are bonded to the substrate by either mechanical or metallurgical bonding.

HVOF

Coatings sprayed by HVOF have superior characteristics in comparison with other thermal spray processes. In particular they show a high density, high bond strength, a great wear resistance (for carbides), an excellent corrosion resistance (for metal coatings) and high thicknesses. Flame Spray uses a liquid fuel High Pressure HVOF to spray almost oxide free metal coatings and carbides with excellent wear-resistance behaviour.

ID-HVOF

The application of wear resistant coating in internal diameter (ID) has always been a big challenge for thermal spray techniques. Being Line of Sight technologies, they could coat surfaces that the torch can “see”. In the past, only APS and Flame wire torches could be mounted on extenders to enter small ID. Flame Spray is using the latest technology in miniaturize HVOF and can coat a diameter as small as 70 mm. With this ID-HVOF the application of very high quality carbides is no longer an issue.

APS

Air Plasma Spray (APS) is the most flexible of all of the thermal spray processes as it has enough energy to melt or plasticize any material from polymers to ceramics. As feedstock it uses powder that permits the application of any kind of material. A mixture of gases (normally Argon or Nitrogen and Hydrogen) is ionized to become a plasma plume with a temperature of 6,600 °C to 16,600 °C. When the coating powder is injected into the plume, it is melted and accelerated towards the substrate. Even if it could spray almost all types of materials, APS is commonly used for spraying ceramics, abradable coatings and metals with high melting points (like molybdenum).

Flame wire

This is a useful process for applying relatively inexpensive coatings that typically contain high levels of oxides and porosity together with the option of achieving a rough surface finish. The process relies on the chemical reaction between oxygen and a fuel of combustion to produce a heat source. This heat source creates a gas stream with a temperature in excess of 3,000°C with correctly balanced conditions between oxygen and acetylene. Among others, the process is typically used for applying bond coat materials or materials for corrosion resistance applications. Due to the small dimensions of the equipment used, this technique is very useful for on-site applications (boiler, infrastructures, etc.).

Spray & fuse

The spray and fuse process consists of two steps. First, oxygen and a fuel gas, at relatively low pressures, are combined to create the flame, as in the wire-flame spray technique. The coating material (in powder form) is fed into the flame where it is melted, and a flow of compressed air atomizes the material and brings it to the target. During the second step, the coating applied in the first step is fused to the substrate by torch or vacuum furnace, which produces a metallurgical bond. The spray and fuse process results in an absence of interconnected pores in the coating and very high bond strengths. The typical chemical composition of spray and fuse coatings is an alloy of NiCrBSi , with various degrees of blending of the above elements. These coating are very useful against wear problems in corrosion environments.

Arc spray

An electric arc is used to provide the heat source by utilising two current carrying wires. As the wires are fed towards each other the electric current short circuits between the wires creating a temperature of around 4,000°C. This temperature causes the tips of the wire to melt and once molten, compressed air or inert gas is used to atomise and accelerate the feed metal towards the substrate. One of the advantages of this system is that two different wires can be used simultaneously to produce a pseudo alloy. Cored wires are also available producing coatings with unique properties.

Cold Spray

Cold Spray is a high-rate coating process that utilizes kinetic rather than thermal energy to build-up layers of metallic materials. During the process, the powder feedstock remains well below its melting point inside supersonic nitrogen flux and are accelerated to very high velocities (200 m/s to 1000 m/s). The bonding between coating and substrate is achieved by plastic deformation through adiabatic shear instabilities of the powder particles that hit the surface at very high speed. Cold Spray is able to produce high thickness metal coatings with high density and adhesion without residual stresses and oxide content. Cold Spray technology is extremely suitable for critical dimensional restoration (up to 1 cm on radius).

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