Coatings are Produced by Thermal Spraying

Compared to many other material processes, coatings produced by thermal spraying generally contain many defects.
(i) The real contact area between the splat and the substrate or the previously deposited layers determines the coating properties. Inter-lamellar pores exist between layered splats, or first splats and substrate; their thickness is between a few hundredths to a few tenths of micrometers. The real contact between splats increases from about 20 to 60 % with particle impact velocities, provided that particles are not either too much superheated or below their melting temperature.

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(ii) Splashing of the melted particles during flattening upon impact can significantly affect the coating properties. Splats deposited on splashed material exhibit lower adhesion and this effect is more significant when spraying metals because the splashed material is oxidized rather fast due to the small droplet sizes.
(iii) Substrate geometry may affect the flow of impacting and splashing particles
(iv) Pores, often called globular, are formed during coating generation because of shadowing effect, narrow holes in valleys between splats not completely filled, un-molten or partially melted particles and exploded particles. These globular pores are distributed more or less homogeneously through the coating and their potential to deteriorate coating properties is proportional to their size. Therefore, to reduce the coating porosity, the incorporation in the coating of large unmelted particles, which are sufficiently heated to stick on the substrate, or of partially melted particles, must be avoided. This can be done by choosing carefully the particle size distribution and optimizing the particle injection. However, sometimes a compromise has to be found between having most particles fully melted but with rather high oxide content, and more un-melted particles with lower oxide content.

(v) Coatings also contain cracks formed during residual stress relaxation and that often contribute to the open porosity of coatings. The micro-cracks within splats result from quenching stress relaxation and are observed in ceramic materials. The macro-cracks are often due to the relaxation of an expansion mismatch stress; they run through layered splats especially at their interfaces and, so, tend to initiate inter-connected porosities. Other stress relaxations can occur but they can be avoided by optimizing spray or service conditions.
(vi) At last, another source of defects is the particle impact angle that reduces the normal impact velocity, resulting in elongated splats. Spraying with an angle above a certain value, depending on particle and substrate materials, will promote splashing even on substrates preheated above the so called “transition temperature”. Above this temperature, the splats exhibit a regular disk shape on a smooth substrate while below they have an irregular shape. Typically, depending on the spray process used and the material sprayed total porosity varies between 0.5 and 15 %. It does not mean necessarily that porosities are inter-connected, but it can happen.

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