Wear behavior of plasma and HVOF sprayed WC-12Co + 6% ETFE coatings on AA2024-T6 aluminum alloy

Abstract

In this study, WC-12Co+6% ethylene trifluoroethylene (ETFE) coatings were formed on the surface of an AA2024-T6 aluminum alloy using both plasma spray and high velocity oxygen fuel (HVOF) processes. The characterization of the coatings was made by microscopic examinations, thickness, porosity, contact angle and hardness measurements and X-ray diffraction (XRD) analysis. The coefficient of friction and wear resistance of coatings were obtained using a reciprocating wear tester by rubbing a 10mm diameter Al2O3 ball on the coatings in dry and acid environments. It was observed that the WC-12Co+6% ETFE coating sprayed using HVOF method exhibited higher hardness and contact angle, better tribological performance and higher amount of retained WC when compared to the plasma sprayed WC-12Co+6% ETFE coating. The wear resistance of the HVOF sprayed WC-12Co+6% ETFE coating tested in acid environment was almost one and a half times bigger than that of dry sliding condition, but the plasma sprayed WC-12Co+6% ETFE coating tested in acid environment showed a lower wear resistance when compared to the result of the experiment performed on dry sliding condition.

Full text

Wear behavior of plasma and HVOF sprayed WC-12Co +6% ETFE coatings on
AA2024-T6 aluminum alloy
Harun Mindivan ⁎
Ataturk University, Engineering Faculty, Department of Metallurgy Engineering, 25240 Erzurum, Turkey
abstractarticle info
Available online 10 November 2009
Keywords:
HVOF
Plasma spraying
WC-12Co+6% ETFE coating
Wear
In this study, WC-12Co + 6% ethylene trifluoroethylene (ETFE) coatings were formed on the surface of an
AA2024-T6 aluminum alloy using both plasma spray and high velocity oxygen fuel (HVOF) processes. The
characterization of the coatings was made by microscopic examinations, thickness, porosity, contact angle
and hardness measurements and X-ray diffraction (XRD) analysis. The coefficient of friction and wear
resistance of coatings were obtained using a reciprocating wear tester by rubbing a 10 mm diameter Al
2
O
3
ball on the coatings in dry and acid environments. It was observed that the WC-12Co + 6% ETFE coating
sprayed using HVOF method exhibited higher hardness and contact angle, better tribological performance
and higher amount of retained WC when compared to the plasma sprayed WC-12Co+ 6% ETFE coating. The
wear resistance of the HVOF sprayed WC-12Co +6% ETFE coating tested in acid environment was almost one
and a half times bigger than that of dry sliding condition, but the plasma sprayed WC-12Co +6% ETFE coating
tested in acid environment showed a lower wear resistance when compared to the result of the experiment
performed on dry sliding condition.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
A unique combination ofhigh specific strengthand good appearance
makes aluminum and its alloys particularly suitable candidates for
aerospace and automotive applications [1]. However, aluminum alloys
exhibit very poor tribological properties (low hardness and low
resistance to friction wear) and moderate resistance to aggressive
environments [2]. Especially in landing gear components of the aircraft,
wear and corrosion control of many components are accomplished by
surfacetreatments of chromiumelectroplatingor anodizing [3,4].Oneof
the most interesting environmentally safer and cleaner alternatives for
the replacement of hard chromium plating or anodizing is tungsten
carbide (WC) thermal spray coating that combines excellent wear
resistance with exceptional corrosion resistance [3,5–12].
WC thermal spray coating is a well-known candidate for wear/
corrosion resistant applications because of its low friction coefficient,
exceptional hardness and environmental friendliness. The coatings
obtained by the addition of a Co binder phase in WC matrix can
present high hardness together with sufficient ductility [13]. In recent
years, fabrication of thermal spray coatings containing microscale
fluoropolymer particles has become more attractive technologically,
because of their excellent properties (good adhesion, high tempera-
ture stability, self-lubricative, release and non-stick properties and
specific resistance to damage) compared to the conventional ones
[14,15]. Unlike the conventional coatings, thermal spray coatings with
fluoropolymer exhibit a low friction coefficient at low temperature
under unlubricated sliding in air [14,16] and chemical resistance [15].
Among the group of fluoropolymer particles available for use, ETFE
particles have excellent mechanical properties (like high strength and
stiffness) as well as low density [17]. Since the resistance towards
mechanical and chemical damages of the coatings for the components
used in severe and aggressive service environment is highly desirable
[18], WC-Co coatings with ETFE can be a good candidate material to
meet the requirements under corrosive tribological service condi-
tions. In the present study, the tribological performance of WC-Co/
ETFE coatings has been examined in dry air and in 5 vol.% sulfuric acid
solution conditions. One study has demonstrated the improvement in
hardness and scratch resistance of WC-Co coatings containing
fluoropolymer more than 10 wt.% [15]; however, this study has
focused on the tribological performance of WC-Co coatings with low
ETFE (≤10 wt.%) prepared by plasma and HVOF spray processes on
NiCr layer deposited on AA2024-T6 alloy. For the wear tests
conducted in acid solution, a particular attention is devoted to the
contribution of the ETFE particles on tribological performance of the
coatings depending on the deposition process, even though acid may
have some corrosive effect on the coatings. In fact, wear [6,8,12,19]
and corrosion [9,10,20–23] behaviors of WC-Co based coatings are
separately available in the literature, published works dealing with
corrosion and/or wear in acid are rather limited in number [2,15].
Although the interaction between corrosion and wear is the utmost
importance for engineering applications, corrosion of the WC-Co
coatings with ETFE in acid has not been examined in this work.
Surface & Coatings Technology 204 (2010) 1870–1874
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E-mail address: hmindivan@hotmail.com.
0257-8972/$ –see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.surfcoat.2009.10.050
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2. Experimental details
This study was carried out with samples of an AA2024-T6 aluminum
alloy that weremachined from the bars of ∼25 mm diameter. Before the
spraying process, the surface of the aluminum samples was roughened
by sand blasting and then coated with anintermediary Ni–Cr layer. This
was due to the fact that the Ni–Cr layer provided an overall increase in
coating thickness and corrosion resistance in conjunction with
enhanced fracture toughness [2,20,23]. Song and Zhang [24] also
reported that the friction coefficient of the coatings with fluoropolymer
decreases considerably when the content of fluoropolymer is below
10 wt.%. Therefore, in order to obtain optimum wear performancefrom
a coating, the mixtures of 6 wt.% ETFE with WC-Co powders were
sprayed over the bond layer by plasmaand HVOF spray processes. In the
micrograph of the powders (Fig. 1), the WC-Co powders in lightcontrast
contain 12 wt.% Co and 88 wt.% WC with a particle size of approximately
32 µm, while the ETFE particles(∼20 μm) in dark contrastare composed
of 80 wt.% C and 20 wt.% F. The optimized spraying conditions are listed
in Table 1. These parameters are routinely used by coating manufac-
turers and were originally selected to obtain high deposition efficiency.
The characterization of the coatings was made by microscopic
examinations, thickness, porosity, contact angle and hardness measure-
ments and X-ray diffraction analysis. Microscopic examinations were
conducted on the surface of the coated samples by a scanning electron
microscope (SEM) coupled with an energy dispersive spectroscopy
(EDS), after grinding and polishing in standard manner. The porosity of
the coatings was calculated by linear intercept method [25].The
hydrophobic performance of the coatings was measured with KSV CAM
101 contactangle meter. The thickness, the porosityand the hardness of
all coatings were measured on the polished cross-sections. Thehardness
was measured by micro Vickers hardness tester with a load of 500 g. For
each coating, at least five measurements were made. XRD analysis was
made by a Rigaku X-ray diffractometre with Cu Kαradiation.
Wear performance of the coatings was determined by utilizing a
reciprocating wear tester described in ASTM G133 standard [26].Wear
tests were carried out by applying a load of 15 N on the surfaces of the
coatings with an Al
2
O
3
ball (diameter 10 mm and hardness 1577 HV
0.5
)
in dry conditions (20 °C and 30% RH) and acid (5 vol.% H
2
SO
4
)
environment. During the tests, the sliding speed of the balls on the
surfaceswas 0.026 m/s for a total slidingdistance of 200 m. The values of
frictioncoefficient that are measured by wear tester weredetermined as
a functionof time. Tests were interrupted in order to measure wear track
area using a stylus profilometer. Afterthe wear test, worn surfaces of the
coatingswere examined by SEM.The deviation of the wear test datawas
10% based on three experiments with the same test conditions. The test
results reported are for one experiment only.
3. Results and discussion
Fig. 2 illustrates representative surface SEM micrographs of the
deposited coatings along with the typical chemical composition of
different places inside the coatings analyzed by EDS. In some regions of
both the plasma and HVOF sprayed coatings, porosities were observed.
Quantitative metallographic studies revealed that the average porosity
values of the plasma and HVOF sprayed coatings were 3% and 1%,
respectively. At the same time, it has been found that the coatings are
composed of three different brightness contrast microzones that
represent the different constituent phases marked as X, Y and Z in
Fig. 2. In general, the bright-colored (marked as X) microzone contains
elements such as C, Co and W depending on the composition of the
initialpowder, while the gray-colored(marked as Z) microzone is rich in
O, C, Co and W content. All coatings show a successful composite
structure with a Co matrix (surrounds WC particles) in which ETFE
particlesare randomly dispersed in the rounded shape (marked as Y) as
shown in Fig. 2. According to Mateus et al. [14], the polymer particles
with the rounded shape in the coatings are possibly due to the
inadequate melting associated with thelimited internal heatconduction
into the organic particles. XRD patterns obtained from the coatings are
presented in Fig. 3. The XRD results showed that WC-12Co+ 6% ETFE
coatings obtained by HVOF spraying exhibited higher amount of
retained WC than those of the ones obtained by plasma spraying
(Fig. 3). HVOF spraying of WC-Co powders without fluoropolymer
addition shows a less decomposition of the WC compared to the other
methods such as plasma spraying [7,8,12]. Although the presence of
fluorine element in the coatings was not clearly identified by the XRD
analysis, the composition of the rounded dark colored phase was rich in
fluorine and carbon according to the results of EDS analysis (Fig. 2).
The plasma sprayed coating has a thickness of 400 μm and a
contact angle of 100° whereas the HVOF sprayed coating has a
thickness of 100 μm and a contact angle of 115°. When the hardness
results have been analyzed, the hardness value of the uncoated
AA2024-T6 aluminum alloy was measured as 125 HV
0.5
. The results
also showed that the hardness increased to 1200 HV
0.5
with HVOF
spraying and 1000 HV
0.5
with plasma spraying. The hardness of the
HVOF sprayed deposit is greater than that of the plasma sprayed
deposit. This can be related to less intensive carbides decomposition
at processing [5,7,11] and also can be verified by the results of XRD
curves comparing the plasma and HVOF sprayed WC-12Co+ 6% ETFE
coatings (Fig. 3). XRD patterns proved that the decarburization
(defined here as the carbon loss from WC) during plasma spraying
because of the high temperature of the plasma flame and the oxidizing
spray atmosphere was obvious by the appearance of W
2
C, Co
3
W
3
C
and WC
1−x
phases besides WC. The decarburization and oxidation
during plasma spraying result in a decrease in mechanical properties,
e.g., the loss of inter-splat adhesion that is responsible for the
degradation of mechanical properties [7]. However, because of HVOF's
low flame temperature and high particle velocity, the pattern of the
HVOF sprayed coating shows peaks indexed to WC and W
2
C. It is
Fig. 1. Powder morphology of WC-12Co + 6% ETFE.
Table 1
Spray parameters.
Plasma spray parameters
a
Electric current (A) 500
Electric voltage (V) 75
Powder feed rate (g/min) 65
Carrier gas flow rate (l/min) 4
Ar/H
2
40/10
Injector diameter (mm) 1.8
Injection angle (°) 75
Nozzle diameter (mm) 6
Spray distance (mm) 150
HVOF spray parameters
b
Flow rate of O
2
(m
3
/h) 42
Ratio of O
2
/kerosene 1.1
Carrier gas Argon
Spray distance (mm) 200
a
Carried out with Metco 3MB equipment.
b
Carried out with Tafa JP-5000 equipment.
1871H. Mindivan / Surface & Coatings Technology 204 (2010) 1870–1874

proposed that high levels of retained WC are indicative of a quite
homogeneous and dense coating with low porosity (1%).
Fig. 4 shows the curves representing the evolution of the friction
coefficient as a function of the sliding time for coatings against the
Al
2
O
3
ball at 15 N under dry and acid conditions. The common aspect
of the coatings used in this work is that, friction coefficient is always
much lower in acid environment than that of dry test condition
because the wear products were removed by acid attack. This result is
in agreement with those obtained by Kusoglu et al. [27]. It is obvious
from Fig. 4 that the friction coefficients of both coatings tested in acid
environment have very similar characteristics with negligible fluctua-
tions when compared to those tested under dry conditions. On the
other hand, the friction coefficient of the plasma sprayed coating
tested under dry conditions is significantly greater than that of the
HVOF sprayed coating. Measuring surface roughness is also an
effective way to reveal the variation of the worn surface [27,28]. The
surface roughness values of the plasma and HVOF sprayed coatings
were measured as 3.38 and 2.63, respectively. As depicted in Fig. 5, the
surface roughness of the worn surface of the plasma sprayed coating
tested under both test conditions slightly decreased as a function of
sliding time, while the surface roughness of the HVOF sprayed coating
did not exhibit such a change in both test conditions.
The progress of the wear during sliding in dry and acid environ-
ments is presented in Fig. 6. Since the wear track area increased
linearly with sliding time, wear rates were calculated from the slope
of the plots in Fig. 6 as the unit of “mm
2
/min”.Table 2 lists the wear
rates of the plasma and HVOF sprayed coatings along with their
relative wear resistance (RWR) values after dry and acid wear tests.
Since the maximum wear rate was obtained from the plasma sprayed
coating tested in acid environment, this condition is taken as the
reference point during the calculation of RWR values. The RWR values
given in Table 2 were obtained by dividing the wear rate of the plasma
sprayed coating tested in acid environment to the wear rates of other
coatings tested under both test conditions.
Table 2 indicates that the HVOF sprayed coating in both test
conditions yielded a better wear resistance than the plasma sprayed
coating. The wear rate of the plasma sprayed coating tested in acid
environment is almost two times bigger than that of the dry sliding
condition (Table 2). It should be noted that, the acid environment did
not provide a considerably beneficial effect on the wear resistance of
the plasma sprayed coating, even though the friction coefficient was
reduced (Fig. 4). However, the HVOF sprayed coating behaved
differently in dry and acid environments. It is interesting to note
that the HVOF sprayed coating was more resistant to wear in acid
environment, when compared to dry sliding condition.
SEM images of wear tracks developed on the surfaces of the
coatings tested under both test conditions are given in Fig. 7. When
compared to the HVOF sprayed coatings, an important ETFE loss was
Fig. 2. SEM micrographs along with EDS analysis results in the coatings deposited with both (a) plasma and (b) HVOF processes. Arrows indicate the location of EDS analyses.
Fig. 3. XRD patterns of the plasma and HVOF sprayed WC-12Co +6% ETFE coatings.
1872 H. Mindivan / Surface & Coatings Technology 204 (2010) 1870–1874

observed on the worn surface of the coatings deposited by plasma
spray process. In the wear track of the plasma sprayed coatings,
microcracks were observed. These microcracks, that indicate the
brittleness of carbide particles, were much more apparent in acid
environment. Also, the surface roughness values measured through
the worn surface (Fig. 5), wear rate (Table 2) and partial carbide
dissolution associated with the rapid removal of cobalt binder phase
in the wear track of the plasma sprayed coating (Fig. 7) increased in
acid environment compared to those in dry conditions. Thus, it is
concluded that under acid condition, the absorption and osmosis of
acid into the microcracks in the plasma sprayed coating reduces the
wear resistance and load-supporting capacity of the plasma sprayed
coating. Therefore, the acid does not make a layer which is capable of
hydrostatic lift but still lubricates as an adhesive layer (Fig. 4) in this
situation. This observation is in good agreement with the findings of
Shipway and Howell [29]. The wear behavior of the WC-Co based
coatings was commonly controlled by the brittleness and the easy
detachment of carbide particles from the worn surface. In other
words, the higher is the degree of decomposition of WC-Co powders
during spraying, the lower is the wear resistance [5–7,12]. Poor wear
resistance of the plasma sprayed coating with a low hardness and
more porous structure to tribo-chemical effects may be due to the
higher decarburization suffered in the coating (Fig. 3).
Contrary to the plasma sprayed coatings, wear track topographies of
the HVOF sprayed coatings in both test conditions are quite different in
appearance suggesting that wear progressed by different mechanisms.
The HVOF sprayed coating was slightly worn under dry sliding
conditions without any evidence of plastic deformation and cracking.
EDS analysis revealed thatthe worn surface of the HVOFsprayed coating
was still covered with an adherent carbon and fluorine-rich film
originated from the ETFE particles.During dry friction, ETFE particles act
as a solid lubricant leading to a noticeable increase in wear resistance
(Table 2) along with a decrease in coefficient of friction (Fig. 4)ofthe
HVOF sprayed deposit. Akin et al. [16] and Luo et al. [30] proved the
lubricating action of polymer particles during sliding tests. Similarly,
the acid environment considerably improved the friction properties of
the HVOF sprayed coating. Since any cracking and carbide dissolution
did not occuron the worn surfaceof the HVOF sprayedcoatings tested in
acid environment, the HVOF sprayed coating exhibited an excellent
wear resistance along with a reduction in the level of decomposition in
the presentstudy. The surface roughness (Fig. 5) and wear rate (Table2)
of the HVOF sprayed coating during sliding in acid environment were
lower than the ones obtained under dry test condition. The HVOF
sprayed coating with a high contact angle (115°) indicates that the
coating surface has better hydrophobicity due to the existence of
fluorine group. The worn surface of the HVOF sprayed coating had no
adsorption of acid. Moreover, the color and microstructure morpholo-
gies on the worn surface had no changes. It is concluded that the
presence of the acid prevents a direct contact between the coating and
Fig. 4. Evolution of the friction coefficient vs. sliding time under both test conditions.
Fig. 5. Surface roughness vs. sliding time under both test conditions.
Fig. 6. Progress of wear in dry and acid environments.
Table 2
Wear rates and RWR values of the coatings examined.
Process Plasma HVOF
Wear rate (mm
2
/min)
Dry 26.0 × 10
−2
17.8 × 10
−2
Acid 44.6 × 10
−2
12.6 × 10
−2
Relative wear resistance (RWR)
Dry 1.7 2.5
Acid 1.0 3.5
1873H. Mindivan / Surface & Coatings Technology 204 (2010) 1870–1874

the Al
2
O
3
ball due to a good hydrophobic property of HVOF sprayed
coating. The superior improvement in the wear resistance of the HVOF
sprayed coatings in dry and acid environments canbe attributed to their
relatively dense structures with lowporosity due tothe high amount of
WC retained in the coating(Fig. 3) as well as increase in theeffective role
of the ETFE particles during wear testing.
4. Conclusions
WC-12Co+ 6% ethylene trifluoroethylene (ETFE) powders were
sprayed onto an AA2024-T6 aluminum alloy by plasma spray and high
velocity oxygen fuel (HVOF) processes and the following conclusions
were obtained.
1. The HVOF sprayed WC-12Co + 6% ETFE deposits exhibited an
excellent resistance to wear in dry and acid environments, lower
coefficient of friction, less intensive carbides decomposition, higher
hardness and contact angle when compared to the plasma sprayed
WC-12Co+ 6% ETFE deposits.
2. The wear resistance of the HVOF sprayed WC-12Co +6% ETFE
coating tested in acid environment was almost one and a half times
bigger than that of dry sliding conditions. However, in “5 vol.%
H
2
SO
4
”solution, the plasma sprayed WC-12Co+ 6% ETFE coating
showed a lower wear resistance than the one tested under dry
condition.
Acknowledgments
The author would like to thank Senkron Metal & Ceramic Coating
Company of Turkey for performing plasma and HVOF spraying. The
author is also grateful to Dr. Ramazan Samur for his assistance during
the study.
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1874 H. Mindivan / Surface & Coatings Technology 204 (2010) 1870–1874