Thermal spray

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Thermal Spray techniques are coating processes which involve spraying melted (or heated) materials onto a surface. As such it is a line of sight process. The energy to heat the feedstock is supplied by electrical (plasma or arc), or chemical means (combustion flame). Coating thicknesses range between approximately 20 micrometers and several millimetres depending on the process and feedstock.

Coating quality is usually assessed by measuring its porosity, oxide content, macro and microhardness, bond strength and surface roughness. Generally the coating quality increases with increasing particle velocities.

The materials to be deposited as the coating are typically fed into the spray gun in powder or wire form where they may be atomized before being accelerated towards the substrate ^[1]. "As the sprayed particles impinge upon the surface, they cool and build up, splat by splat, into a laminar structure forming the thermal spray coating."^[2]

Spray coatings may either be applied manually or by machine, depending on complexity, cost, and todays environmental^[3] and safety concerns.

Some common reasons for spray coating are:^[4]

• corrosion protection
• fouling protection
• increase conductivity
• increase surface hardness to reduce wear
• to repair damaged surfaces - rebuild
• high temperature protection (thermal barrier coatings)
• medical implants

Some common types of thermal spray are:

• Flame Spraying (with powder and wires)

• High Velocity Oxygen-Fuel (HVOF)
• Detonation gun (D-Gun)

• Arc Spraying with wires
• Plasma Spraying with powders under atmospheric and vacuum or low pressure conditions

• DC-arc plasma spray
• RF induction plasma spray
• Wire-arc spray
• Plasma transferred arc spray

[edit] References

1. ^ Sulzer Metco website describing the processes and applications[1]
2. ^ Army manual describing spray coating - http://www.usace.army.mil/usace-docs/eng-manuals/em1110-2-3401/c-2.pdf - Accessed 20 June 2006.
3. ^ Article discusses briefly several environmental concerns and provides links at the bottom to industrial polution papers - http://www.p2pays.org/ref/03/02454/altmeth.htm - Accessed 20 June 2006.
4. ^ Army manual