Study of the influence of particle velocity on adhesion strength of Cold Spray deposits
R. Huang and H. Fukanuma, Saitama/J
The adhesion mechanism of deposits/substrate interface prepared by cold spray method has not been fully
understood up to now. It seems that the adhesion strength is mainly determined by the mechanical (including theplastic deformation of particle and substrate) and thermal interaction between the particle and substrate when theparticles impact onto the substrate with a high velocity. In order to understand the adhesion mechanism, theinfluences of particle impact velocity on the adhesion strength were investigated in this study. The particle velocity
was obtained with DPV-2000 measurement and CFD simulation. The relationships between the adhesion strengthof deposits/substrate interface and particle velocity were discussed. The results show that the more strong
adhesion strength can be obtained with the increase of particle velocity. There are two available ways to improve
the adhesion strength. One is to increase the temperature of working gas, and another is to employ helium gas as
the working gas instead of nitrogen gas.
1

Introduction

Cold spray is a emerging spray coating technologythat was first developed in the mid 1980’s at theInstitute of Theoretical and Applied Mechanics informer Soviet Union [1]. In Cold spraying process,spray particles are injected into a supersonic jet ofcompressed gas and accelerated to a high velocity(300-1200 m/s). The deposition of particles takes
place through the intensive plastic deformation upon
impact in solid state at a temperature well below themelting point of spray materials [2]. As a result, sprayparticles experience little oxidation or decompositionin cold spray [3 and 4]. So far, cold spray was used tospray not only ductile materials such as copper [5 and6], aluminium [Ref], nickel [8], nickel based alloys [9],zinc [10] but also metal matrix composites [11],cermets [12] and ceramic materials [13].
Previous studies suggested that particle deposition
depends on the impact velocity and only the particles
with a velocity higher than a critical velocity can bedeposited. Below the critical velocity, impacting
particles would only cause erosion of the substrate [14and 15]. The experimental and theoretical results
showed that the critical velocity is dependent on theproperties of powder and substrate materials [16and17], particle size and geometry [18], particle
temperature [19], particle oxygen content [20] and thesubstrate preparation [21]. This may partially explainthat even for the same powder materials the reported
critical velocity was somewhat different [20 and 22].
Adhesion strength of coatings prepared by cold spraydetermins its applications in the industual field.
Therefore, many researchers are forcus to thebonding mechanism in the last few years. Mäkinen
and et al presented the influences of power, substrateand heat treatment on the adhesive strength[23].
Moreover, recently numerical simulation also help tofind out the bonding mechanism [24-27]. Thesestudies on the bonding mechanism of cold spraysuggested that the the adhesive strength is mainly
effected by the mechanical interlock [24 and 25] andimpact molten for a physical-chemical adhesion[10and 27] based on the shear instability[26],. But so far,the underlying mechanism of bonding of cold spray
has not been well clarified [19].
In the present study, A novel methode was employed
to measure the higher adhesive strength of copper
coatings instead of the conventional method using theepoxy resin adhesive. In order to control the particle
impact velocity,the coatings for adhesive strength
experiment were prepared on three types of substrate,
A5052, A6063 and copper, by a change of theworking gas pressure and temperature. The velocity
and temperature of working gas and in-flight particle
were calculated by the commerical CFD software of
FLUENT. The particle velocity was also measured byan on-line diagnostic system of DPV-2000. The
relationships between the impact velocity and theadhesive strength of copper coatings were also
discussed.
2

Experimental procedure

2.1
Feedstock Powder and Cold Spray
Process
Owing to commercial availability of copper particles, inthis study copper particles diameters ranging from 5 to45ȝm are used. The morphology of the powder ispresented in Fig.1(a). The powder size distribution
was characterized by the laser diffraction particle size
analyzer (Seishin Trading Co., Ltd. Kobe, Japan). The
volume and number distributions of diameters areshown in Fig.1(b). The volume average diameter isabout 30 µm, and the number average diameter isabout 18 µm.
In this study, a commercial cold spray system, model
number of PCS-305 designed by PLASMA GIKEN CO.
LTD., was used to prepare the coatings for adhensive
strength. The cold spray gun utilized in thisexperiment employed a water cooling, converging–
diverging (De-Laval) nozzle allowing the particles toreach the high velocities required in the process. The
spray conditions for cold spraying were shown in
Table 1.

DVS 276

2.2
The Measurement of In-flight Particle
Velocity
The in-flight particle velocity under the conditions ofpreparing coatings was measured at the centre line of
the flow, using the DPV-2000 system (Tecnar
Automation Ltd., St-Bruno,Québec, Canada). The
substrate was removed during the particle velocity
measurement process. For cold spray process, theradiation intensity emitted from the in-flight particles is
too weak to be detected by the optical sensor
because of low temperature of the particles. Therefore,a high-power diode laser system of CPS-2000 was
equipped in the DPV-2000 system to beam the inflight particles. By detecting the monochromatic lightscattered by particles, the velocity of particles can bemeasured by DPV-2000 system. In this study, thevelocity measurements were taken at a point of 30
mm from the spraying gun exit.

epoxy resin adhesive is not more than 70 MPa, andconsequently the testing method cannot be employed
to measure the high strength coatings more than thestrength of adhesive.

Fig.2. The conventiaonal method to test theadhensive strength of coatings.
For cold spray, a suitable spray condition can prepare
the coatings with high adhensive strength. Therefore,
the conventional testing method become invalid forthe high adhensive strength coatings prepared by cold
spray process. Fortunately, quite thick coatings canbe obtained by the process of cold spray, simply. The
adhensive strength of thick coatings can be measured
using the novel testing method as shown in Fig. 3.
First, the thick coatings are prepared as shown in theFig. 3 (a), and then machined to be the shape asshown in the Fig. 3 (b). Finally, the sample can be
pushed directly with a special jig as shown in the Fig.
3 (c).

(a)10
Volume Distributions
Number Distributions

8

(a)

(b)

6
4
2
0

1

10

100

Particle diameter (Pm)(b)
Fig.1. Morphology (a) and diameter distributions (b) ofcopper powder.
Table 1: The spray conditions
Gas Type

N2

Working gas pressure (MPa)3, 4Working gas temperature (oC)

200-1000

Spray distance (mm)Powder feed rate (g/min)
Substrate

He
2
600
30
200

A5052, A6063, Cu

2.3
The novel method to measure the adhesivestrength
In the conventional adhensive strength test for thermal
spray coatings, a epoxy resin adhesive is commonly
employed to glue the sample with a sample no coated.
The schematic illustration of the method is shown in
the Fig 2. The testing method is tremendously
restricted by the strength of glue. The strength of(c)Fig.3. The novel method to test the cohesive strengthof coatings.
2.4

The Numerical Simulation Method

The CFD code of FLUENT was used to simulate thecold spray process. Due to the axisymmericalcharacteristic of flow in the gun, a two-dimensional
symmetrical steady mode was used in the current
study. According to the previous study, the presenceof substrate had little influence on particle acceleration[28]. Therefore, the substrate was not involved in thissimulation.
The working gas was taken as an ideal andcompressible one. A coupled implicit method wasused to solve the flow field. The realizable K–İturbulence model was utilized in the simulation
because of the high pressure gradients. For the
calculation of particle, a particle with the diameter of
20 µm was fed into the gun at the axisymmetric center.
The accelerating and heating of particles werecomputed using Discrete Phase Modeling (DPM) of
FLUENT [29].

DVS 276

3

Results and Discussions

3.1
Simulation results Compared withmeasured results by DPV-2000
The distributions of temperature and gas velocity
calculated by Fluent are shown in Fig. 4 (a) under theconditions of N2, 3MPa, 1000oC. In this study, theboundary conditions of adiabatic wall were not used
but a constant temperature for nozzle outer wall was
taken into consideration because the water-cooling
gun was used. Therefore, it can be seen that thetemperature of nozzle is low. A copper particle withthe diameter of 20 µm was fed into the gas field at thethe axisymmetric center, and the calculated
temperature and velocity of particle are shown in theFig. 4 (b) and (c). The particle is preheated before
passing through nozzle throat and temperature drops
with fast descend of the gas temperature while
passing through the nozzle throat due to the
expansion of gas. On the contrary, the particle isaccelerated only while passing the nozzle throat until
the location beyond about 100 mm from the nozzle
exit.

(a) Velocity and temperature distributions

seems that the valid ways to obtain a high particle
velocity are to develop a high temperature cold spraysystem or use helium gas instead of nitrogen gas as
the working gas.
1200
1100

900
N2 Gas, 3MPa

800
700

N2 Gas, 4MPa

600
500

200

400

600

800

1000

o

Gas Temperature ( C)

Fig. 5. Calculated particle velocity at the distance of
30 mm beyond nozzle outlet under different spray
conditions.
Depending on the detection of scattering light in theexperiment, DPV-2000 system cannot measureparticle diameter correctly. Therefore, the number
distributions and average of particle velocities can becalculated instead of volume ones. The number
distributions of particle velocity measured by DPV2000 are shown in Fig. 6. Similar to the results ofsimulations, the gas temperature and gas type havegreat influences on the particle velocity, and the gaspressure has little influences on the particle velocity.
The distributions presents that the particle velocity
ranged from 300 to 1000 m/s if the nitrogen gas used,
and from 600 to 1400 m/s if helium gas used.
40

o

o

N2, 3MPa, 1000 C

N2, 3MPa, 1000 C
Cu Particle, 20 Pm

700

o1000 Cu Particle, 20 Pm
800

500
400

Particle Temperature

300

Particle Velocity

200

-300

-200

-100

0

100

200

0

-400

-300

-200

-100

0

100

Distance from the outlet (mm)Distance from the outlet (mm)(b) Gas and particle
temperature(c) Gas and particle
velocity

200

20

15

15

10

10

5

5
500

600

700

800

o

800 C
o

1000 C0300

900 1000

400

Particle Velocity (m/s)

500

600

700

800

Particle Velocity (m/s)(a) N2, 3MPaFig. 4. Results of simulations under the conditions of
N2, 3MPa, 1000oC.

o

600 C

25

20

400

o

400 C
o

200 C

30

o

1000 C

0300

N2, 4MPa

35

o

800 C

25

600
400

Gas Temperature

o

200 C

40

N2, 3MPa

o

600 C

30

Gas Velocity

200
100

400 C

35

600

-400

He Gas, 2MPa

1000

1200

800

0

Cu Particle, 20 Pm
SD=30 mm(b) N2, 4MPa

30
o

25

N2, 2MPa, 800 C

20

Taking the particle velocity calculated at the location
beyond 30mm of nozzle exit, the extracted particle
velocities are show in Fig. 5 under different spray
conditions. It reveals that the particle velocity rises
with the increase of working gas temperature from
200 to 1000oC. However, only little increase of particle
velocity was observed with the increase of working
gas pressure from 3 to 4 MPa. When helium gas
utilized as working gas, higher particle velocities are
obtained even under a lower gas temperaturecompared with the ones when it employed N2 gas. It

DVS 276oHe, 2MPa, 800 C

15
10
5
0
400

600

800

1000

1200

1400

Particle Velocity (m/s)(c) Compared with He gas
Fig. 6. Particle velocity number distributions
measured by DPV-2000.

900 1000

From the distubutions of particle velocity above, thenumber average velocities can be obtained.The
comparision between the number average velocities
measured by DPV-2000 and the ones calculated by
Fluent is shown in Fig. 7. It reveals that the particle
velocities calculated are very close to the ones
measured, and moreover the same change tendencyof particle velocity to the gas temperaure can beobserved no matter which ways were used.

200

200
A5052 Substrate
A6063 Substrate
Cu Substrate

A5052 Substrate
A6063 Substrate
Cu Substrate

150 N2, 3MP

150

Coatings: Cu

N2, 4MPCoatings: Cu

100

100
Rupture in the deposits

Rupture in the deposits

50

0

50

200

400

600

800

0

1000

200

400

850

850

N2, 3MPa
800 Cu Particle
SD=30 mm

Simulation
DPV-2000

N2, 4MPa
800 Cu Particle
SD=30 mm

Simulation
DPV-2000

600

800

Gas Temperature ( C)(a) N2, 3MPa(b) N2, 4MPa

750

750

300

700

700

250

650

650

600

600

550

550

1000

o

o

Gas Temperature ( C)

Rupture in the depositsoHe, 2MPa, 600 C
Coatings: Cu

200
150

500

200

400

600

800

1000
o

Workng Gas Temperature ( C)(a) N2, 3MPa

500

100

200

400

600

800

1000
50

o

Workng Gas Temperature ( C)(b) N2, 4MPa

0

A5052

A6063

Cu

Substrate(c) He, 2MPa, 600oC

Fig. 7. Compare the particle velocities calculated withthe ones measured.
3.2

Fig. 8. Tensile/adhensive strength of copper coatings
prepared under different conditions.

Adhesive strength of coatings

The adhensive strength of copper coatings on thethree substrate is shown in the Fig. 8. It can be seenthat the tensile strength of coatings rises with theincrease of gas temperature. The tensile strength
have a little increase adjusting the gas pressure from
3 to 4 MPa. While helium gas is employed as working
gas, the tensile strength is much stronger comparedwith the one prepared with nitrogen gas.
If the gas temperature exceeded the temperature of
800oC, almost all the specimens ruptured in thecoatings instead of the interface of coatings/substrate
as shown in Fig. 9. It shows that the adhesivestrength is stronger than the vertical strength of
coatings resulted to the rupture inside the coatings.

(b) He, 2MPa, 600oC,
A6063 substrate

Fig. 9. Photo of ruptured specimens after tensile
testing.

Discussions

300

From the results mentioned above, it seems that thespray conditions have similar influences on theadhensive strength of coatings and the particle
velocity. Figure 10 shows the relationship between
the tensile strength and particle velocity. It reveals thatthe adhesive strength is low with a lower particle
velocity, and sharply increases with the particle
velocity exceeding about 700 m/s. The adhesivestrength is stronger than the strength of coatings withhigh particle velocity, and consequently the adhensive
strength cannot be tested but vertical tensile strengthof coatings due to the rupture position inside coatings.
It can be seen that the increase of particle velocity isalso benefit to the coatings strength although theadhesive strength cannot be obtained for the
specimens which ruptured inside coatings.

N2 Gas, Cu Particle

Tensile Strength (MPa)

3.3(a) N2, 3MPa, 800oC,
A5052 substrate

250

A5052 Substrate

200
150
100
50
0
500

600

700

800

900

1000

1100

1200

Particle Velocity (m/s)

Fig. 10. Relationship between the particle velocity andadhensive/tensile strength.
The spats that copper particles impact to A5052
substrates are shown in Fig. 11. With the particle
velocity increasing, the copper particles deeply
embedded into the A5052 substrate, and more

DVS 276intensive plastic deformation occurred both particles
and substrates. The “jetting” can almost not be
observed under the conditions of N2, 3MPa, 200oC,
and small jetting obstain while the gas temperature
increased to 600oC. If the particle velocity increased
further such as the conditions employing the heliumgas as the working gas, the particle deeply embedded
the substrate and consequently a big jetting can begenerated. Unlike the splat only one particle impacting
on the substrate in the actual cold spray process, thesubsequent particle will impact on the jetting and themechanical interlock will occure between the coatingsand substrate as shown in the Fig. 11 (d). The severe
mechanical interlock resulted from the intensive
plastic deformation results to a high adhensive
strength more than 250 MPa under the conditions of
He gas, 2 MPa, 600oC.

(a) N2, 3MPa, 200oC(b) N2, 3MPa, 600oC(c) He, 2MPa, 600oC(d) He, 2MPa, 600oC

Fig. 11. Cross-section of splat and coatings on A5052
substrate.
4

Conclusions

In this study, the copper particle velocity wasmeasured by DVP-2000 system and calculated by the
CFD software of Fluent. The copper coatings wereprepared using cold spray process under different
spray conditions and the adhesive strength on threesubstrate was also tested. It seems that the simulation
is a valid way to predict the in-flight particle velocity
because the calculated particle velocity was wellidentical to the one measured in this study. The
results showed that, the in-flight particle velocity
increased with the increase of gas pressure andtemperature or using helium gas instead of nitrogengas. The adhesive strength of coatings have the samechange trend to the spray conditions similarly to theparticle velocity. It seems that the particle velocity
plays a important role to improve the adhesivestrength base on the plastic deformation. A higher
particle velocity increases the mechanical interlock
effect resulting to a excellent bonding between thecoatings and substrate.

5

References[1]. P. Richer, B. Jodoin, K. Taylor, E. Sansoucy, M.
Johnson, L. Ajdelsztajn and Davis, Effect ofparticle geometry and substrate preparation incold spray, Thermal Spray 2005: Internal
Thermal Spray Conference[2]. W.-Y. Li and C.-J. Li, Effect of annealing
treatment on the microstructure and properties ofcold-sprayed Cu coating, Thermal Spray 2005:
Internal Thermal Spray Conference[3]. T. Stoltenhoff, H. Kreye, and H. J. Richter, An
Analysis of the Cold Spray Process and Its
Coatings, J. Therm. Spray Technol., 2002, 11 (4),p 542-550.
[4]. R. C. Mcune, W. T. Donlon, E. L. Cartwright,
Characterization of Copper and Steel Coatings
Made by the Cold Gas-Dynamic Spray Method,
Thermal Spray: Practical Solutions forEngineering Problems, C.C. Berndt, Ed., Oct 711, 1996 (Cincinnati, OH), ASM International,
1996, p 397-403.
[5]. E. Calla, D.G. McCartney, P.H. Shipway,
Deposition of copper by cold gas dynamic
spraying: an investigation of dependence of
microstructure and properties of the deposits on
the spraying conditions, Thermal Spray 2004:
Advances in Technology and Application, May
10-12, 2004 (Osaka, Japan), ASM International,
2004, p 382-387[6]. T. Xiong, Z. Bao, T. Li, Z. Li, Study on coldsprayed copper coating’s properties andoptimizing parameters for the spraying process,
Thermal Spray 2005: Explore its surfacing
potential, May 2-4, 2005 (Basel, Switzerland),ASM International, 2005, p 178-184[7]. R. Morgan, P. Fox et al., Analysis of cold gas
dynamically sprayed aluminum deposits, Mater.
Lett., 2004, 58, p 1317-1320.
[8]. M. K. Decker, R. A. Neiser, D. Gilmore, H.D.
Tran, Microstructure and Properties of Cold
Spray Nickel, Thermal Spray 2001: New
Surfaces for a New Millennium, C.C. Berndt, K.A.
Khor, and E.F. Lugscheider, Ed., May 28-30,
2001 (Singapore), ASM International, 2001, p
433-439.
[9]. F. Raletz, G. Ezo’o, M. Vardelle, M. Ducos,
Characterization of cold-sprayed nickel-basecoatings, Thermal Spray 2004: Advances inTechnology and Application, May 10-12, 2004
(Osaka, Japan), ASM International, 2004, p 344349[10]. C-J. Li, W-Y Li, Impact Fusion Phenomenon
during Cold Spraying of Zinc, Thermal Spray
2004: Advances in Technology and Application,
May 10-12, 2004 (Osaka, Japan), ASM
International, 2004, p 356-361[11]. D. Morelli, A. Elmoursi, T. Vansteenksite, D.
Gorkiewicz, B. Gillispie, Kinetic Spray of
Aluminum Metal Matrix Composites for ThermalManagement Applications, Thermal Spray 2003:
Advancing the Science and Applying the

DVS 276

Technology, B.R. Marple and C. Moreau, Ed.,
May 5-8, 2003 (Orlando, FL), ASM International,
2003, Vol. 1, p 85-90[12]. J. Karthikeyan, C. M. Kay, J. Lindemann, Cold
Sprayed nanostructured, WC-Co, Thermal Spray
2001: New Surfaces for a New Millennium, C.C.
Berndt, K.A. Khor, and E.F. Lugscheider, Ed.,May 28-30, 2001 (Singapore), ASM International,
2001, p 383-387.
[13]. C-J. Li, G-J. Yang, X-C. Huang, W-Y. Li, A.
Ohmori, Formation of TiO2 photocatalyst
Through cold spraying, Thermal Spray 2004:
Advances in Technology and Application, May
10-12, 2004 (Osaka, Japan), ASM International,
2004, p 315-319[14]. C-J. Li, W-Y. Li, Y-Y. Wang, Effect of Spray
Angle on Deposition Characteristics in Cold
Spray, Thermal Spray 2003: Advancing theScience and Applying the Technology, B.R.
Marple and C. Moreau, Ed., May 5-8, 2003
(Orlando, FL), ASM International, 2003, Vol. 1, p
91-95[15]. N. Papyrin, S. V. Klinkov, V. F. Kosarev,
Modeling of Particle-Substrate Adhesive
Interaction Under the Cold Spray Process,
Thermal Spray 2003: Advancing the Science andApplying the Technology, B.R. Marple and C.
Moreau, Ed., May 5-8, 2003 (Orlando, FL), ASM
International, 2003, Vol. 1, p 27-35[16]. D. L. Gilmore, R. C. Dykhuizen, R. A. Neiser, T.
J. Roemer, M. F. Smith, Particle Velocity andDeposition Efficiency in the Cold Spray Process,J. Therm. Spray Technol., 1999, 8 (4), p 576-582[17]. D. Zhang, P. H. Shipway, D. G. McCartney,
Particle-Substrate Interactions in Cold Gas
Dynamic Spraying, Thermal Spray 2003:
Advancing the Science and Applying theTechnology, B.R. Marple and C. Moreau, Ed.,
May 5-8, 2003 (Orlando, FL), ASM International,
2003, Vol. 1, p 45-51[18]. P. Richer, B. Jodoin, K. Taylor, E. Sansoucy, M.
Johnson, L. Ajdelsztajn, Effect of particle
geometry and substrate preparation in cold
spray, Thermal Spray 2005: Explore its surfacing
potential, May 2-4, 2005 (Basel, Switzerland),ASM International, 2005, p 193-198[19]. F. Gärtner, C. Borchers, T. Stoltenhoff, H. Kreye,
H. Assadi, Numerical and Microstructural
Investigations of the in Cold Spraying, Thermal
Spray 2003: Advancing the Science and Applying
the Technology, B.R. Marple and C. Moreau, Ed.,
May 5-8, 2003 (Orlando, FL), ASM International,
2003, Vol. 1, p 1-8[20]. C-J. Li, W-Y. Li, Examination of the criticalvelocity for deposition of particles in cold
spraying, Thermal Spray 2005: Explore its
surfacing potential, May 2-4, 2005 (Basel,
Switzerland), ASM International, 2005, p 217-224[21]. N. Papyrin, Albuquerque, S. V. Klinkov, V. F.
Kosarev, Novosibirsk, Effect of the Substrate
Surface Activation on the Process of Cold Spray
Coating Formation, Thermal Spray 2005: Explore

its surfacing potential, May 2-4, 2005 (Basel,
Switzerland), ASM International, 2005, p 145-150[22]. T. Stoltenhoff, J. Voyer and H.Kreye, Hamburg,
Cold spray- state of the art and applicability,
International Thermal Spray Conference, E.
Lugscheider and C.C. Berndt, Ed., March 4-6,
2002 (Essen, Germany), DVS Deutscher
Verband für Schweißen, 2002, p 366-374[23]. H. Mäkinen, J. Lagerbom, P. Vuoristo, Adhesionof Cold Sprayed Coatings: Effect of Powder,
Substrate, and Heat Treatment, Thermal Spray2007[24]. M. Grujicica, J.R. Saylor, D.E. Beasley, W.S.
DeRosset, D. Helfritch, Computational analysis of
the interfacial bonding between feed-powder
particles and the substrate in the cold-gas
dynamic-spray process, Applied Surface Science
219 (2003) 211–227[25]. S. Kumar, Gyuyeol Bae, Changhee Lee,
Deposition characteristics of copper particles onroughened substrates through kinetic spraying,
Applied Surface Science 255 (2009) 3472–3479[26]. M. Grujicic, C. L. Zhao, W. S. Derosset, D.
Helfritch, Adiabatic shear instability based
mechanism for particles/substrate bonding in the
cold-gas dynamic-spray process, Mater. Des.,
2004, 25, p 681-688[27]. S. Guetta, M. H. Berger, F. Borit, V. Guipont, M.
Jeandin, M. Boustie, Y. Ichikawa, K. Sakaguchi
and K. Ogawa, Influence of Particle Velocity on
Adhesion of Cold-Sprayed Splats, Journal of
Thermal Spray Technology, Volume 18, Number
3, 331-342[28]. W.-Y. Li, C.-J. Li, Optimization of spray
conditions in cold spraying based on the
numerical analysis of particle velocity, Trans.
Nonferrous Met. Soc. China, 2004, 14 (S2), p
43–48[29]. Ansys Inc., FLUENT 6.3 User’s Guider, NH, 2006

DVS 276