JTTEE5 25:897–919
DOI: 10.1007/s11666-016-0415-7
1059-9630/$19.00 ASM International
Review
A Review of Thermal Spray Metallization
of Polymer-Based Structures
R. Gonzalez, H. Ashrafizadeh, A. Lopera, P. Mertiny, and A. McDonald
(Submitted September 23, 2015; in revised form April 8, 2016)
A literature review on the thermal spray deposition of metals onto polymer-based structures is presented. The deposition of metals onto polymer-based structures has been developed to enhance the
thermal and electrical properties of the resulting metal-polymer material system. First, the description of
the thermal spray metallization processes and technologies for polymer-based materials are outlined.
Then, polymer surface preparation methods and the deposition of metal bond-coats are explored.
Moreover, the thermal spray process parameters that affect the properties of metal deposits on polymers
are described, followed by studies on the temperature distribution within the polymer substrate during
the thermal spray process. The objective of this review is devoted to testing and potential applications of
thermal-sprayed metal coatings deposited onto polymer-based substrates. This review aims to summarize
the state-of-the-art contributions to research on the thermal spray metallization of polymer-based
materials, which has gained recent attention for potential and novel applications.
Keywords
electrical conductivity, metallization, polymers,
thermal conductivity, thermal spraying
1. Introduction
Thermal spraying is a material deposition process
whereby a heat source is used to increase the temperature
of feedstock particles that are accelerated in a fluid stream
through a spray nozzle or torch, for consolidation upon
impact on a substrate. The thermal spray deposition process is characterized by heat, mass, and momentum
transfer phenomena (Ref 1-3). These phenomena are
functions of the thermal spray process parameters (Ref 2)
and affect the properties and structure of the resulting
thermal-sprayed coatings (Ref 1). Thermal energy is
transferred from the thermal spray gas, flame, or plasma to
the substrate by gas convection and the impingement of
solid or molten particles (Ref 4). The kinetic energy of the
impacting particle is used to overcome surface tension
energy, deform the particles, and/or is converted to heat
due to viscous dissipation or adiabatic shear instabilities
during flattening to form a splat (Ref 5-7). As a result of
the thermal spray deposition onto the substrate, a lamellar
structure of splats develops into a coating layer, which
exhibits different physical properties from those of the
coating bulk material (Ref 1). These differences in properties can be the result of phase changes during the
deposition process, residual stresses in the solidified splats,
and the presence of porosities and other microstructural
R. Gonzalez, H. Ashrafizadeh, A. Lopera, P. Mertiny, and
A. McDonald, Department of Mechanical Engineering,
University of Alberta, Edmonton, AB T6G 1H9, Canada.
Contact e-mail: andre.mcdonald@ualberta.ca.
Journal of Thermal Spray Technology
defects within the coating (Ref 8). Thermal spray deposition has been performed using different particle-substrate material systems, including metal particle and
polymer-based substrates (Ref 2, 4, 9-12). The thermal
spray deposition efficiency is determined as the ratio between the weight of the deposited material and feedstock
particles sprayed in a given time (Ref 13, 14).
Thermal spray deposition of metal particles onto
polymer-based structures has gained increasing attention
through studies on the metallization of fiber-reinforced
polymer composite (FRPC) (Ref 15-20), polymer matrix
composite (PMC) (Ref 21, 22), and neat polymer substrates (Ref 23-28). Polymer-based materials, such as PMC
structures, can provide enhanced mechanical properties
(i.e., high specific strength and high specific stiffness) with
respect to single-phase (i.e., metallic) structures. Moreover, surface coating technology has been recognized as a
method to improve the service life and functional performance of advanced engineering structures (Ref 9, 29,
30). Thus, potential structural and operational advantages
may be achieved through the application of thermalsprayed metal coatings to polymer-based substrates. Even
though polymer-based materials can have high mechanical
strength and corrosion resistance (Ref 19, 31), low operational temperatures (Ref 19), low electrical conductivity,
and potential flammability (Ref 32, 33) may limit their
application in regulated industries. However, flammability
requirements for polymer-based structures can be different depending on the specific sector regulations (e.g.,
aerospace, marine, and rail industry sectors) (Ref 32).
Considering the potential advantages in practical application, the deposition of metal particles onto polymerbased substrates has been investigated to improve the
thermal and electrical properties of the polymer system
(Ref 17, 22, 30, 34, 35), which can allow a wider adoption
of polymer-based materials in engineering applications.
Even though alternative surface metal deposition
Volume 25(5) June 2016—897
techniques, such as physical vapor deposition (PVD) (Ref
36, 37), chemical vapor deposition (CVD) (Ref 38), and
plasma-enhanced CVD (PECVD) (Ref 39), may be applied to polymer-based structures, these techniques are
relatively expensive to use (Ref 40) and not suitable to
fabricate thick metallic coatings (over 100 lm) at high
deposition rates (Ref 29, 41), which is feasible by thermal
spray metallization. On the other hand, thermal-sprayed
metal coatings can be engineered to provide friction
coefficient adaptation, dimensional control, dimensional
restoration, and enhanced electromagnetic and thermal
properties to functionalize polymer-based substrates in
service (Ref 2, 8, 19, 30). The thermal spray process also
allows the deposition of coatings onto various substrate
geometries and the ability to recoat damaged coatings
(Ref 4). However, thermal degradation of the polymerbased substrate and formation of char may occur, which
depends on the temperature of the metal particles and
high-temperature gases during thermal spray deposition
(Ref 42). Research on the thermal-physical behavior of
metallized polymer materials can expand the scope and
applications of several polymer-based materials, including
structural PMC components.
The objective of this article is to present a review of the
current knowledge of the thermal spray deposition of
metals onto polymer-based substrates (e.g., neat polymers,
and PMC) and the potential applications of the metallized
polymer-based structures. In addition, the concepts associated with the thermal spray coating deposition onto
polymer-based substrates, including, surface preparation,
temperature distribution, and evaluation and characterization of metal coatings will be discussed in the context of
specific applications where using thermal spray technology
may be of benefit. The subject matter and discussion are
restricted to the thermal spray deposition of pure metals
or metal alloys onto polymer-based substrates.
2. Process Description and Technologies
2.1 Thermal Spray Metallization of Polymer-Based
Structures
Thermal spray metallization of polymer-based structures is defined as the application of thermal spray processes to deposit metals onto a polymer substrate.
Thermal spray deposition can be accomplished by
embedment or adhesion of metal particles onto the polymer-based substrate in the form of deformed splats, as
illustrated in Fig. 1 (Ref 43). The deposition of the metal
splats is affected by the velocity and temperature of the
impacting particles, the roughness and temperature of the
substrate surface, and the relative angle between the
particle trajectory and the substrate (Ref 4, 8), among
others. The deposited metal particles may embed into
relatively softer polymer substrates, such as nylon, polyurethane (PU), and polyethylene (PE) (Ref 23, 44, 45).
Alternatively, the particles may deform and interlock
upon contact with the polymer substrate surface, as shown
in previous studies involving basalt and glass fiber-reinforced epoxy thermosets (Ref 17, 18) and pre-treated
thermoplastic PU (Ref 20). Table 1 shows an overview of
the thermal spray processes and material systems reported
on the metallization of polymer-based structures to date.
The microstructure and physical properties of metal
coatings deposited onto polymer-based substrates are
influenced, in part, by the kinetic energy of the sprayed
particles and the temperature of the fluid stream in which
the deposited particles were entrained, which is usually a
gas (Ref 2, 9). Current thermal spray metallization processes of polymer-based substrates can be classified into
four categories according to the primary energy source
used for particle acceleration and heating (Ref 2, 3, 46).
They are (i) cold spraying, which consists of kinetic-based
Fig. 1 Schematic of the thermal spray process (Ref 43)
898—Volume 25(5) June 2016
Journal of Thermal Spray Technology
Journal of Thermal Spray Technology
Table 1 Overview of processes and material systems used in the thermal spray metallization of polymer-based substrates
Thermal spray process
Substrate material
Powder cold spraying
Carbon fiber-reinforced PEEK (thermoplastic)
Unidirectional carbon fiber-reinforced
epoxy tape (thermoset)
Carbon fiber-reinforced epoxy weave
(thermoset)
Commercial thermoplastic blend of
Polycarbonate and Acrylonitrile Butadiene Styrene
Polyamide-6 (thermoplastic)
Polypropylene (thermoplastic)
Polystyrene (thermoplastic)
Carbon fiber-reinforced epoxy (thermoset)
Carbon fiber-reinforced PEEK450CA30
(thermoplastic)
Polyamide 66 (thermoplastic)
Powder cold spraying
Powder cold spraying
Powder cold spraying
Powder plasma spraying
Powder cold spraying
Powder cold spraying
Powder cold spraying
Powder cold spraying
Powder cold spraying
Polyvinyl chloride (thermoplastic)
HDPE (thermoplastic), nylon 6
(thermoplastic)
Aluminum
100-500 lm
None
(Ref 49)
Zinc
Up to 1053 lm
(Ref 16)
Tin
45-100 lm
A layer of copper particles was
co-cured onto the surface of
the neat polymer during
manufacturing
None
(Ref 47)
Aluminum
Aluminum (bond-coat)
Aluminum (bond-coat)
Less than 30 lm
Less than 15 lm
~500 lm
None
(Ref 48)
None
(Ref 21)
Aluminum
183-635 lm
(Ref 24)
Copper (bond-coat)
Tin (bond-coat)
Copper
800-1000 lm
~150 lm
Not applicable (embedment)
Temperature control of the
neat polymer during deposition
None
Copper
Copper (bond-coat)
Tin
(bond-coat)
Titanium
Not applicable (embedment)
~1000 lm
~150 lm
(Ref 25)
Volume 25(5) June 2016—899
None
(Ref 26)
Copper
Not applicable (embedment)
None
(Ref 44)
Aluminum
45-550 lm
(Ref 27)
Polyester woven fabric (thermoset)
Aluminum
Glass fiber-reinforced epoxy (thermoset)
Basalt fiber-reinforced epoxy (thermoset)
Aluminum
75-100 lm (optimal for conductive and flexible fabrics)
100-200 lm
Cleaning of the cured polymer
with isopropanol
None
Polyurethane (thermoset)
Aluminum-12silicon
274 lm-381 lm
Polyetheretherketone
(thermoplastic)
High density polyethylene (thermoplastic)
Polypropylene (thermoplastic)
Nylon 6 (thermoplastic)
Polytetrafluoroethylene (thermoplastic)
Polycarbonate (thermoplastic)
Polyurethane (thermoplastic)
Polycarbonate (thermoplastic)
Powder flame spraying
References
~1000 lm
Powder cold spraying
Powder flame spraying
Wire flame spraying
Powder flame spraying
Substrate preparation
(Ref 45)
(Ref 68)
Polyurethane (thermoplastic)
Polyvinyl chloride (thermoplastic)
Epoxy (thermoset)
Powder cold spraying
Deposition thickness
The cured polymer was washed
with a detergent solution
Decon 90, rinsed with distilled water, and air dried
None
None
Powder cold spraying
Powder cold spraying
Powder cold spraying
Deposition material
Grit blasting of the cured
composite with alumina particles
A layer of copper particles was
co-cured onto the surface of
the neat polymer during
manufacturing
(Ref 23)
(Ref 34, 40)
(Ref 17)
(Ref 53)
900—Volume 25(5) June 2016
Table 1 continued
Thermal spray process
Substrate material
Deposition material
Deposition thickness
Powder flame spraying
Glass fiber-reinforced epoxy composite
(thermoset)
Aluminum-12silicon
172 ± 14 lm
Powder flame spraying
Glass fiber-reinforced epoxy (thermoset)
Aluminum-12silicon
~200 lm
Powder flame spraying
Glass fiber-reinforced epoxy (thermoset)
Aluminum-12silicon
385 ± 20 lm
Powder flame spraying
Polyurethane (thermoset)
Aluminum-12silicon
~100 lm
Powder flame spraying
Glass fiber-reinforced epoxy (thermoset)
Aluminum-12silicon
290 ± 10 lm
Powder cold spraying
Carbon fiber-reinforced PEEK (thermoplastic)
Unidirectional carbon fiber-reinforced
epoxy tape (thermoset)
Carbon fiber-reinforced epoxy weave
(thermoset)
Commercial thermoplastic blend of
Polycarbonate and Acrylonitrile Butadiene Styrene
Polyamide-6 (thermoplastic)
Polypropylene (thermoplastic)
Polystyrene (thermoplastic)
Carbon fiber-reinforced epoxy (thermoset)
Carbon fiber-reinforced PEEK450CA30
(thermoplastic)
Polyamide 66 (thermoplastic)
Aluminum
100-500 lm
Zinc
Up to 1053 lm
Tin
45-100 lm
Aluminum
Aluminum (bond-coat)
Aluminum (bond-coat)
Powder cold spraying
Powder cold spraying
Journal of Thermal Spray Technology
Powder cold spraying
Powder plasma spraying
Powder cold spraying
Powder cold spraying
Powder cold spraying
Powder cold spraying
Powder cold spraying
Polyvinyl chloride (thermoplastic)
HDPE (thermoplastic), nylon 6 (thermoplastic)
Substrate preparation
A layer of 220-grit (~63 lm)
garnet sand was co-cured
onto the surface of the substrate during manufacturing
A layer of 80-grit (~165 lm)
garnet sand was co-cured
onto the surface of the substrate during manufacturing
A layer of 80-grit (~165 lm)
garnet sand was incorporated
onto the surface of the cured
composite using an epoxy
adhesive
A layer of Aluminum-12silicon
particles was co-cured onto
the surface of the neat polymer during manufacturing
A layer of 80-grit (~165 lm)
garnet sand was incorporated onto the surface of
the cured composite using
an epoxy adhesive
None
References
(Ref 11, 19)
(Ref 18)
(Ref 117)
(Ref 20, 28, 65, 79)
(Ref 118)
(Ref 49)
A layer of copper particles was
co-cured onto the surface of
the neat polymer during
manufacturing
None
(Ref 47)
Less than 30 lm
Less than 15 lm
~500 lm
None
(Ref 48)
None
(Ref 21)
Aluminum
183-635 lm
(Ref 24)
Copper (bond-coat)
Tin (bond-coat)
Copper
800-1000 lm
~150 lm
Not applicable (embedment)
Temperature control of the
neat polymer during deposition
None
The cured polymer was washed
with a detergent solution
Decon 90, rinsed with distilled water, and air dried
(Ref 16)
(Ref 25)
(Ref 23)
Journal of Thermal Spray Technology
Table 1 continued
Thermal spray process
Substrate material
Deposition material
Powder cold spraying
Powder cold spraying
Polyurethane (thermoplastic)
Polyvinyl chloride (thermoplastic)
Epoxy (thermoset)
Powder cold spraying
Polyetheretherketone
(thermoplastic)
Unidirectional glass fiber-reinforced
epoxy composite (thermoset)
Powder flame spraying
Deposition thickness
Not applicable (embedment)
~1000 lm
~150 lm
None
None
(Ref 45)
(Ref 68)
~1000 lm
None
(Ref 26)
NiCrAlY
Ni-20Cr
~100 lm
~80 lm
A layer of 80-grit (~165 lm)
garnet sand was incorporated
onto the surface of the cured
composite using an epoxy
adhesive
Cleaning of the cured composite was performed using
acetone followed by grit
blasting with corundum particles
Grinding of the cured composite was performed using
sand papers
Cleaning of the cured composite was performed using
acetone, followed by grit
blasting with corundum particles, and preheating in an
oven for 10 min
A layer of epoxy thermoplastic
bond-coat was consolidated
onto the surface of the cured
composite
None
(Ref 66, 67)
Graphite fiber-reinforced
(thermoset)
polyimide
Zinc (bond-coat)
~100 lm
Electric arc wire spraying
Graphite fiber-reinforced
(thermoset)
polyimide
Zinc (bond-coat)
Aluminum (bond-coat)
Zinc
(bond-coat)
Aluminum (bond-coat)
~50 lm
Powder plasma spraying
Unidirectional carbon fiber-reinforced
polymer (thermoset)
Copper
50-160 lm
Powder plasma spraying
Unreported polymer composite (thermoset)
Carbon fiber-reinforced unsaturated
polyester (thermoset)
Unreported metal
Not reported
Aluminum (bond-coat)
~40 lm
Powder plasma spraying
References
Copper
Copper (bond-coat)
Tin
(bond-coat)
Titanium
Electric arc wire spraying
Powder plasma spraying
Substrate preparation
Volume 25(5) June 2016—901
Powder plasma spraying
Carbon fiber-reinforced epoxy (thermoset)
Aluminum
~100 lm
Powder plasma spraying
Quartz fiber-reinforced polyimide (thermoset)
Aluminum (bond-coat)
30 lm, 55 lm, and 70 lm
Cleaning of the cured composite was performed using
acetone, followed by grit
blasting with alumina particles, and second cleaning
with acetone
Cleaning of the cured composite was performed using
acetone, followed by grit
blasting with alumina particles, and second cleaning
with acetone
Surface grinding of the cured
composite was performed
using abrasive papers, followed by cleaning with ethanol and water, and drying
(Ref 56)
(Ref 57)
(Ref 60)
(Ref 15)
(Ref 61)
(Ref 62)
(Ref 22)
(Ref 59)
(Ref 64)
Carbon fiber-reinforced epoxy (thermoset)
Powder plasma spraying
Copper
50-60 and 100-120 lm
Surface grinding of the cured
composite was performed
using abrasive papers, followed by cleaning with ethanol and water, and drying
Chemical treatment of the
cured composite was performed in 4 stages: (1) aqueous solution treatment in 25
vol.%
2-(2-butoxyethoxy)
ethanol, (2) application of
potassium permanganate, (3)
exposure to solution of
trichlorotriazine in toluene,
and (4) alkali water solution
treatment in iminodiacetic acid
The cured composite was exposed to the plasma plume at a
stand-off distance of 200 mm
Grit blasting of the cured
composite was performed with
steel particles
55 lm
Quartz fiber-reinforced polyimide (thermoset)
Powder plasma spraying
CoNiCrAlY (bond-coat)
Zinc (bond-coat)
Aluminum (bond-coat)
References
Substrate preparation
Deposition thickness
Deposition material
Substrate material
Thermal spray process
Table 1 continued
902—Volume 25(5) June 2016
deposition of metal or metal alloy particles at relatively
low temperatures; (ii) flame spraying, comprising particle
deposition using a combustion flame jet; (iii) electric arc
wire spray, which utilizes an electric discharge from electrodes around a carrier gas stream to generate a thermal
jet; and (iv) plasma spraying, involving ionized gas jets
generated by either direct current (DC) or radio-frequency (RF) current. Each of the thermal spray processes
will generate different magnitudes of energy in the fluid
heat source that is used to heat and accelerate the particles
for deposition. Figure 2 shows the approximate range of
gas temperature and particle impact velocity of different
thermal spray metallization processes (Ref 2, 4). The spray
parameters of the reported thermal spray metallization
processes are presented in Table 2. A summary of
potential applications and characteristic properties of
thermal-sprayed metal deposits consolidated onto polymer-based substrates is shown in Table 3.
2.2 Cold Spraying
Cold spraying is a relatively low-temperature thermal
spray process where solid-state particles are accelerated
through a nozzle to high velocities (300-1200 m/s), and the
coating is formed as a result of plastic deformation and
interlocking of the deformed particles upon impact (Ref 2,
9). Cold spray deposition is predominantly based on using
the kinetic energy of the impacting particles to promote
solid-state plastic deformation and adiabatic shear instability heating, since melting of the feedstock powder
material does not occur. However, softening of the
impacting particles may develop due to the temperature
rise upon impact and deformation of the ductile metal
particles (Ref 6, 7). Cold-sprayed particles deform plastically upon impact on the substrate at a critical velocity
that overcomes rebounding and substrate erosion and
produces adhesion (Ref 9, 23, 44). As shown in Table 2,
the process parameters of the cold spray process include
nozzle geometry, nozzle stand-off distance (distance between the nozzle and the substrate surface), feedstock
material, and carrier gas temperature and pressure. Cold
spraying is typically performed using a convergent-divergent De Laval nozzle (Ref 9), which accelerates the carrier gas stream to high velocity through expansion of
compressed gases.
Although the mechanism of cold spray deposition on
polymers is a topic of active research, cold spray metallization has been achieved on polymer-based materials (Ref
9). As adduced from Table 1, cold spray metallization of
polymers has been found to be suitable for ductile metals
with relatively low melting temperature, hardness, and
mechanical strength such as aluminum (Al), zinc (Zn), and
tin (Sn) (Ref 9, 16, 21, 47, 48). The limited thermal energy
required in the cold spray metallization of polymers may
forestall potentially deleterious effects in as-sprayed parts,
such as the accumulation of residual thermal stresses, metal
oxidation, and undesired chemical reactions (Ref 9, 49).
Robitaille et al. (Ref 16) showed that the use of lower
feedstock and carrier gas temperatures can prevent thermal
degradation of prepared polymer-based substrates. As a
Journal of Thermal Spray Technology
Fig. 2 Gas temperature and particle impact velocity map of several thermal spray metallization processes (Ref 2, 4)
Table 2 Process parameters of thermal spray processes
Thermal spray process
Cold spraying
Flame spraying
Electric arc wire spraying
Plasma spraying
Spray parameters
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Feedstock powder
Nozzle geometry
Powder feed rate
Carrier gas pressure
Gas temperature
Stand-off distance
Feedstock powder
Nozzle geometry
Powder feed rate
Carrier gas pressure
Fuel gas flow rate
Oxygen flow rate
Compressed air pressure
Stand-off distance
Arc gun geometry
Wire material
Wire feed rate
Electrical current
Compressed air pressure
Stand-off distance
Nozzle geometry
Feedstock powder
Powder feed rate
Electrical current
Plasma gas flow rate
Compressed air pressure
Stand-off distance
result, cold-sprayed metal coatings may be suited for
applications related to surface protection and bond-coating
(bond-coat) engineering (Ref 21, 48) on temperature-sensitive polymer structures, such as aerospace-grade carbon
fiber-reinforced polymer (CFRP) composites (Ref 16, 49).
On the other hand, limited heating of the powder particles
by the carrier gas can increase the plasticity of the metal
particles (Ref 23, 48), which would, in turn, reduce the
critical velocity required for cold spray deposition and
particle adhesion (Ref 9). As shown in Table 1, most of the
previous studies have utilized low carrier gas temperature to
Journal of Thermal Spray Technology
promote cold spray deposition of metals onto thermoplastic
substrates, such as Al onto polyamide 66 (PA66) (Ref 24),
copper (Cu) and Sn onto polyvinyl chloride (PVC) (Ref 25),
Al onto polycarbonate (PC) (Ref 27), and titanium onto
polyetheretherketone (PEEK) (Ref 26).
The cold spray process parameters that are presented
in Table 2 influence the gas flow velocity during metal
deposition onto polymer-based substrates. Based on the
gas flow velocity during the spray deposition process, three
types of cold spray processes can be used for metallization
of polymers (Ref 2, 9, 16, 48):
(a) Low-pressure cold spray (LPCS), in which the gas
pressure is below 1 MPa and the particle velocities
usually range between 300 and 600 m/s (Ref 9). The
lower kinetic energy of the particles in the LPCS
process can reduce the deposition efficiency of the
metallization process since critical velocities of the
impacting particles may not be achieved (Ref 2, 50).
(b) High-pressure cold spray (HPCS), in which a carrier
gas with low molecular weight such as nitrogen or
helium, is utilized and particle velocities are between
800 and 1400 m/s (Ref 2, 9). Typical upstream gas
pressures range between 1 and 4 MPa (Ref 2, 9, 47).
(c) Pulsed-gas dynamic spraying, in which the carrier gas
stream oscillates by way of a series of fixed-frequency
shock waves (Ref 51). The shock waves propagate
abrupt increments of pressure and temperature in the
carrier gas stream that are higher than those of conventional cold spray systems, which produces
impacting particles with higher velocities (Ref 9).
The higher kinetic energy of cold-sprayed metal particles can also induce structural damage to the polymerbased structures at the expense of coating fabrication.
Robitaille et al. (Ref 16) reported limited substrate
abrasion as a result of the deposition of Zn coatings onto
exposed carbon fiber/epoxy composite thermosets using
Volume 25(5) June 2016—903
904—Volume 25(5) June 2016
Table 3 Potential applications and characteristic properties of thermal-sprayed metal deposits consolidated onto polymer-based substrates
Potential application
Substrate material
Deposition material
Electrical conduction
(conductive fabrics)
Polyester fabric (thermoset)
Electrical conduction
Electrical conduction
(lightning protection)
Glass fiber-reinforced epoxy Aluminum
composite (thermoset)
Basalt fiber-reinforced epoxy
composite (thermoset)
Carbon fiber-reinforced epoxy Aluminum
(thermoset)
Electrical conduction
Polyurethane (thermoplastic)
Thermal shock protection
Quartz fiber-reinforced poly- Aluminum
imide (thermoset)
Glass fiber-reinforced epoxy Aluminum-12silicon
composite (thermoset)
Thermal conduction
Aluminum
Aluminum-12silicon
Thermal spray process
Reported properties
of deposits
References
Wire flame spraying
Powder
flame spraying
Powder flame spraying
Electrical sheet resistivity:
0.002-1.43 X m/m
(Ref 34, 40)
Electrical resistivity:
6 9 10 7-8 9 10 7 X m
(Ref 17)
Powder cold
spraying
Powder plasma spraying
Powder flame spraying
Electrical resistivity:
17.1 9 10 8 to 54 9 10
Powder plasma spraying
Powder flame spraying
Journal of Thermal Spray Technology
Electrical conduction
Polyurethane (thermoset)
Aluminum-12silicon
Powder flame spraying
Electrical conduction
Glass fiber-reinforced epoxy Aluminum-12silicon
composite (thermoset)
Powder flame spraying
Electrical resistance
heating (anti-icing system)
Unidirectional glass fiber-rein- NiCrAlY and Ni-20Cr Powder flame spraying
forced epoxy composite
(thermoset)
8
(Ref 48)
Xm
Electrical resistivity:
1.21 9 10 3-22.8 9 10 3 X m
V
olumetric porosity: 19.8-35%
Micro-hardness: 43.4 HV100g
Thermal conductivity:
250 W/m K approx.
Volumetric
porosity: 20%
Electrical resistivity:
0.9 9 10 5 X m-397.9 9
10 5 X m
Volumetric
porosity: 12-20%
Electrical resistivity:
26 9 10 6 X m
Volumetric
porosity: 12-17%
Electrical resistance: 3.55
and 3.2 X
Volumetric
porosity: 2-6.8%
(Ref 53)
(Ref 22)
(Ref 11, 19)
(Ref 20,
28, 65, 79)
(Ref 118)
(Ref 66, 67)
the pulsed-gas dynamic cold spray process. In another
study, erosion of thermoplastic substrates, namely polyamide-6 and a blend of PC and acrylonitrile butadiene
styrene (ABS), was observed as a result of cold spray
deposition of Cu particles at high pressure (Ref 47).
Alternatively, HPCS has been used to embed metal
particles into polymer-based substrates without the formation of a continuous coating (Ref 23, 44, 45), as highlighted in Table 1. Embedment of Cu particles deposited
into PU and high density polyethylene (HDPE) thermoplastic substrates has shown potential for antifouling
applications. Cold-sprayed embedment studies reported to
date have utilized Cu deposition particles and neat polymer substrates, with carrier gas temperature that ranges
between 100 and 400 C.
2.3 Flame Spraying
Flame spraying is a thermal spray process in which the
combustion of fluid fuels produces a flame jet that melts
and accelerates the feedstock material for deposition onto
the substrate. The combustion gases are usually acetylene
and oxygen, though air may also be added for cooling
purposes (Ref 11). The feedstock particles are propelled
toward the substrate through a nozzle by the flow of a
carrier gas, which is usually an inert gas such as argon. The
feedstock material can be in the form of powder, wire, rod,
or cord. The flame temperature should be sufficiently high
to ensure the melting of the feedstock material (Ref 2).
The flame spray process usually exhibits gas temperatures
between 2000 and 3000 C (Ref 4, 52), whereas the particle impact velocity is usually below 100 m/s (Ref 2). As
shown in Table 2, the flame spray process parameters include nozzle geometry, nozzle stand-off distance from the
substrate, carrier gas pressure, feedstock material, and
combustion gas flow ratio (e.g., fuel-to-oxygen ratio).
Previous studies (Ref 19, 53) have shown that flamesprayed metal coatings can provide functional advantages
to polymer-based structures. These functional advantages
include enhancement of electrical and thermal conduction
properties, as shown in Table 3. Although the flame spray
process has been identified as a relatively inexpensive
thermal spray metallization process for polymers (Ref 19),
the flame-sprayed metal coatings contain levels of higher
porosity and, therefore, lower mechanical strength compared to those coating deposited by cold and plasma
spraying (Ref 4). Consequently, deposition by flame
spraying may require careful execution and optimization
of spray parameters to develop suitable and cost-effective
functional metal coatings on polymer-based structures.
Flame spray deposition of metal coatings onto polymerbased substrates is constrained by the sensitivity of the
substrate to the impact of high-temperature, semi-molten,
and molten particles (Ref 34). In addition, heat transfer
studies indicate that a significant thermal load can be exerted during flame spray metal deposition on to polymerbased substrates (Ref 20).
As shown in Table 1, flame spray metallization of
polymer-based substrates has been investigated using Al
and aluminum-12silicon (Al-12Si) powders, the latter
Journal of Thermal Spray Technology
consisting of 88 wt.% aluminum and 12 wt.% silicon.
Voyer et al. (Ref 34, 40) showed that the surface conductivity of Al coatings deposited onto polyester textile
substrates increased proportionally to the coating thickness, while the flexibility of the polymer fabric was retained and was unchanged. Boyer et al. (Ref 53)
characterized flame-sprayed Al-12Si coatings deposited
onto PU substrates. It was found that Al-12Si coatings
with satisfactory bonding and electrical conductivity can
be deposited onto PU elastomer up to a threshold coating
thickness. Beyond the threshold coating thickness, spalling
(peeling) of the metal coating from the substrate occurred.
Therefore, flame-sprayed coatings can be used to modify
the electrical properties of the polymer substrate, provided that the deposition process does not compromise the
structural integrity of as-sprayed structures.
A review of Table 1 shows that other studies have focused on the mechanical behavior of the flame-sprayed
metal coatings deposited onto polymer-based substrates.
Huonnic et al. (Ref 17) studied the effect of flame-sprayed
Al coatings deposited onto grit-blasted basalt and glass fiber
composite tubes with a [±603]T fiber architecture. Significant damage to the FRPC structure was reported as a result
of the flame spray metal deposition process. Subsequent
studies (Ref 18, 19) utilized a garnet sand interlayer between the FRPC substrate and the flame-sprayed metal
particles to protect the temperature-sensitive substrate
from the flame and particle impingement forces.
2.4 Electric Arc Wire Spraying
Electric arc wire spraying is a thermal spray process
whereby the heat generated in an electric arc discharge
melts feedstock wires to form droplets, which are accelerated in a gas stream through a nozzle, for deposition
onto the substrate (Ref 2, 54). Two feedstock wires with
opposing polarity converge at the nozzle outlet at a constant speed to generate the electric arc discharge. The
electric arc wire spray process can generate gas temperatures in excess of 5000 C (Ref 54), whereas particle impact velocities are usually below 300 m/s (Ref 2).
Feedstock wires can be made of metals or metal-cored
wires (Ref 2). As shown in Table 2, the process parameters of the electric arc wire spray process comprise the arc
gun geometry, electrical current input, arc gun stand-off
distance from the substrate, carrier gas pressure, and wire
material and feed rate (Ref 54, 55).
The electric arc wire spray process has been utilized to
deposit Zn and Al particles onto polymer-based substrates, as shown in Table 1. Liu et al. conducted two
studies (Ref 56, 57) on the arc wire spray deposition of Zn
and Al bond-coats onto PMC substrates. In these studies,
it was found that the metal bond-coat allowed the deposition of a harder and more thermally resistant top coating. As adduced from Table 1, limited research is
available on electrical arc metallization of polymer-based
substrates, and recent studies have focused on the fabrication and characterization of bond-coats using low
melting temperature materials such as Al and Zn. On the
other hand, it should be noted that no studies to date have
Volume 25(5) June 2016—905
been reported on powder arc spray deposition of metal
coatings onto polymer-based substrates.
undertaken in industry (Ref 15). However, the authors
of this study did not disclose the spray parameters that
were used to fabricate the coatings.
2.5 Plasma Spraying
Plasma spraying is a thermal spray process whereby
an ionized gas jet melts and propels the feedstock
powder for deposition onto the substrate. The plasma jet
consists of a carrier gas that is expanded through a
nozzle to create a sub- or supersonic flow (Ref 3, 58)
while being heated to the ionized gas state by DC arc or
RF discharges (Ref 3). Argon, nitrogen, helium, and
hydrogen may be used to produce the plasma jet, which
reaches a temperature up to 14,000 C (Ref 43, 46).
Particle impact velocities associated with the plasma
spray process are usually between 160 and 2000 m/s (Ref
2, 4). As indicated in Table 2, the process parameters of
the plasma spray process include torch geometry, electrical current input, torch stand-off distance from the
substrate, plasma gas flow rate, and feedstock material
(Ref 2). Plasma spraying is a unique process compared
to the other thermal spray processes due to the higher
gas temperatures achieved by the plasma jet, which allow the deposition of metals with high melting temperatures.
Plasma spray metallization of polymers has been
performed using air plasma spraying (APS). The APS
process has been used to deposit Al, Zn, and Cu
coatings onto PMC parts (Ref 22, 57, 59-61), as evidenced in Table 1. Plasma-sprayed Al and Zn bondcoats deposited onto PMC specimens exhibited similar
or higher shear adhesion strength compared to that of
similar coatings deposited by electric arc wire spraying
(Ref 57). On the other hand, it has been noted that the
high thermal load exerted by the plasma spray process
on the polymer-based substrates can degrade the substrates (Ref 9, 57). Therefore, careful execution of
plasma spray metallization of PMC parts should be
considered to avoid structural degradation of the substrate.
Limited studies have been conducted on the plasma
spray deposition of electrically conductive metals onto
polymer-based substrates, as is observed from the
information presented in Table 1. Beydon et al. (Ref 60)
and Ganesan et al. (Ref 59) reported the APS deposition of Cu layers onto carbon fiber-reinforced thermosets using chemical, thermal, and mechanical surface
preparation of the substrate. Alternatively, Huang et al.
(Ref 22) and Guanhong et al. (Ref 61, 62) studied the
deposition of thin Al bond-coats onto fiber-reinforced
polymers. In these studies, the research focus was on the
fabrication, thermal expansion, and mechanical characterization of the metal deposits, as opposed to the
investigation of the electrical properties of as-sprayed
parts. On the other hand, Affi et al. (Ref 48) reported
the deposition of electrically conductive plasma-sprayed
Al coatings onto carbon fiber-reinforced epoxy (thermoset) substrates, as indicated in Table 3. Also, plasma
spray deposition of metal coatings onto polymer-based
air foils for application as de-icing elements has been
906—Volume 25(5) June 2016
3. Surface Preparation of Polymer-Based
Substrates
Preparation of the substrate for thermal spraying is
essential to promote the adhesion of the impacting particles. The surface preparation process increases the
roughness of the substrate surface and promotes the formation of a mechanical bond between the impacting
particles and the substrate (Ref 63). Grit blasting is one of
the most common methods for roughening the substrate
prior to thermal spray deposition. During the grit blasting
process, the roughness of the surface increases as a result
of the erosion and material removal caused by the impact
of high velocity hard grit medium. The application of grit
blasting as a pre-treatment technique for thermal spraying
is not limited to metal substrates and has been employed
to roughen polymer-based substrates. As shown in
Table 1, Liu et al. (Ref 56) grit-blasted PMC substrates
made from graphite fiber-reinforced thermo-setting polyimide prior to plasma spray deposition, in order to promote the adhesion and fabrication of Zn coatings.
Huonnic et al. (Ref 17) used grit-blasted glass and basalt
FRPC tubes to promote the formation of Al coatings.
Guanhong et al. (Ref 61, 62) grit-blasted PMC (carbon
fiber-reinforced unsaturated polyester) substrates to promote the deposition of Al bond-coats using APS. Although grit blasting allowed the successful deposition and
adhesion of metal coatings in previous studies, this process
may not be a suitable method for brittle polymer substrates susceptible to cracking and localized fracture under
the impact of high velocity grit media.
Damage induced in the substrate by grit blasting can
adversely affect the mechanical properties of polymerbased structures. The results of the study by Huonnic et al.
(Ref 17) suggested that grit blasting of FRPC tubes can
decrease the burst pressure of composite pipes by compromising the integrity of the reinforcing fibers of the
composite. The degree of damage is dependent upon the
velocity of the blasting media, which is a function of the
control parameters of the grit blasting unit and physical
properties of the blast medium. Liu et al. (Ref 56) showed
that the level of damage in a graphite fiber-reinforced
thermoset polyimide substrate is likely to increase at
higher pressures of the carrier air of the blasting medium
due to the higher velocity of the impacting particles. On
the other hand, difficulties associated with the grit blasting
of polymer-based substrates are not limited to the risk of
structural damage. Soft and flexible elastomers, such as
PU that exhibit high elongation at yield, may cause the
grit medium to rebound from the surface as it deforms
elastically under the load of the impinging medium.
During grit blasting, the roughness of the substrate surface
increases as a result of the erosive wear caused by the
impact of hard particles. On the other hand, the roughness
Journal of Thermal Spray Technology
of the substrate surface can also be increased via abrasive
wear (i.e., grinding) of the substrate surface (Ref 22, 56,
64). As presented in Table 1, Huang et al. (Ref 22)
showed that grinding of the surface of quartz fiber-reinforced polyimide (thermoset) substrates using abrasive
paper (mesh size 240) can increase the surface roughness
through erosion of the substrate resin and exposure of the
fibers. Liu et al. (Ref 56) roughened the surface of graphite fiber-reinforced thermo-setting polyimide substrates
by both grinding and grit blasting. The grinding was conducted by using a range of sandpaper mesh sizes from 60
to 1000. The maximum shear adhesion strength of the
coating was obtained by using mesh size 100, which was
lower than that of samples roughened with grit blasting at
pressures higher than 0.2 MPa. This suggests that irregular
surface asperities caused by the impact of high velocity
grits can result in mechanical bonds of higher strength
between the surfaces and the deposited splats compared to
those of abraded surfaces produced by grinding. On the
other hand, due to the mechanical damage caused by the
abrasion of polymer-based substrates, grit blasting and
grinding methods may not be suitable for surface pretreatment prior to the thermal spray process and alternative methods may be warranted.
The addition of a granular material on the surface of
the polymer substrate is an alternative method that has
been used to increase the roughness of the substrate surface, in lieu of grit blasting. As shown in Table 1, Gonzalez et al. (Ref 18) roughened the surface of glass fiberreinforced epoxy (thermoset) tubes by the addition a layer
of garnet sand during the curing process prior to the flame
spray deposition of Al-12Si coatings. In contrast to results
reported by Hounnic et al. (Ref 17), where the grit
blasting process was employed, this technique did not alter
the strength of the FRPC tubes and the internal pressure
required to cause fracture and damage of the coated
FRPC specimens remained unchanged from that of the
uncoated FRPC specimens. The addition of a ceramic
granular material, with its low thermal conductivity, also
provides a barrier to heat transfer from the high-temper-
ature flame and particles, protecting the polymer substrate
from high-temperature degradation during the thermal
spray process (Ref 18). Also, Robitaille et al. (Ref 16)
suggested that the granular metal layer can be added to
the substrate surface during the curing process of the
polymer substrate to promote thermal spray deposition. In
this study, a layer of Cu particles was added onto the
unidirectional/weave carbon fiber-reinforced epoxy
(thermoset) substrate during the curing process to enable
the deposition of cold-sprayed Zn coatings without
inducing any damage to the substrate fibers. Deposition on
the substrate with no surface pre-treatment led to erosion
of epoxy and exposure and damage of the carbon fibers.
The idea of incorporating granular material on the surface
of the polymer substrates during the curing process has
also been employed for deposition of conductive coatings
onto elastomers such as PU. Boyer et al. (Ref 53)
roughened the castable PU (thermoset) surface by the
addition of Cu particles during the curing process, as
shown in Table 1. It was found that flame-sprayed Al-12Si
particles interlocked with the Cu particles to form a
continuous electrically conductive coating. In another
study conducted by Ashrafizadeh et al. (Ref 28, 65), the
addition of the same metal powder as that of the spraying
powder was suggested to roughen the thermo-setting
PU substrate during the curing process, which ensured
consistency between the roughening agent and the deposited particles. In another study by Lopera-Valle and
McDonald (Ref 66, 67), it was shown that the layer of
granular sand material can also be added to the thermosetting substrate (glass fiber-reinforced epoxy composite)
surface by applying a layer of adhesive after the curing
process, which allowed the deposition of coatings
consisting of 80 wt.% nickel and 20 wt.% chromium
(Ni-20Cr). Figure 3 shows a cross section image of the
deposited Ni-20Cr coating and the sand layer added onto
the FRPC substrate. As evidenced in Fig. 3, the metal
coating and the granular particles used for roughening
the polymer-based substrate are bound by mechanical
interlocking.
Fig. 3 Backscattered scanning electron microscope (SEM) image of the cross section of a flame-sprayed Ni-20Cr coating deposited onto
a fiber-reinforced polymer composite (FRPC) substrate (Ref 67)
Journal of Thermal Spray Technology
Volume 25(5) June 2016—907
On the other hand, the deposition of thermally sprayed
coatings onto polymer-based substrates without surface
preparation has also been reported in the literature (Ref
21, 25, 40, 47, 68). In cold spraying, the sprayed particles
were heated to temperatures below the melting temperature of the powder material. Thus, the metal particles are
stiffer and may penetrate the soft polymeric substrates
when accelerated to high velocities. Also, process
parameters can allow the hot carrier gas to soften the
polymer-based substrate beyond the threshold of penetration for impinging metal particles, resulting in particle
penetration into the substrate and ultimate adhesion. The
sprayed particles would form a mechanical bond with the
deposited metal particles already attached to the polymerbased substrate.
4. Deposition of Metal Bond-Coat
It is desirable to select a thermal spray process that
allows the deposition of metals with low porosity and
oxide levels, while avoiding the deleterious thermal effects
that are common during deposition on polymer substrates.
Most heat resistant polymers, such as polytetrafluoroethylene (PTFE), have a thermal decomposition temperature not higher than 470 C (Ref 69). The fact that the
temperatures involved in thermal spray processes can be
as high as 14,000 C (see Fig. 2) illustrates the challenge of
performing the thermal spray metallization of structural
polymer-based substrates. Hence, monitoring and control
of the temperature within the substrate during thermal
spray deposition onto polymer-based materials are of
interest.
In the case of deposition of powder particles with high
melting temperatures, regardless of the heat produced by
the spray plume, localized degradation of the polymer
substrate can occur as a result of the impact of hightemperature particles. Under these circumstances, fabrication of an intermediate metal coating with lower melting
temperature particles can be advantageous. As presented
in Table 1, Guanhong et al. (Ref 61) deposited an Al
bond-coat onto carbon fiber-reinforced unsaturated
polyester (thermoset) substrate prior to deposition of the
final aluminum oxide (Al2O3) coating by APS. This
technique allowed for enhanced protection of the polymer
substrate from the high temperatures of the molten and
semi-molten Al2O3 particles and the high temperature of
the plasma spray deposition process. Aluminum bondcoats deposited at shorter stand-off distances and higher
plasma currents were found to have higher adhesion
strength. This was due to the greater deformation exhibited by splats and the overall improvement in morphology
that led to a denser structure of the coating. Similarly,
Table 1 shows that Huang et al. (Ref 22) deposited Al as a
bond-coat prior to spraying the final yttria-stabilized zirconia (YSZ) coating in an effort to protect the quartz
fiber-reinforced polyimide (thermoset) substrate from the
high temperature of the plasma-sprayed YSZ deposition
particles. However, it was further shown by Huang et al.,
(Ref 64) that the use of Zn as a bond-coat provided better
908—Volume 25(5) June 2016
thermal protection for the polymer substrate. This was
likely due to the lower thermal conductivity of Zn of
approximately 116 W/m K in comparison to that of Al of
approximately 237 W/m K (Ref 70). The thermal expansion coefficient of Zn is also lower than that of Al. As a
result of the deposition of the Zn bond-coat, the final
deposited coating showed higher thermal shock resistance
compared to that of Al bond-coat. Thus, deposition of a
suitable bond-coat can reduce the adverse thermal effects
of thermal spray metallization of polymer-based substrates. Liu et al. (Ref 57) studied the effect of the bondcoat material on the microstructure and shear adhesion
strength of plasma-sprayed coatings deposited onto graphite fiber-reinforced polyimide (thermoset) substrates. It
was found that materials with higher melting temperature,
such as Cu (melting temperature of 1083 C) and nickel
(melting temperature of 1453 C), produced thermal
damage on the polymer-based substrate (polyimide with
working temperature of 371 C) upon deposition. This
thermal damage affected the bond strength between the
carbon fibers and the polyimide matrix, which led to
separation of some fibers and a weak bonding at the
coating-substrate interface. The bond-coats of materials
with lower melting temperature, such as Al (melting
temperature of 660 C) and Zn (melting temperature of
418 C), did not delaminate from the substrate. However,
in the case of Al, localized broken fibers with thermally
damaged regions between the substrate and the Al bondcoat were observed, whereas for Zn bond-coats, no evident damage on the substrate was produced. This can be
explained by the lower melting temperature of Zn, which
was only slightly higher than the long-term service temperature of the polyimide-based substrate of 371 C.
5. Thermal Spray Process Parameters
The properties of thermal-sprayed coatings are a
function of the spraying parameters. Thermal spray
parameters can be considered as the process inputs that
affect the particle velocity, particle temperature, and
substrate temperature. The substrate temperature is of
particular importance when depositing on substrates with
a low heat capacity, such as polymer-based materials. In
this case, the spraying parameters not only affect the
morphology of the final deposited coating, they also affect
the level of damage that the substrate might experience
during the thermal spraying process.
Thermal spray processes allow for different ranges of
deposition parameters (see, for example Fig. 2). The
appropriate selection of the thermal spray process
parameters should be considered in the context of thermal
spray metallization of polymer-based materials. Liu et al.
(Ref 57) showed that the arc spraying process is not suitable for metal coating deposition onto graphite fiber-reinforced polyimide (thermoset) substrates given that the
tip of the feeding wire must be heated beyond its melting
point to form droplets for deposition. As a result of overheating of Al droplets by arc spraying, the formation of
Journal of Thermal Spray Technology
thermal damage regions on the substrate has been reported. Therefore, the control on the temperature of the
arc-sprayed particles is limited compared to that of other
thermal spraying processes such as flame spraying, in
which the temperature of the spray plume can be controlled independently to avoid over-heating the impinging
metal particles.
Table 2 summarizes the process parameters of thermal
spray metallization processes for polymer-based structures. Values of these parameters should be selected based
on the physical properties of the sprayed particles, the
required temperature and velocity for particle deposition,
and the substrate heat capacity. Consequently, spraying
parameters such as the electric current input in plasma
and arc spraying, and fuel and oxygen feeding rates in
flame spraying should be apt to ensure that the produced
thermal load will not compromise the integrity of the
polymer substrates. Guanhong et al. (Ref 61) studied the
effect of the plasma spray current on the quality of the
coatings deposited onto polymer substrates. It was shown
that the use of high currents and long spraying times
during plasma spraying of Al2O3 introduced thermal
damage to carbon fiber-reinforced unsaturated polyester
(thermoset) substrates even with pre-deposition of Al as
bond-coat. A higher current increases the energy and
velocity of the plasma jet, which would lead to greater
absorption of the incident energy from the plasma jet and
the impacting high-temperature particles by the substrate.
Other spraying parameters such as air cooling and modifying the stand-off distance between the thermal spray
heat source and the substrate can be employed for further
process control. It has been found that the stand-off distance influences the metal coating deposition onto polymer-based substrates (Ref 34, 40). Shorter stand-off
distances led to shorter in-flight time of the metal particles
and, therefore, they had higher impact velocity and temperature and reduced oxide content. On the other hand,
shorter stand-off distances produce a higher heat flux into
the substrate, increasing the temperature within that
substrate (Ref 71). This increases the risk of imposing
excessive thermal load onto heat sensitive substrates such
as polymer-based materials. Thus, although shorter standoff distance can lead to higher deformation of splats and
improved coating morphology, setting the stand-off distance to very low values is not recommended for thermal
spray deposition onto polymer-based materials, unless
temperature control methods such as air cooling are used.
The injection of compressed air during the thermal
spraying process is an effective method to cool the substrate and limit thermal damage induced by the thermal
spray deposition process. As shown by Floristan et al. (Ref
72), air cooling was utilized to reduce the thermal load on
the substrate and prevent the coating from cracking during
plasma spraying of TiO2 coatings onto glass substrates.
The addition of compressed air can protect the substrate
from residual-stress induced cracking by reducing the
thermal load and temperature within the substrate and
allow thermal spray deposition at shorter stand-off distances. As a result of shorter stand-off distances, coatings
with lower electrical resistance were deposited due to the
Journal of Thermal Spray Technology
reduction in oxidation, higher deformation and improved
interlocking of splats and lower porosity of the TiO2
coating (Ref 72). The technique of employing additional
air cooling during the thermal spraying process to control
the temperature within the substrate has been shown to be
applicable to polymer-based substrates as well. Ashrafizadeh et al. (Ref 28, 65) studied the effect of the addition
of compressed air to the flame spray nozzle for deposition
of Al-12Si coatings on PU substrates. It was shown that
the addition of compressed air decreased the coating
porosity and reduced the electrical resistance of the deposited coatings due to the impact of droplets with higher
velocity and higher deformation of splats. The authors
also showed that temperature control within the substrate
was possible by reducing the stand-off distance while
increasing the pressure and flow rate of the compressed
air. As a result of using shorter stand-off distances, denser
coatings with improved morphology (greater deformation
of splats and improved mechanical interlocking) and
higher electrical conductivity were fabricated. In another
study, Voyer et al. (Ref 34, 40) utilized air cooling of the
substrate during the deposition of flame-sprayed Al coatings onto substrates made of polyester woven fabric
(thermoset), as shown in Table 1. It was shown that by
introducing cool air and optimizing the spraying parameters, the stand-off distance could be reduced without
imposing any evident thermal damage to the polymerbased substrate. This technique allowed the fabrication of
metal coatings with improved morphology as a result of
augmented thermal conductivity at shorter stand-off distances during flame spray deposition. Although the use of
compressed air cooling reduced the temperature within
the unfilled PU (Ref 28, 65) and polyester fabric (Ref 34,
40) substrates during flame spraying, future research to
evaluate the efficiency of this technique in reducing the
thermal load on polymer composite substrates such as fiber-reinforced polymers will be of interest.
Similar to other thermal spray metallization processes,
cold spray parameters play an important role on the final
characteristics of the deposited coating and possible mitigation of damage of the polymer-based substrate. As
shown in Table 1, Ganesan et al. (Ref 25) explored the
feasibility of the cold spray process for the deposition of
Cu and Sn coatings onto PVC (thermoplastic) substrates.
It was shown that the gas temperature affects the coating
deposition efficiency, possibly due to the thermal softening of the PVC substrate. Thermal softening of the substrate occurred under conditions in which the gas
temperature exceeded the glass transition temperature of
the PVC substrate (353 K) and changed the condition of
the substrate from rigid to a rubbery state. It was further
shown that Sn powder particles had higher deposition
efficiency than Cu particles due to the soft nature of Sn
and its lower impact energy compared to that of Cu. In a
subsequent study, Ganesan et al. (Ref 68) evaluated the
effect of substrate type on the deposition efficiency of
cold-sprayed Cu and Sn particles, using thermoset epoxy
and thermoplastic PVC substrates. It was found that
brittle epoxy substrate exhibited a lower coating deposition efficiency compared to that of PVC. Fracture of the
Volume 25(5) June 2016—909
epoxy substrate was observed during cold spray deposition, whereas the soft PVC substrate allowed the penetration of impacting particles, which resulted in higher
deposition efficiency (Ref 68). Thermal softening of carbon fiber-reinforced PEEK450CA30 (thermoplastic) substrates caused by the high temperature of gas during cold
spray deposition of Al powder has also been reported by
Zhou et al. (Ref 21). The softening of the PEEK450CA30
matrix was found to be responsible for the penetration of
Al particles into the FRPC material and the formation of a
mechanical bond between Al particles and the polymerbased substrate (Ref 21). Thus, depending on the
mechanical properties of the substrate and the cold spray
parameters, particles with high velocity either rebound
from or penetrate into the substrate (Ref 44). In a comprehensive study by King et al. (Ref 44), the embedment
of copper particles into a series of thermoset substrates,
namely polyamide (nylon), PU, HDPE, PTFE, PC, and
polypropylene (PP) was investigated. The Cu particles
were embedded into the polymer substrates by increasing
the gas temperature from 150 to 350 C. The maximum
penetration depth occurred in the PU (softest material)
and HDPE (lowest melting temperature). However, due
to the softening of the substrates that were studied, the Cu
particles did not deform properly to form a continuous
coating. Similarly, in a study conducted by Gardon et al.
(Ref 26), it was found that due to the softening of the
substrate (bio-compatible PEEK thermoplastic), the coldsprayed titanium particles penetrated (anchored) into the
substrate in the absence of coating build-up. A balance
between the carrier gas temperature and the thermal
softening of the polymer substrate was experimentally
found to allow the deposition of titanium coatings. Another study conducted by Lupoi et al. (Ref 47) showed
that Cu and Al powder particles are not suitable for cold
spray metallization of polymer-based substrates such as
PC/ABS blend, polystyrene, polyamide-6, and PP substrates by the cold spray process. The high density of Cu
particles led to high impact energy and erosion of the
polymer substrates, resulting in higher erosion rates at
higher gas pressure. Fabrication of Al coatings was
unattainable since the required critical velocity for cold
spray deposition of Al particles was not achieved for the
given substrates and process parameters. On the other
hand, Sn, with lower specific weight and melting temperature was successfully deposited onto all tested polymerbased substrates of the same study (Ref 47). Therefore,
alongside the carrier gas temperature, the feedstock particle material properties have a significant effect on the
deposition of cold-sprayed metal coatings onto polymerbased substrates. If the gas temperature is not sufficiently
high so as to soften the impacting particles, the high
velocity, rigid particles may either penetrate or erode the
substrate upon impact. However, introducing excessive
heat during the cold spray deposition process can soften
the substrate and limit the particle impact energy that
remains available for particle deformation, adiabatic shear
instability heating, and ultimate adhesion. In general,
knowledge of the particle velocity and substrate temperature distribution during the thermal spray deposition
910—Volume 25(5) June 2016
process can facilitate the experimental approach to find
optimum spray parameters for the deposition of metal
coatings onto polymer-based substrates.
6. Temperature Distribution Within
the Polymer-Based Substrate During
the Thermal Spray Process
Most studies focused on thermal spray metallization of
polymers have not examined the temperature distribution
within the substrate during the spraying process and the
possible effect of spraying parameters, such as compressed
air pressure and stand-off distance on the temperature
distribution. In fact, the spray parameters have usually
been obtained on a trial-and-error basis by evaluation of
the morphology, characterization of the deposited coatings, and investigation in search of evidence of possible
thermal damage in the substrate. Models that can predict
the temperature distribution within the polymer-based
substrate during the thermal spray deposition process can
be used to ensure that the temperature is maintained below the threshold of the thermal load that will cause
substrate degradation. In addition, knowledge of the effect
of spraying parameters, such as cooling air pressure and
stand-off distance on the substrate temperature distribution can facilitate optimization of the thermal spray process. This section provides a summary of the reported
models for the determination of the temperature distribution within the substrate during the thermal spray process, considering substrate materials with homogenous
thermal properties.
Irrespective of the type of substrate material, the
determination of the temperature distribution within the
substrate during the thermal spray process requires the
solution of a transient heat conduction problem. The heat
sources in the thermal spray processes are the heat from
the spray plume and the deposited particles. The finite
element (FE) method has been used to determine the
temperature distribution within the substrate during the
thermal spraying process. Hugot et al. (Ref 73) developed
a two-dimensional FE model to determine the temperature distribution within the substrate and the resulting
residual stresses in the coating during the plasma spraying
process, whereas Chen et al. (Ref 74) used FE modeling to
model the electric arc spraying process. Moreover, the finite difference numerical method has been used to simulate the heat transfer in thermal spray processes (Ref 7577). Using the finite difference method, Zhu et al. (Ref 75)
modeled the electric arc spray deposition process, whereas
Bao et al. (Ref 76) modeled flame spraying. To avoid the
limitations of numerical methods that involve meshing,
Wu et al. (Ref 78) developed a model based on a meshless
local Petrov-Galerkin approach to simulate the plasma
spray process. The model was verified experimentally and
the influence of the stand-off distance on the substrate
temperature was shown. Although the proposed numerical methods have been found to be reliable for the prediction of temperature distribution within the substrate
Journal of Thermal Spray Technology
during spraying, analytical models have been shown to be
simpler and more efficient in application. Ashrafizadeh
et al. (Ref 20, 79) presented an analytical model based on
the GreenÕs function approach to determine the temperature distribution within a polymer-based substrate during
flame spray deposition. The effect of stand-off distance
and air cooling of the flame on the temperature distribution within the substrate was investigated. Non-dimensional parameters for the determination of the
temperature distribution within the substrate during the
thermal spraying process were also proposed, and the results are illustrated in Fig. 4 (Ref 20, 79). The graph was
determined based on the assumption of an insulated surface on the back of the substrate. The model was experimentally verified and was found to be valid for the periods
of non-dimensional time, t*, smaller than 0.25. In Fig. 4,
T*, x*, and t* are defined as
T ¼
T
T0
q00 L
k
t ¼ Fo ¼
a¼
at
L2
ðEq 1Þ
ðEq 2Þ
k
qCp
ðEq 3Þ
x
;
L
ðEq 4Þ
x ¼
where T, T0, q00 , L, k, a, q, Cp, t, and x are the transient
temperature, initial temperature, torch heat flux, substrate
thickness, thermal conductivity, thermal diffusivity, density, specific heat capacity, time and position from the
substrate surface, respectively. The fact that polymerbased materials have low thermal conductivity (less than
10 W/m K even for most of the composite materials with
high thermal conductivity fillers) (Ref 80-84) suggests that
the non-dimensional parameter of time (t*) may also be
smaller than 0.25 for many spray conditions on polymerbased materials. This may allow for the determination of
the temperature distribution within polymeric substrates
during the thermal spray metallization process by using
the graph presented in Fig. 4.
The proposed modeling techniques can be used for
determination of temperature distribution within substrates with homogenous thermal properties during the
thermal spraying process. On the other hand, the proposed models may be limited in their representation of
materials exhibiting anisotropic thermal properties. This
condition can occur in fiber-reinforced polymer matrix
composites based on the arrangement of fibers and differences between the thermal conductivity of the matrix
and fiber materials. The temperature distribution in anisotropic substrates may not be one-dimensional and the
spatial variation of the thermal conductivity should also be
considered in the model formulation. Thus, additional
research is required to provide accurate models of the
temperature distribution within anisotropic polymerbased substrates during the thermal spraying process.
7. Testing and Characterization
of Thermal-Sprayed Metal Coatings
Deposited on Polymer-Based
Substrates
The testing and characterization of thermal-sprayed
coatings are important parameters in the evaluation of the
quality and properties of the deposited materials, and
therefore, are important to determine the performance of
coatings in the various applications in which they are used.
A combination of materials science and engineering
techniques is used to evaluate the mechanical, thermal,
Fig. 4 Curves of the non-dimensionalized temperature as a function of non-dimensionalized time for different positions within
a polyurethane substrate during flame spray heating (Ref 20)
Journal of Thermal Spray Technology
Volume 25(5) June 2016—911
structural, and chemical properties of the thermal-sprayed
metal coatings that are deposited on polymer-based substrates. Various techniques have been utilized for the
characterization of thermal-sprayed coatings deposited
onto polymer-based substrates, including hardness test
(Ref 85), Scanning Electron Microscope (SEM) (Ref 46),
and x-ray diffraction (XRD) (Ref 86), to name a few. In
addition, experimental techniques have been developed to
assess the performance of the thermal-sprayed coatings
based on specific applications, such as thermal shock
protection (Ref 64) and antifouling resistance (Ref 23).
Testing and the characterization of thermal-sprayed metal
coatings can be accomplished by using general characterization methods and application-based techniques. This
section focuses on the techniques that have been used in
the characterization of thermal-sprayed coatings deposited onto polymer-based substrates.
7.1 General Characterization Methods
7.1.1 Hardness Test. Hardness tests measure the
resistance of materials to deformation under a localized
load (Ref 46, 87, 88). Given that hardness is a widely used
property in materials evaluation, several studies have focused on hardness evaluation methods for most engineering materials, including thermal-sprayed metal
coatings (Ref 46, 88, 89). In addition to the measurement
of hardness, mechanical properties such as YoungÕs modulus, bond strength, and fracture toughness can be measured through hardness tests that are coupled with various
analytical techniques and models (Ref 87, 89-91). ASTM
Standard E384 (Ref 85) describes guidelines for performing hardness tests on common materials, including
metals and polymers, using Knoop and Vickers indenters.
The Vickers micro-hardness test constitutes the most
frequently used technique for measuring the hardness and
elastic constants of thermal-sprayed metal coatings, using
loads that are between 100 and 500 g (Ref 46, 88, 89, 92).
The Vickers micro-hardness equipment consists of a
pyramidal-shaped diamond indenter with an angle between faces of 136 that produces a square-shaped
indentation. The test is done at a pre-set load and dwell
time on the surface of the polished coating (Ref 85). The
distance between the corners across the square-shaped
indentation is used to determine the hardness of the
coating. Vickers micro-hardness has been previously used
in the characterization of thermal-sprayed coatings over
polymer-based composites. For instance, Zhou et al. (Ref
21) performed Vickers micro-hardness testing to characterize a bi-layer cold-sprayed Al/Cu coating deposited
onto carbon fiber-reinforced PEEK. Vickers micro-hardness tests were performed in the Al bond-coat layer first
and following the deposition of the subsequent Cu top
layer. It was noted that the hardness of the Al layer increased by more than 20%, from 42 HV0.1 to 52 HV0.1,
after the deposition of the Cu layer. This was attributed to
the work hardening (compressive) effect of the impacting
Cu particles with high kinetic energy on the Al bond-coat
during the deposition of the top coating layer. The observed reduction in the porosity of the Al bond-coat layer,
912—Volume 25(5) June 2016
from 2.9 to 1.1%, following the deposition of the Cu top
layer also explains the increase in hardness. The hardness
of the Cu layer was found to be 140 HV0.1, which was
reported to be approximately on the same order of that of
rolled copper (Ref 21).
Micro-hardness indentation differs from macro-hardness testing, which involves indenter loads between 1 and
100 kg, and exhibit an indentation depth on the same order of magnitude as that of the thickness of the thermalsprayed coating (Ref 46). Therefore, macro-hardness
measurements made on the top surface of thermalsprayed coatings may be affected by the mechanical
properties of the substrate (Ref 93, 94). As a result, the
reliability and repeatability of macro-indentation measurements on the top surface of thermal-sprayed metal
coatings is constrained (Ref 85). In this regard, ASTM
Standard E384 specifies that the thickness of the material
exposed to hardness testing should be at least ten times
the indentation depth (Ref 85). Given that typical thermal-sprayed coatings have thicknesses between 100 and
1000 lm, the indentation resulting from a macro-hardness
test would likely exceed one-tenth of the coating thickness, which contravenes the requirements ASTM Standard E384. Therefore, macro-hardness characterization of
thermal-sprayed metal coatings deposited onto polymerbased substrates is limited, and careful execution of
indentation measurements is required. Similarly, nanoindentation may be performed to measure the hardness of
thermal-sprayed metal coatings exhibiting a thickness
lower than 100 lm and to evaluate the local mechanical
properties (i.e., individual deposited splats) in the lamellar
and heterogeneous microstructure of the coating. In this
case, the macroscopic effects of porosity and intra-particle
bonding on the hardness of the thermal-sprayed metal
coating would be less evident from nanoindentation
measurements (Ref 95). Even though multiple nanoindentation measurements may be used to estimate the
hardness of thermal-spayed coatings, this technique has
not been reported in regard to the characterization of
thermal-sprayed metal coatings deposited onto polymerbased substrates.
As previously mentioned, hardness tests can be used in
the measurement of mechanical properties such as
YoungÕs modulus, bond strength, and fracture toughness.
For instance, Beshish et al. (Ref 96) implemented indentation techniques in the evaluation of the fracture toughness of YSZ coatings by measuring the dimensions of
indents in the coatings and the length of the surrounding
fracture cracks. This technique can be applied to metallic
coatings deposited onto polymer-based substrates exhibiting suitable stiffness (i.e., no creeping under the force of
the indenter).
7.1.2 Bond Strength. The bond strength of thermalsprayed metal coatings defines their response to compression and shear loads, and the mechanical integrity of
the coating on the substrate (Ref 89, 97). Various bonding
mechanisms are involved in defining the bond strength of
a coating to the substrate (Ref 90, 97-100), and some include (i) mechanical interlocking when the coating material penetrates into the rough asperities of the substrate
Journal of Thermal Spray Technology
surface (Ref 100, 101), (ii) chemical reaction when the
coating and substrate chemically react to form metallurgical bonds (Ref 90), and (iii) solid-state diffusion, also
forming metallurgical bonds (Ref 90). It is challenging to
quantify the contribution of each mechanism to the total
bond strength of the coating to the substrate; therefore,
the measurement of the bond strength is accomplished by
considering the overall impact of all the mechanisms. The
estimation of the coating bond strength allows for evaluation of the integrity of the coating-substrate interface,
usually referred to as adhesion, and the strength of the
coating within itself, referred to as cohesion (Ref 46, 100).
Several techniques have been used to measure the
bond strength of thermal-sprayed metal coatings deposited on polymer substrates. The stud-pull test is a
commonly used method in which a loading element (stud)
is bonded to the flat surface of the coating, and placed in
tension with a load applied at a controlled rate while
measuring the displacement of the stud until delamination
of the coating occurs (Ref 25, 99, 100, 102). For instance,
Ganesan et al. (Ref 25) evaluated the effect of different
substrate preparation treatments on the bond strength of
plasma-sprayed Cu coatings, using stull-pull tests. Chemical, thermal, and mechanical surface preparation treatments were compared in this study. Aluminum studs, with
6 mm in diameter, were bonded to the coatings and pulled
using a universal tensile test machine. This study showed
that with a strength adhesion of 2.6 MPa, the chemical
surface preparation treatment presented higher adhesion
strength than that presented by the thermal and mechanical treatments, with 1.7 and 2.2 MPa, respectively. It was
found that adhesive failure was predominant, presenting a
complete removal of the coating in the area to which the
studs were bonded. Different studies have found that the
failure of metal coatings on polymer-based substrates is
predominantly adhesive (Ref 25, 60, 100). Nevertheless,
cohesive failure has also been reported (Ref 57). Specific
procedures and guidelines for performing adhesion
strength tests of metallic coatings using portable adhesion
testers are described as part of the ASTM D4541 Standard
(Ref 102).
The bond strength of thermal-sprayed metal coatings
estimated by the tensile adhesion test is based on the
ultimate stress and strain (Ref 49, 57, 103, 104). Other test
methods for evaluation of the bond strength of deposited
coatings are the peel test (Ref 105, 106), laser shock
adhesion test (Ref 24, 107-109), and multi-layer three
point bending test (Ref 60, 89). However, the application
of these other methods on thermal-sprayed coatings deposited on polymer-based substrates has not been widely
explored and further research is needed.
7.1.3 Chemical and Phase Composition. The phase
composition of thermal-sprayed coatings has been evaluated using x-ray diffraction (XRD) technique in several
previous studies (Ref 11, 22, 86, 88, 110, 111). For instance,
Osorio et al. (Ref 111) used XRD spectroscopy to characterize the phase transformation of plasma-sprayed YSZ
after exposure to temperatures as high as 1100 C for up
to 1000 h. In addition, the XRD technique has been
used to evaluate changes in the phase composition of the
Journal of Thermal Spray Technology
deposited material as a result of the high-temperature in
thermal spray process (Ref 88, 110). The comparison between the material phases in the coating and those of the
feedstock material can provide information about any
phase transformations (i.e., chemical reactions or oxidation) that occurred during the thermal spray deposition
process. In a study conducted by Chen et al. (Ref 110),
XRD was used to compare the material phases of the AlAl2O3 powder feedstock and the resulting flame-sprayed
coating deposited onto a polyurethane substrate. The use
of XRD enabled finding a higher content of the Al2O3
phase in the thermal-sprayed coating, which was possibly
due to the oxidation of Al particles during the high-temperature deposition (Ref 110).
In addition to material phase identification, elemental
analysis is important in the characterization of thermalsprayed coatings. Energy dispersive x-ray spectroscopy
(EDX), usually coupled with scanning electron microscopy, can be used to determine the elemental composition of thermal-sprayed coatings (Ref 86).
7.1.4 Microstructural Analysis. Knowledge of the
morphology and distribution of splats, voids, and cracks in
thermal-sprayed coatings can be obtained from microstructural characterization, which is usually performed by evaluating a sample specimen of material using equipment for
microscopy and porosimetry (Ref 112). Microstructural
analysis can be used to evaluate the porosity and lamellar
splat morphology of thermal-sprayed metal coatings deposited onto polymer-based structures. These characteristics
of the coating are affected by the spraying parameters, substrate surface conditions, and feedstock material properties
and morphology (Ref 2, 4, 13).
Measurement of the porosity in thermal-sprayed coatings is a significant element of microstructural characterization. Pores are three-dimensional defects (voids) in the
thermal-sprayed metal coating that can be classified as (i)
inter-splats defects, generated as a result of a gap between
two different particles, (ii) intra-splat defects, inside the
splats, generated from fragmentation of the particles
during the deposition process or as result of thermal
stresses during the deposition and cool-down of the deformed particles or splats, and (iii) coating delamination
defects caused by damage of the coating at either the inter- or intra-splat locations (Ref 9, 46, 88). Some techniques to measure the microstructural porosity within the
coating include the use of a volume of fluid to occupy the
voids spaces as a mean of measuring the volume of the
pores (Ref 46, 88). Examples of the fluid-based measurement methods to estimate the pore volume, and ultimately
the porosity, include mercury infusion porosimetry, gas
pycnometry, and water absorption (Ref 113, 114). However, microscopy and digital image analysis are more
frequently used for the characterization of porosity and
the microstructure of thermal-sprayed metal deposits on
polymers. In some respects, microscopy and digital image
analysis can provide additional information about the
coating microstructure including the distribution of pores
and location of cracks (Ref 46, 88). For instance, SEM
allows qualitative identification of the splat morphology,
the degree of interlocking between the coating and
Volume 25(5) June 2016—913
substrate, and the existence of contaminants and oxides;
additionally, high-resolution SEM images can be used to
measure the thickness of the thermal-sprayed coatings (Ref
46). For example, Floristan et al. (Ref 72) studied the
thickness and porosity of plasma-sprayed TiO2 coatings by
conducting digital image analysis on images taken by SEM.
7.2 Application-Based Techniques
In order to assess the functional characteristics of the
thermal-sprayed metal coatings, the use of methodologies
that are focused on specific responses of the coating-substrate material system is required. This section describes
some evaluation techniques that have been developed for
specific applications of thermal-sprayed coatings deposited onto polymer-based substrates.
7.2.1 Thermal Shock Resistance. Thermal shock is the
sudden change in temperature that can cause the failure of
a component due to stresses induced by high-temperature
gradients. The resistance of materials to a sudden change
in temperature without presenting failure is called thermal
shock resistance, and assessing this property in multi-layer
coated-polymers is of interest. As presented in Table 1,
Huang et al. (Ref 22, 64) studied the feasibility of APS
deposition of cobalt-nickel-chromium-aluminum-yttrium
(CoNiCrAlY), Al, and Zn bond layers and a ceramic YSZ
top layer to serve as a protective coating system on
polyimide matrix composites. The thermal shock resistance of the a-sprayed specimens was evaluated by thermal shock testing, in which the samples were exposed to
abrupt changes in temperature. The coated samples were
heated to 400 C in a furnace. Thereafter, the samples
were quenched in water at room temperature, producing a
high-temperature gradient in the coated samples. Cracks
and delamination due to the abrupt change in temperature
were examined under a SEM. The use of thermal shock
resistance tests in this study allowed concluding that the
Zn bond-coat exhibited higher thermal shock resistance
than the Al and CoNiCrAlY bond-coats.
7.2.2 Fouling Resistance. Biofouling comprises the
accumulation of organic materials on a surface, which has
been shown to decrease the lifetime of marine structures
(Ref 23, 45, 115, 116). The design of techniques to avoid
biofouling has been the main interest of several research
studies (Ref 23, 45). Vucko et al. (Ref 23) used the cold
spray process to embed copper-based particles onto
HDPE and nylon composites. The suitability of copperembedded composites as a marine antifouling system was
evaluated for 250 days. As-sprayed and unsprayed
140 mm x 170 mm samples were placed 200 mm below the
water surface of the Townsville Yacht Club Lake
(Queensland, Australia) in mid-December (Summer) and
photographs were taken each week to track fouling
development. This study showed that copper-embedded
HDPE plates have similar antifouling properties to pure
Cu plates after 250 days underwater. The coatings limited
soft fouling formation (algae) and deterred the deposition
of hard fouling (carbonate sediments), which, from a
financial perspective, is the most detrimental to marine
and water-based enterprises.
914—Volume 25(5) June 2016
8. Potential Applications
8.1 Electrically Conductive Structures
The fabrication of electrically conductive thermalsprayed metal coatings onto polymers has gained attention
as a fabrication technique for electrical sensors (Ref 30,
53, 117-119). However, the development of suitable thermal-sprayed electrically conductive coatings for engineering applications is currently an area of research (Ref
120). Several fundamental studies (Ref 17, 53, 121-123)
have shown that the porous structure of metal thermalsprayed coatings results in electrical properties that differ
from that of the bulk metal. Even though Ohmic resistive
behavior can be observed in thermal-sprayed metal coatings, such as Ni-Al (Ref 124) and Al-12Si (Ref 118), the
porosity and morphology of the coating strongly affect the
ability of thermal-sprayed metal coatings to conduct
electrical charges. For instance, Sampath et al. (Ref 121)
reported that dense cold-sprayed Cu coatings can exhibit a
25% increase in electrical resistivity with respect to that of
the bulk Cu, while the resistivity of porous plasma-sprayed
Cu coatings can increase by 300-400%. Therefore, the
development of thermal-sprayed metal coatings that are
tailored to has electrical performance that approach those
of the bulk metal requires precise control of the process
parameters and knowledge of the cross-property relationships, such as porosity and electrical resistivity. Also,
pores and oxide lamellas strongly affect the electrical
behavior of thermal-sprayed metal coatings, leading to
microscopic anisotropy. In contrast, the electrical resistivity of thin metallic coatings fabricated using vapor
deposition methods, such as CVD and PECVD, exhibit a
relatively homogeneous behavior due to the low level of
microscopic defects (i.e., material grain boundaries) and
higher uniformity and density of the deposits (Ref 38).
As shown in Table 3, limited studies exist on the electrical characterization of thermally sprayed metal coatings
deposited onto polymer-based structures for electrical
applications. Voyer et al. (Ref 34, 40) studied the deposition of electrically conductive Al coatings onto polyester
fabrics deposited via the flame spraying process. Optimal
flame spray deposition resulted in coatings that were 75100 lm thick with a sheet resistivity of at least 0.002 X m/
m (Table 3). It was found that an electrically conductive
Al coating can be fabricated without compromising the
flexibility of the polyester fabric. Huonnic et al. (Ref 17)
deposited flame-sprayed Al coatings onto cured glass and
basalt FRPC tubes and measured the electrical resistance
and resistivity of the metal layer to characterize the
quality of the deposits. A minimum of two flame spray
torch passes was required to produce a uniform and
electrically continuous metal layer on the polymer-based
tubes. No evidence of phase changes or oxidation was
observed in the flame-sprayed coatings. However, the
values of electrical resistivity of the porous Al coating
were measured to be 6 9 10 7 and 8 9 10 7 X m
(Table 3), which are one order of magnitude greater than
the resistivity of annealed Al alloys (3 9 10 8 X m).
Affi et al. (Ref 48) compared the electrical resistivity of
Journal of Thermal Spray Technology
cold- and plasma-sprayed Al coatings deposited onto
carbon fiber-reinforced epoxy (thermoset) substrates. It
was noted that the plasma-sprayed Al deposits exhibited
higher resistivity values compared to those of cold-sprayed
Al coatings, which may be a consequence of increased
oxidation of sprayed particles at higher carrier gas temperatures. In another study, Gonzalez et al. (Ref 118)
studied the electrical behavior of porous (12-17% volumetric porosity) flame-sprayed Al-12Si coatings for damage detection in cross-ply FRPC substrates, as shown in
Table 3. The change in the electrical resistance of the
Al-12Si coating was measured during the application of a
linear tensile load to the coated FRPC specimens. It was
found that the change in resistance across the Al-12Si
coating due to early matrix damage in the cross-ply FRPC
was limited, but increased significantly as a result of fiberdominated failure and delamination in the substrate.
8.2 Thermal Systems
Typical thermal properties of the polymer-based
materials do not allow for quick heat conduction through
the material. Thermal-sprayed metal coatings can be used
to improve the conduction of heat in polymer-based
material structures. For instance, Therrien et al. (Ref 19)
utilized flame-sprayed Al-12Si coatings on glass FRPC
parts to study the heat conduction efficiency along the
surface area of as-sprayed polymer-based plates. Al-12Sicoated samples were heated up with a resistance heating
wire while thermocouples were used to measure the
temperature distribution on the coated and bare sides of
the polymer. As presented in Table 3, this study found
that the high thermal conductivity of the flame-sprayed
Al-12Si coating, at approximately 250 W/m K, allows a
more efficient conduction of heat to the surface of the
FRPC substrate.
Additionally, methods that protect polymers from the
adverse effects of high temperature are also required in
the aerospace industry. Huang et al. (Ref 22) fabricated a
multi-layer Al-YSZ thermal barrier coating onto quartz
FRPC using APS. An Al bond layer and YSZ top layer
were successfully deposited onto the polymer composite
and the thermal shock resistance of the coating was
evaluated. It was found that thermal spray deposition of
an Al bond-coat with an YSZ top layer can serve as a
thermal barrier to protect the polymer-based structure.
Prevention of ice accretion on the surface of polymerbased structures is another potential application of the
metal coatings that are thermally sprayed onto polymer
substrates. Ice accretion is the formation and accumulation
of ice due to exposure to a super-cooled fluid. Rooks (Ref
15) proposed the use of plasma spraying in the fabrication of
resistive heater elements for ice protection of the leading
edge portion of the main rotor blade of a military helicopter.
Lopera-Valle and McDonald (Ref 66, 67) investigated the
possible application of flame-sprayed nickel-chromiumaluminum-yttrium (NiCrAlY) and Ni-20Cr coatings deposited onto FRPC parts for de-icing applications. As
shown in Table 3, the coatings electrical resistance of the
NiCrAlY and Ni-20Cr coatings was estimated between 3.2
Journal of Thermal Spray Technology
and 3.55 X. Electrical current was applied to coated samples
to increase the surface temperature by resistive (Joule)
heating. The surface temperature profiles of the coatings
were measured under free and forced convection conditions
at different ambient temperatures, ranging from 25 to
23 C. It was found that at ambient air temperatures below
0 C, the surface temperature of the coating, and in some
cases, that of the FRPC remained above 0 C for both the
forced and free convection conditions. In addition, there
was a nearly homogeneous temperature distribution over
the coating surface. This suggested that flame-sprayed
coatings may be used as heating elements to mitigate ice
accretion on polymer-based structures, without the presence of areas of localized high temperatures (Ref 66, 67). In
a subsequent study, Lopera-Valle and McDonald (Ref 125)
evaluated the performance of NiCrAlY flame-sprayed
coatings that were deposited onto FRPC substrates as deicing elements. The coated FRPC samples were placed
inside a cold room at temperatures between 5 and
25 C, and water was sprayed over the specimens, simulating light to moderate rain, and leading to the formation
of an ice layer of up to 4.5 mm thick. As in previous studies
(Ref 66, 67), electrical current was applied to the coated
samples, which generated an increase in the temperature of
the ice-covered sample, and melted the ice layer by way of
Joule heating. This study showed that the heat generated
by the NiCrAlY flame-sprayed coatings successfully melted the ice, providing evidence of the feasibility of implementing flame-sprayed coatings on FRPC as embedded deicing elements (Ref 125). An existing implementation of
this concept is the de-icing system integrated into the
Boeing 787 Dreamliner CFRP composite wing that is
based on a proprietary thermal spray deposition technique
developed by GKN Aerospace (Redditch, UK) (Ref 35).
This application comprises thermal-sprayed metal coatings deposited onto a fiber-reinforced polymer ply to create a resistive heater mat, which is embedded into the
aircraft wings. However, the scientific literature available
on thermal spray metallization of FRPC structures for
aerospace applications is scarce, as indicated in Table 3. To
date, only a preliminary preview by Bheekhun et al. (Ref
126) has been presented on the thermal spray metallization
of PMC substrates in relation to gas turbine engine applications.
9. Conclusion and Suggestions for Future
Work
Polymer-based materials can have excellent mechanical
and physical properties, such as enhanced resistance to
corrosion, resistance to wear, and high strength-to-weight
ratio. However, the low thermal and electrical conductivities of polymer-based materials limit their use in electrical charge conduction applications and elevated
temperature environments. Metallization of polymerbased materials by the deposition of metals is a possible
means to augment their effective thermal and electrical
conductivities. Also, the deposited metal coatings can
Volume 25(5) June 2016—915
provide new functionality for the new metal-polymer
system such as resistive (Joule) heating applications.
In this study, research contributions on the thermal
spray metallization of polymer-based materials were presented. One of the main challenges with thermal spraying
on polymer materials is the inability to control the substrate temperature during the deposition process since
most polymers have low thermal conductivity and heat
capacity and may degrade or experience significant changes in properties at high temperatures. The review of
different thermal spray deposition processes demonstrated
that cold spraying, flame spraying, electric arc wire
spraying, and plasma spraying have been employed in
previous studies for the metallization of polymer materials. Although these processes can expose the polymer
substrates to high heat loads, which depends on the
spraying parameters and feedstock material, successful
metal deposition onto polymer materials can be achieved.
The use of compressed air cooling and deposition of low
melting temperature metals as bond-coat prior to deposition of the final top coating were introduced as two
methods to control the thermal load on the polymer-based
substrate from the spray torch.
Knowledge of the temperature distribution within the
substrate during the spray process can facilitate the sensitivity analysis to spray parameter changes and prevent
thermal overload within the polymer substrate. Analytical
models and numerical techniques have been used to
model the thermal spraying process in an effort to predict
the temperature distribution within the substrate during
the spraying process. Substrate temperature distribution
models that have good agreement with experiments were
introduced. On the other hand, the number of studies
focused on the temperature distribution within the polymer-based materials is limited and future work is needed
in this area.
Pre-treatment of the polymer surface prior to thermal
spraying was discussed and the previous research in this
area was reviewed. Based on previous studies, grit blasting
of polymer materials may not be the optimal pre-treatment option for thermal spray metallization due to the
risk of inducing structural damage in the polymers.
Roughening of the substrate by the inclusion of powder
materials on the surface of the polymer substrates, either
during the curing process or by an adhesive agent, was
recommended. The protection provided to the polymerbased substrate by the low thermal conductivity roughening agent from the heat load during thermal spray
metallization was briefly discussed. However, fundamental
study and analysis in this area are needed and future research is required.
The review of the methods for evaluation and characterization of thermal-sprayed metal coatings deposited
onto polymer substrates showed that most of the methods
such as measurement of hardness, bond strength, chemical
composition, and digital image analysis are similar to
those used in respect of non-polymer substrates. However,
novel techniques based on the final application of the
polymer-metal system have been employed.
916—Volume 25(5) June 2016
Improvement of thermal and electrical conductivities
of the polymers has been the focus for deposition of the
metal coating onto polymer-based substrates in most
studies. On the other hand, it has been shown that the
combination of the metal coating and the polymer can
produce a material system that is capable of being employed in novel applications. The applications discussed in
published studies included the use of the deposited coating
as a heating element for de-icing purposes, antifouling,
and structural damage detection. Overall, future work is
needed in these areas to establish and test the possible
novel applications of metal deposits onto polymer-based
substrates. Some application-relevant areas that can be
considered for future work and study are given below:
(1) The use of metal coatings as heating elements on
coated FRPC pipes to decrease the viscosity of flowing
fluids and minimize pump power requirements during
low ambient temperature events;
(2) The use of metal coatings as heat tracers to improve
the wear performance of coated protective elastomer
liners;
(3) Structural health monitoring of polymer-based structures by controlling the impedance of the metal
coating deposited on the outer surface of the structure
or embedded in a laminated architecture; and
(4) The investigation of the temperature distribution in
substrates exhibiting anisotropic thermal properties
(e.g., fiber-reinforced polymers) during the thermal
spray process, and development of analytical/numerical models that can account for substrate anisotropy.
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