Thermal spraying technology

Thermal spraying, also known as spray metallization, is used to apply metal coatings. The technology has been known for more than a century and is constantly evolving both in terms of equipment design and the materials applied. Thermal spraying technologies are used in the reconditioning of machine parts, in surface refinement, as well as in corrosion protection of steel structures.

Thermal spraying technology involves the application of layers of metallic, ceramic, cermetallic or plastic materials, which are in a partially or completely molten state due to the action of a heat source, in order to obtain a strongly adherent coating with special required operating properties. Thermal spraying techniques are characterized by the fact that materials are applied to metallic and non-metallic substrates. The spraying process does not overmelt the substrate material, and the sprayed coating is bonded to the substrate adhesively, and in some cases diffusively.

The heat source used to bring the sprayed materials into a liquid state is a gas flame of oxygen-burning acetylene or propane, an electric arc or a plasma arc, which depends on the physical properties of the sprayed materials.

Thermal spraying methods

Wire flame spraying

Material in the form of a wire, with the help of transport rollers, is delivered to the place of action of the acytelene-oxygen flame. As a result of the flame’s heat, the material is brought to a plastic state, and compressed air fed into the gun causes material particles to be stripped from the wire and coating material to be applied to the previously prepared surface.

The wire thermal spraying method has found application in the application of anti-corrosion coatings, as well as utility coatings, in the restoration of dimensional surfaces. The flame spraying method has found application in spraying steel, bronze, zinc, copper, molybdenum, among others.

Flame spraying with powder

Material in powder form, with the help of transport rollers, is delivered to the place of action of the acytelene-oxygen flame. As a result of the heat of the flame, the material is brought to a plastic state, and compressed air fed to the gun causes the coating material to be applied to the previously prepared surface.

The flame thermal spraying method has found application in the application of anti-corrosion coatings, as well as utility coatings, in the restoration of dimensional surfaces.
The flame spraying method has found application in spraying steel, bronze, zinc, copper, molybdenum, ceramic coatings, among others.

Arc spraying

During the arc spraying process, the coating material is in the form of wires. From two spools, the wire is fed at a set speed into an arc gun. An arc is ignited at the point where the wires meet, which as a heat source causes the material to melt. Compressed air fed into the gun imparts acceleration to the molten material particles and applies them to the previously prepared surface, forming a coating

The arc thermal spraying method is the most efficient method of all thermal spraying methods, and thanks to this it has found application in the performance of anti-erosion protection (sheet piling walls of power boilers), anti-corrosion Zn, Al, e.g. steel structures. In addition, the arc method makes it possible to produce coatings of steel, bronze, copper used in the regeneration of machine parts.

HVAF supersonic thermal spraying

Depending on the requirements, supersonic thermal spraying (HVAF) systems can use propylene, propane, hydrogen or natural gas as fuel. As a result of the high kinetic energy transferred to the particles in HVAF processes, the coating material generally does not need to be completely melted. Instead, the powder particles are in a molten state and flatten plastically when they strike the surface of the workpiece. The resulting coating has a very homogeneous and fine grain structure. HVAF coatings are similar and generally comparable to coatings produced by HVOF. HVAF is a “warm spray” process, which is cooler than HVOF. HVAF guns use axial injection of powder into an air-fuel nozzle at a temperature of about 1900-1950 ° C. As a result, the process is able to effectively apply carbide-based materials, and because the air-fuel jet produces far fewer oxides than high-temperature oxy-fuel nozzles, the HVAF process can also apply metals with almost zero oxidation, similar to Cold Spray. All common thermal spray powders can be applied using HVAF, except ceramics.

Properties of thermal sprayed coatings

Typical properties of thermal sprayed coatings depending on the materials used and their operating conditions include:

• Corrosion resistance – e.g.: zinc , aluminum, acid-resistant steel,
• Heat resistance – e.g.: aluminum oxides and titanium oxides,
• Tribological properties – e.g.: bearing alloy (babbit), bronze,
• Abrasion resistance – e.g.: tungsten carbides, chromium carbides, chromium oxide,
• Thermal and electrical conductivity – e.g.: copper,
• Thermal and electrical insulation – e.g.: polyamide, polyester
• Good machinability – e.g.: low carbon steels, low alloy steels
• Decorative properties – e.g.: zinc, bronze, brass.