Surface & Coatings Technology 261 (2015) 278–288

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

The effects of heat treatment on the mechanical properties ofcold-sprayed coatings
Renzhong Huang, Michiyoshi Sone, Wenhua Ma, Hirotaka Fukanuma
Plasma Giken Co. Ltd., Toshima, Tokyo, Japana r t i c l e

i n f oArticle history:
Received 7 March 2014
Accepted in revised form 5 November 2014
Available online 14 November 2014
Keywords:
Cold spray
Tensile strength
Elongation
Fracture surface
Heat treatmenta b s t r a c t
In the cold spray process, deposition of particles takes place through intensive plastic deformation upon impact ina solid state at temperatures well below their melting point. Therefore, spray particles experience little oxidation
or decomposition during this process. As a result, cold-sprayed coatings have excellent mechanical, electrical andthermal properties. In this work, pure Al, Cu, Ti and stainless steel 316 were deposited by cold spray. The tensile
strength and elongation of these coatings were also measured. The results showed that the as-sprayed coatings ofthe four materials have poor ductility and almost no elongation. However, heat treatment can improve themechanical properties of the cold-sprayed coatings to some extent. Here, the effects of heat treatment conditions
on the mechanical properties of the four cold-sprayed materials are discussed.
© 2014 Elsevier B.V. All rights reserved.

1. Introduction
Cold spray is an emerging spray coating technology that was ﬁrst
developed in the mid 1980's at the Institute of Theoretical and Applied
Mechanics in the former Soviet Union [1]. Compared to the traditional
thermal spray processes, the most distinct characteristic of cold sprayprocessing is its lower processing temperature; consequently lower
thermal effects to the processed materials. Therefore, cold spray isparticularly suitable to prepare coatings that are sensitive to oxidation
or applications in the ﬁelds in which oxidation and thermal inﬂuences
during the coating process have to be avoided [2]. So far, cold sprayhas been used to spray not only ductile materials such as copper [3,4],
aluminum [5], nickel [6], nickel-based alloys [7], zinc; [8] but also
metal matrix composites [9,10], cermets [11,12] and ceramic materials[13].
The most prominent property of cold-sprayed coatings is its low
oxidation. Moreover, cold-sprayed coatings can be extremely dense
under suitable spray conditions where the particles experience intensive plastic deformation. Dense coatings with low-oxidation will alsomean that they have excellent mechanical, thermal and electrical properties [14,15]. Therefore, the coating deposited by cold spray is a strong
candidate to apply as a structural component in the industrial ﬁeld.
However, the temperature history of particles during deposition processing generates some residual tensile stress due to thermal effects[15,16]. Moreover, the severe impact deformation generates someresidual compressive stress due to the kinetic effects [16–19]. The intensive plastic deformation of particles also decreases the ductility of cold-

E-mail address: rz_huang@plasma.co.jp (R. Huang).

http://dx.doi.org/10.1016/j.surfcoat.2014.11.017
0257-8972/© 2014 Elsevier B.V. All rights reserved.

sprayed coatings due to work hardening effects. The existence of residual stress and work hardening in cold-sprayed coating decreases its
mechanical, thermal and electrical properties. Some researches about
the improvement of cold-sprayed coating via heat treatment werereported [14,15,20–24]. Heat treatment can effectively change themicrostructure of cold-sprayed Cu [14,15,20], Al [21], stainless steel
coatings [22,23] and Inconel 718 [24]. The thermal and electric conductivity of cold-sprayed coatings increased owing to the improved contactof particle-to-particle interface and the micro-hardness decreased
owing to the removal of work hardening after heat treatment [14,20].
Moreover, the mechanical properties (tensile strength, elongation andtec.) of cold-sprayed coatings improved via heat treatment [15,23,24]
and even demonstrated similar performance as bulk materials [15].
The previous studies paid major attention on the improvement of themechanical properties via heat treatment. However, the determinantsof the coatings' mechanical properties are still undeﬁned up to now.
In the present study, four typical materials of Al, Cu, Ti and stainlesssteel were deposited by cold spray and their mechanical properties
were tested. Moreover, the effects of heat treatment temperature on
coating microstructures and mechanical properties were also investigated. The relationship between the porosity of cold-sprayed coatings
and their mechanical properties was discussed.
2. Experimental procedures
2.1. Feedstock powder and cold spray process
Commercially available Al (N99.7%), Cu (N99.7%), Ti (Grade 2) andstainless steel 316 powders are used in the study and their morphologies are presented in Fig. 1(a)–(d). The pure Al and Ti powders show

R. Huang et al. / Surface & Coatings Technology 261 (2015) 278–288

279(a) Al(b) Cu(c) Ti(d) Stainless Steel 316

25
Al
Cu
Ti
Stainless SteelDistribution (%)

20
15
10
5
0

1

10

100

Diameter (µm)(e)
Fig. 1. Morphology of Al (a), Cu (b), Ti (c), stainless steel 316 powder and their diameter distributions (e).

perfect spherical shapes. The pure Cu and stainless steel 316 powders
show near spherical shapes. The volume distributions of the particle
diameter are shown in Fig. 1(e). Al powder has the narrowest distribution of particle diameter among the four types, with a volume average
diameter of about 38 μm. The powder diameter of pure Cu and Ti ranges
from 5 to 80 μm, and both of their volume averages are about 30 μm. The
powder diameter of stainless steel 316 ranges from 10 to 80 μm, and thevolume average of particle diameter is about 35 μm.
A commercial cold spray system, model number PCS-1000 designedby PLASMA GIKEN CO. LTD., was used to prepare the coatings. A convergent–divergent (De-Laval) nozzle was conﬁgured to accelerate theworking gas to supersonic speed. Nitrogen gas was used as the propellant gas in this test. The spray conditions for cold spray are shown in
Table 1. Al alloy cylinder with 100 mm of both outer diameter andlength was utilized as the substrate, and more than 5 mm of coatings

were deposited in order to satisfy the dimension of tensile specimen.
Fig. 2(a) illustrates the coating preparation process. The cold-sprayedcoatings were cut from the substrate, and then machined to a tensile
specimen as shown in Fig. 2(b). The detailed dimension of tensile
specimen is shown in Fig. 3, and it followed the standard of No. 14B
according to JIS Z2201.

Table 1
Spray conditions.
Materials

Al

Cu

Ti

SS316

Gas pressure (MPa)Gas temperature (°C)
Powder feed rate (g/min)
Spray distance (mm)

3
380
25
30

800
100

900
60

800
100

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R. Huang et al. / Surface & Coatings Technology 261 (2015) 278–288

(a)

(b)
Fig. 2. Preparation of the tensile specimen.

2.2. Heat treatment and tensile test
Heat treatment for the tensile specimen was performed in an argon
atmosphere. Table 2 shows the heat treatment conditions. The tensile
specimens of as-sprayed and heat-treated coatings were measured
with the tensile testing equipment: AG-X (100KN), manufactured by
Shimadzu Co., Ltd. Two cameras were equipped with the tensile
machine in order to measure the extension of the specimen within
the gage length during tensile test.
2.3. Coating characterization
Cross-sections of as-sprayed and heat-treated coatings were prepared vertically to their surfaces by a conventional mechanical polishingmethod with SiC paper and diamond suspensions. The mirror-polishedcross-sections were examined with a digital microscope (VHX-900,
Keyence, Japan). The porosity of coatings was measured by image
analysis with the cross-section photos. Cu and Ti coatings were etchedwith 2% nitric acid and 1% ﬂuoric acid respectively before observation.
In order to understand the failure mechanism of the cold-sprayedcoatings, the fracture surface of the tensile specimens were observed
by a ﬁeld emission scanning electron microscope (JSM-5200LV, JEOL,Japan).
3. Results
3.1. Mechanical properties
Fig. 4 shows the tensile properties of cold-sprayed Al, Cu, Ti andstainless steel 316 coatings. It can be seen that all the as-sprayed coatings have poor ductility and their tensile elongations are less than
0.5%. All the mechanical properties (especially elongation) improved
with increase in heat treatment temperature.
For the cold-sprayed Al coatings, the as-sprayed coating has a similartensile strength as the bulk in spite of the poor elongation. After heat
treatment at 200 °C, the cold-sprayed Al coating becomes even stronger,

but still with poor ductility. Further improving the heat treatment
temperature to 300 °C, the strength of coating decreases compared tothat of heat-treated coating at 200 °C. Nevertheless, it is higher thanthat of the bulk Al. The coating began to have some ductility with anelongation of about 1.3%. With the increase of heat treatment temperature to more than 400 °C, the coating showed decent ductility andgained more than 10% of elongation. However, the tensile strength of
coating decreased and became a little lower than the bulk Al after
heat-treating at 400 °C. The most excellent properties of Al coating is
the one that was heat-treated at 600 °C. The coating possessed elongation of about 25% (half of the bulk material) and ultimate tensile
strength of about 70 MPa (close to the bulk material). However, theyield strength of Al coating after treatment temperature at 600 °C
decreased to far lower than the bulk material.
For the cold-sprayed Cu coatings, the as-sprayed coating has a higher
tensile strength than the bulk Cu. After heat-treating at 300 °C, the coldsprayed Cu coating becomes stronger, but with poor ductility (less than1%). Further raising the heat treatment temperature to 400 °C and 500°C, excellent mechanical properties were obtained. The tensile strengthand elongation of coating even exceed that of bulk Cu. When the heat
treatment temperature is increased to 700 °C, the tensile strength andelongation of the coating decreased compared with the ones heattreated at 400 and 500 °C. It seems that excessive heat treatment cangenerate adverse effect of improving mechanical properties for coldsprayed Cu coatings.
For cold-sprayed Ti coatings, the tensile strength and elongation are
extremely low compared with that of the bulk Ti (even when heattreated at a higher temperature of 1000 °C). Generally, the tensile
strength of Ti coatings is less than 200 MPa and their elongation is lessthan 0.5%. It seems that the heat treatment did not signiﬁcantly improvethe mechanical properties of cold-sprayed Ti coatings.
For cold-sprayed stainless steel 316 coatings, the tensile strengthand elongation have no improvement compared to that of the
as-sprayed coating if the heat treatment temperature is less than 600 °C.
When the heat treatment temperature is increased to 800 °C, the tensile
strength increased to 300 MPa. However, it still revealed poor ductility(less than 0.5%) because the specimen failed before the plastic deformation stage. With the heat treatment temperature of 1000 °C, thestrength of stainless steel 316 coating further increased and someductility starts to appear. However, strength and elongation are still

Table 2Heat treatment conditions.

Fig. 3. Dimensions of tensile specimen.

Materials

Al

Cu

Ti

Temperature (°C)

200, 300, 400,600
4300, 400, 500,
700

400, 600, 800, 400, 600, 800,
1000
1000

Time (h)SS316

R. Huang et al. / Surface & Coatings Technology 261 (2015) 278–288(b) 400(a) 120

Pure Al Bulk
As-sprayed Coatings
Heat Treatment 200°C
Heat Treatment 300°C
Heat Treatment 400°C
Heat Treatment 600°C

100
80
60
40
20

300
250
200
150
100
50
0

0

0.2 0.4 0.6 0.8

1

10

20

30

40

50

0

0.2 0.4 0.6 0.8

Normal Strain (%)

1

5 10 15 20 25 30 35 40 45

Normal Strain (%)(d) 700(c) 500
400
300
Pure Ti Bulk
As-sprayed Coatings
Heat Treatment 400°C
Heat Treatment 600°C
Heat Treatment 800°C
Heat Treatment 1000°C

200
100

Normal Stress (MPa)Pure Ti

Normal Stress (MPa)Pure Cu Bulk
As-sprayed Coatings
Heat Treatment 300°C
Heat Treatment 400°C
Heat Treatment 500°C
Heat Treatment 700°C

Pure Cu

350

Normal Stress (MPa)Normal Stress (MPa)Pure Al

0281

Stainless Steel 316

Stainless Steel 316 Bulk
As-sprayed Coatings
Heat Treatment 400°C
Heat Treatment 600°C
Heat Treatment 800°C
Heat Treatment 1000°C

600
500
400
300
200
100
0

0
0

0.2 0.4 0.6 0.8

1

5 10 15 20 25 30 35 40 450

0.2 0.4 0.6 0.8

1

10 20 30 40 50 60 70 80

Normal Strain (%)

Normal Strain (%)

Fig. 4. Tensile properties of cold-sprayed Al (a), Cu (b), Ti (c) and stainless steel 316 (d) coatings.

lower than the bulk since it failed at the early stage of plastic deformation during the tensile test.

3.2. Microstructure
The heat treatment signiﬁcantly improved the mechanical properties of cold-sprayed Cu. Nevertheless, almost no effects of heat treatment on the mechanical properties of cold-sprayed Ti can be observed(as shown in Fig. 4). To understand the phenomena, microstructuresof Cu and Ti were analyzed.
Fig. 5 shows the microstructure of cold-sprayed Cu coatings before
and after heat treatment. It can be seen that extremely dense Cu coating
was obtained by cold spray. The as-sprayed coating shows that Cu
particle deformed intensively during the deposition process owing to
the high particle velocity and its excellent ductility. At the same time,
grain of Cu particle also deformed during the impact process as shownin Fig. 5(a). After heat treatment of 500 °C, the interface between particles disappeared. The grain of Cu coating after heat treatment at 500 °C
looks very uniformed and ﬁne. Further improving the heat treatment
temperature to 700 °C, the grain of Cu coating becomes coarse
compared to that of 500 °C.
Fig. 6 shows the microstructure of cold-sprayed Ti coatings before
and after heat treatment. It can be seen that the cold-sprayed Ti coating
looks very porous. The as-sprayed coating shows that Ti particle hardly
deforms during the deposition process. It seems that Ti particle is too
hard to deform during impact. After being heat-treated at 600 °C,
some of the interface between particles disappeared, but some still
remained. It seems that effective diffusion between the particle interface only happens at closer cohesion of particles. Further improving
the heat treatment temperature to 1000 °C, the interface of particles
almost disappeared, but some pores inside the coating still remained.

3.3. Fracture surface
After the tensile test, fracture surface was observed by SEM in orderto understand its fracture mechanism. Fig. 7 shows the observation
location of the fractured specimens.
Fig. 8 shows the fracture surface of Al coatings. For the as-sprayed Al
coating, the fracture occurred between the interfaces of particles. No
dimple can be observed according to the fractography. It seems thatde-cohesive rupture occurred to the as-sprayed Al coating since its
cohesion of particles is quite poor without heat treatment. Al coating
still fractured at particle interface after heat treatment at 300 °C, evenwith some diffusion between the particle interfaces as shown in
Fig. 8(b). This failure can still be considered as a de-cohesive rupture.
Further improving the heat treatment temperature to 600 °C, some
dimples can be observed at the fracture surface as shown in Fig. 8(c).
It seems that the fracture of the coating belongs to micro-void coalescence rupture because some plastic deformation can be observed. However, some defects can be observed at the fracture surface. This makes
the coating fail at its early stage of tensile test and possesses lower
tensile strength than that of bulk material. It can be seen that heat treatment cannot ﬁx all of the defects or pores inside the Al coating evenwith higher heat treatment temperatures.
Fig. 9 shows the fracture surface of Cu coatings. For the as-sprayed
Cu coating, the fracture occurred between the interfaces of particles asshown in Fig. 9(a). No dimple can be observed according to thefractography and it seems that particle interface is relatively weak.
Consequently, a de-cohesive rupture occurred here for the as-sprayed
Cu coating. It can be seen that like Al, brittle failure occurred to theas-sprayed Cu coating, even though it was comparatively dense. After
heat-treated at 400 °C, plenty of dimples were observed at the fracturesurface. Further improving the heat treatment temperature to 500 °C,
dimples at the fracture surface look more uniformed. It seems that

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R. Huang et al. / Surface & Coatings Technology 261 (2015) 278–288

Fig. 5. Microstructure of as-sprayed Cu coatings (a), heat-treated at 500 (b) and 700 °C (c).

fracture in the Cu coatings heat-treated at 400 and 500 °C is due to plastic rupture of micro-void coalescence. When the heat treatment temperature was improved to 700 °C, it can be seen that the size of
dimple became larger, and this drops yield and ultimate strength.
Fig. 10 shows the fracture surface of Ti coatings. For the as-sprayedcoating, the fracture occurred between the particle interfaces. No
dimple can be observed according to the fractography and it can beseen that the brittle de-cohesive rupture occurred to the as-sprayed Ti
coating. It seems that the cohesion of particles in the coating is quitepoor without heat treatment. The Ti coating still fractured at the particle
interface after heat-treated at 600 °C as shown in Fig. 8(b). Compared tothe as-sprayed Ti coating, it can be seen that particle interface becomesless visible after heat treatment at 600 °C due to diffusion. However,fracture of the coating can still be considered as a de-cohesive rupture.
Further raising the heat treatment temperature to 1000 °C, dimples
cannot be observed at the fracture surface. Defects can still be observedat the fracture surface of Ti coating heat-treated at 1000 °C. These
defects made the coating fail at an early stage (before entering theplastic deformation stage) during the tensile test. Therefore, the fractureof Ti coating still belongs to brittle de-cohesive rupture after heat treatment at 1000 °C. It seems that heat treatment cannot ﬁx all of defects or
pores inside the Ti coating even when the heat treatment temperature is
raised to 1000 °C.
Fig. 11 shows the fracture surface of stainless steel 316 coatings. For
the as-sprayed stainless steel coating, the fracture occurred between theparticle interfaces like the other three materials. The fracture belongs tobrittle de-cohesive rupture because no dimple was observed (as shownin Fig. 11(a). Stainless steel 316 coating still fractured at its particle
interface after heat-treating at 600 °C. This time, the fracture surface
became rougher than that of the as-sprayed coating. It is possible thatfracture went between the weak particle interface and diffusion

happened between some compact particles during heat treatment.
The fracture bypassed the diffused particle interface and consequently,
made the fracture surface rougher. Further improving the temperature
to 1000 °C, some small dimples can be observed at the fracture surface.
It seems that fracture of coating belongs to micro-void coalescence rupture because some plastic deformation can be observed. However, some
defects can be observed at the fracture surface and this decreases thecoating tensile strength. It seems that heat treatment of 1000 °C is still
not enough to ﬁx all of the defects inside the stainless steel 316 coating.
4. Discussion
In general, almost no ductility can be observed with the as-sprayedcoatings and heat treatment can improve their mechanical properties
to a certain extent. In order to discuss the relationship between densiﬁcation and mechanical properties, porosity of the four as-sprayed coatings were measured. Fig. 12 shows the microstructures of as-sprayed Al
and stainless steel coatings. With Figs. 5, 6 and 12, porosity of the assprayed coatings can be measured and the results are shown in
Table 3. There are two typical coatings: dense (Cu coating) and porous(Ti Coating). These two types of coatings exhibit different mechanicalproperties compared to their bulk materials. Both the different behaviors of mechanical properties for the four cold-sprayed coatings andtheir improvements of mechanical properties after heat treatment canbe explained by their evolving microstructures.
The evolution of cold-sprayed coatings' microstructure during heat
treatment can be summarized as shown in Fig. 13. Two typical coldsprayed coatings (dense and porous) are shown. If particle velocity ishigher and material is ductile, particles deform more and the coating
itself becomes dense. In contrast, a porous coating is formed whenparticle velocity is low and material is hard. The Cu coating can be

R. Huang et al. / Surface & Coatings Technology 261 (2015) 278–288

283

Fig. 6. Microstructure of as-sprayed Ti coatings (a), heat-treated at 600 (b) and 1000 °C (c).

considered as a typical dense coating as shown in Fig. 5. Few defects canbe observed in the coating and extremely low porosity of 0.04% was
measured. Additionally, intensive work hardening is generated during
the impact process of particles. This leads to excellent tensile strengthof Cu coating that is even higher than bulk and almost no elongation
can be obtained with the as-sprayed Cu coating even though it is extremely dense. The tensile strength of as-sprayed Cu coating is almost
determined by the particles interface strength because almost all failure
happens here as shown in Fig. 9(a). Cold-sprayed Ti coating can be considered as a relatively porous coating as shown in Fig. 6(a) and porosityof 14.2% was measured. Plenty of defects can be observed in the coating,and this leads to a de-cohesive rupture owing to the failure happened in
the early stage of elastic deformation during the tensile test. Its tensile

Fig. 7. Observation location of the fractured specimens.

strength is much less than bulk as shown in Fig. 4(c). For the assprayed Al and stainless steel 316 coatings, some defects existed inside
as shown in Fig. 12. Consequently, tensile strength of the two materials
was decreased by defects. Compared with the bulk materials, the decreased tensile strength of stainless steel coatings is more signiﬁcantthan that of the Al coating as shown in Fig. 4(a) and (d). This is due toporosity in the coatings as shown in Fig. 12 that the stainless steel has
more porosity (2.14%) than that in the Al coating (0.87%). The more defects that the coatings may have, the earlier they will fail during the tensile test, consequently a lower tensile strength compared to the bulk
materials.
At a lower heat-treating temperature, some diffusion between particles will happen as shown in Fig. 13(b). The interface of particles will
become obscure for the dense coating and only the compactly connected interfaces will disappear for the porous coating. This improves thestrength of particles interface. However, the work hardening generated
during cold spray process still remains in the coatings with lower heat
treatment temperature since there are no changes in the grains. Therefore, dense coatings will become stronger but with low ductility. The
improved tensile strength of Cu coating heat-treated at 300 °C can attribute to diffusion in the particle interfaces. The same phenomenon canalso be observed in Al heat-treated at 200 °C. For stainless steel 316
coating heat-treated at 400 and 600 °C, it is comparatively low andno improvement of tensile strength can be observed (in Fig. 4(d)).
Comparing the morphologies of as-sprayed stainless steel coatings
with the one heat-treated at 600 °C (in Fig. 11), no signiﬁcant change
can be found. Therefore, sufﬁcient diffusion at particles interface canhappen only when the heat treatment temperature is more than 600°C for stainless steel. The improvement of tensile strength heat-treated
at 800 °C proves that there was a signiﬁcant diffusion at particle interface. In contrast, no signiﬁcant change of the mechanical properties

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R. Huang et al. / Surface & Coatings Technology 261 (2015) 278–288

Fig. 8. Fracture surface of as-sprayed Al coatings (a), heat-treated at 300 (b) and 600 °C (c).

Fig. 9. Fracture surface of as-sprayed Cu coatings (a), heat-treated at 400 (b), 500 (c) and 700 °C (d).

R. Huang et al. / Surface & Coatings Technology 261 (2015) 278–288

Fig. 10. Fracture surface of as-sprayed Ti coatings (a), heat-treated at 600 (b) and 1000 °C (c).

Fig. 11. Fracture surface of as-sprayed stainless steel 316 coatings (a), heat treatment at 600 (b) and 1000 °C (c).

285

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R. Huang et al. / Surface & Coatings Technology 261 (2015) 278–288

Fig. 12. Microstructure of as-sprayed Al (a) and stainless steel 316 (b) coatings.

will be observed for the porous coatings compared to the as-sprayedcoatings because diffusion cannot occur when particles are not bonded
strongly. Comparing the microstructures of as-sprayed Ti coating withthe one heat-treated at 600 °C in Fig. 6(b), some diffusion were generated at the compact particles interface because some particles interface
became less visible. However, the defects in the Ti coating caused thetensile strength to remain still low and there are no signiﬁcant changesof the tensile strength after heat-treated at 400 and 600 °C.
Further improving the heat treatment temperature, diffusion at theparticle interfaces became more intensive and the interfaces completely
disappeared. Work hardening and intensive plastic deformation in thecoatings generated in the cold spray processing will gradually beremoved due to recrystallization during heat treatment (as shown in
Fig. 13(c). Recrystallization for the plastically deformed coating will
generate ﬁne grains as shown in Fig. 5(b). From the fractured surface
in Fig. 9, it can be seen that recrystallization for Cu coating happened
when the heat treatment temperature is over 400 °C; in whichcorresponded to Kwon's research that recrystallization temperature of
Cu is higher than 400 °C [25]. The ﬁne grain of coating leads to goodstrength and excellent elongation. As shown in Fig. 4(b), it can be better
than the bulk in case for Cu. By removing work hardening along withrecrystallization, the material restored its ductility and generated a
micro-void coalescence rupture during the tensile test. Therefore, plenty of dimples can be observed at the fracture surface as shown in
Fig. 9(b) and (c). For stainless steel 316 coating, recrystallization wasseen with heat treatment temperature of 1000 °C, since some ﬁne dimples can be observed at the fracture surface (in Fig. 11(c)). Recrystallization temperature of stainless steel 316 in the present studies seems to
be a little higher than Hirota's research (from 800 to 950 °C) [26]. It is
possible that the porosity in the stainless steel coating has obstructed
its diffusion between the particle interfaces during heat treatment andmade the recrystallization temperature become a little higher. Althoughrecrystallization brought some elongation to the stainless steel 316
coating heat-treated at 1000 °C, the remained defects led to its rupture
at an early stage of plastic deformation during the tensile test since thecurrent heat treatment condition cannot ﬁx all the defects in the coating
as shown in Fig. 11(c). Therefore, both tensile strength and elongationof coating are lower than that of the bulk stainless steel. For Al coatings,
recrystallization starts at 300 °C. This is where the tensile strength starts

to decrease and some elongation appears as shown in Fig. 4(a). This
result is corresponded to Johnson's research that recrystallization temperature of Al is about 300 °C [27]. Unlike Cu, the grain in the Al coating
did not become signiﬁcantly ﬁne because the original grain did notdeform much during the deposition process (shown in Fig. 8). Even
when the recrystallization happened to the Al coating when heattreated at 300 °C, the work hardening did not completely remove
from the coating. Therefore, coating should have a higher tensile
strength and lower elongation compared to the bulk material. Like thestainless steel 316, some defects still remained in the Al coatings asshown in Fig. 8. This will decrease the mechanical properties of Al coating. As a result, Al coating heat-treated at 300 °C has a slightly higher
tensile strength and much lower elongation than the bulk material(see Fig. 4(a)). For Ti coating, recrystallization temperature is about
700 °C [28] and heat treatment at 800 and 1000 °C should make thecoatings change their mechanical properties. However, since only few
particles are deforming, it leads to a small change with the grain during
the recrystallization stage as shown in Fig. 6. Diffusion increased theconnection strength between the close contact particles, but theTable 3Porosity of the as-prayed coatings.
Materials

Porosity (%)

Al
Cu
Ti
SS 316

0.87
0.04
14.2
2.14

Fig. 13. Schematic of the evolving cold-sprayed coating during heat treatment.

R. Huang et al. / Surface & Coatings Technology 261 (2015) 278–288

improvement effect becomes quite poor owing to the residual defects(example: big pores). This consequently leads to a brittle de-cohesive
rupture as shown in Fig. 10(c). With Ti coating heat-treated at 800
and 1000 °C, poor elongation was obtained and the tensile strength
improved a little compared to the as-sprayed coating and far lower
than the bulk Ti as shown in Fig. 4(c).
If the heat treatment temperature is too high, the grain will grow asshown in Fig. 13(d). The obvious grain growth happened to the Cu coating heat-treated at 700 °C as shown in Fig. 5. The grown grain can meanthat materials can plastically deform easily at lower yield strength.
Dimples at the fracture surface became bigger owing to the grain
growth as shown in Fig. 9. It can be seen that the grain growth decreased
the mechanical properties for dense coatings. For the Al coating, grain
growth happened due to the decrease in yield strength with heat treatment of 600 °C. However, Al coating is not as dense as the Cu coating
since some porosity existed in the coating. When the heat-treating temperature is higher, it further recovered the defects in the coating andcompensated the decrease of mechanical properties caused by thegrain growth to some extent. As a result, Al coating heat-treated at
600 °C possesses better elongation than that of 400 °C. For Ti and stainless steel 316 coatings, the heat treatment temperature of 1000 °C is still
low to experience the grain growth stage since they have a higher melting point. It can be proved in the Fig. 4(c) and (d) that no decrease inyield strength can be observed for Ti and stainless steel coatings.
Besides mechanical properties of the yield, ultimate strength andelongation, the elastic properties can also be calculated with thestress–strain curve. Table 4 shows the Young's modulus of coldsprayed coatings derived from Fig. 4. According to Phani and Niyogi's
researches, the elastic properties of porous materials are determinedby the porosity [29]. The porosity of materials leads to the Young's modulus of porous materials reduced [29,30]. Meanwhile, the cold-sprayedcoating always shows lower Young's modulus due to the existence ofseparated inter-splat boundaries (tiny cracks) [31,32]. For Cu coating,although almost no pores and defects can be observed in the coating(see Fig. 5), its Young's modulus of as-prayed coating is still about 20%
lower than the corresponding bulk material due to the existence ofmany unbonded inter-splat boundaries as reported in Ref. 31 to 33.
After heat treatment at 300 °C, the Young's modulus of coating becomesclose to the bulk Cu since the tiny cracks are repaired during heat treatment via diffusion at the inter-splat boundaries. It seems that the tiny
cracks in cold-sprayed Cu coatings are almost completely repaired if
the heat treatment temperature is higher than 300 °C since the Young's
modulus hardly change any more with the higher heat treatment
temperatures. For Al coating, the tiny cracks in the coating added its porosity (0.87%) make the Young's modulus of as-sprayed Al coating lower
about 25% compared with its bulk material. After heat treatment at 200
and 300 °C, the Young's modulus increases a little owing the disappearance of some tiny cracks. Further improved the heat treatment temperature to 400 and 600 °C, the yield strength decreases signiﬁcantly and it
is difﬁcult to calculate the Young's modulus owing to the short-timeelastic stage during the tensile test. For the stainless steel coatings,
some pores (2.14%) and separated inter-splat boundaries can be observed (see Fig. 12(b)). Therefore, the as-sprayed coating has a lower

287

Table 5Porosity of cold-sprayed Ti coatings.
Heat treatment conditions

Porosity (%)

As-sprayed
Heat treatment at 600 °C
Heat treatment at 1000 °C

14.2
10.5
7.6

Young's modulus than only half of its bulk material as shown in
Table 4. The Young's modulus of stainless steel coatings gradually
increases with the increase of heat treatment temperature from 400 to1000 °C. This is caused by the gradual recovery of defects during theheat treatment process, which reduces the density of separated intersplat boundaries. Heat treatment can also reduce the pores in the
cold-sprayed stainless steel 316 coating [23], and this can also increase
Young's modulus. However, the Young's modulus of coating after heat
treatment at 1000 °C is still a little lower than its bulk material since
the bigger cracks (pores) cannot be completely repaired as shown in
Fig. 10(c). For Ti coatings, they are so porous that the Young's modulusof as-sprayed coating is far lower than the bulk material as shown in
Table 4 (one third), and its Young's modulus gradually rises with theincrease of heat treatment temperature like the stainless steel coatings.
Table 5 shows the porosity (from Fig. 6) of cold-sprayed Ti coatings. It
can be seen that porosity of cold-sprayed Ti coatings decreases after
heat treatment because some defects are recovered and some particle
boundaries disappeared (see Fig. 6). The Young's modulus in the Ti coating improves when its pores and inter-splat boundaries decreased.
However, the Young's modulus of Ti coating heat-treated at 1000 °C is
still lower than the corresponding bulk material. Heat treatment candecrease some of the defects inside the Ti coatings, but it cannot eliminate the larger pores (see Fig. 6). Therefore, the Young's modulus cannot
reach to the bulk material properties even after heat-treated at 1000 °C.
5. Conclusions
In this study, the mechanical properties of cold-sprayed Al, Cu, Ti andstainless steel 316 coatings were measured and the effects of heat treatment on their mechanical properties were also discussed. It seems thatthe mechanical properties of cold-sprayed coatings are mainly determined by its microstructure that was formed in cold spray process
and changed by the subsequent heat treatment process.
For dense as-sprayed coating such as Cu, the tensile strength is very
high, but elongation is poor compared with the bulk material. With
lower heat treatment temperature, the cohesive strength of coating
improved due to the diffusion between particles, but elongation wasless because the work hardening generated during the cold spray process still remained. By increasing the heat treatment temperature, thestrength deceased since the work hardening was removed. Also,
excellent elongation was possible to obtain because recrystallization
was able to restore the grain deformation. It is possible to obtain its
best mechanical properties at this stage and it is even better than bulk
material. If the heat treatment temperature is too high, it can lead tograin growth. Consequently, lowers the yield or tensile strength.

Table 4Young's modulus of cold-sprayed coatings and corresponding bulk.
Materials

Young's modulus

Al

Heat treatment temperature (°C)
Young's modulus (GPa)Heat treatment temperature (°C)
Young's modulus (GPa)Heat treatment temperature (°C)
Young's modulus (GPa)Heat treatment temperature (°C)
Young's modulus (GPa)

Cu
SS316
Ti

As Sprayed52
As Sprayed
84.3
As Sprayed85.1
As Sprayed
32.5

200
57.2
300
104
400
103
400
45.8

300
54.6
400
103
600
117
600
51.8

400
–
500
100
800
145
800
57.6

600
–
700
111
1000
154
1000
72.8

Bulk
69.3
Bulk
107
Bulk
183
Bulk
104

288

R. Huang et al. / Surface & Coatings Technology 261 (2015) 278–288

For porous coatings with some defects (like cold-sprayed Al andstainless steel 316 coatings), even if the working hardening generated
during cold spray processing improved coating's strength, the tensile
strength is still lower than that of bulk materials owing to the defects
within the coatings. The phenomenon is more signiﬁcant with stainlesssteel 316 coating than that of Al since there were more defects in stainless steel than that in Al coatings. Higher heat treatment temperatureeffectively ﬁxed the defects in the coatings, which led to obtain better
tensile strength. Furthermore improving the heat treatment temperature, the ductility of coatings increased owing to the recrystallizationof coating after heat treatment.
For porous coatings with plenty of defects such as Ti, it is difﬁcult toﬁx all the defects inside the coatings, especially the big pores. The effectsof heat treatment on the mechanical properties also look quite poor
excluding Young's modulus that is mainly determined by the cohesionof particles. Consequently, the coating's failure always happens at theearly elastic deformation stage during the tensile test even when diffusion had been generated. It seems that the tensile strength of Ti coating
is mainly determined by these defects. Therefore, the current heat treatment conditions are not sufﬁcient to restore the all the defects andimprove the mechanical properties of the porous Ti coating.
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