Thermal spraying consists in metling a consumable (most often powder or wire) and project it as molten particles onto the substrate. Upon impact with the substrate, the molten particle flatten and solidify very rapidly. The adhesion is primarily mechanical (for this reason, sprayed MCrAlY coatings are often given a diffusion heat-treatment to obtain good adherence). A sprayed coating will typically include voids (i.e. have some percentage of porosity), and oxide particles. The amount of both depends on the method employed.

All thermal spraying processes are line-of-sight processes, that is to say, only the parts of the component which are directly in the line of the spray are coated.

Schematic microstructure of thermal spray coating, showing only a few layers of particles.

The adhesion of the coating depends on the cleanliness of the substrate surface, and its area (a high surface roughness is desirable for good adhesion), on the velocity of the particles, etc.

There are various types of thermal spraying methods, some of which are briefly introduced:

• Flame spraying: the first method employed uses an oxyacetylene flame (about 2700 \degc) to melt and project the coating, fed as wire, rod or powder. Typical particle velocity is about 40 m/s, porosity 10-15 %, identical oxide content, for a deposition rate of 1 to 10 kg/h.
• Plasma spraying: similar but uses a ionised gas plasma to melt and propel the coating (powder), temperature of the plasma can exceed 16,000 \degc, while the substrate itself will seldom heat beyond 150 \degc. Particle velocity can reach 200-300 m/s, the porosity is reduced to 5-10 %, the oxide content to 1-3 % for deposition rates of 1-5 kg/h.
• High velocity oxyfuel(HVOF): spraying uses oxygen and hydrogen with a fuel gas such as methane, particle velocity ranges between 600-1000 m/s, oxide and porosity are reduced to 1-2 % for a deposition rate similar to that of plasma spray.

A couple of interesting variants of the conventional plasma spraying are known as low pressure plasma spraying (LPPS) and vacuum plasma spray (VPS), which, as indicated by their names, are identical to plasma spraying except for it being done in inert gas at low pressure, or in vacuum. This reduces slowing and cooling the particles by air, and avoids oxidation during spraying. The traditional method is usually referred to as APS (air plasma spray).

Schematic illustration plasma torch.

APS and LPPS are widely used to apply MCrAlY coatings. TBC can be applied by this method, but it has been shown that the use of EBPVD increases by 7 to 13 times the life of the coating, therefore easily compensating for the higher cost of the latter process. This has been discussed here.

Bibliography

A good part of the above informations comes from:

1. TWI: flame spraying, HVOF spraying, arc spraying, plasma spraying.
2. There is also a good introduction to plasma spraying here.
3. and some information here.