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Protection and Restoration
Łatka et al. (2020) underline that coating quality can be controlled by particle inflight temperature, while the most important parameters influencing the deposition of
FGC are spray distance and flow rate of working gases, fuel, and oxygen. Moreover,
they suggest paying attention to control of the powder with the lowest melting point
in order to avoid excessive evaporation.
Low-pressure cold spraying (LPCS) and High-pressure cold spraying (HPCS)
processes both utilize the main advantage of cold spraying, namely, the low processing temperature compared to other thermal spray processes. Cold gas dynamic spray
is a solid-state process, based on the kinetic energy of incident particles to deposit
coatings (Sabard and Hussain, 2019). In the cold spray processes, metallic powders
are accelerated by a supersonic gas flowing through a convergent–diverging nozzle.
Impact of particles on a surface causes combined plastic deformation of the feedstock material and the substrate and thus formation of a coating. Sprayed materials
are kept below melting temperature, so that any deleterious effect of the temperature
during deposition is avoided. Most probably, the accumulation of dislocations during
severe plastic deformation is responsible for various grain structure formations of
cold-sprayed deposits in the interfacial regions, from elongated grains between 200
nm and 1 μm to ultrafine grains (50–200 nm) (Sabard and Hussain, 2019).
Łatka et al. (2020) categorize cold spraying methods according to the initial pressure of working gas:
1. High-pressure cold spraying at a gas pressure above 1 MPa, usually with
nitrogen or helium as the working gas
2. Low-pressure cold spraying at a gas pressure below 1 MPa with nitrogen
or air
In both the methods, the working gas is heated before reaching the nozzle. The
LPCS equipment and processing costs are much lower than in HPCS, but since the
sprayed particles reach much lower velocity in the former, its application is significantly limited to deposition of easily plastic deformable materials such as tin, zinc,
copper, aluminum, nickel, and some composite materials. Especially metal matrix
composites (MMCs) can be fabricated for various purposes, such as regeneration of
surfaces, increase of electrical conductivity, corrosion and wear resistance improvement, biomedical applications, etc. (Łatka et al., 2020).
Lazorenko et al. (2021) in their review emphasize that thermal spray coating techniques have been developed recently to protect rail steel from atmospheric corrosion, mainly due to its simplicity, relatively low cost, and high efficiency. They give
an example of effects of atmospheric corrosion in a coastal zone on carbon steel
with Zn, Al, and Zn-Al thermally sprayed coating after 33 years, where corrosion
of steel with a zinc coating was effectively slowed down by zinc corrosion products
formed on the surface, while long-term corrosion resistance of aluminum coatings
was facilitated by a thin surface oxide film. They provide similar examples of good
condition of 40- and 50-year-old bridges with thermal spray zinc duplex coatings,
stressing that lower durability of the coatings (20 years) was caused by some errors,
such as pinholes, spitting, and low coating thickness.
Protection and Restoration
Łatka et al. (2020) underline that coating quality can be controlled by particle inflight temperature, while the most important parameters influencing the deposition of
FGC are spray distance and flow rate of working gases, fuel, and oxygen. Moreover,
they suggest paying attention to control of the powder with the lowest melting point
in order to avoid excessive evaporation.
Low-pressure cold spraying (LPCS) and High-pressure cold spraying (HPCS)
processes both utilize the main advantage of cold spraying, namely, the low processing temperature compared to other thermal spray processes. Cold gas dynamic spray
is a solid-state process, based on the kinetic energy of incident particles to deposit
coatings (Sabard and Hussain, 2019). In the cold spray processes, metallic powders
are accelerated by a supersonic gas flowing through a convergent–diverging nozzle.
Impact of particles on a surface causes combined plastic deformation of the feedstock material and the substrate and thus formation of a coating. Sprayed materials
are kept below melting temperature, so that any deleterious effect of the temperature
during deposition is avoided. Most probably, the accumulation of dislocations during
severe plastic deformation is responsible for various grain structure formations of
cold-sprayed deposits in the interfacial regions, from elongated grains between 200
nm and 1 μm to ultrafine grains (50–200 nm) (Sabard and Hussain, 2019).
Łatka et al. (2020) categorize cold spraying methods according to the initial pressure of working gas:
1. High-pressure cold spraying at a gas pressure above 1 MPa, usually with
nitrogen or helium as the working gas
2. Low-pressure cold spraying at a gas pressure below 1 MPa with nitrogen
or air
In both the methods, the working gas is heated before reaching the nozzle. The
LPCS equipment and processing costs are much lower than in HPCS, but since the
sprayed particles reach much lower velocity in the former, its application is significantly limited to deposition of easily plastic deformable materials such as tin, zinc,
copper, aluminum, nickel, and some composite materials. Especially metal matrix
composites (MMCs) can be fabricated for various purposes, such as regeneration of
surfaces, increase of electrical conductivity, corrosion and wear resistance improvement, biomedical applications, etc. (Łatka et al., 2020).
Lazorenko et al. (2021) in their review emphasize that thermal spray coating techniques have been developed recently to protect rail steel from atmospheric corrosion, mainly due to its simplicity, relatively low cost, and high efficiency. They give
an example of effects of atmospheric corrosion in a coastal zone on carbon steel
with Zn, Al, and Zn-Al thermally sprayed coating after 33 years, where corrosion
of steel with a zinc coating was effectively slowed down by zinc corrosion products
formed on the surface, while long-term corrosion resistance of aluminum coatings
was facilitated by a thin surface oxide film. They provide similar examples of good
condition of 40- and 50-year-old bridges with thermal spray zinc duplex coatings,
stressing that lower durability of the coatings (20 years) was caused by some errors,
such as pinholes, spitting, and low coating thickness.