explanation about all major thermal coating processes

1. 1
Basics of Thermal Spray Technology
I Processes
Andreas Wank
GTV Verschleiss-Schutz GmbH, Luckenbach, Germany
1. Introduction
The importance of coating technologies and among them thermal spraying is
constantly rising, because coatings permit to exploit cost saving potentials by
combination of base materials that fulfill the structural demands (stiffness and
strength) with coating materials that fulfill the demands on the components surfaces
(corrosion resistance, wear resistance, electrical isolation or conductivity, thermal
isolation, catalytic function, decorative effect, etc.). E.g. for components that are
subject to severe stress conditions such material compounds can be provided at
significantly lower cost than bulk material solutions that cover both structural and
surface demands. Additionally repair of damaged or worn surfaces of costly
components is an important application field for thermal spray processes.
The standard DIN EN 657 comprises the different processes that belong to the group
of thermal spray processes. Therein, thermal spraying is defined as a process, in
which the feedstock is fed inside or outside a spraying gun and heated up to molten
state or at least to a state of high plastic deformability. The feedstock is propelled
unto a prepared surface that is not molten during the spraying process. The most
important distinction refers to the applied energy source:
• thermal spraying by atomization of a melt, i.e. melt bath spraying (MBS)
• thermal spraying by gaseous or liquid fuels, i.e. wire flame spraying (WFS),
high velocity wire flame spraying (HVWFS), powder flame spraying (PFS),
high velocity oxy fuel spraying (HVOF) and detonation gun spraying (DGS)
• thermal spraying by expansion of compressed gases without combustion, i.e.
cold gas spraying (CGS)
• thermal spraying by electrical arc or gas discharge, i.e. arc spraying (AS),
shrouded arc spraying (SAS), atmospheric plasma spraying (APS), shrouded
plasma spraying (SPS), vacuum plasma spraying (VPS), high pressure
plasma spraying (HPPS), liquid stabilized plasma spraying (LSPS), inductively
coupled plasma spraying (ICPS)
• thermal spraying by high energy density beams, i.e. laser spraying (LS)
Typically, thermal spray coatings are produced with a thickness between 30 µm and
2 mm. However, in some special applications coating thickness can be up to 10 mm.

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Thermal Spray processes feature inherent advantages:
• All materials with liquid phase or sufficient ductility below their decomposition
temperature can be sprayed.
• Thermal stress of the substrate can be kept low (coating of polymers or wood is
possible).
• Coatings can be deposited both on large or locally very limited areas.
• Adapted thermal spray equipment can be used on-site, e.g. within power plant
boilers, at bridges, marine landing stages or other steadily mounted constructions.
However, in general there is a comparatively weak bonding between substrate and
coating and the coatings contain pores. Post treatment, e.g. by remelting, hot
isostatic pressing (HIP) or shot peening, can improve bond strength and / or
decrease coating porosity. Usually the exact coating thickness and desired surface
roughness is achieved by machining.
2. Melt bath spraying (MBS)
In the MBS process the feedstock material is molten inside a crucible and then
atomized through a nozzle (figure 1). The atomizing gas, usually compressed air, is
most commonly applied in pre-heated state. The mass flow of the melt can be
controlled by adjusting the pressure inside the crucible. The size of the atomized
droplets depends on the temperature dependent viscosity and surface energy of the
molten feedstock material, the atomizing gas type and flow rate and the nozzle
geometry. The particles impacting on the substrate form the coating.
Figure 1: Principle of the melt bath spraying process [1]
These days the process principle is used for production of bulk material (known as
spray forming or Osprey process) more often than for production of coatings. The
MBS process features outstanding deposition rates, but the heat transfer to the
substrate is usually very strong. Thereby metallurgical bonding between coating and
substrate can be achieved, but temperature sensitive substrates are easily
overheated. Details concerning commonly applied materials and applications are
presented in [2].

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3. Flame spraying
3.1 [High velocity] wire flame spraying ([HV]WFS)
For wire flame spraying the feedstock material is applied in wire, rod or flexible cord
shape. The diameter of the feedstock can vary in the range between less than 1 mm
and 8 mm. Most commonly the feedstock is fed axially into a flame (figure 2), but
there are also guns with radial feed of one or more wires. At the wire tip the feedstock
is melted and then atomized by means of an atomizing gas. Usually acetylene,
propane or hydrogen are applied as combustion gases and compressed air as
atomizing gas.
Figure 2: Principle of the wire flame spraying process [1]
WFS is used e.g. for deposition of aluminum or zinc coatings for cathodic corrosion
protection of steel structures or for repair of worn component surfaces using similar
coating materials like the substrate material. Despite the strong increase in material
price in the recent decades molybdenum is still widely applied for production of wear
protective coatings, e.g. on automotive piston or gear synchronizing rings, because
the coatings feature excellent resistance to galling and fretting. These coatings
density and hardness strongly depends on the process parameters. In HVWFS, i.e.
WFS at increased process gas pressures and flow rates, there is an extremely fine
atomization of the molybdenum melt from the wire tip. Due to the large specific
surface area the droplets react strongly with the oxygen of the spray jet atmosphere,
which results in coating hardness of up to 1,000 HV0.3. Fine droplets are accelerated
very effectively and therefore impinge at high velocity on the substrate, which results
in high density coatings. Molybdenum is also used as bond coat material. In that case
process parameters are adapted in order to achieve large droplets, because bond
strength to both steel and aluminum substrates increases with heat content of the
impinging particles as a result of metallurgical interaction. WFS is also used for
processing of noble materials like platinum. According coatings are used for
protection of components in glass fabrication due to their outstanding durability [3].
Rods and flexible cords are mainly applied for production of ceramic coatings. Al2O3
based coatings are used for wear protection, partially in combination with corrosion
protection and ZrO2 based coatings provide excellent thermal insulation properties at
high thermal shock resistance. Production of rods by sintering is relatively expensive.

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Therefore flexible cords filled with the according oxide powder permit significant cost
savings. Also, cored wires expand the spectrum of applicable coating materials.
However, the structural and metallurgical design of these wires need to fit the
process characteristics [4-5].
3.2 Powder flame spraying (PFS)
The energy source for melting and acceleration of powder feedstock in PFS is a
gaseous fuel - oxygen flame. Inside the combustion gas jet the powder particles are
heated up and accelerated (figure 3). Depending on the required heat transfer to the
applied feedstock different gun designs with and without injection of additional gases
that provide stronger acceleration of powder particles and / or lead to a more focused
spray jet are applied.
Figure 3: Principle of the powder flame spraying process [1]
The most important material group for PFS are so called self-fluxing alloys based on
the system Ni-Cr-B-Si-C. These alloys feature relatively low melting temperature with
a wide range between solidus and liquidus temperature and high melt viscosity.
Therefore the relatively high porosity of PFS coatings in the as sprayed state can be
removed by a subsequent fusion process for this type of coatings. Diffusion
processes during coating fusion also result in metallurgical bonding and accordingly
high bond strength. The fusion can be carried out in furnaces, by inductive heating or
by use of the spraying guns flame without injection of powders, which is especially
suitable for on-site applications. Depending on the chemical composition coating
hardness between 20 HRC and 62 HRC can be achieved. Additionally reinforcement
with fused tungsten carbide (approximately eutectic mixture of W2C and WC) or
WC/Co is applied to respond to demands of extreme wear like in decanters and
separators as well as in mining applications. Besides wear protection also corrosion
protection capability of these coatings is excellent for a lot of environments. However,
heat affection can cause strong distortion of the coated components, which affords
subsequent straightening.
Besides metals also ceramics can be processed by PFS. However, deposition rate is
significantly lower. Even the manufacturing of thermoplastic polymer coatings is
possible by PFS for use of guns with adapted design that permit to prevent

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overheating of the feedstock. Thereby the axially fed granules are shrouded by a
coaxial gas, which is heated by an external coaxial flame.
3.3 Detonation gun spraying (DGS)
DGS was developed in the 1950´s in the United States and is the progenitor of the
HVOF processes. It is based on a discontinuous combustion process that is
characterized by a sequence of charging, in which the combustion gases - usually
acetylene and oxygen - and the powder feedstock with the nitrogen carrier gas are
injected into the tube, the ignition of the combustible and a flushing of the tube with
nitrogen (figure 4). The ignition frequency can vary from 4 - 8 Hz for early developed
systems up to about 100 Hz for newly developed spraying guns, which work with fluid
dynamical control of the gas injection (HFPD - High Frequency Pulse Detonation).
Figure 4: Principle of the detonation gun spraying process [1]
Detonation gun spraying is characterized by relatively high process gas tempera-
tures, which can be up to 4.000 °C, and high particle velocities, i.e. up to 900 m/s.
Oxidation of metallic spray particles is limited due to the short interaction time
between the hot combustion gases with the powder particles. The high particle
velocities result in high coating density and high bond strength.
DGS is mainly applied for production of wear protective cermet coatings based on
WC or Cr3C2, but metallic or ceramic coatings can also be manufactured. In
comparison to HVOF DGS shows economical advantages due to significantly lower
gas consumption. But noise levels of more than 140 dB(A) even exceed the high
noise level of HVOF. Especially for guns operating at very low frequency the spraying
needs to be carried out in special sound and explosion proof rooms. Additionally the
comparatively high weight makes the handling of DGS guns more difficult compared
to HVOF guns. Therefore mainly simple geometries that do not require extensive
movement of the gun are coated.
3.4 High velocity oxy fuel spraying (HVOF)
The HVOF process has been developed as an advancement of the conventional PFS
process, when it was found out that increased particle velocities - like in DGS - permit
improved coating density, cohesion and adhesion to the substrates. Therefore gas

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flows were increased to provide high combustion gas velocities. Thereby particle
velocities that are typically in the range of 50 m/s for PFS could be increased to about
450 m/s for use of the first HVOF guns in the early 1980´s. These guns showed a
convergent-cylindrical nozzle design. Therefore inside the gun the combustion gases
could reach a maximum of sound velocity only. Modern HVOF guns that feature a de-
Laval, i.e. convergent-divergent, contour (figure 5) permit mean particle velocities up
to 850 m/s and development is not yet finished.
Figure 5: Principle of the HVOF spraying process for advanced gun concepts [1]
Besides gaseous fuels like hydrogen, methane, acetylene, ethylene, propylene or
propane also liquid fuel can be applied. The choice of the combustible determines
the maximum achievable flame temperature (figure 6). By adjusting the ratio between
combustible and oxygen flow rate the actual flame temperature can be influenced
additionally. For cooling of the flame injection of water or additional gases like
nitrogen is possible. Use of compressed air instead of pure oxygen is equivalent to
additional injection of nitrogen and so results in decreased flame temperature
(figure 6).
The heat transfer to the powder feedstock does not only depend on the flame
temperature of the applied mixture, but also on the location of injection and the
injection boundary conditions like injection angle, injector internal diameter, carrier
gas flow rate and powder feed rate. For axial injection into the combustion chamber
there is strong heat transfer, while radial injection in the divergent part of de-Laval
nozzles results in significantly lower heat transfer, because the combustion gases
have cooled down significantly at this location and the overall time of interaction with
the hot environment is strongly reduced. Finally the length and shape of the
expansion nozzle influence the heat transfer to the powder particles. As the heat
transfer inside the expansion nozzle is stronger compared to the free expanding jet
long nozzles result in strong heat transfer. Divergent expansion nozzles result in
faster combustion gas jets compared to cylindrical nozzles, which causes reduced
dwelling time of particles inside the spray jet and therefore reduced heat transfer to
them.

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Figure 6: Flame temperature depending on fuel and combustion stoichiometry [6]
The choice of the combustible must be taken under consideration of the required
supply pressure that must be higher than the combustion chamber pressure. The
combustion chamber pressure in modern HVOF guns can exceed 1 MPa.
Liquefaction prohibits the use of propane under these conditions. Due to safety
reasons (exothermic dissociation reaction) acetylene can only be provided at
maximum pressures as low as 0.15 MPa. Therefore use of methane, hydrogen and
ethylene as gaseous fuels for advanced HVOF guns is advised.
Like for DGS the main application field of HVOF processes is the manufacturing of
wear protective cermet coatings based on WC or Cr3C2. The relatively low particle
temperatures and the short dwelling time of particles inside the hot combustion gas
jet permit to avoid melting of the composite powder particles and the formation of
brittle mixed carbides can be limited to low degrees. In optimized HVOF sprayed
WC/Co coatings formation of mixed carbide can be kept below the detection limit of
X-ray diffraction (XRD). Additionally HVOF replaces VPS sprayed hot gas corrosion
protective MCrAlY coatings for turbine blades more and more. Also iron, nickel or
cobalt based materials are sprayed for corrosion protection, sometimes also in
combination with wear protection.
Due to flame powers up to 250 kW there is a strong heat transfer to the substrates.
This necessitates effective cooling of the components that have to be coated.

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HVOF spraying is accompanied by high noise levels of up to 140 dB(A) and the
expansion of the combustion gases results in a strong rebound. Therefore and
because of the strong heat and dust evolution HVOF spraying is mainly carried out
inside sound insulating cabinets with robot handling systems. On-site application with
manual handling, e.g. for deposition of corrosion protective coatings in power plant or
waste incinerator boilers, is generally possible, but significant effort for occupational
safety and health is required.
Coating formation by high velocity particle impact of spray particles on the substrate
in solid state also results in compressive residual stress state, which is beneficial
concerning life time of coated components under dynamical load. Both for low alloyed
steel and also for aluminum alloy substrate materials improvement of fatigue strength
and limit by deposition of HVOF coatings has been proved. The existence of a fatigue
limit for HVOF coated aluminum parts in contrast to uncoated parts eases their
design substantially. So, for some base materials there is high potential to exploit
weight savings by light weight concepts considering HVOF coatings. However, for
high strength steel substrates fatigue properties are not affected only for very careful
pre-treatment by grit blasting and cleaning.
4. Cold gas spraying
Cold gas spraying is a relatively new spraying process that consequently follows the
trend to increased particle velocity and decreased particle temperature in order to
prevent in-flight oxidation of metallic feedstock materials and to provide compressive
residual stress state in the formed coatings. Powders are heated up and accelerated
by a process gas that expands through a de-Laval type nozzle with inlet pressures up
to 4 MPa at a maximum inlet temperature of 800 °C (figure 7). Typically nitrogen is
used as process gas, but helium or compressed air are applied alternatively, also.
While the use of compressed air permits cost savings at increased risk of feedstock
oxidation helium provides the highest velocities of the expanding process gas due to
its high sound velocity and therefore permits higher particle velocities compared to
nitrogen. Electrical resistance heating in helical tubes is common. As the powder is
fed in the convergent part of the nozzle for standard gun concepts the powder feeder
needs to operate at pressures exceeding the process gas inlet pressure.
Figure 7: Principle of the cold gas spraying process [1]

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In contrast to other thermal spray processes in cold gas spraying spreading of spray
particles at impact on the substrate due to molten or at least strongly plastified state
is not possible. Particle velocity must exceed a critical material dependent value in
order to permit sticking instead of rebounding. Typically particle velocity significantly
exceeds 500 m/s. For spraying of ductile materials like pure copper or aluminum with
adequate size fraction, i.e. 5 µm < d < 25 µm (Cu) and 10 µm < d < 45 µm (Al)
respectively, transformation of kinetic to thermal energy upon impact on the substrate
permits formation of well adherent coatings with nearly theoretical density by cold
welding mechanisms. However, low ductility materials like ferritic stainless steels or
hardmetals cannot be sprayed economically and spraying of ceramics is not possible
at all [7-8].
The main advantage of cold gas spraying is the avoidance of oxidation both of the
spray material and the substrate. The electrical conductivity of cold gas sprayed
copper coatings can be as high as 90% of cast material. Coating thickness can be
several millimeters and even on polished glass surfaces well adhering conductive
layers can be deposited. The small divergence of the particle trajectories permits the
manufacturing of free standing complex structures. Presently the most important
application of cold gas spraying is the deposition of thin copper coatings on extruded
aluminum parts in order to provide low thermal resistance coatings that can be
soldered to copper plates. Thereby heat sinks with advanced efficiency for cooling of
computer processors are produced [9].
5. Arc spraying (AS)
In arc spraying processes wire shaped feedstock is melted by means of an electrical
high current arc. The formed melt at the wire tip is propelled by means of an
atomizing gas, most commonly compressed air, unto the surface of the substrate.
Due to the process principle only electrically conductive materials that are available in
wire shape can be applied. There are single wire arc spraying guns with the arc
burning between a constantly fed wire and a non-consuming electrode that also acts
as nozzle for the atomizing gas [10]. However, in industries only twin wire arc
spraying has found broad acceptance. In according guns two wires, which act as
electrodes, are continuously fed towards each other under a defined angle and
consumed by melting at their tips (figure 8). The potential difference between the two
wires is usually in the range between 15 - 50 V. In order to keep oxidation of the
sprayed material as low as possible the lowest voltage providing steady processing is
applied, because superheating of the feedstock melt increases with voltage.
Superheating and oxidation of the melt at the wire tip determine the melts viscosity
and surface energy. The size of atomized droplets depends on these parameters as
well as on the atomizing gas type and flow rate and the nozzle design. In-flight
oxidation of spray particles mainly depends on the droplet size dependent specific
surface and the oxygen content in the spray jet. Even for use of inert atomizing gases

10. 10
like nitrogen or argon entrainment of atmospheric gases into the free expanding
spray jet results in considerable oxidation.
Figure 8: Principle of the (shrouded) arc spraying process [1]
For use of inert atomizing gases in combination with inert shrouds (figure 8, green),
i.e. a shielding gas jet surrounding the spray jet, oxidation of droplets can be kept
low. The sheath gas can also be used to minimize the spray jet divergence, which is
particularly important for minimized overspray during coating of small components.
High quality coatings of metals with strong affinity to oxygen like titanium alloys are
only achieved, if the arc spraying process is carried out inside a closed chamber with
controlled inert atmosphere.
With the aim to produce coatings with maximum bond strength atomizing gas flow is
kept low in order to produce large droplets that do not oxidize as much as small
droplets and provide high heat content for localized metallurgical interactions with the
substrate surface forming microscopic diffusion zones. However, porosity and
roughness is relatively high for spraying of coarse droplets. Fine structured high
density coatings are achieved for high atomizing gas flows and accordingly small
atomized droplets.
Among the thermal spray processes arc spraying features outstanding energetic
efficiencies and deposition rates. Therefore this process is particularly interesting
from the economical point of view, if the application dependent requirements on the
coating quality can be met. Additionally robust and mobile equipment that permits on-
site application is available. Arc spraying is applied for large area coating of steel and
concrete structures with corrosion protective aluminum or zinc based coatings and for
(on-site) repair of damaged components. Due to economical advantages compared
to alternative spraying processes and high bond strength also bond coats like Ni5Al
are often arc sprayed. Zinc sputter targets are produced by shrouded arc spraying
with argon as atomizing gas.

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Due to the necessity to provide the coating material in electrically conductive wire
shape the spectrum of coating materials is rather small, but cored wires permit a
significant extension of the producible material spectrum, e.g. by addition of hard
phases as filler material for production of wear resistant coatings [11].
6. Plasma spraying
6.1 Atmospheric [DC] plasma spraying (APS)
In DC plasma spraying a high current arc is used to generate a thermal plasma jet.
The arc is started by high frequency high voltage ignition and maintained by DC
electrical power supply. Inert gases passing the gap between the electrodes are
heated up by the arc. Thereby monoatomic gases like argon or helium are partially
ionized and molecular gases like hydrogen or nitrogen are dissociated prior to
ionization. During recombination of ions and electrons and re-formation of molecules
energy is set free and results in temperatures of the plasma jet up to 10,000 K in the
core at the gun exit. Due to the extreme increase in temperature there is strong
acceleration of plasma gases inside the plasma torch.
These days most DC plasma torches contain a pin cathode and an anode with axially
symmetrical shaped inner contour that acts as nozzle (figure 9). Most commonly both
electrodes consist of tungsten doped with thoria or rare earth oxides for improved
electron emission properties. Both electrodes are actively cooled to prevent thermal
destruction. Mixtures of argon, helium, hydrogen and nitrogen are applied as plasma
gas. The powder feedstock is fed into the thermal plasma jet radially inside the
nozzle or directly behind the nozzle exit.
Figure 9: Principle of the (shrouded) DC plasma spraying process [1]
The heat transfer to the cathode is significantly lower than that to the anode. Thermal
overload of the anode is only avoided because of arc root movement. For
conventional inner nozzle contour, i.e. convergent-cylindrical, there is axial as well as
circumferential movement of the arc root. However, axial movement causes
undesirable deviations of arc length and therefore also of voltage and power. Newly
developed nozzles with convergent-divergent inner contour permit axial fixation of the
arc without thermal overload because circumferential movement is maintained.

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Besides minimized plasma power fluctuations significant reduction of noise level is
achieved. Plasma gas velocity is also strongly reduced which results in increased
dwell time of powder particles inside the plasma jet and increased heat transfer.
Therefore refractory oxide coatings can be sprayed at increased deposition efficiency
[12]. However, for spraying of metals careful adjustment of process parameters and
feedstock powders is necessary to avoid excessive oxidation and / or evaporation.
Due to the extremely high temperature of the thermal plasma jet any desired material
with liquid phase can be melted, if it is fed into the plasma jet with adequate powder
size. Therefore the most important application field of atmospheric plasma spraying is
the production of refractory oxide coatings. Al2O3/TiO2 or Cr2O3 coatings are used for
wear and corrosion protection. Al2O3 coatings provide good electrical and ZrO2 based
coatings (additives: Y2O3, CeO2, MgO, CaO) good thermal insulation properties. But
high quality metallic and cermet coatings can also be produced. So, plasma spraying
is the most versatile spraying process in terms of coating material spectrum.
Like for arc spraying the use of gas shrouds (figure 9, green) has proven to be
effective to prevent excessive oxidation during spraying of reactive materials. Near
the torch exit a fast flowing, coaxial laminar stream of inert gases is utilized to prevent
entrainment of environmental gases into the plasma jet. The sheath gas also
effectively cools the substrate.
Single cathode DC plasma spraying guns with axial powder feeding have not found
industrial acceptance due to poor reliability. These days the only commercially
available plasma spraying gun with axial feeding of the powder feedstock is the
Axial III. It contains three separate plasma jets generated by separate electrode pairs
that unite in front of an axial powder injector inside the gun. This gun can be operated
economically for large area coating production due to outstanding high deposition
rate and efficiency as a result of the small divergence of the particle trajectories and
therefore the improved homogeneity of heat transfer in comparison to conventional
radial injection in single cathode systems. The major disadvantage of this torch is the
high energy and gas consumption due to the operation of three separate plasma jets,
each with a comparable power and gas flow like a conventional single cathode torch.
Triple cathode torches permit to overcome the drawbacks of high energy and gas
consumption. In such guns the electrical power is divided by three arc columns. So,
thermal load of the electrodes is reduced and therefore lifetime increased. Each arc
has its fixed anode root attachment. This results in a stable process state, which
permits homogeneous heat transfer to the sprayed particles despite of a radial
injection in front of the three anode roots. The improved heat transfer conditions
permit increased deposition rates with simultaneous increase of powder feed rate.
However, coating properties (porosity, hardness, bond strength) remain comparable
to use of single cathode APS guns. Also, triple cathode torches show the drawback
that the circumferential position of the arc roots on the anode ring vary with process

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parameters (plasma current, plasma gas composition and flow rate). For each set the
injection locations need to be adapted according to the actual arc root positions.
The most recent development in the field of APS torch design is a triple anode torch
[13]. This type of gun features a single pin cathode and three separate anodic ring
segments. Due to the relatively low thermal load on the cathode over-heating is
securely avoided even for application of indirect contact cooling. In addition to the
advantage of timely fixed arc roots like in triple cathode guns the triple anode gun
features constant arc root positions corresponding to the anode ring attachments that
do not change their position depending on the process parameters. Therefore, triple
anode torches permit the highest possible process stability among DC plasma
torches these days. Like for triple cathode torches increase of deposition rate with
simultaneous increase of the powder feed rate is possible. Further research is
required to decide, if the improved process stability will make improved coating
properties accessible, also.
Spraying of wire feedstock with DC plasma torches has been a research topic since
the 1980´s, but has not reached industrial acceptance, yet.
6.2 Vacuum [DC] plasma spraying (VPS)
The plasma spraying process has been advanced for use in controlled environments.
One example is the vacuum plasma spraying (VPS) process that is also known as
low pressure plasma spraying (LPPS) process. Typically chamber pressures are
higher than 2 kPa during the coating operation. However, in order to achieve well
inert environment the chamber is typically evacuated to a pressure of roughly 1 Pa
before it is flooded with an inert gas to the desired working pressure. The process is
traditionally used for the production of hot gas corrosion protective MCrAlY coatings,
which feature high density and a very homogeneous, oxide free microstructure, on
gas turbine blades. Low oxygen content is decisive in order to achieve optimal
oxidation resistance. These coatings also act as bond coats for subsequently applied
ZrO2 based thermal insulation coatings.
VPS is also important in the field of medical applications, e.g. for production of
porous and rough titanium coatings on various types of implants - from hip to tooth
implants. Due to the extremely high affinity of titanium to oxygen and nitrogen, it
cannot be sprayed under atmospheric conditions. Also hydroxyapatite, i.e. bone
material, is processed by VPS in order to achieve implant surfaces that provide
optimal preconditions for long-term stable connection between implant and original
bone material [14]. There are also some special applications of VPS, e.g. for
deposition of tantalum corrosion protective coatings, for production of high quality
sputter targets that are used for deposition of thin films from vapor phase or for
production of refractory metal free standing bodies [15].

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6.3 High pressure [DC] plasma spraying (HPPS)
Besides VPS also plasma spraying in high pressure environments, i.e. high pressure
plasma spraying (HPPS), also known as controlled atmosphere plasma spraying
(CAPS) has been developed. Typically chamber pressure is below 0.3 MPa and
various gas (mixtures) can be employed with the aim to exploit reactions of spray
particles with the environmental gases, e.g. nitriding of titanium particles in order to
form TiN. Also high chamber pressure results in low plasma gas velocity and high
energy density of the thermal plasma jet, which eases heat transfer to spray particles
and is therefore particularly well suited for spraying of high melting point materials.
However, up to now HPPS has not found significant industrial acceptance.
6.4 Liquid stabilized plasma spraying (LSPS)
In liquid stabilized plasma spraying torches the plasma gas is produced from a
process liquid. Most commonly water is applied, but use of e.g. methanol or ethanol
is also possible. Typically the arc is burning between a graphite anode and a rotating,
water cooled copper anode behind the nozzle exit (figure 10). The arc is confined
and stabilized by the interaction with the vortex liquid flow inside the torch. Due to
constant feeding of the liquid the liquid jacket is maintained and acts both as coolant
and as a thermal and electrical isolation to the chamber. The thermal plasma is
generated as a consequence of partial vaporization, dissociation and ionization of the
stabilizing liquid. Plasma temperatures and enthalpies achieved in this type of
torches are substantially higher than in common gas stabilized torches, but plasma
density is lower [16].
Figure 10: Principle of the liquid stabilized plasma spraying process [1]
Like in most gas stabilized plasma torches the powder feedstock is fed radially into
the plasma jet at the torch exit. Due to the strongly oxidizing environment inside the
high enthalpy plasma jet liquid stabilized plasma spraying is particularly suitable for
high rate spraying of oxide ceramic coatings. For these materials relatively long
exposure to oxidizing environments at very high temperature does not result in
undesired reactions like oxidation of metallic materials. Despite the robust design of

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respective torches and the relatively low costs for process media - especially for use
of water - the rather limited spectrum of suitable coating materials has led to rather
seldom use of LSPS in industries.
Hybrid plasma torches with combined gas and liquid stabilization of the arc that result
in intermediate processing conditions are still in the stage of laboratory research [16].
6.5 Inductively coupled plasma spraying (ICPS)
Inductively coupled plasma spraying (ICPS), also known as high frequency plasma
spraying, utilizes a plasma jet that is generated by inductive coupling of electrical
energy into a gas stream (figure 11). The main advantage of this technique is the
generation of the thermal plasma jet without contact of the generating components
with the plasma gases. Therefore even highly reactive gases, including oxygen, and
gas mixtures can be applied and incorporation of eroded electrode material in the
coatings is ruled out. Additionally the large plasma volume permits high deposition
rates and the particles undergo homogeneous heat treatment due to the axial
injection inside the torch through water cooled probes. These days systems with
plate powers typically between 25 and 200 kW are available.
Figure 11: Principle of the inductively coupled plasma spraying process [1]
The temperature inside the plasma jet usually does not exceed 10,000 K. In contrast
to thermal plasma jets in DC torches the highest temperature is not achieved in the
plasma jet core. With increase of the applied frequency the temperature maximum
shifts to the jet fringes as a consequence of the skin effect. For spraying under

16. 16
atmospheric conditions gas and therefore also particle velocities are relatively low
(< 50 m/s). Gravity is commonly exploited to achieve the highest possible velocity.
For vacuum plasma spraying with inductively coupled plasma torches that are
equipped with adequate nozzles even supersonic flow conditions are possible.
However, thereby the plasma jet diameter is reduced accordingly and the reduced
chamber pressure also results in significantly decreased energy density.
The main disadvantages of ICPS torches are the poor flexibility concerning handling
of the torch due to the dependence of the particle trajectories on the angle between
torch and gravity axis for conventional torches, the stiff power supply design and the
strong electromagnetic fields, that prevent use of electronically controlled substrate
handling systems. Therefore the most important application field of inductively
coupled thermal plasma jets are the spheroidization of powders [17-18] and the
analysis of materials by emission spectroscopy after complete evaporation in the
thermal plasma jet [19].
7. Laser spraying (LS)
The laser spraying process proceeds by injection of a feedstock in powder shape
through a suitable powder injector into a laser beam. The laser beam melts the
powder particles that approach the substrate surface under the influence of the
carrier gas flow and gravity (figure 12). Typically the spray jet is covered by an inert
sheath gas jet.
Figure 12: Principle of the laser spraying process [1]
In contrast to laser cladding during laser spraying the substrate surface is not melted
or only melted to a negligible degree. Therefore bond strength is comparatively low.

17. 17
Due to the rather small volume of the free high energy density laser beam only low
deposition rates can be achieved. So, laser spraying is only suitable for well defined,
small area coating of component surfaces that must not be melted and the use in
industry is not very common. However, the process attracts some interest in research
topics that require well defined heat transfer to the material to be processed [20-21].
8. Related processes
8.1 Thermal Plasmajet CVD (TPCVD)
Although conventional plasma spray equipment is used, Thermal Plasmajet CVD
does not belong to the thermal spray processes, because coating formation does not
proceed by impact of particles on the component to be coated but via gas phase
reactions on the components surface. If the precursor for coating formation is not
injected into the thermal plasma jet in gaseous state already, it is fully vaporized
before the substrate surface is reached. All kinds of plasma spray torches can be
applied. As the gas phase composition is decisive for coating formation the process
needs to be carried out in controlled atmosphere. Usually relatively low chamber
pressure is applied in order to achieve improved homogeneity of properties within the
plasma jet and to achieve a small boundary layer thickness on top of the surface.
TPCVD is particularly suitable for high rate
deposition of coating materials that cannot be
processed by conventional thermal spray methods,
because they show neither a liquid phase nor
sufficient ductility below their decomposition
temperature, i.e. diamond, c-BN, SiC or Si3N4. The
simplest and best studied TPCVD coating process
is diamond synthesis with methane as precursor
[22] (figure 13). In contrast to alternative processes
for pure diamond coating synthesis deposition
rates exceeding 100 µm/h can be achieved. For
SiC and Si3N4 coating formation even 1,500 µm/h
is possible [23].
The major drawback of this process is related to
the strong gradients in the properties of the plasma
jet acting on the substrate surface. Therefore
phase composition and thickness of the produced
coatings varies strongly between plasma jet axis
and the jets fringes. Also heat flux to the substrate
is very strong and for diamond coating synthesis
substrate temperature is typically > 800 °C.
Figure 13: TPCVD of a diamond coating at the Institute of Materials Technology,
University of Dortmund

18. 18
8.2 Hypersonic Plasma Particle Deposition (HPPD)
Hypersonic Plasma Particle Deposition (HPPD) is in between TPCVD and
conventional thermal spraying, because it proceeds via gas phase on the one hand,
but on the other hand coatings are formed by impact of particles on a component to
be coated. For this process nearly usual VPS technology is applied. The major
difference concerns the chamber pressure which is typically in the range of 100 Pa.
Low chamber pressure results in hypersonic velocities of the plasma gases leaving
the torch and accordingly fast quenching. Gaseous precursors are added inside a
specially designed nozzle and strong heat transfer by the thermal plasma jet results
in full dissociation of the injected molecules. During quenching the vapor becomes
supersaturated and nanosized particles are precipiated. Due to their small size
particles are accelerated effectively. Depending on their size inertia is sufficient to
cross the boundary layer on top of the substrate to be deposited and to form a
coating by superposition of individual impacting particles. Too fine particles are
sucked off by the vacuum pumps together with the plasma gas.
Due to the fact that supersaturated vapor is required precursor feed rate and
resultant deposition rate can be significantly higher compared to TPCVD. For silicon
deposition more than 3,000 µm/h is possible [24]. The process is still at the stage of
laboratory research, but it has already been shown that HPPD is not only capable to
produce different nanostructured metallic and ceramic coatings at considerable
deposition rate [25]. Also, use of aerodynamic lenses permits production of micro-
patterns like micro-towers or micro-walls [26]. So, the process has also high potential
to be used for micropart production.
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