Report of plasma spraying

Plasma Coating
SNJB’s Late Sau. Kantabai Bhavarlalji Jain college of Engineering, Chandwad
1
1. INTRODUCTION
A coating is a relatively thin layer of material which is applied to cover a substrate.
Coatings are applied for a variety of reasons. One of the most common reasons is to
improve the surface properties of a substrate. By using different coating processes,
coating materials and the process parameters, the coating properties may differ. In the
industrial applications the process properties include thickness, porosity, adhesion,
deposition rate and surface finish. The optimum coating process is selected on the
basis of desired coating properties. Coating acts as a protective layer in the different
infrastructure like pipelines, mining equipments, tunnels where durability is the major
importance as well as in many industries like automotive, aerospace, oil and gas
mining, shipbuilding etc. In some cases the coatings are used to improve the surface
properties like adhesion, corrosion resistance, wear resistance.
Plasma spray is one of the most versatile techniques of the thermal spray processes.
Plasma is capable of spraying all materials that are considered sprayable.
In plasma spray devices, an arc is formed in between two electrodes in a plasma
forming gas, which usually consists of either argon/hydrogen or argon/helium. The
particles are accelerated and heated in the plasma jet and then impact onto the
substrate where the sudden deceleration causes a pressure build-up at the particle–
surface interface that forces liquid material to ﬂow laterally. The liquid spreads
outward from the point of impact, solidiﬁes and forms a lamella; the coating is built by
the piling of such lamellae. In air plasma spraying, the size of the injected particles is
generally between 0.1 to l mm and the resulting lamellae have a thickness of a few
micrometers and a diameter ranging from a few tens to hundreds of micrometers. As
the plasma gas is heated by the arc, it expands and is accelerated through a shaped
nozzle, creating velocities up to mach 2. Temperatures in the arc zone approach
20,000°K. Temperatures in the plasma jet are still 10,000°K several centimetres from
the exit of the nozzle. The powder particles get impacted on the surface which is
known as the substrate.
This process finds many applications in the engineering field such as the internal
coating of pipelines, condenser tubes, IC engine parts etc. and it also uses for glazing
purpose as well as it acts as the anti corrosive method.

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2. LITERATURE REVIEW
Dr. Satrugna Das[4], the past decade has seen a rapid development in the range of
techniques which are available to modify the surfaces of engineering components. In
the last two decades this in turn has led to the emergence to the new field of surface
modification. It describes the about the aluminium-aluminide coatings. Surface
Engineering is the name of the discipline and surface modification is the philosophy
behind it. The object of surface engineering is to up- grade their functional capabilities
keeping the economic factors in mind.
Christian Moreau et.al. [5], Plasma spraying is often assumed to be a mature
technology in which all the important phenomena have been observed and described
adequately. These technologies include the spraying of liquid feedstock in the form of
sub micrometric particles or chemical precursors in a solvent and, coatings formed by
vapour condensation onto the substrate. This paper attempts to dine some of the
current important issues like arc dynamics, recent designs of spray guns and research
priorities in the plasma spray field.
Kannan et.al. [3], Cold spray process is a new technique of thermal spray process
which is used in industries and very limited data is available. This paper presents an
investigation on the powder stream characteristics in cold spray supersonic nozzles.
This work describes a detailed study of the various parameters, namely applied gas
pressure, gas temperature, size of particles, outlet gas velocity, dimensions of the
nozzle on the outlet velocity of the particles. A model of a two-dimensional axis
symmetric nozzle was used to generate the flow field of particles (copper or tin) with
the help of a carrier gas (compressed) stream like nitrogen or helium flowing at
supersonic speed.
Hemant Panchal et.al. [1], Thermal Spray is a generic term for a group of processes in
which metallic, ceramic, cermets, and some polymeric materials in the form of
powder, wire, or rod are fed to a torch or gun with which they are heated to near or
somewhat above their melting point. The resulting molten or nearly molten droplets of
material are accelerated in a gas stream and projected against the surface to be coated.
These papers covers design of different plasma guns for different processes.

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3. GENERAL BACKGROUND OF THERMAL COATING
PROCESS
Different coating processes are as follows:-
3.1. Overlay Coating
This type of coating is performed by the application of new materials onto the surface
of a component. A major issue of overlay coating is the adhesion of the coating to the
substrate. There will be a great chances of the breakage of adhesion due to the
different properties possesses by the coating material as well as the substrate.
3.2. Diffusion Coating
In this category, chemical interaction of the coating elements with the substrate by
diffusion is involved. New element is diffused onto the substrate surface. The main
advantage of this type of coating is that the uniformity in the coating process is
obtained.
Fig.3.1. Schematic Diagram of Thermal Coating Process [1]
3.3. Thermal Spray Coating
It is the process that involves the deposition of the molten or semi-molten droplets of
powder onto the surface of a substrate to form a coating. For protective coating to

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material surfaces, thermal spraying is widely used in the industrial process. It exhibits
a very good wear resistance property but its corrosion resistance is not good as good as
its wear resistance [2]. As the Fig.3.1. shows schematic diagram shows general
classification of the different thermal coating processes
Different thermal coating processes are as follows
3.3.1. Plasma Spraying
Plasma arc spraying is one of the most sophisticated and versatile thermal spray
methods. As the plasma spray process uses a DC electric arc to generate a stream of
high temperature ionized plasma gas, which acts as the spraying heat source. The
plasma gun comprises a copper anode and tungsten cathode, both of which are water
cooled. A high frequency arc is ignited between them. Plasma gas (i.e., He, H2, N2 or
mixtures) flows around the cathode and through the anode which is shaped as a
constricting nozzle and ionized such that a plasma plume several centi-meters in
length develops. The temperature within the plume can reach as high as 16000° K. The
spray material is injected as a powder outside of the gun nozzle into the plasma plume,
where it is melted, and hurled by the gas onto the substrate surface.
3.3.2. Electric Arc Wire Spray
In the electric arc spray process, two consumable wire electrodes connected to a high-
current direct-current (dc) power source are fed into the gun and meet, establishing an
arc between them that melts the tips of the wires. The process is energy efficient
because all of the input energy is used to melt the metal. The molten metal is then
atomized and propelled toward the substrate by a stream of air. Spray rates are driven
primarily by operating current and vary as a function of both melting point and
conductivity. Substrate temperatures can be very low, because no hot jet of gas is
directed toward the substrate. Electric arc spraying also can be carried out using inert
gases or in a controlled-atmosphere chamber.
3.3.3. Flame Spray
Flame spraying is the oldest of the thermal spraying processes, characterized by low
capital investment, high deposition rates and efficiencies, and relative ease of
operation and cost of equipment maintenance. Flame spray uses combustible gas as a

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5
heat source to melt the coating material. A wide variety of materials can be deposited
in rod, wire, or powder form as coatings using this process Flame spray guns and vast
majority of components are sprayed manually, so the quality of coating depends upon
the person to person according to handling.
3.3.4. HVOF
In the HVOF process, fuel and oxygen are introduced to the combustion chamber
together with the spray powder. The combustion of the gases produces a high
temperature and high pressure in the chamber, which causes the supersonic flow of the
gases through the nozzle. The powder particles melt or partially melt in the
combustion chamber and during the flight through the nozzle. The flame temperature
varies in the range of 2500 °C to 3200 °C, depending on the fuel, the fuel gas/ oxygen
ratio and the gas pressure.
Table 3.1. Comparison of Different Thermal Coating Process [1]
Process
Coating
Material
Form
Heat
Source
Flame
temperature
oc
Gas
Velocity
m/s
Poros
-ity
%
Coating
Adhesion
Mpa
Plasma
Spray Powder
Plasma
Flame 12000-16000 500-600 2-5 4070
Wire
Arc
Spray
Wire
Electric Arc
5000-6000 <300 5-10 28-41
Wire
Flame
Spray
Wire
Oxy-fuel
Combustion 3000 <300 5-10 14-21
HVOF Powder
Oxy-gas
fuel
Combustion
3200 1200 1-2 >70

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4. INTRODUCTIONOF PLASMA
Plasma is one of the four fundamental states of matter, the others being solid, liquid,
and gas. Yet unlike these three states of matter, plasma does not naturally exist on the
earth under normal surface conditions, and can only be artificially generated from
neutral gases. The term was first introduced by chemist Irving Langmuir in the 1920.
Plasma was first identified in a Crookes tube, and so described by Sir William Crookes
in 1879. Plasma is an electrically neutral medium of unbound positive and negative
particles. Although these particles are unbound, they are not ‘free’ in the sense of not
experiencing forces. Moving charged particles generates an electric current within a
magnetic field; and any movement of a charged plasma particle affects and is affected
by the general field created by the motion of other charges. In turn this governs
collective behaviour with many degrees of variation.
Fig.4.1. Plasma Production [10]
It can simply be considered as a gaseous mixture of negatively charged electrons and
highly charged positive ions, being created by heating a gas or by subjecting gas to a
strong electromagnetic field. However, true plasma production is from the distinct
separation of these ions and electrons that produces an electric field, which in turn,
produces electric currents and magnetic fields.
The positive charge in ions is achieved by stripping away electrons from atomic
nuclei. The number of electrons removed is related to either the increase in
temperature or the local density of other ionised matter. This also can be accompanied
by the dissociation of molecular bonds, though this fundamental process is distinctly
different from chemical processes of ion interactions in liquids or the behaviour of

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ions existing in metals. A significant number of highly charged particles together make
plasma electrically conductive.
Electric propulsion achieves high specific impulse by the acceleration of charged
particles to high velocity. The charged particles are produced by ionization of a
propellant gas, which creates both ions and electrons and forms what plasma. Plasma
is then a collection of the various charged particles that are free to move in response to
fields they generate or fields that are applied to the collection and, on the average, is
almost electrically neutral. This means that the ion and electron densities are nearly
equal, ni = ne , a condition commonly termed “quasi-neutrality.”[7]
Non-equilibrium plasma or cold plasmas, more popularly known as glow-discharge
plasmas, are low-pressure plasmas characterized by high electron temperatures ( Te )
and low ion and neutral particle temperatures ( Ti ). They are widely used in lighting,
surface cleaning, etching, film deposition and polymerization. Thermal plasmas or hot
plasmas are characterized by the electron temperature being approximately equal to
the gas temperature ( Tg ) and the plasma is said to be in local thermal equilibrium.
Normally, plasmas in the temperature range of 2,000 – 30,000 K and with charged
particle density of 1019-1021 m-3 are termed thermal plasmas. Thermal plasma
processing has been successfully applied to develop advanced ceramic coatings,
synthesis of nano crystalline materials, processing of minerals and ores, and treatment
of hazardous wastes.
Table 4.1. Plasmas of technological interest [4]
Sr. Category of plasma Applications
1 Low-pressure plasmas
(10-4 - 10-2 torr)
& ( Te >Ti >Tg )
Sputtering and surface modification
processes, plasma source for ion
implantation
2 Medium pressure plasmas
(10-2 - 1 torr)
& (Te > Ti = Tg)
Etching, microelectronic processing
3 Sub atmospheric pressure plasmas
(1- 100 torr)
& (Te > ~ Ti = Tg )
Plasma chemistry, plasma
polymerization
4 Atmospheric plasmas
(100+ torr)
& ( Te = Ti = Tg )
Plasma spraying, plasma melting,
material synthesis

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5. PRINCIPLE OF WORKING
Plasma spray deposition or plasma spraying is a process that combines particle
melting, quenching and consolidation in a single operation. The process involves
injection of powder particle into the plasma jet created by heating an inert gas in an
electric arc confined within a water-cooled nozzle. The particles injected into the
plasma jet undergo rapid melting and at the same time are accelerated [4]. These
molten droplets moving at high velocities (exceeding 100 meters/second) impact on
the surface of the substrate forming adherent coating.
Fig 5.1. Principle of plasma coating process [4]
Thermal plasma for plasma spray deposition is generated using plasma spray gun or
plasma spray torch. The spray system also includes DC power supplies, cooling water
system, gas feeding system, powder feeder and control console. The coating material
may be in the form of the thin wires or in the powder form and having the constant
feed rate. The plasma torch consists of a cathode, made of thoriated tungsten and a

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nozzle shaped copper anode. Both the electrodes are water-cooled. The electrodes are
separated by an insulating block made of nylon that has provision for gas injection.
Powder to be spray deposited is injected through an injection port located at the nozzle
exit which also accelerates the speed of the powder.
A DC arc is struck between the anode and the cathode and the arc energy is extracted
by the plasma gas, usually argon, which issues out of the nozzle with high temperature
(10,000-15,000K) and high velocity. The material can be introduced in the form of
wires or rods (Arc wire spraying and rod spraying) or powders, which is the most
widely used variant of the process. Metal or ceramic powder is injected into the
plasma jet, where the powder particles melt and the molten droplets are accelerated
towards the substrate and get deposited.
Table 5.1. General process parameters of Plasma Spray [6]
Sr Parameters Range
1 Torch input power 10-20 KW
2 Plasma gas (Ar) flow rate 85 LPM
3 Secondary gas(H 2 ) flow rate 25 LPM
4 Powder feed rate 50 g/min
5 Powder carrier gas(N2) flow rate 38 LPM
6 Torch to base distance 100 mm
This results in a typical lamellar structure. The coating-substrate interface bond
mechanism is purely mechanical. Plasma spray deposits typically have lamellar
structure with fine-grained microstructure within the lamellae. Atmospheric plasma
sprayed coatings also contain varying amounts of retained porosity and inclusions
depending on the deposition parameters. The process is controlled by the different
controlling parameters affecting the process [6].

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6. EXPERIMENTALSETUP
The details of the processes are outlined here.
Fig.6.1. Setup diagram of plasma coating process [3]
6.1 SPRAY TORCH SYSTEM
The plasma spray system developed at the Laser & Plasma Technology Division,
Bhabha Atomic Research Centre, Mumbai, has been used for plasma spray
experiments. The experimental set up is shown in Figure 6.1. The spray system
consists of [3]
1. DC Plasma Spray Torch
2. Power Supply
3. Control Console,
4. Gas Feeding System
5. Water Cooling Arrangement And
6. Powder Feeder.
6.1.1 DC Plasma Torch
The plasma torch used in the experiment is a non-transferred DC arc type (Fig.6.1).
Plasma torches used for plasma spray deposition operate in the non-transferred mode
at high currents and gas flow rates. The arc energy is extracted by the plasma gas,

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which issues out of the torch nozzle with high velocity (600-800 m/s) and high
temperature (15,000K). Metallic or ceramic powder introduced in the plasma jet melts
and the molten droplets strike the substrate surface with high velocity forming
adherent coating.
6.1.2. DC Plasma Torch and Accessories
The cathode consists of a tungsten rod with a conical tip. About 2% thorium oxide is
added to tungsten to improve the thermionic emission characteristics of tungsten. The
nozzle is made of copper and is designed in the form of a nozzle. An insulating block
of nylon separates the electrodes. The plasma gas, usually argon, is injected into the
inter-electrode region through a side port in the insulator. The electrodes are intensely
water-cooled. The nozzle has a port near its edge for feeding carrier gas and powders.
When an electric arc is struck between the cathode and anode, the plasma gas extracts
the energy from the arc and issues out of the nozzle as a high temperature, high
velocity jet. A thermal pinch effect is produced by the combined action of the cold
wall of the nozzle and the cold gas sheath around a very high temperature, conducting
core of the arc column.
Fig.6.2. Plasma Torch [1]
Any powder introduced into the plasma jet melts and the molten particles, travelling at
high velocity (about 100 m/s) are projected onto the substrate surface, where they
solidify forming an adherent coating. The dimensions are: nozzle diameter: 6 mm, gap
between the cathode and anode fixed at 12 mm and cathode length: 50 mm.

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6.2. Power supply
The torch is energized by a power supply with an open circuit voltage of 80 V. The
maximum current drawn could be 800 A DC. It is cooled by forced air. The power
supply has a full control HF unit consisting of a HF (1 MHz) transformer, tuned
circuit, spark gap and 5 stage timer circuits for initiating pilot arc. The power supply is
connected to control console through cables.
6.3. Gas feeding system
The gas feeding system consists of gas cylinders, pressure gauges and gas tubes. The
cylinders each have 7m3 capacities. The pressure was maintained at 75 kg/cm2. There
is a gas feeding arrangement for primary gas, secondary gas and carrier gas.
Appropriate gas flow rates can be selected depending on the operating power and
nature of the material to be coated.
6.4. Water cooling system
Water cooling system consists of 2 HP electric mono-block pump set, reservoir,
cooling tower and pipelines. Water-cooling is made for power cables, power supply
unit, cathode and anode separately. There are water flow meters and thermometers for
measurement of water flow rate and inlet and outlet temperature respectively.
6.5. Powder feeding system
Powder flow rate could be varied by motor speed. The carrier gas flow rate was
chosen such that the powder particles enter the plasma core. At lower flow rate, the
particles may not be able to enter the core of the plasma leading to poor coating
quality. On the other hand, if the carrier gas flow is very large, the powder particles
will cross the central plasma zone without proper melting leading to poor quality of
coating. The carrier gas flow rate needs to be optimized for each particular powder.
6.6. Control console
The control console displays the arc current, arc voltage, flow rate of primary gas and
secondary gas, test/run mode switches. It has also water flow and gas flow indication
lamp. Appropriate flow meters were used to monitor the plasma forming gas flow

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rates. It also consists of the relays and solenoid valves and other interlocking
arrangements essential for safe running of the equipment. For example, the arc can
only be started if the cooling water supply is on and water pressure & flow rate is
adequate.
Table 6.1. Process Parameters
Sr Parameter Value
1 Plasma Torch DC Arc type
2 Arc Temperature 150000 K
3 Gas Velocity 600-800 m/s
4 Substrate Aluminium alloy
5 Coating Powder Copper
6 Nozzle Copper
7 Insulator Nylon
8 Plasma Gas Argon
9 Arc Velocity 100 m/s
10 Cathode Tungstan with 2% thorium oxide
11 Nozzle Diameter 6 mm
12 Gap Between the Cathode And Anode 12 mm
13 Cathode Length 50 mm
14 Current 800 A DC
15 Applied Voltage 80 V
16 Gas Pressure 75 kg/cm2
17 Input Gas Cylinder Volume 7m3

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7. FACTORS AFFECTING THE PROCESS
7.1. Roughness of the substrate surface
A rough surface provides a good coating adhesion. A rough surface provides enough
room for anchorage of the splats facilitating bonding through mechanical interlocking.
A rough surface is generally created by shot blasting technique. The shorts are kept
inside a hopper, and compressed air is supplied at the bottom of the hopper. The shorts
used for this purpose are irregular in shape, highly angular in nature, and made up of
hard material like alumina, silicon carbide, etc. Upon impact they create small craters
on the surface by localized plastic deformation, and finally yield a very rough and
highly worked surface. The roughness obtained is determined by shot blasting
parameters, i.e., shot size, shape and material, air pressure, standoff distance between
nozzle and the job, angle of impact, substrate material etc.[3]
7.2. Cleanliness of the substrates
The substrate to be sprayed on must be free from any dirt or grease or any other
material that might prevent intimate contact of the splat and substrate. For this purpose
the substrate must be thoroughly cleaned with a solvent before spraying. Spraying
must be conducted immediately after shot blasting and cleaning. Otherwise on the
nascent surfaces, oxide layers tend to grow quickly and moisture may also affect the
surface. These factors deteriorate the coating quality drastically.
7.3. Cooling water
For cooling purpose distilled water should be used, whenever possible. Normally a
small volume of distilled water is re-circulated into the gun and it is cooled by an
external water supply from a large tank. Sometime water from a large external tank is
pumped directly into the gun.
7.4. Arc power
It is the electrical power drawn by the arc. The power is injected in to the plasma gas,
which in turn heats the plasma stream. Part of the power is dissipated as radiation and
also by the gun cooling water. Arc power determines the mass flow rate of a given
powder that can be effectively melted by the arc. On the contrary, once a complete

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15
particle melting is achieved, a higher gas temperature may prove to be harmful. In the
case of steel, at some point vaporization may take place lowering the deposition
efficiency.
7.5. Plasma gas
The most commonly used gases for plasma generation are argon, nitrogen, helium,
hydrogen and air. Plasma gas flow rate and the electric power to the plasma torch must
be properly balanced in order to get a stable arc. The choice of plasma gas depends on
many factors, such as the design features of the torch, in particular the electrode
materials. In the case of plasma torches employing tungsten cathode, the choice of
plasma gas is limited to inert gases and non-oxidizing gases. Gas enthalpy is another
important factor deciding the choice of the gas. The major constituent of the gas
mixture is known as primary gas and the minor is known as the secondary gas. The
neutral molecules are subjected to the electron bombardment resulting in their
ionization.
7.6. Mass flow rate of powder
Ideal mass flow rate for each powder has to be determined. Spraying with a lower
mass flow rate keeping all other conditions constant results in under utilization and
slow coating build up. On the other hand, a very high mass flow rate may give rise to
an incomplete melting, resulting a high amount of porosity in the coating. The un-
melted powders may bounce off from the substrate surface as well keeping the
deposition efficiency low.
7.7. Torch to base distance
It is the distance between the tip of the gun and the substrate surface. A long distance
may result in freezing of the melted particles before they reach the target, whereas a
short standoff distance may not provide sufficient time for the particles in flight to
melt. The relationship between the coating properties and spray parameters in spraying
alpha alumina has been studied in details. It is found that the porosity increases and the
thickness of the coating (hence deposition efficiency) decreases with an increase in
standoff distance. From mechanical point of view, adhesion can be estimated by the
force corresponding to interfacial fracture and is macroscopic in nature. Coating

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adhesion tests have been carried out by many investigators with various coatings. It
should be noted that the fracture mode is adhesive if it takes place at the coating-
substrate interface and that the measured adhesion value is the value of practical
adhesion, which later is strictly an interface property, depend exclusively on the
surface characteristics of the adhering phase and the substrate surface condition. The
variation of interface bond strength of Fe-Al and Ni-Al coatings on selected substrates
with respect to different power level as well as at various torch to base (i.e. substrate)
distances are shown in fig.7.7.
Fig.7.7. Strength and TBD Relationship [4]
Maximum adhesion strength of ~15 MPa and of ~23.5 MPa was found in the case of
Fe-Al bond coatings, deposited at a torch to base distance (TBD) of 100mm both at
10kW and at 16 kW input power levels respectively. It is observed that, with increase
in torch input power level; there is an increase in the coating adhesion strength. But
increase in the spraying distance (TBD), the adhesion strength decreases.
7.8. Spraying angle
This parameter is varied to accommodate the shape of the substrate. In coating
alumina on mild steel substrate, the coating porosity is found to increase as the
spraying angle is increased from 300 to 600. Beyond 600 the porosity level remains
unaffected by a further increase in spraying angle. The spraying angle also affects the

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adhesive strength of the coating. The influence of spraying angle on the cohesive
strength of chromia, zirconia-8 wt%, and molybdenum has been investigated, and it
has been found that the spraying angle does not have much influence on the cohesive
strength of the coatings.[4]
7.9. Substrate cooling
During a continuous spraying, the substrate might get heated up and may develop
thermal-stress related distortion accompanied by a coating peel-off. This is especially
true in situations where thick deposits are to be applied. To harness the substrate
temperature, it is kept cool by an auxiliary air supply system. In additions, the cooling
air jet removes the un-melted particles from the coated surface and helps to reduce the
porosity.
7.10. Preheating of the substrate
The nascent shot blasted surface of the substrate absorbs water and oxygen
immediately after shot blasting. Before spraying, the substrate should be preheated to
remove moisture from the surface and also for a sputter cleaning effect of the surface
by the ions of the plasma.
7.11. Angle of Powder Injection
Powders can be injected into the plasma jet perpendicularly, coaxially, or obliquely.
The residence time of the powders in the plasma jet will vary with the angle of
injection for a given carrier gas flow rate. The residence time in turn will influence the
degree of melting of a given powder. For example, to melt high melting point
materials a long residence time and hence oblique injection may prove to be useful.
The angle of injection is found to influence the cohesive and adhesive strength of the
coatings as well.
7.12. Gas Pressure
In an experiment performed the effect of the gas inlet pressure on the deposition
efficiency was investigated and the results showed that deposition efficiency increases
with increase in the gas pressure. A separate gas compressor is required in these
systems and gases such as helium is used in the system because of its low molecular

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weight. In a low pressure system a powder stream is injected into the nozzle at the
point where gas has expanded to low pressure. Since no pressurized feeder is required
in this system, it is often used in portable cold spray systems.
Fig.7.12. Effect of gas pressure [4]
7.13. Type of Gas
In cold spray processes the type of gas used to spray powder particles plays an
important role in the acceleration of particles. Plasma spray process parameters were
also developed with nitrogen to reduce the costs while maintaining satisfactory coating
performance. In one-dimensional flow theory the Mach number at the throat is
assumed to be unity and the velocity of gas can be calculated from:
𝑉 = 𝛾𝑅𝑇
Where 𝛾=specific heat ratio
T is temperature of gas
R=Specific gas constant
The above equation shows why it is often found that helium makes a better carrier gas
for cold spraying. It has a smaller molecular weight and higher specific heat ratio. The
specific heat ratios of nitrogen and helium are 1.4 and 1.66 respectively. The specific
gas constants for nitrogen and helium are 296.8 J/Kg K and 2,077 J/Kg K respectively.

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7.14. Particle Size
The powder is fed into a gas stream flowing through the nozzle. The acceleration of
each particle depends upon its size. Small particles achieve high acceleration and large
particles achieve less acceleration. For making a successful powder deposit only the
particles with a velocity greater than a critical velocity can contribute to the coating.
The particles size distribution can be expressed by the following Rosin-Rammler
formula:-
𝑌 𝑑 = 𝑒(-d/d’)n
Where 𝑌 𝑑=mass fraction
n=the size distribution
At high temperatures more plastic deformation occurs in particles when they strike a
substrate and this improves deposition efficiency. A previous study shows that the
particle temperature reaches maximum value when the diameter of the particle is
10μm.[4]
7.15. Deposition Efficiency
It is an important factor that determines the techno- economics of the process. And it is
calculated using the relation as mentioned below.
η = (Gc / Gp) x 100 %
Where η is the deposition efficiency
Gc is the weight of coating deposited on the substrate
Gp is the weight of the expended feedstock
Plasma spray deposition efficiency of a given materials depends on its melting point,
thermal heat capacity and particle size of the powder. At lower input power to the
plasma torch, the plasma jet temperature is not high enough to melt the entire powder
particles that enter the plasma jet. As the power is increased, the average plasma
temperature increases melting a larger fraction of the powder. The spray efficiency,
therefore, increases with plasma power. However, beyond a certain power level of
plasma, the temperature of the plasma gas is very high, leading to vaporization or the

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Fig.7.15. Deposition Efficiency of Fe-Al Bond-coat on metal substrates [4]
dissociation of the powder particles. This causes the deposition efficiency to decrease
at higher power levels. This tendency is observed in all materials. However, the
plasma power above which the efficiency decrease depends on the chemical nature of
the powder and its particle size. In the present investigation the deposition efficiency
increased from 20% to 50% on aluminium substrates, from 32% to 60% on copper,
from 28% to 54% on mild steel and from 24% to 48% on stainless steel substrates
(with input power to plasma torch increasing from 10 kW to 20 kW) in case of Fe-Al
bond coating.
.

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8. APPLICATIONS, ADVANTAGES AND LIMITATIONS:-
8.1. Applications
Plasma spraying is extensively used in hi-tech industries like aerospace, nuclear
energy as well as conventional industries like textiles, chemicals, plastics and paper
mainly as wear resistant coatings in crucial components.
Table 8.1. Applications of Plasma Coating
Sr Industrial Sector or Coated
Component
Coating Material
1 Textile industry:
Thread guiding & distribution rollers,
ridge thread brakes, distribution plates,
driving & driven rollers, gallets, tension
rollers, thread brake caps
Al2O3 + 3% TiO2, Al2O3 + 13%
TiO2, Cr2O3, WC + Co
2 Paper and printing industry:
Paper drying rolls, sieves, filters, roll
pins etc. in paper machines, printing
rolls, tension rolls
Oxide layers composed of Al2O3
with 3 to 13 % additions of TiO2,
Cr2O3 or MnO2
3 Automotive Industry and the
production of Combustion engines:
 Steel piston rings
 Gear-shift forks for gear
boxes
 Diesel engine pistons
 Ship engine valves
 Piston crown & cylinder head in
adiabatic Diesel engines
 Water pumps
 Brake drum
 Bronze valves
 Mo+NiCrBSi
 Coated with a layer of bronze
0.4mm over a bond coat
NiAl.
 ZrO2+MgO or ZrO2+Y2O3
 NiCrAl on valve heads
 CoCrAlY of 500μm
 NiCrAl or NiCrAlY
 Al2O3+TiO2 or Cr2O3
 Mixture of ceramic material
 NiCrBSi + CuSn + MoS2

Plasma Coating
22
4 Hydraulic Equipment:-
 Turbine blades
 Bronze valves in the production
of hydrostatic transducer
 WC, Al2O3
 Coatings of Mo or special
alloys type NiCrBSi + CuSn
+ MoS2
5 Chemical Industry:-
 Blades of a chemical mixer
 Roll for the production of
plastic foils
 Fan blades
 Induction Flow meter
 NiCrBSi
 Al2O3
 Al2O3
 ZrSiO4 on the internal
surfaces
6 Aircraft Industry:-
 Stationary blades of jet engines
 Combustion chambers and
guide blades
FeCrAlY, CoNiCrAlY
ZrO2+MgO,ZrO2+Y2O3
8.2. Benefits/ Advantages
1. Plasma coating finds wide range of applications in industrial sector
2. Plasma coating is more flexibility than the other traditional coating process.
3. It increases the life of component by 20 to 30% than the other traditional
coating processes.
4. It indirectly affects on the reduction in idle timing as well as in the process of
replacement of worn out parts.
5. It ultimately reduces the corrosion rate as well as the cavitations of different
automobile parts.
6. The plasma coating offers the shiny as well as the glazed look and
simultaneously improves the aesthetic look.
7. Plasma coating processes have low running cost.
8. It gives good adhesion strength as well as the good case hardening strength.
9. In case of Hi-tech applications the process is fully automated.

Plasma Coating
23
10. As the process involves automation so the homogeneous coatings are obtain.
8.3. Limitation
1. The process requires high initial setup cost.
2. The process is only suitable for the batch production.
3. Due to the less interference of worker may lead to unemployment issues.
4. Due to the high initial cost it is impossible implement such techniques by small
scale industries.

Plasma Coating
24
9. CONCLUSION/SUMMARY
As according to the adjoined research papers, internet sources as well as the different
information gathered regarding this seminar work the following conclusions are made.
1. The plasma coating is highly developing and mature technology among the
other coating processes.
2. The quality of coating is greatly affected by the different process parameters
like deposition efficiency, TBD, particle size, gas pressure and temperature etc.
3. As according to the use of automation techniques the wide range of coatings
are available with different thickness size with the use of different coating
materials.
4. The results show that by increasing the gas temperature increases the gas
velocity more significantly than the gas pressure, which shows that the gas
velocity is a function of gas temperature and not the gas pressure. The result
shows the particle size distribution plays a major role in the particle exit
velocity i.e. lighter particles travels much faster than heavier particles.
5. Surface coating improves the life of the component and reduces the cost of
replacement. The purpose of surface technology is to produce functionally
effective surfaces. A wide range of coatings can improve the corrosion, erosion
and wear resistance of materials.
6. By using plasma spray technique uniform coating thickness, continuous layer
of coating and high hardness can be obtained. It has more advantages over the
high strength, hardness, porosity, wear and corrosion as compared to other
process.

Report of plasma spraying

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