Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.8, pp.717-726, 2011
jmmce.org Printed in the USA. All rights reserved
717
Thermal Sprayed CNT Reinforced Nanocomposite Coatings – A Review
Manoj Kumar Singla1*, Harpre et Si ng h2 , Vikas Chawla3
1) Mechanical Engineering Department, RIEIT,Railmajra Distt. SBS Nagar, India.
2) School of Mechanical Engineering, Materials & Energy Engineering, IIT Ropar, (PB), India.
3) Mechanical Engineering Department, FCET, Ferozeshah, Ferozepur, India.
*Corresponding Author: [e-mail]
ABSTRACT
This review is done essentially t o s tu dy results in the field of synthesis and characterization of
Carbon Nanotubes (CNT’s) reinforced nanocomposite coatings using thermal sprayed coatings.
CNT reinforced nanocomposite coatings produced by thermal spray process are being
developed for a wide variety of applications, e.g. aerospace, automotive and sports equipment
industries. It is anticipated that, if properly deposited, nanocomposite ceramic coatings
could also provide improved properties like wear resistance and thermal barrier coatings.
These results clearly demonstrate that the significant improvement in coating performance
can be achieved by utilizing proper thermal sprayed nanocomposite coatings. Thermal
sprayed nanocomposite coatings shows improvement of resistance to wear, erosion, corrosion
and mechanical properties. The purpose of this paper is to review CNT reinforced
nanocomposite coa t i ngs usi ng
thermal spray by various researchers .
Keywords: Thermal spraying, nanocomposite coatings, Feedstock material, Erosio n, Wear
resistance, Corrosi o n resistan c e , Ball
milling.
1. INTRODUCTION
The materials having high strength to weight ratio is the need of the hour. There are different
techniques of fabricating Metal Matrix Composites to have tailor made properties depending on
the particular application. Metal matrix composites (MMCs) have been widely recognized to
have relatively superior mechanical properties, such as better wear resistance, higher elastic
modulus and yield strength, as compared to the unreinforced monolithic metal. As compared to
fiber reinforced MMCs, particulate reinforced MMCs are gaining popularity due to their ease of

718 Manoj Kumar Singla, Harpreet Singh , Vikas Chawla Vol.10, No.8
fabrication, high throughput and lower manufacturing cost. If the one phase of mixture is of nano
size these are called nanocomposites.
Thermal spraying is an effective and
low cost method to apply thick coatings to change surface
properties of the component [1]. Coatings are used in a wide range of applications
including automotive systems, boiler components, and power generation equipment,
chemical process equipment, aircraft engines, pulp and paper processing equipment,
bridges, rollers and concrete reinforcements, orthopedics and dental, land-based and marine
turbines, and ships [1]. Plasma spraying has been around for more than four decades and has
been used to deposit a wide range of metals, ceramics and even composite materials for many
different applications [2]. Despite this long and successful history, there has still been a great
interest among the engineers and scientists in developing new coating materials and researching
phenomenon associated with the formation and application of coatings [3].
Carbon nanotubes (CNTs) have evolved tremendous amazement after its invention by Iijima in
1991. The researchers experience several discriminating applications by virtue of its
remarkable mechanical, electrical and thermal properties [4–9]. Depending on their length and
diameter, chirality and orientations, carbon nanotubes exhibit almost five times elastic modulus
(1 TPa) and closely 100 times tensile strength (150 GPa) than those of high strength steels [10–
12]. The unimaginable high strength of CNTs makes them potential reinforcement for the
composite materials. Besides, the nanosized carbon tubes also provide superior dispersion
strengthening to the composite structures.
2. THERMAL SPRAYING
It is coating processes in which melted (or heated) materials are sprayed onto a surface. The
"feedstock" (coating precursor) is heated by electrical (plasma or arc) or chemical means
(combustion flame). Thermal spraying can provide thick coatings (approx. thickness range is 20
micrometers to several mm, depending on the process and feedstock), 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.

Vol.10, No.8 Thermal Sprayed CNT Reinforced Nanocomposite Coatings 719
Lech Pawlowski has summarized the following thermal spray processes that have been
considered for the deposition of coatings: Flame spraying, Plasma spraying, Detonation Gun,
High Velocity Oxy-fuel (HVOF), Arc Spraying.
2.1 Flame Spray
Flame spray is divided into three subcategories, based on the form of the feedstock material,
either powder, wire or rod-flame spray. Flame spray utilizes combustible gasses to create the
energy necessary to melt the coating material. Combustion is essentially unconfined, in that there
is no extension nozzle in which acceleration can occur. Common fuel gases include hydrogen,
acetylene, propane, natural gas, etc. The lower temperatures and velocities associated with
conventional flame spraying typically result in higher oxides, porosity, and inclusions in
coatings. Fig. 1 shows the cross section of a Flame Gun.
Fig. 1 Flame Gun Cross Section [13]
2.2 Plasma Spray
Plasma spray (Fig. 2) is the most versatile 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. 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
36,000°F (20,000°K). Temperatures in the plasma jet are still 18,000°F (10,000°K) several
centimeters form the exit of the nozzle.

720 Manoj Kumar Singla, Harpreet Singh , Vikas Chawla Vol.10, No.8
Fig. 2 Plasma Gun Cross Section [14]
2.3 Detonation Gun Spray
The D-gun™, shown schematically in Figure 3, includes a long, water cooled barrel with an ID
of about 25mm (Schwarz, 1980). A mixture of oxygen (4) and acetylene (5) is fed into the barrel,
together with a charge of powder (1). The gas is ignited, explodes and its detonation wave
accelerates the powder. In order to avoid ‘backfiring’, i.e. explosion of the fuel gas supply, an
inert gas, such as nitrogen, is used between the portions of exploding mixture.
Fig. 3 Schematic of D-gun process: 1. powder injection 2. Spark plug 3. Gun barrel 4.
Oxygen input 5. Nitrogen input [15]
Nitrogen also purges the barrel. The detonation process therefore has the following cycles
(Kadyrov and Kadyrov, 1995):
• Injection of oxygen and fuel into the combustion chamber;
• Injection of powder and nitrogen to prevent ‘backfiring’;

Vol.10, No.8 Thermal Sprayed CNT Reinforced Nanocomposite Coatings 721
• Ignition of mixture and acceleration of powder;
• Purging of barrel by nitrogen.
There are 1–15 detonations per second with purges of nitrogen between them.
2.4 High Velocity Oxy-fuel (HVOF) Spray
High-velocity, oxy-fuel, (HVOF) Fig. 4 devices are a subset of flame spray. There are two
distinct differences between conventional flame spray and HVOF. HVOF utilizes confined
combustion and an extended nozzle to heat and accelerate the powdered coating material.
Typical HVOF devices operate at hypersonic gas velocities, i.e. greater than MACH 5. The
extreme velocities provide kinetic energy which help produce coatings that are very dense and
very well adhered in the as-sprayed condition.
Fig. 4 HVOF Gun Cross Section [16]
2.5 Arc Spray
Like flame spray, electric-arc spray (Fig. 5) was invented in the early 20th century. Even though
the technology has been around for a long time, it still remains a very powerful thermal spray
technology. Electric-arc spray uses a simple, low power arc drawn between two electrically
charged wires. Arc spray equipment resembles GMAW (MIG) welding equipment, in the power
source and wire feeding units. Common arc spray units are capable of spraying iron and copper
alloys at rates up to 40 lbs./hr (18 Kg/hr.) using only 12 kW (42 MJ) of electricity. Electric-arc
spraying produces the fastest coating rates of any technology. Electric-arc spray devices are
thermally efficient and, because there is no flame or plasma, little heat are transferred to the part
being coated.

722 Manoj Kumar Singla, Harpreet Singh , Vikas Chawla Vol.10, No.8
Fig. 5 Electric Gun Cross Section [17]
3. CARBON NANO TUBES
Carbon nanotubes can be visualized as a graphene sheet that has been rolled into a tube with
hemispherical caps at both ends [18]. Unlike diamond, where a 3-D diamond cubic crystal
structure is formed with each carbon atom having four nearest neighbours arranged in a
tetrahedron, graphite is formed as a 2-D sheet of carbon atoms arranged in a hexagonal array. In
this case, each carbon atom is linked to three nearest neighbours. Rolling the sheets of
graphite into cylinders form carbon nanotubes [19]. The properties of nanotubes depend on
atomic arrangement (how the sheets of graphite are ‘rolled’), the diameter and length of the
tubes.
Nanotubes exist as either single-walled or multi-walled structures. Multi-walled nanotubes
(MWNTs) are simply composed of concentric single-walled nanotubes (SWNTs) Fig. 6(a &
b). Primary synthesis methods to prepare single walled and multi walled nanotubes include
methods of arc discharge, laser ablation, gas phase catalytic growth from carbon monoxide
and similar carbon sources and chemical vapour deposition. Considering the application
of carbon nanotubes as reinforcements in composites which requires production of large
amount of carbon nanotubes economically, gas phase techniques like chemical vapour
deposition (CVD) offers the greatest potential for optimization of nanotube production [20].
Fig. 6 (a) Single walled [21] Fig. 6(b) Multi-walled Nanotube [22]

Vol.10, No.8 Thermal Sprayed CNT Reinforced Nanocomposite Coatings 723
The purpose of this paper is to review the CNT reinforced nanocomposite coatings using thermal
spraying techniques by various researchers.
4. STUDIES RELATED TO THERMAL SPRAY COATINGS
T. Laha et al. [23] synthesized and characterized carbon nanotubes as reinforced composites of
unique properties. In his endeavor, free standing structures of Al-based nanostructured composite
with carbon nanotubes as second phase particles has been synthesized by plasma spray forming
technique. Optical microscopy, scanning electron microscopy, X-ray diffraction, transmission
electron microscopy has been carried out to analyze the composite structure and to verify the
retention of carbon nanotubes. Besides, density and microhardness measurements have been
performed to understand the effect of carbon nanotube reinforcement on the mechanical
properties of the composite. It has been observed that microhardness increased from 85 to 146
The characterization affirms the presence of unmelted and chemically unreacted carbon
nanotubes in the composite. Moreover, the composite experienced an increase in relative
microhardness due to the presence of Carbon Nano tubes.
T. Laha et al.[24] fabricated free standing structures of hypereutectic aluminum-23 wt% silicon
nanocomposite with multiwalled carbon nanotubes (MWCNT) reinforcement by two different
thermal spraying technique viz Plasma Spray Forming (PSF) and High Velocity Oxy-Fuel
(HVOF) Spray Forming. Comparative microstructural and mechanical property evaluation of
the two thermally spray formed nanocomposites has been carried out. Presence of nanosized
grains in the Al–Si alloy matrix and physically intact and undamaged carbon nanotubes were
observed in both the nanocomposites. Excellent interfacial bonding between Al alloy matrix and
MWCNT was observed. The elastic modulus (82.8+9.3 GPa) and hardness (1.93+0.14 GPa) of
HVOF sprayed nanocomposite is found to be higher than PSF sprayed composites.
K. Balani et al. [25] evaluated wear behavior of plasma-sprayed carbon nanotube (CNT)-
reinforced hydroxyapatite (HA) coating in the simulated body fluid environment. Apart from
enhancing the fracture toughness and providing biocompatibility, CNT-reinforced HA coating
demonstrated superior wear resistance compared with that of hydroxyapatite coating without
CNT. Initiation and propagation of micro- cracks during abrasive wear of plasma-sprayed
hydroxyapatite coatings was suppressed by CNT reinforcement. Surface characterization and
wear studies have shown that in addition to acting as underprop lubricant; CNTs provide
reinforcement via stretching and splat- bridging for enhanced abrasion resistance in vitro.
K. Balani et al. [26] dispersed CNTs are grown on Al2O3 powder particles in situ by the catalytic
chemical vapor deposition (CCVD) technique. Consequently, 0.5 wt.% CNT-reinforced Al2O3
particles were successfully plasma sprayed to obtain a 400 µm thick coating on the steel
substrate. In situ CNTs grown on Al2O3 shows a promising enhancement in hardness and
fracture toughness of the plasma-sprayed coating attributed to the existence of strong

724 Manoj Kumar Singla, Harpreet Singh , Vikas Chawla Vol.10, No.8
metallurgical bonding between Al2O3 particles and CNTs. Hardness increased from 806 to 906
VH Fracture toughness from 4.14 ± 0.22 to 4.62 ± 0.27 MPa m-1/2 . In addition, CNT tentacles
have imparted multi-directional reinforcement in securing the Al2O3 splats. High-resolution
transmission electron microscopy shows interfacial fusion between Al2O3 and CNT and the
formation of Y-junction nanotubes.
K. Balani et al. [27] studied plasma sprayed Al2O3 –CNT nanocomposite, he observed that
molten Al2O3spreads uniformly on the CNT surface by forming a thin (20–25 nm) ceramic layer
without any cracks. The wettability of the Al2O3–CNT system is associated with the surface
tension and capillary forces as captured from the evolution of microstructure. The dynamic
equilibrium between melting and solidification of Al2O3 was deduced from the meniscus
height, curvature, and contact perimeter and projection area of solidified Al2O3 on the CNT
surface. This interfacial phenomenon illuminates the mechanisms of microstructure evolution
from Al2O3 -coated CNT bridge structures to CNT mesh formation. Consequent ab initio
modeling depicted distorted iso-surface contours at the interface, suggesting partial bonding and
good wettability of Al2O3 on the CNT surface.
K. Balani et al. [28] has developed a process map for plasma sprayed aluminum oxide (Al2O3)
ceramic nanocomposite coatings with carbon nanotube (CNT) reinforcement in varying content
and spatial distribution. The process map was constructed using the temperature and velocity
data of the in-flight powder particles exiting from the plasma plume. Process map elucidates the
interdependence of powder feedstock pre-treatment, CNT content and dispersion behavior on the
in-flight particle thermal history and subsequently evolving microstructure and coating
properties. High thermal conductivity of CNTs alters the heat transfer characteristic during the
splat formation. Microstructure of the coatings consists of fully melted zone (FM), partially
melted or solid-state sintered zone (PM) and porosity. Process map provides a processing control
tool for plasma spraying of Al2O3–CNT nanocomposite coatings.
S.R. Bakshi et al. [29] prepared Multiwalled carbon nanotube (CNT) reinforced aluminum
nanocomposite coatings using cold gas kinetic spraying. Spray drying was used to obtain a good
dispersion of the nanotubes in micron-sized gas atomized Al–Si eutectic powders. Spray dried
powders containing 5 wt.% CNT were blended with pure aluminum powder to give overall
nominal CNT compositions of 0.5 wt.% and 1 wt.% respectively. Cold spraying resulted in
coatings of the order of 500 μm in thickness. Fracture surfaces of deposits show that the
nanotubes were uniformly distributed in the matrix. Nanotubes were shorter in length as they
fractured due to impact and shearing between Al–Si particles and the Al matrix during the
deposition process. Nanoindentation shows a distribution in the elastic modulus values from 40–
229 GPa which is attributed to microstructural heterogeneity of the coatings that comprise the
pure Al, Al–Si eutectic, porosity and CNTs.

Vol.10, No.8 Thermal Sprayed CNT Reinforced Nanocomposite Coatings 725
A.K. Keshri et al. [30] compared the interaction of carbon nanotubes (CNTs) with the
flame/energy sources during different thermal spray processes viz. plasma spraying (PS) (E=
325-420 GPa, thickness of coating = 500µm), high-velocity oxy fuel spraying (HVOF) (E=
82.8+9.3 Gpa, thickness of coating = 1240µm), cold spraying (CS) (E=40-229GPa, thickness
of coating = 400µm), and plasma spraying of liquid precursor (PSLP). CNTs were
successfully retained as reinforcement in metal and ceramic composite coatings in all thermal
spray processes except PSLP. The retention of CNT structure is attributed to micron size
metal/ceramic powder which acts as a carrier and thermal shield against high heat in plasma
spraying (PS) and high-velocity oxy fuel spraying (HVOF). However, vaporization of CNTs
occurred in PSLP under the intense heat of the plasma which is attributed to phase
transformation in unshielded CNTs.
5. CONCLUSIONS
1. Studies show that CNT reinforced nanocomposite coatings can be successfully
synthesized using Thermal Spray Coatings.
2. It has been also concluded that CNT reinforced nanocomposite coatings possesses better
mechanical properties like high elastic modulus, hardness, wear properties.
3. Retention of CNT in Matrix is also good.
4. CNT reinforced coatings have strong industrial potential and rapid industrial growth is
expected in next decade.
5. Although much work of reinforcing CNT in Aluminium matrix is discussed but CNT
reinforcement in other matrix could be explored using thermal spray coatings.
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