The influence of heat treatment on the microstructure and mechanical properties of cold sprayedcoatings
Bo Sun, Renzhong Huang, Hirotaka Fukanuma
Plasma Giken Co., Ltd., Saitama 369-1214, JapanIn this paper Cu and Ti coatings were deposited by cold spray process. The influence of heat treatment temperature on thecoating microstructure and mechanical property were investigated. The experiment results show that heat treatment
temperature give significant influence on Cu coating microstructure. The Cu coating with stress-strain characteristiccomparable to the bulk material can be obtained at the heat treatment temperature of 500oC. However, the Ti coatings exhibit
no significant change on the stress-strain characteristics against the heat treatment temperature although the coating
microstructure changed. The different effects of heat treatment to the Cu and Ti coating mechanical property were discussed by
combining the change of coating microstructure.
1Introduction
In cold spray process spray particle impact to the substratein the solid state. The characteristics of high particle impact
velocity as well as the very little oxidation and no phasetransformation contribute to form a coating with goodmechanical properties. However, the coating microstructure
and mechanical properties are also significantly influenced
by the spray conditions. The residual stress and defects inthe coating such as pores will also lowering coatingproperties. Heat treatment is a feasible method to improvethe coating microstructure and properties. In this paper, theCu and Ti coatings were deposited by the cold spray. The
effect of heat treatment temperature on the coating
microstructure and mechanical property were investigated.
2Experimental procedures
The SEM images of Cu and Ti powers used in theexperiment are shown in figure 1. The cold spray systemused in the experiment was developed by Plasma Giken Co.,
Ltd. Nitrogen gas was used as the propellant gas. To
characterize the coating mechanical property, the influenceof heat treatment temperature on the coating stress-strain
characteristic was investigated. Fig. 2 show the preparationof the tensile specimen. The coating with thickness over

5 mm was deposited on an aluminum tube with a size of φ
100 x L100 mm. A strip was cut from the tube as shown infigure 2 and then machined to a tensile specimen according
to the JIS Z2201. Fig. 3 shows the shape and size of thetensile specimen. The heat treatment to the tensile specimen
was performed in an argon atmosphere. Table 1 shows thecold spray and heat treatment conditions.

Fig.3. The shape and size of tensile specimen
Table: Cold spray and heat treatment conditions
Gas pressure (MPa)Gas pressure (oC)Heat treatment temperature (oC)Heat treatment time (h)

Cu
3.0
800
500, 700
4Ti
3.0
900
600,1000
43
Experimental results and discussions
The tensile stress-strain curves of Cu coatings and Cu bulk
are shown in figure 4.
350
As-sprayed
500oC 4h700oC 4hCu-bulk

CuFig. 1. Powder SEM images.

Ti

Normal Stress (MPa)

300
250
200
150
100
50000.2 0.4 0.6 0.8

15 1015202530354045

Normal Strain (%)

Fig.4. Cu coatings and Cu bulk tensile stress-strain curves

Fig. 2. Preparation of the tensile specimen

The as-sprayed Cu coating has a high tensile strength of290 MPa while it exhibits almost no strain. For the heat
treatment temperature of 500oC, the coating tensile strength
decreased to 245 MPa but the strain increased dramaticallyto 40% which is even higher than that of the Cu bulk
material. With the further increase of heat treatment
temperature to 700oC the coating tensile strength and strain

inversely decreased to 180MPa and 24% respectively
compared to the tensile specimen of 500oC heat treated.
Fig. 5 shows the tensile stress-strain curves of Ti coatings
and Ti bulk. Coating tensile strength increased from
122MPa of the as-sprayed coating to 133MPa and 192MPa
respectively corresponds to the heat treatment temperatureof 600oC and 1000oC. However the coating strain exhibit nosignificant change against the heat treatment temperature.
Compared to the tensile strength of 400MPa and strain of40% for the Ti bulk, the mechanical property of as-sprayed
and the heat treated Ti coatings is insufficient.
500

As-Sprayed
600oC
1000oCTi-bulk

Normal Stress (MPa)

400

300

200

100

000.2 0.4 0.6 0.8

18 16 24 32 40 48

Normal Strain (%)

Fig. 6. Cu coatings cross section microstructure andFracture surface morphology

Fig.5. Ti coating and Ti bulk stress-strain curves
The tensile test results show that the effect of heat treatment
to the Cu and Ti coating stress-strain characteristic isdifferent. Fig. 6 and 7 show the Cu and Ti coating crosssection microstructure and fracture surface morphology
respectively. For the Cu coatings the particle in the assprayed coating exhibit well deformed shape due to highparticle impact velocity. There is almost no obvious poresor cracks in the coating which contributed to a high coating
tensile strength. While the fracture surface morphology
indicated clearly that the fracture occurred at the particle
interface and the particles exhibit no sigficant elongation
along the tensile direction. At the heat treatment
temperature of 500oC the interface between the particles
disappeared and the grain size in the coating is obviously
fine than that of the as-sprayed coating. It may be attributed
to the diffusion and recrystallization of the grains. The
fracture surface morphology exhibit typical dimple patternwith features of plastic fracture. As a result the coatingexhibit good combination of tensile strength and ductility atheat treatment temperature of 500oC. For the heat treatment
temperature of 700oC the grain grows obviously,
meanwhile the dimple size increased but the number ofdimples decreased which lead to the reduction of coating
tensile strength and ductility.
Due to the high elastic modulus of the Ti material, the Ti
particle impact velocity drove by N2 gas is not high enoughto provide sufficient particle deformation. As a result theparticles in the as-sprayed coating keep approximately
spherical shapes like the feedstock powders as shown infigure 1and 7. It exists many visible interface and a few
pores between the particles. With the increase of heat
treatment temperature from 600oC to 1000oC the amount ofinterface significantly decreased but the pores numbers
exhibit no obvious decrease. For the as-sprayed coating thefracture surface morphology show that the fracture occurredat the particle interface. At the heat treatment temperature
to 600oC and 1000oC, although a fraction of fracture
occurred in the particle there is no obvious dimple patternFig. 7. Ti coatings cross section microstructure andFracture surface morphology
can be observed. There are still some defects in the coatings.
All the fracture patterns for the Ti coatings exhibit brittlefracture. As a result the coating tensile strength increased
but the coating ductility did not change appreciably.
3Conclusions
The influence of heat treatment temperature on Cu and Ti
coatings stress-strain characteristic were investigated. As tothe Cu coatings the heat treatment temperature of 500 oC
improve the coating mechanical property due to diffusion,
grain refinement and transition to ductile fracture pattern.
As to the Ti coating the heat treatment give significant
influence on the coating tensile strength but not coatingstrains whicn can be attributed to the similar brittle fracture
pattern even at heat treatment temperature of 1000 oC.
4References （ omitted）