Thermal spraying techniques are coating processes in which melted materials are
sprayed onto a surface. The "feedstock" is heated by electrical or chemical
means. Thermal spraying can provide thick
coatings, over a large area at high deposition rate as compared to other
coating processes such as electroplating, physical and chemical
vapor deposition. Coating materials available for thermal spraying include
metals, alloys, ceramics, plastics and composites. They are fed in powder or
wire form, heated to a molten or semimolten state and accelerated towards
substrates in the form of micrometer-size particles. Combustion or
electrical arc discharge is usually used as the source of energy for thermal
spraying. Resulting coatings are made by the accumulation of numerous sprayed
particles. The surface may not heat up significantly, allowing the coating of
flammable substances. Coating quality is usually assessed by
measuring its porosity, oxide content, macro and micro-hardness, bond strength
and surface roughness. Generally, the coating quality increases with
increasing particle velocities. Several variations of thermal spraying
are distinguished: Plasma spraying
Detonation spraying Wire arc spraying
Flame spraying High velocity oxy-fuel coating spraying
Warm spraying Cold spraying
In classical but still widely used processes such as flame spraying and
wire arc spraying, the particle velocities are generally low (15000 K,
which makes it possible to spray refractory materials such as oxides,
molybdenum, etc. System overview
A typical thermal spray system consists of the following:
Spray torch - the core device performing the melting and acceleration of the
particles to be deposited Feeder - for supplying the powder, wire
or liquid to the torch through tubes. Media supply - gases or liquids for the
generation of the flame or plasma jet, gases for carrying the powder, etc.
Robot - for manipulating the torch or the substrates to be coated
Power supply - often standalone for the torch
Control console(s) - either integrated or individual for all of the above
Detonation thermal spraying process The detonation gun consists of a long
water-cooled barrel with inlet valves for gases and powder. Oxygen and fuel is
fed into the barrel along with a charge of powder. A spark is used to ignite the
gas mixture, and the resulting detonation heats and accelerates the
powder to supersonic velocity through the barrel. A pulse of nitrogen is used
to purge the barrel after each detonation. This process is repeated
many times a second. The high kinetic energy of the hot powder particles on
impact with the substrate results in a buildup of a very dense and strong
coating. Plasma spraying
In plasma spraying process, the material to be deposited — typically as a powder,
sometimes as a liquid, suspension or wire — is introduced into the plasma
jet, emanating from a plasma torch. In the jet, where the temperature is on the
order of 10,000 K, the material is melted and propelled towards a
substrate. There, the molten droplets flatten, rapidly solidify and form a
deposit. Commonly, the deposits remain adherent to the substrate as coatings;
free-standing parts can also be produced by removing the substrate. There are a
large number of technological parameters that influence the interaction of the
particles with the plasma jet and the substrate and therefore the deposit
properties. These parameters include feedstock type, plasma gas composition
and flow rate, energy input, torch offset distance, substrate cooling, etc.
= Deposit properties = The deposits consist of a multitude of
pancake-like 'splats' called lamellae, formed by flattening of the liquid
droplets. As the feedstock powders typically have sizes from micrometers to
above 100 micrometers, the lamellae have thickness in the micrometer range and
lateral dimension from several to hundreds of micrometers. Between these
lamellae, there are small voids, such as pores, cracks and regions of incomplete
bonding. As a result of this unique structure, the deposits can have
properties significantly different from bulk materials. These are generally
mechanical properties, such as lower strength and modulus, higher strain
tolerance, and lower thermal and electrical conductivity. Also, due to
the rapid solidification, metastable phases can be present in the deposits.
= Applications = This technique is mostly used to produce
coatings on structural materials. Such coatings provide protection against high
temperatures, corrosion, erosion, wear; they can also change the appearance,
electrical or tribological properties of the surface, replace worn material, etc.
When sprayed on substrates of various shapes and removed, free-standing parts
in the form of plates, tubes, shells, etc. can be produced. It can also be
used for powder processing. In this case, the substrate for deposition is
absent and the particles solidify during flight or in a controlled environment.
This technique with variation may also be used to create porous structures,
suitable for bone ingrowth, as a coating for medical implants. A polymer
dispersion aerosol can be injected into the plasma discharge in order to create
a grafting of this polymer on to a substrate surface. This application is
mainly used to modify the surface chemistry of polymers.
= Variations = Plasma spraying systems can be
categorized by several criteria. Plasma jet generation:
direct current, where the energy is transferred to the plasma jet by a
direct current, high-power electric arc induction plasma or RF plasma, where the
energy is transferred by induction from a coil around the plasma jet, through
which an alternating, radio-frequency current passes
Plasma-forming medium: gas-stabilized plasma, where the plasma
forms from a gas; typically argon, hydrogen, helium or their mixtures
water-stabilized plasma, where plasma forms from water or other suitable
liquid hybrid plasma - with combined gas and
liquid stabilization, typically argon and water
Spraying environment: air plasma spraying, performed in
ambient air controlled atmosphere plasma spraying,
usually performed in a closed chamber, either filled with inert gas or
evacuated variations of CAPS: high-pressure plasma
spraying, low-pressure plasma spraying, the extreme case of which is vacuum
plasma spraying underwater plasma spraying
Another variation consists of having a liquid feedstock instead of a solid
powder for melting, this technique is known as Solution precursor plasma spray
= Vacuum plasma spraying = Vacuum plasma spraying is a technology
for etching and surface modification to create porous layers with high
reproducibility and for cleaning and surface engineering of plastics, rubbers
and natural fibers as well as for replacing CFCs for cleaning metal
components. This surface engineering can improve properties such as frictional
behavior, heat resistance, surface electrical conductivity, lubricity,
cohesive strength of films, or dielectric constant, or it can make
materials hydrophilic or hydrophobic. The process typically operates at 39–120
°C to avoid thermal damage. It can induce non-thermally activated surface
reactions, causing surface changes which cannot occur with molecular chemistries
at atmospheric pressure. Plasma processing is done in a controlled
environment inside a sealed chamber at a medium vacuum, around 13–65 Pa. The gas
or mixture of gases is energized by an electrical field from DC to microwave
frequencies, typically 1–500 W at 50 V. The treated components are usually
electrically isolated. The volatile plasma by-products are evacuated from
the chamber by the vacuum pump, and if necessary can be neutralized in an
exhaust scrubber. In contrast to molecular chemistry,
plasmas employ: Molecular, atomic, metastable and free
radical species for chemical effects. Positive ions and electrons for kinetic
effects. Plasma also generates electromagnetic
radiation in the form of vacuum UV photons to penetrate bulk polymers to a
depth of about 10 µm. This can cause chain scissions and cross-linking.
Plasmas affect materials at an atomic level. Techniques like X-ray
photoelectron spectroscopy and scanning electron microscopy are used for surface
analysis to identify the processes required and to judge their effects. As
a simple indication of surface energy, and hence adhesion or wettability, often
a water droplet contact angle test is used. The lower the contact angle, the
higher the surface energy and more hydrophilic the material is.
= Changing effects with plasma = At higher energies ionization tends to
occur more than chemical dissociations. In a typical reactive gas, 1 in 100
molecules form free radicals whereas only 1 in 106 ionizes. The predominant
effect here is the forming of free radicals. Ionic effects can predominate
with selection of process parameters and if necessary the use of noble gases.
Wire arc spray Wire arc spray is a form of thermal
spraying where two consumable metal wires are fed independently into the
spray gun. These wires are then charged and an arc is generated between them.
The heat from this arc melts the incoming wire, which is then entrained
in an air jet from the gun. This entrained molten feedstock is then
deposited onto a substrate. This process is commonly used for metallic, heavy
coatings. = Plasma transferred wire arc =
Plasma transferred wire arc is another form of wire arc spray which deposits a
coating on the internal surface of a cylinder, or on the external surface of
a part of any geometry. It is predominantly known for its use in
coating the cylinder bores of an engine, enabling the use of Aluminum engine
blocks without the need for heavy cast iron sleeves. A single conductive wire
is used as "feedstock" for the system. A supersonic plasma jet melts the wire,
atomizes it and propels it onto the substrate. The plasma jet is formed by a
transferred arc between a non-consumable cathode and the type of a wire. After
atomization, forced air transports the stream of molten droplets onto the bore
wall. The particles flatten when they impinge on the surface of the substrate,
due to the high kinetic energy. The particles rapidly solidify upon contact.
The stacked particles make up a high wear resistant coating. The PTWA thermal
spray process utilizes a single wire as the feedstock material. All conductive
wires up to and including 0.0625" can be used as feedstock material, including
"cored" wires. PTWA can be used to apply a coating to the wear surface of engine
or transmission components to replace a bushing or bearing. For example, using
PTWA to coat the bearing surface of a connecting rod offers a number of
benefits including reductions in weight, cost, friction potential, and stress in
the connecting rod. High velocity oxygen fuel spraying
During the 1980s, a class of thermal spray processes called high velocity
oxy-fuel spraying was developed. A mixture of gaseous or liquid fuel and
oxygen is fed into a combustion chamber, where they are ignited and combusted
continuously. The resultant hot gas at a pressure close to 1 MPa emanates through
a converging–diverging nozzle and travels through a straight section. The
fuels can be gases or liquids. The jet velocity at the exit of the barrel
exceeds the speed of sound. A powder feed stock is injected into the gas
stream, which accelerates the powder up to 800 m/s. The stream of hot gas and
powder is directed towards the surface to be coated. The powder partially melts
in the stream, and deposits upon the substrate. The resulting coating has low
porosity and high bond strength. HVOF coatings may be as thick as 12 mm.
It is typically used to deposit wear and corrosion resistant coatings on
materials, such as ceramic and metallic layers. Common powders include WC-Co,
chromium carbide, MCrAlY, and alumina. The process has been most successful for
depositing cermet materials and other corrosion-resistant alloys.
Cold spraying In the 1990s, cold spraying was
introduced. The method was originally developed in Russia, with the accidental
observation of the rapid formation of coatings. This occurred while
experimenting with particle erosion of a target exposed to a high velocity flow
loaded with fine powder in a wind tunnel. In cold spraying, particles are
accelerated to very high speeds by the carrier gas forced through a
converging–diverging de Laval type nozzle. Upon impact, solid particles
with sufficient kinetic energy deform plastically and bond mechanically to the
substrate to form a coating. The critical velocity needed to form bonding
depends on the material's properties, powder size and temperature. Metals,
polymers, ceramics, composite materials and nanocrystalline powders can be
deposited using cold spraying. Soft metals such as Cu and Al are best suited
for cold spraying, but coating of other materials by cold spraying has been
reported. The deposition efficiency is typically
low for alloy powders, and the window of process parameters and suitable powder
sizes is narrow. To accelerate powders to higher velocity, finer powders (