T. F. KUBATÍK et al.: MECHANICAL PROPERTIES OF PLASMA-SPRAYED LAYERS OF ALUMINIUM AND ...
323–327
MECHANICAL PROPERTIES OF PLASMA-SPRAYED LAYERS OF
ALUMINIUM AND ALUMINIUM ALLOY ON AZ 91
MEHANSKE LASTNOSTI S PLAZMO NANE[ENIH PLASTI
ALUMINIJA IN ALUMINIJEVE ZLITINE NA AZ 91
Tomá{ Franti{ek Kubatík, Pavel Ctibor, Radek Mu{álek, Marek Janata
Institute of Plasma Physics CAS, v.v.i., Za Slovankou 1782/3, 182 00 Prague, Czech Republic
kubatik@ipp.cas.cz
Prejem rokopisa – received: 2015-10-29; sprejem za objavo – accepted for publication: 2016-02-09
doi:10.17222/mit.2015.326
Magnesium alloys are promising materials thanks to their high specific strength and stiffness. However, their wide application is
discouraged by their low resistance to wear and very low resistance to corrosion. Plasma-spraying technology makes it possible
to prepare protective metal and non-metal coatings. In this work, 450 μm thick and 490 μm thick plasma-spray coatings of
commercially pure (CP) aluminium and aluminium alloy AlCr6Fe2 were prepared on magnesium alloy AZ91. The adhesion
strength of plasma-sprayed aluminium was about 19 MPa, while the aluminium alloy showed an adhesion strength of about
12 MPa. Plasma-sprayed coatings improved the wear resistance of the AZ91 surface up to four times.
Keywords: plasma spraying of aluminum, adhesion of coating, wear, magnesium alloy AZ91
Magnezijeve zlitine so obetajo~ material, zahvaljujo~ njihovi veliki specifi~ni trdnosti in togosti. Vendar pa njihovo {iroko
uporabo zavira majhna odpornost na obrabo in zelo slaba korozijska odpornost. Tehnologija plazemskega nabrizgavanja
omogo~a izdelavo za{~itnih kovinskih in nekovinskih nanosov. V tem delu so bili pripravljeni plazemsko nabrizgani, 450 μm in
490 μm debeli nanosi komercialno ~istega (CP) aluminija in aluminijeve zlitine AlCr6Fe2 na magnezijevi zlitini AZ91.
Adhezijska trdnost plazemsko nabrizganega aluminija je bila okrog 19 MPa, aluminijeva zlitina je pokazala adhezijsko trdnost
okrog 12 MPa. Plazemsko nabrizgani nanosi so izbolj{ali obrabno odpornost zlitine AZ91 za 4×.
Klju~ne besede: plazemsko nabrizgavanje aluminija, adhezija nanosa, obraba, magnezijeva zlitina AZ91
1 INTRODUCTION
Magnesium and its alloys have an excellent ratio of
density and strength; they are suitable for lightweight
construction in the automotive and aerospace industries,
and also for bio-medical applications due to their high
specific strength and stiffness.1,2 Magnesium and magne-
sium alloys are very susceptible to galvanic corrosion,
which can result in severe pitting, especially in a wet and
salty environment. This disadvantage can be utilized in
biomedicine for endoprosthetics, which may dissolve at
a controlled speed.3 Alloying of magnesium with alu-
minium or manganese can improve not only its mecha-
nical properties but also the corrosion resistance.
Aluminium in the contents of 4–9 % by weight can
rapidly improve the corrosion resistance thanks to the
formation of the intermetallic phase Mg17Al12, which
precipitates along the grain boundaries and serves as a
corrosion barrier.4 Magnesium and its alloys can be also
protected by a deposition of surface-protective coatings.
There are several technologies for producing protective
coatings, metal and non-metal ones (in particular the
technology of plasma spraying) making it possible to
prepare anticorrosion coatings, improving the surface
abrasive resistance as well as the hardness. In refe-
rences5–8 the authors devote their studies to the plasma-
sprayed coatings of aluminium, NiAl5 and Al2O3 on the
magnesium alloy AZ91.8 Plasma spraying of pure Al on
the AZ 91 alloy provided coating adhesion strengths of
6–12 MPa, and when spraying NiAl5 on the AZ91 alloy,
the adhesion strengths of 17–25 MPa were reported.8
This work deals with the study of mechanical properties,
the adhesion strength and the resistance to wear of the
coatings made of commercially pure (CP) aluminium
and an alloy thermally stable up to 300 °C, i.e.,
AlCr6Fe29 on the magnesium alloy AZ 91 using a hybrid
water-stabilized plasma torch WSP-H 500© which
potentially provides a rather high throughput for
spraying large areas.
2 EXPERIMENTAL PART
Two powders were used as a feedstock for plasma
spraying using a hybrid water-stabilized torch WSP-H
500© (Institute of Plasma Physics CAS, v.v.i, CZ)
operated at the power of 160 kW, namely, a commer-
cially pure (CP) aluminium powder with grain sizes from
45 μm to 90 μm and aluminium alloy AlCr6Fe2 (ICT-
Prague, CZ) with grain sizes from 80 μm to 180 μm. The
aluminium alloy powder was prepared with the atomi-
zation of the melted aluminium alloy AlCr6Fe2 using
compressed Ar and the subsequent sieving to achieve the
proper fraction. During plasma spraying, the powder in
the quantity of 110 g/min was injected with compressed
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Professional article/Strokovni ~lanek MTAEC9, 51(2)323(2017)
air into the plasma jet, and the spraying distance was 330
mm. The substrates of (70 × 20 × 5) mm were
manufactured from magnesium alloy AZ91. Prior to
plasma spraying, the substrates were grit blasted with
alumina grit and degreased in acetone. The original
feedstock powder as well as both the free surface and the
metallographic cross-sections of the sprayed layers were
observed with a scanning electron microscope EVO MA
15 (Carl Zeiss SMT, D). The cylindrical specimens (the
substrate of magnesium alloy AZ91) with a diameter of
25 mm, a height of 50 mm and with an impassable
tapped screw M10 were prepared for the evaluation of
the coating adhesion strength, which was conducted in
accordance with the ASTM C 633 standard. The coating
deposition was carried out on the blasted cylinder ends
with a roughness of approximately Ra 9.5 μm, pre-
heated to 180 °C and with a target thicknesses of
approximately 500 μm. This thickness guaranteed that
the coating-adhesion test was not influenced by the glue
penetrating into the coating microstructure.10 The
counterpart, i.e., a steel cylinder of the same dimensions,
was glued with the heat-hardenable one-component
epoxy adhesive E1100S (Gupex, UK) on the plasma-
coated surface. After the heat hardening of the adhesive,
tension tests were conducted using an Instron 1362 (In-
stron, UK) electro-mechanical tensile testing machine.
The presented adhesion-strength result is the average of
six measurements.
Table 1: Average values of the adhesion strength for CP-Al and alu-
minium alloy
Tabela 1: Povpre~na vrednost adhezijske trdnosti za CP – Al in alu-
minijevo zlitino
Sample Adhesion strength
CP-aluminium coating 18.94 ± 1.29 MPa
Aluminium-alloy coating 12.15 ± 1.22 MPa
In order to obtain a compact reference sample for
abrasive-wear testing, the feedstock powders, used for
plasma spraying, were sintered by means of the spark-
plasma-sintering (SPS) technique.11 The sintering began
with evacuation of the chamber. Afterwards, the sample
was pre-loaded to 20 MPa, followed by heating with a
rate of 100 °C/min. After reaching the desired sintering
temperature of 550 °C, the sample was loaded with the
final compression pressure of 80 MPa. The dwell time at
this temperature and pressure was 5 min. After the dwell
time, the DC pulse source was turned off and the sample
was cooled down to the room temperature by free
cooling. After aeration of the chamber, the sample was
removed from the graphite tool.
The wear resistance of the samples was evaluated by
the slurry abrasion response (SAR) test following the
ASTM standard.12 The test was carried out in four
increments (runs) with the mass loss being measured at
the end of each run. The applied force was 22 N per
specimen. In the case of the studied materials, the runs
were shortened to one quarter of the standard duration
because of the relatively low thickness and resistance of
the coatings. After each run, the specimens were ultra-
sonically cleaned, dried and weighted. The slurry con-
sisted of 150 g of organic oil (the water recommended in
the ASTM standard had to be avoided because of its
reactivity with the studied materials) and 150 g of
alumina powder with a size of 40–50 μm. The accuracy
of the measurement is typically ± 5 %.12
Microhardness was measured using a Hanemann
microhardness head (Zeiss, Germany) mounted on an
optical microscope with a fixed load of 0.5 N and a
Vickers indenter. Twenty indentations made at randomly
selected spots on the cross-section of each sample were
analyzed.
3 RESULTS AND DISCUSSION
3.1 Microstructure
Figure 1 depicts the microstructures of the used
powders: a) CP aluminium and b) aluminium alloy
T. F. KUBATÍK et al.: MECHANICAL PROPERTIES OF PLASMA-SPRAYED LAYERS OF ALUMINIUM AND ...
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Figure 2: Free surface of the layer prepared with plasma spraying:
a) aluminium powder CP-Al and b) aluminium alloy (SEM – BSE)
Slika 2: Prosta povr{ina nanosa, izdelanega s plazemskim napr{e-
vanjem: a) aluminija v prahu CP-Al in b) aluminijeva zlitina (SEM –
BSE)
Figure 1: Feedstock powder: a) aluminium CP-Al, fraction 90 + 45
μm, b) aluminium alloy AlCr6Fe2, fraction 180 + 80 μm (SEM –
BSE)
Slika 1: Izhodni prah: a) aluminij CP-Al – 90 + 45 μm frakcija, b)
aluminijeva zlitina AlCr6Fe2, frakcija 180 + 80 μm (SEM – BSE)
AlCr6Fe2. It can be seen that the used aluminium
powder on Figure 1a was of an irregular shape, most
often consisting of elongated straight or twisted particles.
The aluminium alloy from Figure 1b consisted of
regular spherical particles with the mean size of about
100 micrometers. It is also visible on Figure 2a that the
surface of the sprayed coatings is mainly composed of
irregular splats and small spherical particles. In the case
of the aluminium alloy from Figure 2b the surface is
dominantly composed of irregular splats.
Figure 3 shows the cross-sectional microstructure of
the plasma coating of CP aluminium on magnesium
alloy AZ 91. The microstructure is typical for plasma-
sprayed metallic coating sprayed in an open-air atmo-
sphere, containing splats, spherical particles and pores.
The layer was prepared using plasma spraying with the
average thickness of 448±31 μm. The plasma-sprayed
layer of the aluminium alloy on magnesium alloy AZ-91
is shown in Figure 4. The layer contains large particles
and splats because almost a double grain size of the
powder was used. The prepared layer has an average
thickness of 489±26 μm.
3.2 Adhesion testing
The average value of the tensile adhesion strength of
CP Al coating is 18.94±1.29 MPa, and the one of the
aluminum alloy is 12.15±1.22 MPa. In the case of CP Al
coatings, all samples were fractured near the coating-
substrate (Figure 5) interface in combined cohesion/
adhesion failure. In the case of Al alloy coating on
AZ91, the whole layer was detached from the surface at
the substrate-coating interface in all six measured
samples (adhesion failure of Al alloy coating, cross-
section, Figure 6). The values of adhesion strength for
CP aluminium coating ranging from 17 to 20.5 MPa are
significantly higher than in the case of the strength of Al
T. F. KUBATÍK et al.: MECHANICAL PROPERTIES OF PLASMA-SPRAYED LAYERS OF ALUMINIUM AND ...
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Figure 6: Residue of aluminium alloy coating on AZ 91 after the
adhesion test (SEM – BSE)
Slika 6: Ostanki aluminijeve zlitine na AZ91 po preizkusu adhezije
(SEM – BSE)
Figure 4: Microstructure of the cross-section of the aluminium alloy
sprayed on magnesium alloy AZ 91 (SEM – BSE)
Slika 4: Mikrostruktura prereza aluminijeve zlitine nabrizgane na
magnezijevo zlitino AZ91 (SEM – BSE)
Figure 3: Microstructure of the cross-section of CP-Al sprayed on
magnesium alloy AZ 91 (SEM – BSE)
Slika 3: Mikrostruktura prereza nabrizganega CP-Al na magnezijeve
zlitine AZ91 (SEM – BSE)
Figure 5: Residue of CP-Al coating on AZ 91 after the adhesion test
(SEM – BSE)
Slika 5: Ostanki CP-Al nanosa na AZ91 po preizkusu adhezije (SEM-
BSE)
Figure 7: SAR test result, weight-loss dependence on the travel
distance
Slika 7: Rezultati SAR-preizkusa, odvisnost izgube te`e od dol`ine
poti
alloy coatings, and also higher than published by Maria
Parco et al.,8 who published values ranged from 6 MPa to
12 MPa for the substrate without pre-heating, and
adhesion strength of 15 MPa for the substrate preheated
to 160 °C and with a blasted surface.
The results of the wear-resistance measurement,
namely, the dependence of mass losses for path lengths
(576, 1152, 1728 and 2304) m, are plotted in the graph in
Figure 7. The results of the wear-resistance measure-
ment are in line with the expectations, i.e., the
SPS-sintered pure aluminium showed the highest wear
loss followed by the uncoated magnesium alloy AZ 91. It
is clearly visible in the graph that both plasma coatings
have an improved wear resistance of the substrate
material (AZ 91). The best results are achieved when
aluminium alloy is used as a coating, and the wear values
within the distance of 1152 m are similar to those of the
sintered aluminium alloy. The sintered aluminium alloy,
followed by the aluminium-alloy coating, are the most
resistant to wear for the whole path. The measured
roughness is listed in Table 2. The roughness is rather
high for the as-sprayed coatings but after 576 m of the
wear test, it is greatly reduced and not changed signifi-
cantly during the further wear testing. The compact
sintered aluminium alloy is tougher and more resistant to
abrasion and there is a similar effect of the plasma
spraying technology when used for improving the
abrasion resistance.
Table 2: Roughness Ra (μm) of the free surfaces of test samples before
and during the wear test
Tabela 2: Hrapavost Ra (μm) treh prostih povr{in na preizku{ancih,
pred in po preizkusu obrabe
Sample 0 576 m 1728 m 2304 m
uncoated AZ-91 2.3 1.9 2.0 2.4
CP Al-coated AZ-91 14.8 5.9 5.0 7.0
Al-alloy coated AZ-91 20.2 10.6 9.0 8.2
Table 3: Microhardness of plasma-sprayed layers
Tabela 3: Mikrotrdota plazemsko nabrizganih plasti
Sample Microhardness (HVm)
CP Al coating 65±11
Al-alloy coating 223±48
Substrate 120±42
Plasma-sprayed CP-Al, Figure 8a, has the surface
textured in the direction of the movement of the test
(deep vertical grooves). The plasma-sprayed aluminium
alloy, Figure 8b, has, thanks to its higher hardness
(Table 3), very homogeneous wear damage concentrated
only in small areas. The highest weight loss during the
wear test was shown on sintered pure aluminium and the
uncoated magnesium alloy AZ 91. Their surfaces are
displayed in Figures 8c and 8e. The surfaces are
scratched by the abrasive medium but the damage is very
homogeneous. The appearance of the surface of the
SPS-sintered aluminum alloy, Figure 8d, is similar.
T. F. KUBATÍK et al.: MECHANICAL PROPERTIES OF PLASMA-SPRAYED LAYERS OF ALUMINIUM AND ...
326 Materiali in tehnologije / Materials and technology 51 (2017) 2, 323–327
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Figure 9: Details of the microstructures of plasma-sprayed layers with
indenter imprint after the measurement of microhardness (SEM –
BSE): a) aluminium alloy, b) CP-aluminium
Slika 9: Detajl mikrostrukture s plazmo nabrizganega sloja, z odtis-
kom trdote po merjenju mikrotrdote (SEM – BSE): a) aluminijeva
zlitina, b) CP-aluminium
Figure 8: Free surfaces of the samples after the measurement of
abrasion resistance: a) plasma-sprayed CP-Al, b) plasma-sprayed
aluminium alloy, c) SPS-sintered CP-Al, d) SPS-sintered aluminium
alloy, e) uncoated AZ-91 (SEM – SE)
Slika 8: Prosta povr{ina vzorcev po merjenju odpornosti na abrazijo:
a) plazemsko nabrizgan CP-Al, b) plazemsko nabrizgana aluminijeva
zlitina, c) SPS sintran CP-al, d9 SPS sintrana aluminijeva zlitina, e)
AZ-91 brez nanosa (SEM – SE)
3.3 Hardness
The hardness-measurement results are listed in Table 3
for the plasma-sprayed layers and the substrate. The
measured microhardness values are comparable with the
microhardness of the Al coatings deposited with the
cold-spray technique.13 The average microhardness of
the plasma-sprayed aluminium alloy (223 HVm) is
nearly four times higher than that of the sprayed CP
aluminium (65 HVm). In reference9 dealing with the
thermal stability of a similar alloy, a hardness of
160–195 HV is listed. Figure 9 shows micrographs of
the indents. In both cases, the coatings are very plastic,
without any cracks (in the case of a small load) initiating
from the indent corners and also without any distortion
of the adjacent pores.
4 CONCLUSION
In this work, plasma-sprayed coatings of commer-
cially pure (CP) aluminium and aluminium alloy
AlCr6Fe2 were applied on the magnesium alloy AZ 91
by means of a hybrid water-stabilized torch WSP-H
500©. Layers with a thickness of 450 μm for CP alumi-
nium and 490 μm for the aluminium alloy were prepared.
Both obtained plasma-sprayed coatings are relatively
inhomogeneous and porous. The micrographs (Figures 3
and 4) show incomplete metallurgical bonds between
individual powder particles and molten droplets (splats),
respectively. The prepared plasma coatings have a rela-
tively good adhesion, microhardness and abrasion resis-
tance in comparison with the relevant literature. Plasma
coating of CP aluminium showed the average adhesion
strength of 19 MPa, and the aluminium alloy showed an
adhesive strength of 12 MPa. During the pull-off test,
combined adhesive-cohesive fracture of the CP-alumi-
num coating occurred. In the case of the aluminium-alloy
coating, the whole layer was pulled off, leading to the
adhesion mode of failure. The results also clearly show
that the abrasion resistance of the plasma-sprayed
CP-aluminum coating is better than that of the sintered
CP aluminum. In case of the aluminium alloy, the
plasma-sprayed coating and the sintered material have a
similar abrasion resistance.
Acknowledgements
This work was supported by the Czech Science
Foundation through the project with number 14-31538P
"Evaluation of bonding interface during and after plasma
spraying of metallic materials on magnesium and mag-
nesium alloy."
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