58
Remanufacturing and Advanced Machining
reaction rates and also allow chemical reactions to occur, impossible in
other conditions.
• Ions can be drawn out from a plasma surface at energies of tens to hundreds
of eV, making possible anisotropic etching and deposition.
• Due to the low density of charged plasma species, plasma particles can
interact with a surface without heating it significantly.
• Low-pressure operation allows plasmas to utilize process reactants with
high efficiency, which reduces waste and pollution.
Basically, thermal plasma jets can be generated in devices called plasmatrons by
direct current (DC), alternating current (AC), radio frequency (RF), or other discharges (Cao et al., 2016). Among DC methods, two main types are distinguished,
namely, free-burning arcs (transferred arcs) and plasma torches (non-transferred
arcs), which are widely used in materials processing (steel production, metal cutting and coating). The basic design of a plasma torch consists of two electrodes
and a cylindrical discharge chamber where a plasma-forming gas is introduced
under pressure. In general, there is a button-type cathode, a gas distributor, and
a nozzle anode which forms a discharge chamber and usually acts as an arc constrictor in non-transferred DC plasma torches. The arc is initiated between the
cathode and the anode by a high voltage discharge, while the plasma-forming
gas is supplied through the gas distributor radially or tangentially. Typically, the
cathode consists of a water-cooled copper holder with a press-fitted rod of alloyed
tungsten, hafnium, or graphite (Mostaghimi et al., 2017). In free-burning arcs
(transferred torches), a treated conductive material serves as an electrode, so that
the plasma jet is formed in the arc column, as shown in Figure 1.14 (Dragobetsky
et al., 2012).
Variations of DC plasmatron designs include axial and coaxial plasma generators, designs with toroidal electrodes, with outer plasma arcs, and with eroding electrodes (Gevorkyan et al., 2016). Among AC plasma generators, high-frequency and
ultrashort-wave dischargers find a wide range of applications (Wegman et al., 2010),
as shown in Figure 1.15.
High-frequency capacitive (HFC) plasmatrons work in the range of frequencies
between 10 and 50 MHz (Toumanov, 2003). The value of their discharge current is
limited due to the resistance of the capacitive coupling, so that operational frequency
should not be lower than 10 MHz. The specific intensity of the electric constituent
of the electromagnetic field is in the range of 100 to 400 V/cm, the voltage on electrodes is within 5–15 kV, and the currents are within 3–15 A. The permissible range
of operation frequencies lies between 13.56 and 27.12 MHz. An important feature
of HFC discharges is a low value of minimal power necessary for sustaining the
discharge, so that the range of practically assimilated powers is 1–100 kW. In a highfrequency inductive (HFI) plasmatron, a discharge chamber (5) is a tube made of a
dielectric material placed in a coil (4), as shown in Figure 1.15a. The magnetic field
is oscillating due to a high-frequency current in the coil and induces a vortex electric
field, which in turn ignites and sustains a discharge. HFI plasmatrons can vary in
design: some include one grounded end of the coil, some a coil supplied according
Remanufacturing and Advanced Machining
reaction rates and also allow chemical reactions to occur, impossible in
other conditions.
• Ions can be drawn out from a plasma surface at energies of tens to hundreds
of eV, making possible anisotropic etching and deposition.
• Due to the low density of charged plasma species, plasma particles can
interact with a surface without heating it significantly.
• Low-pressure operation allows plasmas to utilize process reactants with
high efficiency, which reduces waste and pollution.
Basically, thermal plasma jets can be generated in devices called plasmatrons by
direct current (DC), alternating current (AC), radio frequency (RF), or other discharges (Cao et al., 2016). Among DC methods, two main types are distinguished,
namely, free-burning arcs (transferred arcs) and plasma torches (non-transferred
arcs), which are widely used in materials processing (steel production, metal cutting and coating). The basic design of a plasma torch consists of two electrodes
and a cylindrical discharge chamber where a plasma-forming gas is introduced
under pressure. In general, there is a button-type cathode, a gas distributor, and
a nozzle anode which forms a discharge chamber and usually acts as an arc constrictor in non-transferred DC plasma torches. The arc is initiated between the
cathode and the anode by a high voltage discharge, while the plasma-forming
gas is supplied through the gas distributor radially or tangentially. Typically, the
cathode consists of a water-cooled copper holder with a press-fitted rod of alloyed
tungsten, hafnium, or graphite (Mostaghimi et al., 2017). In free-burning arcs
(transferred torches), a treated conductive material serves as an electrode, so that
the plasma jet is formed in the arc column, as shown in Figure 1.14 (Dragobetsky
et al., 2012).
Variations of DC plasmatron designs include axial and coaxial plasma generators, designs with toroidal electrodes, with outer plasma arcs, and with eroding electrodes (Gevorkyan et al., 2016). Among AC plasma generators, high-frequency and
ultrashort-wave dischargers find a wide range of applications (Wegman et al., 2010),
as shown in Figure 1.15.
High-frequency capacitive (HFC) plasmatrons work in the range of frequencies
between 10 and 50 MHz (Toumanov, 2003). The value of their discharge current is
limited due to the resistance of the capacitive coupling, so that operational frequency
should not be lower than 10 MHz. The specific intensity of the electric constituent
of the electromagnetic field is in the range of 100 to 400 V/cm, the voltage on electrodes is within 5–15 kV, and the currents are within 3–15 A. The permissible range
of operation frequencies lies between 13.56 and 27.12 MHz. An important feature
of HFC discharges is a low value of minimal power necessary for sustaining the
discharge, so that the range of practically assimilated powers is 1–100 kW. In a highfrequency inductive (HFI) plasmatron, a discharge chamber (5) is a tube made of a
dielectric material placed in a coil (4), as shown in Figure 1.15a. The magnetic field
is oscillating due to a high-frequency current in the coil and induces a vortex electric
field, which in turn ignites and sustains a discharge. HFI plasmatrons can vary in
design: some include one grounded end of the coil, some a coil supplied according