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Remanufacturing and Advanced Machining
oxy-fuel and high-velocity air-fuel spraying, low-pressure cold spraying, and highpressure cold spraying.
Atmospheric plasma spraying (APS) is perhaps one of the most frequently used
methods of coating manufacturing in the field of TS. A detailed description of this
method can be found in Heimann (2008) or Pawłowski (2008). During processing,
an electrical arc is ignited between a copper anode and a thoriated tungsten cathode
ionizing plasma gases, heating and expanding them to form a plasma jet. The powder
particles are transported by a carrier gas and injected into the hot stream of plasma
jet, where they are heated and accelerated by drag force. Directed to the substrate
surface, these particles hit it with a relatively high kinetic energy and form splats that
solidify and build up the coating. Commercially available low-power plasma torches
feature powers up to 80 kW, but typical torches can operate with an electric power
input of up to 200 kW, ensuring a powder feed rate of up to 100 kg/h.
The microstructure of sprayed coatings results from the phenomena occurring
inside a powder particle in plasma jet dependent on various process parameters.
Composition of a working gas is the fundamental process parameter, where two
main groups of gases are used: (1) primary gases to stabilize arc inside the torch nozzle, frequently argon Ar and sometimes nitrogen N 2 ; and (2) secondary gases added
in order to increase heat conductivity of plasma. Other important APS parameters
listed by Fauchais et al. (2014) include electrical power, flow rate of the plasma gases,
feed rate of the powder, particle size distribution of the powder (usually between 20
and 90 μ), and speed of plasma torch relative to the substrate. According to Rico et al.
(2018), power is the key parameter governing the microstructure and mechanical
performance of the coating, but other parameters, such as the arc current, plasma gas
flow rate, carrier gas flow rate, or stand-off distance can also have effect and thus
should undergo optimization.
A proper APS process leads to melting of injected particles before their impact
on the substrate through an appropriate choice of spray parameters. However, the
powders usually include small and large particles, which somewhat disrupts the
process and affect coating properties. On the one hand, small particles may get
molten and start to evaporate in plasma, weakening the coating effect, on the other
hand, large ones may remain solid and form weakening inclusions in a formed
structure. Generally, the powder is injected radially, with injection angles ranging
from 75° to 120° relative to the torch axis, with injectors placed outside the plasma
torch, though some torches are designed to feature injection inside the torch (Łatka
et al., 2020).
APS is the preferred method for spraying high-temperature ceramic oxides and
the large number of adjustable process parameters allows adaptation of the coating process to a wide range of materials and substrates with different properties
(Fauchais et al., 2014). However, the excessive number of process parameters (up
to 60) also becomes a disadvantage since proper control of all these parameters
requires special effort. According to Pawłowski (2008), a typical adhesion strength
of APS-obtained coatings is in the range from 15 up to 30 MPa, but application of a
bond coat, such as Ni-Al or Ni-Cr or Mo, may increase the adhesion strength up to
70 MPa. APS coatings are typically a few hundreds of micrometers thick.
Remanufacturing and Advanced Machining
oxy-fuel and high-velocity air-fuel spraying, low-pressure cold spraying, and highpressure cold spraying.
Atmospheric plasma spraying (APS) is perhaps one of the most frequently used
methods of coating manufacturing in the field of TS. A detailed description of this
method can be found in Heimann (2008) or Pawłowski (2008). During processing,
an electrical arc is ignited between a copper anode and a thoriated tungsten cathode
ionizing plasma gases, heating and expanding them to form a plasma jet. The powder
particles are transported by a carrier gas and injected into the hot stream of plasma
jet, where they are heated and accelerated by drag force. Directed to the substrate
surface, these particles hit it with a relatively high kinetic energy and form splats that
solidify and build up the coating. Commercially available low-power plasma torches
feature powers up to 80 kW, but typical torches can operate with an electric power
input of up to 200 kW, ensuring a powder feed rate of up to 100 kg/h.
The microstructure of sprayed coatings results from the phenomena occurring
inside a powder particle in plasma jet dependent on various process parameters.
Composition of a working gas is the fundamental process parameter, where two
main groups of gases are used: (1) primary gases to stabilize arc inside the torch nozzle, frequently argon Ar and sometimes nitrogen N 2 ; and (2) secondary gases added
in order to increase heat conductivity of plasma. Other important APS parameters
listed by Fauchais et al. (2014) include electrical power, flow rate of the plasma gases,
feed rate of the powder, particle size distribution of the powder (usually between 20
and 90 μ), and speed of plasma torch relative to the substrate. According to Rico et al.
(2018), power is the key parameter governing the microstructure and mechanical
performance of the coating, but other parameters, such as the arc current, plasma gas
flow rate, carrier gas flow rate, or stand-off distance can also have effect and thus
should undergo optimization.
A proper APS process leads to melting of injected particles before their impact
on the substrate through an appropriate choice of spray parameters. However, the
powders usually include small and large particles, which somewhat disrupts the
process and affect coating properties. On the one hand, small particles may get
molten and start to evaporate in plasma, weakening the coating effect, on the other
hand, large ones may remain solid and form weakening inclusions in a formed
structure. Generally, the powder is injected radially, with injection angles ranging
from 75° to 120° relative to the torch axis, with injectors placed outside the plasma
torch, though some torches are designed to feature injection inside the torch (Łatka
et al., 2020).
APS is the preferred method for spraying high-temperature ceramic oxides and
the large number of adjustable process parameters allows adaptation of the coating process to a wide range of materials and substrates with different properties
(Fauchais et al., 2014). However, the excessive number of process parameters (up
to 60) also becomes a disadvantage since proper control of all these parameters
requires special effort. According to Pawłowski (2008), a typical adhesion strength
of APS-obtained coatings is in the range from 15 up to 30 MPa, but application of a
bond coat, such as Ni-Al or Ni-Cr or Mo, may increase the adhesion strength up to
70 MPa. APS coatings are typically a few hundreds of micrometers thick.