Thermal Spray processes
In plasma powder spraying, the powder inside the plasma gun is melted by a plasma jet and accelerated toward the part being coated. The plasma is generated by an electric arc burning in a gaseous environment. This dissociation and ionization of the gases occurs, resulting in high velocity at the exit. The electric arc burns between a central cathode and a water-cooled anode. This method can be used in normal atmospheres, in a protective gas (e.g., argon), in a vacuum, and underwater. The main advantage is the high energy concentration in the plasma jet (plasma jet temperatures up to 30,000°C), which allows for the application of coatings of any chemical composition, including refractory alloys and ceramics.
Applications: aerospace industry (e.g., turbine blades, inlet surfaces), medicine (implants), thermal barrier coating.
Process Characteristics:
- Thermal Energy: Up to 30,000°C
- Kinetic Energy: Up to 450 m/sec
- Productivity: 4-8 kg/hour
- Ability to apply coatings of any chemical composition, including refractory alloys and ceramics
High-velocity oxygen spraying (HVOF) involves continuous combustion of gas at high pressure inside a combustion chamber, into which the powdered spray material is fed. A mixture of combustible gas or kerosene with oxygen creates high pressure in the combustion chamber and ensures the required high gas flow velocity in a profiled nozzle. This accelerates the sprayed particles to high speeds, resulting in extremely dense, dimensionally accurate coatings with excellent adhesion. The coating structure also contains significantly fewer oxidized particles. Application: This method is most commonly used for spraying tungsten carbide, various sliding surfaces in steam generators, various rollers, and petrochemical and chemical equipment components, such as pumps, gate valves, ball valves, mechanical seals, etc.
Process Characteristics:
- Thermal Energy: Up to 3160 °C
- Kinetic Energy: Up to 550 m/sec
- Productivity: 2-8 kg/hour
- High coating density and adhesion
In flame spraying, the sprayed material is continuously fed with wires or rods into the center of an oxy-fuel flame, where it melts. Using an atomizing (dispersing) gas, such as compressed air or nitrogen, the molten droplets are blown out of the melting zone and accelerated toward the prepared part. Flame spraying is one of the most common methods, producing very high-quality coatings. It is inexpensive to implement and operate, widely used for restoring the geometry of parts and tools, and for protecting large tools and metalworking facilities from corrosion.
Process Characteristics:
- Thermal Energy: Up to 3160°C
- Kinetic Energy: Up to 200 m/sec
- Capacity: 6-8 kg/hour
- The most common and affordable method
In arc spraying, the coating is formed from droplets of liquid metal moving in a stream of carrier gas. Heating and melting of the sprayed material occurs due to the heat of an electric arc burning between consumable wires—the electrodes—that form the sprayed material. The liquid metal is blown from the electrode ends, fragmented by gas-dynamic and electromagnetic forces, and moves in the form of droplets toward the sprayed surface. Arc spraying is a high-performance spraying process, but is only suitable for spraying electrically conductive materials. Applications are extremely wide, including tank coating, corrosion protection of metal structures, restoration of worn machine parts, etc.
Process Characteristics:
- Thermal Energy: Up to 4000°C
- Kinetic Energy: Up to 150 m/sec
- Capacity: 2–20 kg/hour
- Very high productivity
- Requires the use of only electrically conductive materials
