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The Process

Thermal spraying, also commonly known as metal spraying is a surface engineering / coating process where a wide range of metals and ceramics can be sprayed onto the surface of another material.

Thermal spraying is widely used to provide corrosion protection to ferrous metals or to change the surface properties of the sprayed items, such as improve the wear resistance or thermal conductivity.

The range of thermal spray applications is vast and a selection of them are summarised in our solutions section.  If you can’t find your exact application, it's possible that we can still help you.  We'd be pleased to discuss your specific requirement with you so please contact us or one of our resellers who will be pleased to help.

The Process Basics

All methods of thermal spraying involve the projection of small molten or softened particles onto a prepared surface where they adhere and form a continuous coating. To create the molten particles, a heat source, a spray material and an atomisation/projection method are required. Upon contact, the particles flatten onto the surface, freeze and mechanically bond, firstly onto the roughened substrate and then onto each other as the coating thickness is increased.

The metal spray equipment falls into four main categories, Flame SprayArc SprayPlasma Spray and High Velocity Oxygen Fuel (HVOF).  A fifth, more recent process is Laser Cladding.  More detailed information on each process can be viewed below.

Flame Spray Process

Heat Source

Gas fuel* and oxygen flame (*commonly propane or acetylene).

Material

Wire or powder or ceramic rods

Transfer

Compressed air (wire flame)

Process

The gas fuel and oxygen are mixed and ignited to produce a flame. The material, either a wire or powder is fed into the flame. For wire flame spray, the material is melted and the compressed air, passing through a spray nozzle atomises the molten metal and sprays it onto the work piece. The larger the wire diameter, the higher the spray rate. For powder flame spray, the powder particles (metal or ceramic ) are softened in the flame and the speed of the flame gases through the nozzle sprays the softened powder onto the work piece.

Arc Spray Process

Heat Source

Electric Arc

Material

Wire

Transfer

Compressed air

Process

Two wires (hence a common term for the process is Twin Wire Arc Spray), are fed into the pistol and electrically charged, one positive and one negative. The wires are forced together and form an electric arc, melting the wire. Compressed air, passing through a nozzle, atomises the molten metal and sprays it onto the work piece. There are three methods of wire feeding, push, pull and push/pull. The higher the current rating of the system, e.g. 350A, 700A etc., the higher the spray rate.

Plasma Spray Process

Heat Source

Plasma arc

Material

Powder (ceramic, metal, plastics)

Transfer

Via plasma jet

Process

Plasma is the term used to describe gas which has been raised to such a high temperature that it ionises and becomes electrically conductive. In the case of Plasma spraying, the plasma is created by striking an electric arc between the nozzle and the electrode inside the plasma gun. The plasma jet then emerges from the nozzle. Powder particles are injected into this jet where they soften and then strike the surface at high velocity to produce a strongly adherent coating. The work piece remains cool because the plasma is localised at the gun.

HVOF Spray Process

Heat Source

Fuel (liquid or gas) and oxygen flame

Material

Powder (metal)

Transfer

Via the flame

Process

The fuel (liquid kerosene in our system) is mixed with oxygen and ignited. The combustion gases pass through a converging/diverging nozzle and accelerate to around 1,500m/sec. The powder is injected into the accelerated flame where it softens and gathers speed. The high impact speed of the particles produce a highly adherent, dense coating structure.

Laser Spray Process

Heat Source

Fuel (liquid or gas) and oxygen flame

Material

Powder (metal)

Transfer

Via the flame

Process

The fuel (liquid kerosene in our system) is mixed with oxygen and ignited. The combustion gases pass through a converging/diverging nozzle and accelerate to around 1,500m/sec. The powder is injected into the accelerated flame where it softens and gathers speed. The high impact speed of the particles produce a highly adherent, dense coating structure.

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