Surface & Coatings Technology 258 (2014) 832–841

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Surface & Coatings Technology
journal homepage: www.elsevier.com/locate/surfcoat

Development of ultra-strong adhesive strength coatings using cold spray
Renzhong Huang ⁎, Wenhua Ma, Hirotaka Fukanuma
Plasma Giken Co., Ltd., Toshima, Tokyo, Japana r t i c l e

i n f oArticle history:
Received 25 March 2014
Accepted in revised form 24 July 2014
Available online 4 August 2014
Keywords:
Cold spray
Adhesive strength
Copper coating
Splat
Fracture surface

a b s t r a c t
In the cold spray process, adhesive strength between coatings and substrates is considered to be the most crucial
mechanical property. Bonding is an important factor in determining if cold spray can be used for an application.
Therefore, development of effective bonding between coatings with various substrates is essential to cold sprayprocessing. In this study, Cu coatings were deposited on to three different substrates: Al5052, Al6063, and stainlesssteel 316 L. The adhesive strength of Cu coatings on these three substrates was investigated. The experimental results showed that effective bonding could be generated only when particle velocity exceeds 500 m/s for Al alloy
substrates, and 800 m/s for stainless steel 316 L substrates. Based on the present studies, an ultra-strong bonding(more than 200 MPa) between dissimilar materials has been developed with cold spray processing.
© 2014 Elsevier B.V. All rights reserved.

1. Introduction
Cold spray is an emerging spray coating technology that was ﬁrst
developed in the mid-1980s at the Institute of Theoretical and Applied
Mechanics in the former Soviet Union [1]. Cold sprayed coatings canbe achieved only when the velocity of in-ﬂight particles exceed a certain
critical velocity [2,3]. Therefore, the velocity prior to impacting thesubstrate is the most important parameter in cold spray [4]. Compared
to traditional thermal spray processes, particles go through intensive
plastic deformation upon impact in solid states at temperatures well
below their melting point. Therefore, cold spray is particularly suitableto prepare coatings that are sensitive to oxidation [5].
Adhesive strength of coatings prepared by cold spray determines
its applications in the industrial ﬁeld. Many researches have been
performed to focus on the bonding mechanism of cold spray, but it
still remains unclear [6–19]. Mäkinen et al. presented the inﬂuences ofpowder, substrate and heat treatment on adhesive strength [6]. Donneret al. examined the effects of chemical afﬁnities and hardness of substrate on adhesive strength [7]. Moreover, recent numerical simulation
also has helped to explore the bonding mechanism [8–15]. Thesestudies on bonding mechanism of cold spray suggested that adhesivestrength is mainly affected by mechanical interlock [7–9,16] and metallurgical bonding caused by impact molten or diffusion bonding [11–19]⁎ Corresponding author.
E-mail address: rz_huang@plasma.co.jp (R. Huang).

http://dx.doi.org/10.1016/j.surfcoat.2014.07.074
0257-8972/© 2014 Elsevier B.V. All rights reserved.

based on the shear instability [10]. One of the purposes to study thebonding mechanism is to prevent coating de-bonding from substrate.
De-bonding of coating in thermal spray is mainly caused by its residual
stress, and the increased thickness aggravates the de-bonding tendency
owing to the accumulation of residual stress [20,21]. For cold spray,
Fukanuma and Ohno also experimentally demonstrated that by increasing coating thickness, it increases the risk of separation between coating
and substrate due to the accumulated internal stress [22]. Effective
bonding can be formed between coating and substrate only in the casewhere there is adequate adhesive strength to overcome the internal
stress [21–23].
Adhesive strength measurements for cold spray coatings are carriedon by pull tests, e.g. according to the standard: EN582 Thermal Spraying.
In the conventional test, an epoxy resin adhesive is employed to glue
specimen with a loading block. The reliability of the test depends on
this glue's strength. Due to the restriction of test method and cold
spray equipment's capability, the adhesive strength of cold-sprayedcoatings reported has no more than 70 MPa. Even though Smurov et al.
put forward a new method by pulling a specially machined specimen
to break through the restriction of the conventional adhesive strength
test, the adhesive strength was still less than 30 MPa due to the nonoptimized spray conditions [24]. Studies about ultra-strong adhesivestrength of cold-sprayed coating have not been reported up to now.
In our previous study, an adhesive strength test method similar tothe one of Smurov was put forward [25,26]. In the present study, theadhesive strength of Cu coatings on Al5052, Al6063 and stainlesssteel 316 L substrate was measured. Particle velocities were calculated
by one-dimensional isentropic model. Moreover, relationships between
the particle velocity and the adhesive strength of Cu coatings wereR. Huang et al. / Surface & Coatings Technology 258 (2014) 832–841

833

(b)

(a)

Volume Fraction(%)

15

10

5

0

1

10

100

Particle Diameter (µm)
Fig. 1. Morphology (a) and diameter distributions (b) of Cu powder for adhesive strength coatings.

discussed. In order to understand the bonding mechanism of cold sprayprocess, splats were also observed. A way to develop the ultra-strong
adhesive strength of cold-sprayed coating was established.
2. Experimental procedures
2.1. Feedstock powder and cold spray process
Commercially available water atomized Cu powder (Cu-Atw350 mesh, Fukuda Metal Foil & Powder Co., Ltd.) was used to prepare
the coatings for adhesive strength test. The morphology of the powder
is presented in Fig. 1(a) and the volume distributions of particle diameterare shown in Fig. 1(b). Powder is near spherical shaped and its diameter
ranges from 5 to 80 μm with a volumetric average diameter of 28 μm.
In order to restrict the deviation of splat, customized high-pressure
water atomized Cu powder (Mitsui Mining & Smelting Co., Ltd.) wasused as the feedstock to deposit the splat. The morphology of the powder
is shown in Fig. 2(a). Volumetric distributions of particle diameter are
shown in Fig. 2(b). The powder presents a perfectly spherical shape andthe diameter ranges from 20 to 50 μm with a volumetric average diameterof 33 μm.
Cold spray system PCS-1000 was used to prepare the splat for observation and coatings for adhesive strength test. A convergent–divergent(De-Laval) nozzle was conﬁgured in the cold spray system to accelerate
the working gas to supersonic speed. This nozzle is cooled by water inorder to alleviate nozzle clogging which highly improves the reliability

(b)
30
25

Volume Fraction(%)(a)

of this system. In the current cold spray system, the preheated workinggas (high temperature) is mixed with the powder feeding gas (roomtemperature) before ﬂowing into the convergent section of the nozzle.
The mixed gas temperature is regard as the inlet gas temperature
ﬂowed into the nozzle that determines the gas speed at the nozzle
throat. In order to control the mixed gas temperature (or the gas
velocity at the nozzle throat), powder feeding gas to working gas ﬂow
rate ratio was set to about 1/4–1/5. Both helium and nitrogen gas
were employed as working gas. The working gas pressures were controlled ranging from 2 to 5 MPa and the working gas temperatures ranging from 473 to 1273 K to achieve different particle velocities. A planar
robot program was used to spray adhesive test coatings with transverse
speed of 300 mm/s and step of 1 mm. Splats were sprayed by a line
movement robot program with the speed of 800 mm/s. Substrates
were placed about 30 mm away from the gun exit for both spraying
processes of the coatings and the splats.
Al5052, Al6063 and stainless steel 316 L were utilized as substrates.
According to the previous studies, sandblasting did not beneﬁt the bonding of cold-sprayed coating [16,27]. The coatings for adhesive strength
test were deposited on machined substrates without sandblasting.
Tensile strength of substrates was measured with the tensile testing
equipment AG-X (100KN), manufactured by Shimadzu Co., Ltd. Dimensions of the substrate tensile specimens are shown in Fig. 3(a) following
the standards of No. 14B according to JIS Z2201 and the measured
mechanical properties are shown in Fig. 3(b). It can be seen that thetensile strength of stainless steel 316 L substrate exceeds 600 MPa,

20
15
10
5
0

1

10

Particle Diameter (µm)
Fig. 2. Morphology (a) and diameter distributions (b) of Cu powder for preparing splat.

100

834

R. Huang et al. / Surface & Coatings Technology 258 (2014) 832–841(a)acceleration and heat transfer to the particles can be calculated. In thepresent study, particle velocity at the nozzle exit was considered asa typical particle velocity instead of its impacting velocity onto the substrate in order to simplify the calculation. The detailed method to calculate the particle velocity and temperature are given in Refs. [29] and [30].
2.3. Splat characterization
In this study, the cross sections of splatted Cu particles were prepared perpendicularly to their surfaces by a commercially available
apparatus called Cross Section Polisher (SM-09020CP, JEOL, Japan)
using a broad ion beam. The method minimizes the artifacts such as
exaggerated densiﬁcation and/or reduced porosity, which can be
induced by the smearing of sprayed metallic powder particles during
sample preparation [31]. The acceleration voltage and milling speed of
the Cross Section Polisher were 4 kV and 50 μm/h respectively, under
the chamber pressure of 2 × 10− 3 Pa. Morphologies and preparedcross sections of the splatted copper particles were examined by anemission scanning electron microscope (JSM-5200LV, JEOL, Japan).

(b)
700
A5052
A6063
316L

Normal Stress (MPa)

600
500
400
300
200

2.4. Adhesive strength test

100
0

0

10

20

30

40

50

60

70

80

Normal Strain (%)
Fig. 3. Dimension of tensile specimen (a) and tensile strength (b) of substrate.

which is more than twice of the other two Al alloys. The tensile strengthof Al6063 is a little stronger than the Al5052 substrate.

2.2. Particle velocity
In our previous study, the steady-state one-dimensional isentropic
model can predict the particle velocity for the water-cooling cold spraysystem with an acceptable accuracy [25]. Therefore, in this study, thesteady-state one-dimensional isentropic model was used to calculate
the gas ﬂow inside the De-Laval nozzle in order to simplify the calculation. Gas is considered as perfect and compressible gas, and the nozzle
is regarded as adiabatic wall so that no heat loss occurs to the surroundings. The dimension of nozzle calculated in this study is similar to the one
given in Ref. [26]. Based on the above assumptions, Mach number, gasvelocity and temperature can be obtained with nozzle dimension [28].
Once the gas temperature and velocity are characterized, the particle(a)In the conventional adhesive strength test for thermal spray coatings,
an epoxy resin adhesive is employed to glue the sample to a loading
block. The test results are tremendously restricted by the strength ofglue. The adhesive strength of epoxy resin is generally not higher
than 70 MPa. Consequently, the testing method cannot be employed
to measure coating adhesions with a higher adhesive strength thanthat of the adhesive.
For cold spray process, a suitable spray condition can prepare coatings
with high adhesive strength [26]. Therefore, the conventional testing
method became invalid for the high adhesive strength coatings preparedby cold spray process. Fortunately, it is possible to build thick coatings bycold spray process. The cold-sprayed coating can be machined and pulled
to measure the adhesive strength [24–26]. In this study, the same tensile
test method to our previous studies was adopted. The detailed dimensions and test procedures were given in Refs. [25] and [26].
3. Results
3.1. Particle velocity
With the powder average diameter of 28 μm, the calculated particle
velocities (at the nozzle exit) under different working gas temperaturesand pressures are shown in Fig. 4. It can be seen that particle velocity
ranged from about 500 to 700 m/s with nitrogen gas and from about

(b)
1200

1200

1100

Particle Velocity (m/s)

Particle Velocity (m/s)

1100

N2-3MPa
N2-4MPa
N2-5MPa

1000
900
800
700
600
500
400
400

Cu, 28 mm
Nitrogen Gas
600

800

1000

1200

Working Gas Temperature (K)1400

Cu, 28 mm
Helium Gas

1000
900
800
700
600

He-2MPa
He-3MPa
He-4MPa
He-5MPa

500
400
400

600

800

1000

1200

Working Gas Temperature (K)Fig. 4. Particle velocity accelerated by N2 (a) and He (b).

1400

R. Huang et al. / Surface & Coatings Technology 258 (2014) 832–841

800 to 1200 m/s with helium gas. Particle velocity increased as both gas
temperature and pressure increased. Higher gas temperature leads tohigher sonic speed at the nozzle throat (consequently higher gas
velocity inside the nozzle). Higher gas pressure leads to higher gas
density inside the nozzle (consequently a larger gas drag force to theparticle). Both increases in gas velocity and density beneﬁt to improving
particle velocity. Since the sonic speed of helium gas is much higher
than that of the nitrogen gas, particle velocities accelerated by heliumgas are far higher than those of nitrogen.

835

3.2. Splat
Fig. 5 shows the morphologies and cross-sections of the coldsprayed Cu splats on Al5052 substrate under different working gas temperature, keeping the gas pressure at 3 MPa. It can be seen that particle
re-bounded from the substrate and only some craters remained in thesubstrate when the gas temperature is less than 473 K. It seems that itis difﬁcult to deposit to Al5052 substrate at N2, 3 MPa, 473 K condition,
since the particle velocity is low. With the increase of working gas(a) Surface at 473 K(b) Cross-section at 473 K(c) Surface at 673 K(d) Cross-section at 673 K(e) Surface at 873 K(f) Cross-section at 873 K(g) Surface at 1073 K(h) Cross-section at 1073 K

Fig. 5. Morphologies (a, c, e, and g) and cross-sections (b, d, f, and h) of Cu splats sprayed on Al5052 substrate at working gas pressure of 3 MPa and different gas temperatures.

836

R. Huang et al. / Surface & Coatings Technology 258 (2014) 832–841(a) Surface at 4MPa(b) Cross-section at 4MPa(c) Surface at 5MPa(d) Cross-section at 5MPaFig. 6. Morphologies (a and c) and cross-sections (b and d) of Cu splats sprayed on Al5052 substrate at working gas temperature of 673 K and different gas pressures.

temperature, not only the Cu particles were deposited on to the substrate, but also they were embedded into the substrate owing to the increased particle velocity. Jetting can be observed when the gas
temperature exceeds 873 K as shown in Fig. 5(f) and (h).
Fig. 6 shows the morphologies and cross-sections of the Cu splats on
the substrate of Al5052 under different working gas pressure keeping
the gas temperature of 673 K. Together with Fig. 5(c) and (d), it canbe seen that the particles are embedded into the substrates more deeply
when the working gas pressure is increased from 3 to 5 MPa. This iscaused by increase in particle velocity and the softening of substrate(more sufﬁcient heat exchange between gas and substrate) at a higher
gas pressure. A small jetting can be observed for the Cu splat at thegas pressure of 4 MPa as shown in Fig. 6(b), and the jetting became
more remarkable when the working gas pressure was increased to
5 MPa as shown in Fig. 6(d).
Fig. 7 shows the morphologies and cross-sections of Cu splats on
Al6063 substrate at working gas pressure of 3 MPa and gas temperatureof 873 K with nitrogen gas. Compared with the Cu splat on Al5052
substrate as shown in Fig. 5(e) and (f), it seems that no signiﬁcant difference can be observed between the two Al alloy substrate of Al5052 and(a) Surface, Al6063 substrate

Al6063. Fig. 8 shows the morphologies and cross-sections of Cu splats
on stainless steel substrate at working gas pressure of 3 MPa and gas
temperature of 673 K with nitrogen gas. It can be seen that Cu particle
underwent severe deformation and almost no deformation with thestainless steel substrate. According to the studies of Bae, plastic energy
is primarily dissipated onto the softer counterpart when a hard material
impacts with a soft material [32]. As a result, the deformation mostly
happens to the softer Cu particle instead of the harder stainless steel
substrate.
Fig. 9 shows the morphologies and cross-sections of Cu splats on
Al5052 and stainless steel substrates with working gas pressure of
3 MPa and temperature of 673 K with helium gas. Compared with the
Cu splats as shown in Figs. 5(c) and (d) and 8, it can be seen that splatson the two substrates are much more embedded when helium is being
used. The particle velocity with helium is much higher than that of thenitrogen gas. As a result, splats were embedded deeper. Even though
the substrate of stainless steel is signiﬁcantly harder, the Cu particle
caused some deformation of the substrate with its higher particle
velocity as shown in Fig. 9(c) and (d), consequently embedded into thesubstrate to some degree.

(b) Cross-section, Al6063 substrate

Fig. 7. Morphologies (a) and cross-sections (b) of Cu splats sprayed Al6063 substrate at working gas temperature of 873 K and pressure of 3 MPa with nitrogen gas.

R. Huang et al. / Surface & Coatings Technology 258 (2014) 832–841(a) Surface, SS substrate

837(b) Cross-section, SS substrate

Fig. 8. Morphologies (a) and cross-sections (b) of Cu splats sprayed on SS substrate at working gas temperature of 673 K and pressure of 3 MPa with nitrogen gas.

3.3. Adhesive strength
Adhesive strength of Cu coatings on the three substrates is shown in
Fig. 10. It can be seen that the tensile strength increased with its gas
temperature. The gas pressure hardly has any effect on the adhesivestrength for Al5052 and Al6063 substrates when nitrogen gas isused (especially under lower working gas temperature as shown in
Fig. 10(a) and (b)). On the other hand, when helium gas was employed
as the working gas, the tensile strength of coatings on Al alloy substrates
was much higher. For stainless steel substrates, the coatings depositedby nitrogen gas were separated from the substrates, and the measured
adhesive strength of helium gas is shown in Fig. 10(c). It is obviousthat the tensile strength increased signiﬁcantly with the helium gas
pressure owing to the increased particle velocity. High particle velocity
can generate an ultra-strong adhesive strength of Cu coatings (morethan 200 MPa) even on stainless steel substrates.
Although the current test method can measure the coatings withhigh adhesive strength, another problem in the test process became
clear. When adhesive strength between the coatings and substrates is
extremely strong, during the tensile test, rupture will then occur inside

the coatings instead of the interface as shown in Fig. 11. One of thereasons to cause failure within the coatings is stress concentration. In
spite of the arc transition used at the inner corner near the interface of
coating/substrate, the stress concentration may have caused some
failures of specimens near the inner corner. Another reason is the inherent defects in the cold-sprayed coatings. These defects are so sensitive tothe tensile stress that the cohesive strength of coatings decreases
intensively. Consequently, when the adhesive strength is stronger
than the cohesive strength of coatings, rupture inside the coatings
appears.
4. Discussion
It has been commonly accepted that the bonding between substrate
and the cold-sprayed coating is based on the intensive plastic deformation of substrate and particle during the impacting process. The intensive localized deformation causes adiabatic shear instability at theinterface, and consequently material extrudes from the interface andforms a metal jet at the rim. Upon impact, the thin oxide ﬁlm at theinterface is broken and clean interface is formed [2,8,10,12,14]. The(a) Surface, Al5052 substrate(b) Cross-section, Al5052 substrate(c) Surface, SS substrate(d) Cross-section, SS substrate

Fig. 9. Morphologies (a and c) and cross-sections (b and d) of Cu splats sprayed on Al5052, and SS substrate at the working gas temperature of 673 K and pressure of 3 MPa with helium gas.

838

R. Huang et al. / Surface & Coatings Technology 258 (2014) 832–841

Fig. 10. Tensile strength of adhesive test on Al5052 (a), Al6063 (b) and SS 316 L (c) substrates.

(a) He, 2 MPa, 1073 K(b) He, 2 MPa, 1073 K(c) He, 5 MPa, 1073 K

Fig. 11. Ruptured specimen photos of Al5052 (a), Al6063 (b) and SS316L (c) substrates.

R. Huang et al. / Surface & Coatings Technology 258 (2014) 832–841

Fig. 12. Relationship between particle velocity and adhesive/tensile strength.

effective bonding attributes to mechanical interlock caused by the
mixture of substrate and coating materials at the interface region [8,26] and the true metallurgical bonding at the clean fresh interface
based on local high temperature caused by the strain heating similar
to the explosive welding [11,14–16]. Even though the increased particle
and substrate temperature beneﬁts to the coating process [13], particle
velocity is the most important parameter to inﬂuence bonding [19].
Fig. 12 shows the relationship between the tensile strength (fromFig. 4) and the calculated particle velocity (from Fig. 10). It reveals
that only when particle velocity exceeds a certain value (critical velocity), effective bonding can be obtained. It seems that the substrate materials tremendously affect effective bonding that higher particle velocity
is necessary for higher strength substrate. Cu coating can effectively bebonded with Al alloy substrate if particle velocity is more than 500 m/s
and stainless steel substrate with particle velocity of more than 800 m/s(as shown in Fig. 12). Fig. 4 reveals that it is difﬁcult to acquire particle
velocity more than 800 m/s with nitrogen gas using the current cold
spray system. Therefore, Cu coatings deposited with nitrogen gas
separated from the stainless steel substrates.
The adhesive strength increases slowly when particle velocity is alittle higher than critical velocity, and then sharply increases with theparticle velocity as shown in Fig. 12. When the adhesive strength
reaches to the point where it is stronger than the cohesive strength of
the inside coating, adhesive strength cannot be tested using the current
test method due to the rupture location inside the coatings as shown in
Fig. 11. Considering the rupture position and the stress concentration at
the inner corner of tensile specimen, Fig. 12 only shows the lower limitof its adhesive strength of coating/substrate at the higher particle
velocity region. The real adhesive strength should be stronger than839

the measured one. Even so, conclusion can be drawn that adhesivestrength beneﬁts from an increase in particle velocity. The results
agree well with Dr. Schmidt's report that particle velocity is the mostimportant parameter to determine the bonding in cold spray process[19].
Ultra-strong adhesive strength can be explained by observing themorphologies of Cu splat on the three substrates. For Al alloy substrate,with the increase in particle velocity, Cu particles get deeply embedded
into the substrate, which causes intensive plastic deformation in boththe particles and the substrates as shown from Figs. 5 to 7. With thedeformation of Al substrate, an inward jetting of substrate is formed at
the rim of the particle. Unlike the splat (which is only one particle
impacting on to the substrate), in the cold spraying process, particle
will impact on jets and mechanical interlock will happen. Fig. 13
shows the cross-sections of Cu coatings on Al alloy substrates. It canbe seen that interlock was formed at Cu/Al alloy interface. Moreover,the interlock effect becomes more remarkable with the increase inparticle velocity (compared at Fig. 13(a) and (b)).
During the impact process of Cu particle on Al alloy substrate, notonly did the deformation and metal jetting occur, but also intermetallic
compounds (Al2Cu and Al4Cu9) were formed at the interface [11]. Theintermetallic compounds of Al and Cu are brittle and low in strength[33]. Therefore, even though the metallurgical bonding based on diffusion was formed at the interface of Cu coating and Al substrate, themechanical interlock is able to account for a large proportion of thetotal bonding strength in most cases [16]. In the present study, it is considered that interlock plays a key role in bonding instead of its metallurgical bonding. Fig. 5 shows that jetting was almost unobservable under
the conditions of N2, 3 MPa, 673 K, and jetting was obtained when thegas temperature was increased to 873 K. Fig. 6 reveals that the increasedgas pressure makes the particle more deeply embedded into the substrate due to the increased particle velocity. Unlike the one when thegas pressure was increased, by increasing the gas temperature, it more
effectively embedded the particle into Al5052 substrate (compared at
Figs. 5 and 6). One reason is because gas temperature affects the particle
velocity more with nitrogen gas (shown in Fig. 4(a)). Another reason isthat by increasing the gas temperature, it softens the particle and thesubstrate by heat. With particles deeply embedding into the substrate,
it increases the material mixing (interlock) at the interface. Consequently, this gives higher interface bonding strength [8]. Therefore, theincrease of gas temperature is more effective in improving the adhesivestrength than that of gas pressure if nitrogen gas is used as shown in
Fig. 10(a) and (b). If particle velocity increased further with helium gas,
particles are embedded more into the Al5052 substrate as shown in
Fig. 9(a) and (b). The severe mechanical interlock resulting from intensive plastic deformation leads to high adhesive strength (more than
250 MPa on Al5052 substrate). For Al6063 substrate, the splat isvery similar to the one from the Al5052 substrate as shown in Fig. 7.

(a) Al6063 substrate, N2, 3MPa, 1273 K (b) Al6063 substrate, He, 2MPa, 1073 K

Fig. 13. Cross-sections of Cu coatings on Al alloy substrates.

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R. Huang et al. / Surface & Coatings Technology 258 (2014) 832–841(a) Al5052 substrate, N2, 5MPa, 1273 K (b) Al6063 substrate, N2, 3MPa, 1073 K

Fig. 14. Cross-sections of fractured Cu coatings on Al alloy substrates.

Therefore, the relationship between particle velocity and adhesivestrength for the two Al alloy substrates looks very similar as shown in
Fig. 12. Fig. 14 shows the cross-sections of fractured Cu coatings on Al
alloy substrates. The Cu particles that remained on the substrates are
deeply embedded and Al alloy substrate is also inlaid in the coatings
between the Cu particles (shown in Fig. 14(a)). This proves that there
is intensive interlock between the Cu coating and Al alloy substrate.
The adhesive strength on Al alloy substrate sharply increased when
the particle velocity exceeded about 600 m/s (as shown in Fig. 12).
Observing the splat on Al alloy substrate, the jetting are not remarkable
if the spray temperature is less than 873 K and particle velocity less than
600 m/s (as shown in Fig. 4). Consequently, the interlock effect will not
be signiﬁcant between the coating and substrate and only weak bonding can be obtained. This weak bonding maybe generate from the friction between substrate and coating [34]. Even though these substrates
were not prepared with sandblasting, the particle impact process
made the substrate surface rough (enough to form a weak mechanicalbonding based from friction). With the increase of impact velocity,
the inward jetting formed on softer Al alloy substrate surface due toparticle's inset. And consequently, particle is almost enclosed by thesubstrate. This generated a signiﬁcant interlock effect at the interfaceof Cu coating and Al alloy substrate (as shown in Fig. 13(b)), and stronger adhesive strength can be observed.
For stainless steel substrates, the substrate is so strong that it is
difﬁcult to make the substrate deform since deformation always happens
to the softer counterpart during impact [32]. Therefore, it is difﬁcult todeposit Cu coating on stainless steel substrates. Even when helium gas
was used to deposit Cu coating on stainless steel substrate, it did notdeform as much as Al alloy substrate (as shown in Fig. 9). Therefore,
the interlock effect for the bonding between Cu particle and stainlesssteel substrate seems difﬁcult. However, a strong adhesive strength(a) He, 2MPa, 1073 K(more than 200 MPa) on stainless steel substrate can still be acquired.
Effective bonding between Cu particle and the stainless steel substrate
may be mainly attributed to their metallurgical bonding. The strain
heat can be generated during the high velocity particle impact of Cu
particle on the stainless substrate and local temperature can be close to
the melting point [14,19]; like pressure welding, strong diffusion bonding or metallurgical bonding can be formed [35–37]. Fig. 15 shows the
cross-sections of fractured Cu coatings on stainless steel substrates. It
can be seen that some Cu particles still remained on the stainless steel
substrate even though they did not deeply embed into the substrate.
Observing the surface of the stainless steel 316 L substrate after thefracture in Fig. 16, it can be also seen that plenty of Cu coatings remainedon the substrate. Also, the fracture occurred in the coating/substrateinterface and partly inside of the Cu coatings. It seems that bonding
strength is stronger than the cohesive strength of Cu coating at part ofthe coating/substrate interfaces and weaker at other interfaces. The
impacting particle velocity being uneven causes the difference in bonding strength for different particles. The different particle velocities attribute to the particle diameter distribution and the different trajectory ofparticle inside the nozzle [11]. The increased gas pressure overall improves particle velocity and increases the bonding between the particles
and the substrates by generating more strain heat and more adequate
diffusion. Therefore, the remained particles sprayed under 3 MPa aremuch more than those sprayed under 2 MPa (as shown in Fig. 16). It
can be also seen from Fig. 16 that few large particles remained on thesubstrate at 2 MPa compared with the ones at 3 MPa because small
particles are easier to get a higher velocity. With the increase of gas
pressure (from 2 to 3 MPa), not only the interface bonding increased,
but also the cohesive strength of Cu coating improved. The increased adhesive and cohesive strength beneﬁted to the measured tensile strength
when helium gas pressure increased from 2 to 3 MPa.

(b) He, 3MPa, 1073 K

Fig. 15. Cross-sections of fractured Cu coatings on SS 316 L substrates.

R. Huang et al. / Surface & Coatings Technology 258 (2014) 832–841(a) He, 2MPa, 1073 K

841(b) He, 3MPa, 1073 K

Fig. 16. Surface of SS 316 L substrates after fracture.

5. Conclusions
In this study, Cu coatings were prepared with cold spray process
under different spray conditions onto three different substrates.
Then, adhesive strength on three substrates was tested. Splat wasalso observed in order to understand the bonding mechanism. An
ultra-strong adhesive strength (more than 200 MPa) of Cu coatings on
Al5052, Al6063 and stainless steel 316 L substrates was obtained withcold spray processing by improving the particle impact velocity.
Plastic energy from the initial impact kinetic energy is primarily dissipated in the soft counterpart of substrate and particle. Therefore, for Al
alloy substrates, the deformation mainly occurred in the substrate, inwhich resulted in the formation of a deeply embedded splat. Jetting
was formed by the substrate deformation with the increase of embedded depth due to the high particle velocity. Higher particle velocity generated stronger bonding strength between the coatings and Al alloy
substrates mainly owing to the improvement of mechanical interlock
effect. In contrast, the deformation mainly generates in the Cu particle
instead of the substrate on stainless steel 316 L substrate. Effective
bonding can be formed based on the diffusion between Cu coating andstainless steel substrate to generate a metallurgical bonding. The ultrastrong adhesive strength between Cu coating and stainless steel canbe possible if high particle velocity is achieved.
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