Thermal spraying technique by using plasma

1. PLASMA SPRAYING
Presented by:
ROLWYN MARIAN CARDOZA
MACHINE DESIGN
RV COLLEGE OF ENGINEERING
BANGALORE.

2. 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.
 Improve the surface properties of a substrate.
 To obtain different properties such as thickness, porosity, , corrosion
resistance, wear resistance. and surface finish.
 Coating acts as a protective layer in the different infrastructure like
pipelines, mining equipment’s and is used in many industries like
automotive, aerospace, oil and gas mining, shipbuilding etc.
 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.

3. PRINCIPLE:
• The process involves injection of powder
particles into the plasma jet created by
heating an inert gas in an electric arc
confined within a water-cooled nozzle.
• The particles are accelerated and heated in
the plasma jet and then impact onto the
substrate
• 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

4. THE EXPERIMENTAL SET UP
The plasma spray system developed at the Laser & Plasma Technology Division,
Bhabha Atomic Research Centre, Mumbai, has been used for plasma spray
experiments.
1. DC Plasma Spray Torch
2. Power Supply
3. Control Panel
4. Gas Feeding System
5. Water Cooling Arrangement
6. Powder Feeder.

5. Plasma Spray Torch
 The cathode consists of a tungsten rod with a conical
tip.
 About 2% thorium oxide is added to tungsten to improve
the thermionic emission.
 The anode is made of copper shaped as nozzle.
 An insulating block of nylon encloses the electrodes.
 The electrodes are intensely water-cooled
 The plasma gas, usually argon, is used
 The nozzle has a port near its edge for feeding
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 of Mach 2.

6.  size of the powder = 10 to 80 um
 plasma jet melts and the molten
particles, travelling at high
velocity about 100 m/s
 The dimensions are:
 nozzle diameter: 6 mm
 gap between the cathode and anode
fixed at 12 mm
 cathode length: 50 mm.

7. Power supply & Water cooling system
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.
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

8. Gas feeding &Powder feeding system
Gas feeding system
 The gas feeding system consists of gas cylinders, pressure
gauges and gas tubes. The cylinders each have 7 m3
capacities.
 The pressure was maintained at 7.5 Mpa
 The carrier gas flow rate was chosen such that the powder
particles enter the plasma core.
Powder feeding system
 At lower flow rate, the particles may not be able to enter
the core of the plasma leading to poor coating quality.
 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

10. Control Panel
 The control panel displays the arc current, arc voltage, flow rate of 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, Plasma
Coating 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.

11. FACTORS AFFECTING THE PROCESS
1) Roughness of the substrate surface
 A rough surface provides a good coating
adhesion.
 A rough surface is generally created by shot
blasting technique.
 The shots are kept inside a hopper, and
compressed air is supplied at the bottom of the
hopper. The shots 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.

12. 2)Gas Pressure
 In an experiment performed the effect of
the gas inlet pressure on the velocity was
investigated and the results showed that
velocity increases with increase in the gas
pressure.
3) Particle Size
 The decrease in particle size decreases
the porosity as the material melts easily.

13. 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
 Arc power determines the mass flow rate of a given
powder that can be effectively melted by the arc.
 Increased Power ,In the case of steel, at some point
vaporization may take place lowering the deposition
efficiency.

14. 5)Plasma gas flow rate
 The most commonly used gases for plasma generation are
argon, helium, hydrogen, nitrogen and air.
 The major constituent of the gas mixture is known as
primary gas and the minor is known as the secondary gas.
6)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.

15. Advantages:
 The plasma coatings are generally denser, stronger, and much cleaner than a
thermal spray process
 It increases the life of component by 20 to 30% than the other traditional
coating processes
 The plasma coating offers the shiny as well as the glazed look and
simultaneously improves the aesthetic look.
 It gives good adhesion strength as well as the good case hardening strength.

16. Disadvantages
1. The process requires high initial setup cost.
2. It is a line-of-sight process, similar to all other thermal spraying processes,
making it difficult to coat internal bores of small diameters or restricted access
surfaces.
3.The process is only suitable for the batch production.
4. Due to the less interference of worker may lead to unemployment issues.
5. Due to the high initial cost it is impossible implement such techniques by small
scale industries.

17. Applications
•Seal ring grooves in the compressor area of aero-engine turbines with tungsten
carbide/cobalt to resist fretting wear.
•Zirconia-based thermal barrier coatings (TBCs) on turbine combustion chambers.
•Wear resistant alumina and chromium oxide ceramic on printing rolls for
subsequent laser and diamond engraving/etching.
•Molybdenum alloys on to diesel engine piston rings.

18. CONCLUSION
 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

19. Case study1: Effects of spray distance on the microstructure and
mechanical properties of plasma sprayed Ti-graphite coatings
Powder materials : Commercial Ti powder and graphite were used with mass ratio of
6:1 were wet-mixed by deionized water, sodium carboxymethyl cellulose and
Gum Arabic, and stirred by an electro-motion for 2 h to form slurry. Then
dispersed slurry was sprayed using feeding system.
Substrate :Mild carbon steel specimens with size of 10 mm × 10mm × 10
mm
Gas used :Argon, Nitrogen.

20. Case 2:Plasma sprayed diamond
reinforced molybdenum coatings
Coating thickness =250-280 µm
Powder: 90wt % of molybdenum powder + 10% mono-crystalline diamond
particles.
Mixing-The ball milling was continued for 36 hours at 500 rpm in a toluene
medium.
Powder obtained:40-70 µm
Substrate: Test coupons of dimension 50mm ×50mm ×6mm (thickness) were cut
off from rolled strips of AISI 52100 bearing steel
Heating Temperature : 3000K

21. References
Case study
1. Effects of spray distance on the microstructure and mechanical properties
of reactive plasma sprayed Ti-gaphite coatings
2. Das, P., Paul, S., & Bandyopadhyay, P. P. (2018). Plasma sprayed diamond
reinforced molybdenum coatings. Journal of Alloys and Compounds, 767, 448–
455.doi:10.1016/j.jallcom.2018.07.088

22. THANK YOU