The document discusses different thermal spraying coating processes. It describes combustion flame spraying, high velocity oxy-fuel spraying, two wire electric arc spraying, plasma spraying, and vacuum plasma spraying. Plasma spraying allows any material to be coated and produces coatings with very low porosity. Carbon nanotubes were added to TiO2 coatings via plasma spraying to improve tribological properties. Test results showed the TiO2-CNT coatings had lower friction and 96% less wear than pure TiO2 coatings due to reinforcement and lubricating effects of the carbon nanotubes.

1. Presented By
Lucky
2019MEZ8417
Ph.D. Student, Production Eng.
COATINGS BY THERMAL
SPRAYING
Centre For Automotive Research and Tribology (CART),
IIT Delhi

2. Contents
■ What is thermal spraying
■ Classification of processes
■ Processes details
■ Paper review/case study
■ Query/feedback
■ References
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3. Thermal spraying
■ Melted or heated elements are
sprayed on substrate
■ Three methods used to heat the
feed stock i.e., plasma, arc and fuel
gas.
■ Coating thickness: 20µm- several
mm
■ Metals, metal oxides, ceramics,
alloys and non-metals can be feed
materials
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Fig. 1- Thermal Spraying
Source: https://metallisation.com/applications/thermal-spray-
engineering-applications/

4. Classifications
They are divided into following types:
■ Combustion flame spraying
■ High velocity oxyfuel spraying
■ Two wire electric arc spraying
■ Plasma spraying
■ Vacuum plasma spraying
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5. Combustion Flame Spraying
■ Relatively low temperature
process.
■ Lower melting point materials.
■ Powder or wire stock materials
are used
■ High level porosity and rough
coating are obtained.
■ Oxidation resistance is poor
■ Higher deposits of metal oxides
and poor bond strength
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Fig. Combustion Flame Spraying
Source: https://www.ep-coatings.com/processes/flame-spraying/

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Fig. combustion spraying in process
Source: https://www.ep-coatings.com/processes/flame-spraying/

7. High Velocity Oxyfuel
Spraying
■ Coating powder is delivered axially in
pre-ignited flame.
■ Supersonic ejection of mixture
towards the substrate surface.
■ Helps to increase erosion and wear
resistance, and protection against
corrosion.
■ Process results in exception hardness
and excellent adhesion properties.
■ Provides low porosity coating i.e.,
increased oxidation resistance.
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Fig. HVOF process; 1. carrier gas with powder
2. fuel gas 3. compressed gas
Source: www.manufacturingguide.com/en/high-velocity-oxygen-fuel-coating-hvof#

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9. Two wire electric arc spraying
■ Arc produced between two wire used to
melt them. As name says , wires are fed
as coatings in molten form.
■ molten material projected towards the
substrate surface. by compressed gas in
atomized form.
■ DC-Constant voltage power source
■ Deposition rate: 3-5 times greater than
flame spraying
■ Denser coating & higher strength than
flame spray coatings
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10. Plasma Spray coatings
■ One of most flexible thermal spray process.
■ Less distortion to substrate and various kinds of substrates can be used.
■ Any material can be coated😃; even ceramics, cermet and refractory materials can also
be deposited!!!
■ Plasma is produced by discharge between two non-consumable electrodes and inert
gas acts as a medium.
■ Temperature of plasma is significantly high up to 25000 centigrade.
■ Filler material is fed towards substrate through plasma.
■ Vacuum plasma sprayed coatings have very less porosity and higher density.
■ The pressure in Vacuum plasma coating varies from 10-50kPa
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Fig. Vacuum plasma spray
Source: Oerlikon Metco
Fig. Air plasma spraying
Source:
https://www.researchgate.net/publication/259
003877_Parametric_Appraisal_of_Process_Par
ameters_for_Adhesion_of_Plasma_Sprayed_Na
nostructured_YSZ_Coatings_Using_Taguchi_Exp
erimental_Design
Photo: Vacuum plasma spraying
Source: Plasma Coating System for Chinese University Institute

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fig. comparison of
different processes
Source: the fabricator

13. Applications
■ Combustion flame spraying- Crank shafts, sucker rod
couplings
■ HVOF spray coatings- Landing Gear Legs, Flap Tracks in
aeronautical applications
■ Arc spray- Anti corrosion coatings, Erosion resistance for
cooking vessels and chemical industry process equipment.
■ Plasma arc spray- used in very versatile industries, for
instance, medical, marine and electronics. Where high
temperature, extreme wear resistance and chemical
inertness is required.
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Sucker rod couplings
A Flap track

14. Paper title: Tribological behavior of plasma
sprayed carbon nanotubes reinforced TiO2
coatings
■ Journal : Journal of the European Ceramic Society
■ Authors: Hai-douWangaPeng-feiHeaGuo-zhengMaabBin-shiXuaZhi-guoXingaShu-
yingChenaZheLiuacYi-wenWanga
■ Other details: Volume 38, Issue 10, August 2018, Pages 3660-3672
■ Aim: to reveal the degradation degree of the CNTs in TiO2 matrix during plasma
spraying.
■ Material used:
– Substrate: AISI 4140 steel
– Materials to deposited: nano-TiO2 (∼60 nm, purity >99.9%), nano-TiO2 and
3 wt.% multi-walled CNTs.
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15. ■ coatings were polished to a surface roughness (Ra) of 0.21–0.38 μm with a
thickness of 250–280 μm
■ Raman spectroscopy, SEM and TEM were used to reveal microstructures.
■ Frictional experiments were conducted with Universal Wear Apparatus: UT-3MT at
room temperature and 25-35% relative humidity
– Applied load: 20N
– Frequency: 10Hz
– Reciprocating distance: 4mm
– Time: 20mins
■ Counter Ball Material: Zirconia
■ LEXT, QLS 4000 laser 3D microscope used to check the topography of worn
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Diameter Grade Density Hardness Fracture
toughness
4 mm G10 6.05 G/CM3 12 GPa 10 Mpa m1/2

16. Results
■ CNT in layer is retained due to:
– Small residence time of CNT in plasma
– Existence of CNTs in partially melted regions
– Covering by molten rutile layer
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Fig.(a) Schematic of plasma
spraying process for fabricating
TiO2-CNTs coating; (b) SEM image
of a CNT shielded by a molten TiO2
film between fully melted (Ⅰ) and
partially melted (Ⅱ) regions [16];
(c) SEM image of an intact CNT
emerging from an unmelted region
(Ⅲ)

17. ■ Co-efficient of friction:
■ Wear resistance:
– Wear volume in coatings with TiO2-CNT is dropped by 96.36%
– Global reduction in depth and width of track profile
– Higher fracture toughness and hardness resulted in good wear residence and the wear track
are provided on next page
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H.-d. Wang et al. Journal of the European Ceramic Society 38
(2018) 3660–3672 Fig. . Typical friction coefficient and
combined height change of the counterparts versus sliding
time for TiO2 and TiO2-CNTs coatings.

18. ■ Comparison of wear tracks:
– Macro cracks observed in TiO2 coating due to intrinsic brittleness of ceramics,
highly rough surfaces and severe abrasive damage.
– No severe delamination of worn TiO2-CNT coating, characterized by smooth
surface & mild abrasion
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Fig.. Typical 3D images of wear
tracks for (a) TiO2 and (b) TiO2-CNTs
coating after wear test; the
corresponding cross-section profiles
of (c) TiO2 and (d) TiO2- CNTs
coating.

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Fig. SEM images of wear scars for (a) TiO2 and (d)
TiO2-CNTs coating after wear test; (b) enlarged view
of area A in (a); (c) enlarged view of area B in (b); (e)
enlarged view of area C in (d); (f) enlarged view of
area D in (e).

20. Conclusions
■ CNTs play an essential role in improving the tribological properties of TiO2 coating
(decrease in friction coefficient by ∼36.8% and wear rate by ∼93.6%), indirectly by
influencing the microstructure of the coating and directly by the tribo-effects of
CNTs during tribotest
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Fig.. Schematic illustration
with TEM, SEM and Raman
map images to reveal the CNTs
induced wear mechanism for
plasma sprayed ceramic
coating: 1) structure
strengthening, 2) tribo-
protruding of CNTs, 3) tribo-
reorientation of CNTs, 4) tribo-
film of CNTs and 5) tribo-
degradation of CNTs.

21. .
■ CNTs can still be firmly embedded inside TiO2 grains and strongly trapped between
the grain boundaries..
■ Since CNTs are easy to crush by shear and impact after pulling out, CNTs roll and
slide between mating surfaces. Consequently reduction in the coefficient of
friction.
■ As the sliding progresses, CNTs, which are crushed or crumbled adhere tightly to
surface of coating and forms a protective lubricating lare.
■ After all, analysis showed that nanotubes crushed by shear stress and plastic
deformation.
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22. Query ?
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23. Questions & Answers!!( 1-2 words)
■ What is the pressure range of vacuum plasma spray coating process?
– It varies from 10-15 kPa
■ Least porosity and highest possible density of coatings is obtained in which spray process?
– It is obtained by Vacuum Plasma Spray coating
■ What are three major sources of heating or melting the coating material in spray coating?
– Fuel gas, Plasma and arc
■ Which spray coating process finds significant applications in aeronautical parts?
– High Velocity Oxy-fuel Coating
■ In wear, the multiwalled carbon Nano-tubes ‘s walls gets separated by mainly_____and____.
– Shear stress, plastic deformation
■ Plasma is produced by discharge between two________ electrodes and medium for discharge
is________ in plasma spray. (S 10)
– Non-consumable, inert gas( Ar/He/Ar+He)
■ In electric arc spraying methods, coating material is fed to torch in_____ ()form before melting.
– Wire
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24. Questions & Answers!!(short Answer type)
1. Enlist the mechanism and properties of plasma spray coating process? (slide no.
10)
2. What are the applications of thermal sprayed coatings?( slide no. 13)
3. What the problems associated with combustion flame process? ( slide no.3)
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25. References
■ http://www.alloysprayengineering.com/electric-arc-spray.html
■ https://www.manufacturingguide.com/en/high-velocity-oxygen-fuel-coating-
hvof
■ http://www.thermalspray.com/thermal-spray-coatings/thermal-spray-coating-
processes/plasma-spray/
■ https://www.oerlikon.com/metco/en/products-services/coating-
equipment/thermal-spray/processes/controlled-atmospheric-plasma/
■ https://www.thermalspray.com/thermal-spray-most-common-uses/
■ https://www.fst.nl/systems/hvof-high-velocity-oxygen-fuel-thermal-spray/
■ https://www.metallisation.com/applications/hvof-engineering-applications/
■ https://twinwirearc.com/
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