Thermal spray processes involve delivery of material usually within the size range of 5 to 200 microns at a high speed towards a surface. Historically this material took the form of liquid droplets that were heated by a flame and transported within a flame towards a surface where the splashing phenomena and subsequent solidification were able to create a holding strength for the deposited droplets and the resulting coating. Today, the size of material transported has reached the bottom limit of the particle size range and may not necessarily be in a molten state. Furthermore, the form of material fed into a thermal spray source may not necessarily only contain powder.
Initially thermal spray processes were classified according to the type of heat source. For example, processes were classed as flame, plasma or HVOF spraying. Modifications to the heat source environment further classed these materials into the precise category by which these processes are known today.
- Flame spraying
- High velocity oxy fuel (HVOF) spraying
- Arc spraying
- Plasma spraying
Materials that can be plastically formed either in the solid or liquid state can be deposited. Where heating is involved, only those materials that remain stable upon heating can be sprayed. Instability may refer to oxidation or decomposition of the material. These materials may, however, be deposited in the form of composites where a secondary material is used to protect the unstable or reactive material. Spraying into a special atmosphere, use of a metallic or polymeric binder to form a composite, or encapsulation of these powder are means of protecting the thermally sensitive materials.
Thermal spray feedstocks can take several forms which are suited to various materials such as:
- Powder – plastic, metal, composite, ceramic
- Wire – metal, composite
- Rods – ceramic
Thermal spraying is used to produce flattened particles, spherical particles (either hollow or dense) and coatings. The most common application is the production of coatings. Coatings can be deposited to such dimensions that freeforms can be produced
- Wear and abrasion resistance coatings
- Thermal barrier coatings
- Corrosion Resistant (Anticorrosion) coatings
- Reclamation of worn components
- Art – glass colouring, bronze application
- Electronic applications
- Able to deposit high melting temperature materials
- Fast coating deposition
- No volatile organics are employed as is the case with many paints
- Fast heating and cooling produced in equilibrium phases and may avoid decomposition of certain materials.
A molten particle or a particle able to deform plastically is transported at high speeds within a heat source towards a surface upon which deposition occurs. The droplet or particle undergoes spreading and may create a chemical bond with the underlying surface. With materials that are not able to produce a chemical bond, the substrate is roughened to create a mechanical bond. Each droplet or particle impacts a roughened surface and mechanically interlocks with the asperities on the underlying surface.
The residual stress remaining within the deposited particles mostly influences ceramic coatings. The cooling of such materials needs to be optimized to avoid excessive residual stress levels.
The coatings thickness is dictated by the size of the feedstock for powders, the size of the droplets for arc spraying or the size of the atomized droplets created by the liquid spray process. Typically, flattening of the material by factor of three of the particle size can be expected. To create thin coatings one requires a very fine particle size, usually at sizes between 10 and 20 microns. It is not uncommon to find coatings as thin as 30 microns. Liquid spray processing is able to decrease the thickness even more. Softer metals can typically be applied very thick. In some cases as thick as ¼”.
Bond strength is dictated by the speed of the particle, temperature within the thermal spray plume, substrate roughness and reaction with the underlying substrate. Bond strength up to 5000psi is not uncommon for thermally sprayed materials. The bonding ability is material and process dependent.
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