A. KRA^UN et al.: EFFECT OF Al2O3 NANOPARTICLES ON THE TRIBOLOGICAL PROPERTIES OF STAINLESS STEEL
607–615
EFFECT OF Al
2
O
3
NANOPARTICLES ON THE TRIBOLOGICAL
PROPERTIES OF STAINLESS STEEL
VPLIV DODAJANJA Al
2
O
3
NANODELCEV NA TRIBOLO[KE
LASTNOSTI NERJAVNEGA JEKLA
Ana Kra~un
1,2
, Franc Tehovnik
1
, Fevzi Kafexhiu
1
, Tadeja Kosec
3
, Darja Feizpour
1
,
Bojan Podgornik
1
1
Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia
2
Jo`ef Stefan International Postgraduate School, Jamova cesta 39, 1000 Ljubljana, Slovenia
3
Slovenian National Building and Civil Engineering Institute, Dimi~eva ulica 12, 1000 Ljubljana, Slovenia
Prejem rokopisa – received: 2019-06-03; sprejem za objavo – accepted for publication: 2019-07-08
doi:10.17222/mit.2019.118
Austenitic stainless-steel specimens reinforced with aluminium oxide (Al2O3) nanoparticles added in (0.5, 1.0 and 2.5) w/%
concentrations were produced by a conventional casting route. The produced composite specimens were analysed in terms of
microstructural properties and wear resistance. The pin-on-disc testing method was used to study the wear behaviour of the
composite at room temperature, including steady-state coefficient of friction, the running-in behaviour and wear resistance. A
hardened stainless-steel ball (X47Cr14) with a diameter of 10 mm and hardness of 490 HV1 was used as a loading
counter-body, sliding against an investigated disk specimen. Microstructure observations revealed that the concentration and
size of the particles have an impact on the distribution of the reinforcement within the matrix as well as on the wear behaviour.
The Al2O3 particles increased the hardness and consequently led to improved tribological properties of the composites.
Keywords: stainless steel, matrix composites, aluminium oxide, friction, wear
Z metodo konvencionalnega litja je bilo izdelano avstenitno nerjavno jeklo z dodatki nanodelcev aluminijevega oksida (Al2O3),
v koncentracijah (0.5, 1.0 in 2.5) w/%. Izdelane kompozitne vzorce smo analizirali z vidika mikrostrukture in odpornosti proti
obrabi. Pri sobni temperaturi so bili izvedeni “pin-on-disc” obrabni testi za preu~evanje obna{anja obrabe kompozita in
dolo~itve obrabne odpornosti, vklju~no z bele`enjem koeficienta trenja v za~etnem in ustaljenem re`imu. Kot protitelo smo
uporabili kaljeno nerjavno jekleno kroglico (X47Cr14) s premerom 10 mm in trdoto 490 HV1, ki je drsela po preiskovanem
vzorcu. Opazili smo, da ima koncentracija in velikost delcev vpliv tako na porazdelitev armature v matrici kot tudi na obrabno
obna{anje kompozita. Dodani Al2O3 nanodelci so pove~ali trdoto in posledi~no izbolj{ali tribolo{ke lastnosti kompozitov.
Klju~ne besede: nerjavno jeklo, kompoziti, aluminijev oksid, trenje, obraba
1 INTRODUCTION
Particulate-reinforced metal-matrix composites
(MMCs) are well known for their higher specific mo-
dulus and strength as well as for their monolithic
counterparts.
1
In recent years, the use of iron-based
alloys or steels as the matrix material for MMCs has
attracted considerable attention from researchers.
Fe-based alloys are by the far the most widely used
metallic materials because of their low cost and good
mechanical properties.
There has been significant research into austenitic
stainless-steel-based MMCs, because of the excellent
corrosion resistance and workability of the steel matrix.
However, austenitic stainless steels have poor wear
resistance due to their low hardness.
2
The ceramic
reinforcement such as borides, carbides, oxides and
nitrides are implemented into steels in order to increase
the elastic modulus and to improve the wear resistance,
creep and strength. Therefore, these composites can be
used for structural applications in the wear industry.
3
In recent years, researches concerning the influence
of the different ceramic particulates (TiB
2
, TiC, Al
2
O
3
,
Y
2
O
3
, SiC) on the tribological properties of stainless
steels were realized. Also, an improvement in the wear
resistance has been observed when intermetallic particles
were used as the reinforcement of the steel-matrix com-
posites. The authors focused mainly on studies of the
effect of reinforcing phases on the wear resistance,
physical, and mechanical properties and the microstruc-
ture of the composites.
4,5
For example, Pagounis et al.
6
investigated the abrasion wear behaviour of the steel-
matrix composites reinforced with TiC, Al
2
O
3
and Cr
3
C
2
.
The results indicate that the incorporation of ceramic
particles into austenitic steel matrices can lead to an
improvement in the abrasive wear resistance. In the case
of composites with stainless-steel matrices, Pagounis at
al. have performed a preliminary study on the three-body
abrasive behaviour of austenitic (316L) and duplex steels
reinforced with Al
2
O
3
particles.
6
They pointed out that
the addition of a low volume fraction of 10 x/% Al
2
O
3
particles significantly increases the wear resistance of
the steel matrices, without impairing the corrosion pro-
perties.
6
Materiali in tehnologije / Materials and technology 53 (2019) 4, 607–615 607
UDK 620.1:666.762.11:620.3:691.714.018.8 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(4)607(2019)
*Corresponding author's e-mail:
anakracun89@gmail.com

Different factors can affect the wear and friction
behavior of MMCs. The basic tribological factors that
can control the wear and friction behavior of MMCs can
be classified into three categories:
7
• material factors, such as type of reinforcement, their
size, shape, volume fraction and the microstructure of
the matrix,
• mechanical or contact factors, such as normal load,
sliding velocity and sliding distance,
• physical factors, such as temperature, humidity and
environmental conditions.
Two factors have a significant effect on the properties
of MMCs. They are (a) distribution of nanoparticles into
the metal matrix and (b) de-agglomeration of the nano-
particles. Researchers have demonstrated that by having
a good distribution of particles in the metal matrix and
avoiding agglomeration of the particles in the matrix, the
properties of MMCs can be significantly improved.
7
There are different routes by which MMCs may be
manufactured, and among all the liquid-stat processes
are considered to have the most potential for engineering
applications in terms of production capacity and cost
efficiency. Casting techniques are economical, easier to
apply and more convenient for large parts and mass pro-
duction with regards to other manufacturing techniques.
However, it is extremely difficult to obtain a uniform
dispersion of ceramic nanoparticles in liquid metals due
to the poor wettability and to the difference in specific
mass between the ceramic particles and the metal
matrix.
5
The aim of the present work is to produce nanopart-
icles-reinforced austenitic stainless steel and identify the
distribution of particles in the steel matrix that were
introduced through a conventional melting and casting
method. Furthermore, tribological testing was focused on
the influence of different concentrations (0.5, 1.0 and
2.5) w/%, sizes (50 and 500) nm and methods of adding
Al
2
O
3
particles on the friction and wear behaviour of the
nanoparticles-reinforced austenitic stainless steel. In
terms of methods of adding nanoparticles the focus was
on the influence of the dispersion medium CaSi (Ca-30
%, Si-70 %) on the distribution homogeneity of the
Al
2
O
3
particles.
2 EXPERIMENTAL PART
2.1 Material
Al
2
O
3
powder with a mean particle size of 500 nm
and 50 nm and austenitic stainless steel (AISI 316) with
the chemical composition given in Table 1 were used as
starting materials. The chemical analysis of the material
was performed by the means of X-Ray fluorescence
(XRF) spectrometry using the Thermo Scientific Niton
XL3t GOLDD+ XRF spectrometer.
Austenitic stainless steel has been used for this work,
mainly due to its excellent corrosion resistance and
distinctive two-phase microstructure of austenite and
ferrite. It also belongs to the most used group of stainless
steels, also when it comes to loaded parts subjected to
sliding and corrosion. They are paramagnetic, have a
face-centred cubic lattice and excel with a good com-
bination of hot and cold workability, mechanical
properties and corrosion resistance.
Table 1: Chemical composition of the base austenitic stainless steel in
mass fractions (w/%)
Elements Si Mn Cr Ni Cu Mo V C
w/% 0.33 1.24 17.4 10.1 0.36 1.29 0.08 0.02
As reinforcement particles, commercial Al
2
O
3
powder from the company US Research Nanomaterials,
Inc. with mean particle sizes of 500 nm (Figure 1a) and
50 nm (Figure 1b) have been used. The Al
2
O
3
particles
were selected due to their high chemical stability to Fe,
good wear resistance and high specific mass. Particu-
larly, it was reported that the wetting angle  between
Al
2
O
3
and molten iron alloy is less than 50°, even at high
temperatures and in many different types of atmo-
spheres.
8
A. KRA^UN et al.: EFFECT OF Al2O3 NANOPARTICLES ON THE TRIBOLOGICAL PROPERTIES OF STAINLESS STEEL
608 Materiali in tehnologije / Materials and technology 53 (2019) 4, 607–615
Figure 1: SEI of Al
2
O
3
powder with: a) a mean particle size of 500 nm
and b) a mean particle size of 50 nm

In the current work commercial CaSi was used as a
dispersing agent due to the fact that it is most commonly
used as a deoxidant element in steelmaking, and does not
cause contamination of the steel melt. In the present case
the aim of the CaSi (Ca 30 %, Si 70 %) addition was to
control the dispersion (de-agglomeration) and distri-
bution of oxide particles added to the steel melt due to
the high reactivity of CaSi.
2.2 Specimens preparation
First, a predefined quantity (20 kg) of base austenitic
stainless steel was melted in an induction furnace under
normal atmospheric conditions followed by casting in
the steel moulds and adding Al
2
O
3
nanoparticles. The
mass of each cast ingot was 2 kg. In the first set of
experiments six different batches were prepared using
two different particle sizes (500 nm and 50 nm) and three
different concentrations (0.5 w/%, 1.0 w/% and 2.5 w/%)
of Al
2
O
3
.Al
2
O
3
powder was wrapped into the aluminium
foil, placed into the mould and the molten metal poured
over it into the mould. During casting the aluminium foil
melts and dissolves in the metal.
The second set of experiments comprised four
batches where two different Al
2
O
3
particle sizes (500 nm
and 50 nm) in concentration of 1.0 w/% were used. In
two cases Al
2
O
3
powder was mixed with the dispersion
media, CaSi, in a 1:1 mass ratio and filled into a cast iron
tube. For the comparison two additional batches were
prepared where only Al
2
O
3
powder sealed into a cast iron
tube was used. The cast iron tube had a length of 400
mm, outer diameter 12 mm and wall thickness 2 mm, as
shown in Figure 2. The ends of the tube were sealed
with pliers and the molten metal was poured over the
iron tube placed into the mould before casting. The iron
tube melts and dissolves in the melt.
2.3 Metallographic characterization
The surface of the investigated material was prepared
and analysed according to Ref.
9
. The microstructural
changes and the dispersion of the ceramic particles in the
steel matrix were observed and analysed by scanning
electron microscopy (SEM). Samples for microstructure
analyses were taken from the middle portion of the cast
ingot, with each position comprising 10 randomly se-
lected specimens in order to analyse the homogeneity of
the Al
2
O
3
particle distribution. Metallographic samples
were prepared by grinding using SiC paper, followed by
polishing using 1-μm diamond suspension and analysed
to reveal the particle distribution. All the specimen
surfaces were cleaned with alcohol and dried in hot air.
2.4 Hardness and wear test
The specimens for hardness measurements and
tribological tests were taken from the center of the cast
ingot at a distance of about 15 mm away from the bottom
side. The surface of the samples was prepared according
to the technique described in Section 2.3. Vickers hard-
ness measurements of the nanoparticles-reinforced auste-
nitic stainless steel were carried out with an Instron
Tukon 2100B machine using a load of 9.807 N (HV1).
Tribological tests were carried out under dry
reciprocating sliding contact at room temperature using a
TRIBOtechnic Pin-on-Disc TRIBOtester. We selected
two sets of test conditions to determine how the material
behaves under different loads and sliding speeds. A
hardened stainless-steel ball (X47Cr14) with a diameter
of 10 mm and hardness of 490 HV1 was used as a load-
ing counter-body, sliding against an investigated disk
specimen under a constant load F
n
of1Nor3N,corres-
ponding to nominal contact pressures of 470 MPa and
675 MPa, respectively (Figure 3a). The sliding ampli-
tude a) was 5 mm, average sliding speed (v
s
) was 2 mm/s
or 20 mm/s and test duration 300 s or 3000 s, resulting in
a total sliding distance of 6 m. In order to establish the
test-to-test repeatability of the data, each friction/wear
test was repeated three times. The coefficient of friction
was measured continuously during the test and the wear
A. KRA^UN et al.: EFFECT OF Al2O3 NANOPARTICLES ON THE TRIBOLOGICAL PROPERTIES OF STAINLESS STEEL
Materiali in tehnologije / Materials and technology 53 (2019) 4, 607–615 609
Figure 3: Tribological test: a) Schematic of the ball-on-disk wear test
system and b) wear scar
Figure 2: Schematic diagram of a cast iron tube filled with nanoparti-
cles

volume determined by the use of a 3D confocal micro-
scope (Alicona Infinite Focus G4) after the completion
of the test. An example of a wear scar is shown in Figure
3b. In the wear measurements we identify the average
cross-section line of the whole wear scar, the 3D surface
profile and the 2D surface profile shown in Figure 4.For
each test, a new ball was used and both specimens
washed in high-purity acetone and dried in air prior to
testing.
3 RESULTS AND DISCUSSION
3.1 Microstructural characterization
The microstructure studies of Kra~un et al.
9
were
largely focused on the microstructure of quenched alloys
of pure austenitic stainless steel. From the LM micro-
graphs it was established that the matrix consists of a
distinctive two-phase microstructure of austenite and
 -ferrites. To show the morphology and size of the Al
2
O
3
particles clearly, the etched samples of as-cast austenitic
stainless steel and Al
2
O
3
particles were examined by
SEM, as shown in Figures 5 to 7. For a representative
analysis of the particle distribution, we took ten images
with a scanning electron microscope for each sample
taken from the bottom, middle and top portions of the
cast piece. In order to conduct the particle analysis
A. KRA^UN et al.: EFFECT OF Al2O3 NANOPARTICLES ON THE TRIBOLOGICAL PROPERTIES OF STAINLESS STEEL
610 Materiali in tehnologije / Materials and technology 53 (2019) 4, 607–615
Figure 4: Identification of: a) average cross-section line of the whole
wear scar, b) 3D surface profile, c) 2D surface profile
Figure 6: Cast microstructure of austenitic stainless steel at: a) 1,000×
magnification and b) 10,000× magnification with6%o f -ferrite
(black arrow) and Al
2
O
3
(50 nm, 1.0 w/%) nanoparticles (white
arrows)
Figure 5: Cast microstructure of austenitic stainless steel at: a) 1,000×
magnification and b) 10,000× magnification with6%of -ferrite and
Al
2
O
3
(500 nm, 1.0 w/%) ultrafine particles (white arrows), without
using the CaSi

efficiently, all the images were taken at the same mag-
nification (1000×) with similar contrast. It can be seen
clearly that the Al
2
O
3
particles are incorporated, but
non-uniformly distributed in the matrix, with pro-
nounced particle clustering being observed.
Size-dependent analyses also showed that Al
2
O
3
powder with a mean particle size of 50 nm is more
homogeneously distributed than the 500-nm powder, as
indicated by Figure 7.
Concentration-dependent analyses showed in Table 2a
that at the nanoparticle mass fractions of (0.5 and 1.0)
w/% the number of Al
2
O
3
particles per unit area (μm
2
)is
0.0087–0.0113 in the case of 50-nm Al
2
O
3
particles and
0.0069–0.0073 in the case of 500-nm Al
2
O
3
particles. At
the nanoparticle mass fractions 2.5 w/% the number of
Al
2
O
3
particles per unit area (μm
2
) is in case of 50-nm
Al
2
O
3
particles 0.0127 and 0.0189 in the case of 500-nm
Al
2
O
3
.
Table 2a: Influence of different concentrations and sizes of Al
2
O
3
particles on their distribution, the first set of experiments
Portion of the
cast ingot
Weight percent
(w/%) of Al
2O3
particles
Number of Al
2
O
3
particles
per unit area (μm
2
)
500 nm 50 nm
Middle
0.5 0.0073 0.0113
1.0 0.0069 0.0087
2.5 0.0189 0.0127
The particle distribution analyses from the second set
of experiments (Table 2b) show that the use of a dis-
persion agent reduces the influence of the particles size
(500 nm or 50 nm) on the particle distribution in the steel
matrix. It results in a difference in the particles’ con-
centration throughout the ingot being reduced from
about 0.01 particles/μm
2
to less than 0.002 particles/μm
2
.
A concentration-dependent analysis shows in Table 2b
that at the nanoparticle mass fraction 1.0 w/% the
number of Al
2
O
3
particles per unit area (μm
2
) is 0.0154
in the case of 50 nm Al
2
O
3
particles and 0.0216 in the
case of 500 nm Al
2
O
3
particles. At the nanoparticle mass
fraction 1.0 w/% mixed with CaSi the number of Al
2
O
3
particles per unit area (μm
2
) is in case of 50-nm Al
2
O
3
particles 0.0116 and 0.0093 in the case of 500-nm Al
2
O
3
.
Table 2b: Influence of different sizes of Al
2
O
3
particles and CaSi on
the distribution of Al
2
O
3
particles (1.0 w/%), the second set of experi-
ments
Portion of
the cast
ingot
Number of Al
2
O
3
particles per unit area (μm
2
)
Al2
O
3
particles
(1.0 w/%)
Al
2
O
3
particles mixed
with CaSi (1.0 w/%;
Al
2O3:CaSi = 1:1)
500 nm 50 nm 500 nm 50 nm
Middle 0.0216 0.0154 0.0093 0.0116
By using CaSi as a dispersion medium and intro-
ducing the Al
2
O
3
/CaSi mixture through a sealed iron
tube, reduced particle clustering and a more homo-
geneous distribution of reinforcement nanoparticles in
the steel matrix was obtained, as shown in Figure 7.W e
A. KRA^UN et al.: EFFECT OF Al2O3 NANOPARTICLES ON THE TRIBOLOGICAL PROPERTIES OF STAINLESS STEEL
Materiali in tehnologije / Materials and technology 53 (2019) 4, 607–615 611
Figure 7: Cast microstructure of austenitic stainless steel at: a) 1,000×
magnification and b) 10,000× magnification with6%of -ferrite and
Al
2
O
3
(50 nm, 1.0 w/%) nanoparticles (white arrows) mixed with CaSi
(1.0 w/%; Al
2
O
3
:CaSi = 1:1)
Figure 8: STEM-line profile of Al
2
O
3
ultrafine particles (500 nm,
1.0 w/%) in the cast microstructure of austenitic stainless steel,
without using the CaSi

used CaSi due to the fact that it is highly reactive and is
most commonly used as a deoxidant element in steel-
making, and the addition would not cause contamination
of the steel melt. During the calcium treatment, the
Al
2
O
3
inclusions are converted to molten calcium alumi-
nates, which are globular in shape because of the
surface-tension effect. The change in inclusion com-
position and shape is known as the inclusion morphology
control. With the addition of CaSi to liquid steel we
could control the shape, size and distribution of oxide
particles added to the steel melt.
In the context of the microstructural changes’ charac-
terization and analysis of the ceramic particles’ incor-
poration into the steel matrix Auger Electron Spectro-
scopy (AES) and Transmission Electron Microscopy
(TEM) were also employed by Kra~un et al.
9
With the
STEM line-profile analyses in Figure 8 it was shown
that the incorporation and coherent bonding of the Al
2
O
3
nanoparticles into the steel matrix was successful. No
discontinues at the particle/matrix interface, modification
of metal matrix or formation of intermetallic phases
could be observed.
3.2 Hardness measurements
Hardness results of the austenitic stainless steel rein-
forced with (0.5, 1.0, and 2.5) w/% Al
2
O
3
particles and
two different particle sizes (50 nm and 500 nm) are
summarised in Figure 9.InFigure 9a for Al
2
O
3
nano-
particles alone and in Figure 9b for Al
2
O
3
nanoparticles
mixed with CaSi. The reference austenitic stainless steel
in the as-cast condition showed a hardness of  155
HV1. Reinforcement particles’ concentration-size de-
pendent analysis shows that austenitic stainless steel
reinforced with 1.0 w/% Al
2
O
3
particles provides the
largest hardness increase, especially when using
50-nm-sized particles, followed by (0.5 and 2.5) w/%.
The austenitic stainless steel with 0.5 w/% Al
2
O
3
with a
mean particles size of 500 nm displays the hardness of
about 195 HV1, at 1.0 w/% the hardness reaches the
maximum value of over 205 HV1, while at 2.5 w/% the
hardness again decreases to  180 HV1. An even higher
maximum hardness is obtained when using smaller
Al
2
O
3
particles of 50 nm. In the case of the austenitic
stainless steel reinforced with 0.5 w/% of 50-nm-sized
Al
2
O
3
nanoparticles the hardness is similar to the larger
particles case of about 195 HV1. However, as the con-
centration of 50-nm-sized Al
2
O
3
nanoparticles was
increased to 1.0 w/% a maximum hardness of almost 220
HV1 is reached, as shown in Figure 9a. On the other
hand, a further increase in the concentration of small
nanoparticles (2.5 w/%) had the opposite effect, with the
hardness being decreased to about 170 HV1.
By mixing Al
2
O
3
nanoparticles with the CaSi disper-
sive medium, which facilitates a more homogeneous and
uniform distribution of nanoparticles in the steel matrix
an additional increase in the hardness is obtained. In the
case of the austenitic stainless steel reinforced with
1.0 w/% of 500-nm-sized Al
2
O
3
particles the hardness
increased from 170 HV1 to about 185 HV1 when adding
CaSi to Al
2
O
3
powder. Also, in the case of smaller
50-nm-sized Al
2
O
3
nanoparticles mixing nanoparticles
with the CaSi dispersive medium has similar effect. It in-
creased the hardness of the austenitic stainless steel with
1.0 w/% of 50-nm-sized Al
2
O
3
nanoparticles from
 165 HV1 to almost 180 HV1, as shown in Figure 9b.
In general, the successful incorporation of the Al
2
O
3
nanoparticles was found to increase the hardness of the
austenitic stainless steel. Furthermore, the addition of
CaSi has a significant effect on the distribution and
de-agglomeration of the particles (Figure 7), thus mo-
difying the hardness of the material. As shown in Figure
9b, for both Al
2
O
3
particle sizes the addition of CaSi
tends to further increase the austenitic stainless steel’s
hardness.
3.3 Wear testing results
Figure 10 shows the variation of the coefficient of
friction for austenitic stainless steel and investigated
reinforced composites, tested for 300 s at a load of 1N
and a sliding speed of 20 mm/s. As shown, the coeffi-
A. KRA^UN et al.: EFFECT OF Al2O3 NANOPARTICLES ON THE TRIBOLOGICAL PROPERTIES OF STAINLESS STEEL
612 Materiali in tehnologije / Materials and technology 53 (2019) 4, 607–615
Figure 9: Results of hardness measurements (HV1) for different con-
tent of Al
2
O
3
particles (w/%) and different particle sizes for: a) 1
st
set
of experiments and b) 2
nd
set of experiments with added CaSi disper-
sive media

cient of friction decreases with the addition of Al
2
O
3
nanoparticles, reducing the steady-state coefficient of
friction for austenitic stainless steel from 0.6 even down
to about 0.4. In the case of 500-nm-sized Al
2
O
3
particles
the lowest and the most stable coefficient of friction of
about 0.45 was obtained at 1.0 w/% concentration (Fig-
ure 10a), which is also true for the 50-nm-sized
nanoparticles (Figure 10b). However, in the case of
500-nm particles the addition of CaSi further improves
the friction behaviour of austenitic stainless steel, as
shown in Figure 10a. The same trends and similar
coefficient-of-friction values are observed also for a
sliding speed of 2 mm/s and a normal load of 3 N, with
the addition of Al
2
O
3
particles reducing the coefficient of
friction and the lowest and the most stable friction being
provided by 500-nm-sized Al
2
O
3
particles added in
1.0 w/% concentration and CaSi used as a dispersion
agent, as shown in Figure 11.
Although the variation in coefficient of friction
values is not large, it shows a reducing trend when the
Al
2
O
3
particles are added to the austenitic stainless steel.
Moreover, the coefficient of friction depends on the con-
tent and size of Al
2
O
3
particles, with 500-nm Al
2
O
3
particles and 1.0 w/% concentration giving the best
results, as shown in Figure 11.
A. KRA^UN et al.: EFFECT OF Al2O3 NANOPARTICLES ON THE TRIBOLOGICAL PROPERTIES OF STAINLESS STEEL
Materiali in tehnologije / Materials and technology 53 (2019) 4, 607–615 613
Figure 11: Coefficient-of-friction curves for the reference austenitic
stainless steel and investigated composites, tested at load 3 N, sliding
speeds 2 mm/s and test time 3000 s
Figure 10: Coefficient-of-friction curves for the reference austenitic
stainless steel and investigated composites, tested at load 1 N, sliding
speeds 20 mm/s and test time 300 s for: a) Al
2
O
3
particles in size
500 nm and b) Al
2
O
3
particles in size 50 nm
Figure 12: Steady-state coefficient of friction and running-in time for
tested composites in samples: a) without CaSi and b) with added CaSi

In Figure 12 the results for the steady-state coeffi-
cient of friction and running-in time required to reach
steady-state conditions are shown for different contents
of Al
2
O
3
particles (w/%) and different particle sizes. The
error bars in Figure 12 represent the scatter of three
parallel tests. As shown in Figure 12a, the best results in
the case of Al
2
O
3
nano-particles being added without any
dispersive media were obtained with 500-nm-sized Al
2
O
3
particles added in 2.5 w/% concentration. In the case of
500-nm-sized Al
2
O
3
particles the lowest steady-state
coefficient of friction was obtained and reached in the
shortest time; after about 20 s for a high sliding speed
(20 mm/s) and after 170 s for a low sliding speed
(2 mm/s).
Also in the case of Al
2
O
3
nanoparticles being mixed
with the CaSi dispersive medium (1:1 ratio), as shown in
Figure 12b, the best results were obtained in the case of
500-nm Al
2
O
3
particles, but this time for a lower concen-
trations of 1.0 w/%. For a high sliding speed (20 mm/s)
steady-state conditions were reached after about 35 s.
However, for a low sliding speed (2 mm/s) the 50-nm-
sized Al
2
O
3
nanoparticles gave a faster running-in, with
the steady-state conditions being reached after about
400 s.
Figure 13 shows the wear rate k, calculated accord-
ing to eqn. 1 for the investigated composite specimens
tested at two different loads (1 N, 3 N) and two sliding
speeds (2 mm/s, 20 mm/s).
k
W
Fs
=
⋅
N
[mm
3
/Nm] (1)
where W is the wear volume, F
N
is the normal load and
s is the sliding distance.
As shown in Figure 13, with the addition of Al
2
O
3
particles the wear rate of austenitic stainless steel has
been reduced by more than an order of magnitude. The
wear rate of the reference austenitic stainless steel has
been reduced from 2.0·10
–3
mm
3
/Nm down to
0.1–0.4·10
–3
mm
3
/Nm when adding Al
2
O
3
reinforcement
nanoparticles (Figure 13a). It is evident that the Al
2
O
3
particles’ addition is beneficial in improving the wear
resistance of stainless steel due to the high hardness and
better wear resistance of the reinforcing Al
2
O
3
ceramic
particles. Reinforcement particles concentration-size de-
pendent analysis shows that the lowest austenitic
stainless-steel wear rate is obtained when reinforced with
0.5 w/% Al
2
O
3
particles with the mean particles size of
500 nm. In the case of the 50-nm-sized Al
2
O
3
particles
the best results in terms of low wear rate are obtained
when using an Al
2
O
3
nanoparticles concentration of
2.5 w/%. However, the level of wear is very similar for
both particle sizes, especially at higher concentrations
( 1.0 w/%). On the other hand, the distribution of nano-
particles does not seem to play a role on the austenitic
stainless steel wear resistance, regardless of the particles’
size and concentration. Either using Al
2
O
3
nanoparticles
with or without the CaSi dispersive medium results in a
very similar wear rate of about 0.25·10
–3
mm
3
/Nm, as
shown in Figure 13b.
4 CONCLUSIONS
The purpose of this research was to assess the
influence of different concentrations, sizes and methods
of adding Al
2
O
3
particles on the friction and wear beha-
viour of the nanoparticles-reinforced austenitic stainless
steel. Furthermore, we focus on the methods and possibi-
lities of the uniform distribution of the particles within
the steel matrix using conventional casting routes.
Through a proper insertion method the nanoparticles can
be successfully introduced into the metal matrix and
improve the wear behaviour and hardness of the stainless
steel by using conventional casting routes. The Al
2
O
3
nanoparticles were successfully incorporated into the
steel matrix with no signs of any intermetallic reactions
taking place between the nanoparticles and the steel
matrix.
A. KRA^UN et al.: EFFECT OF Al2O3 NANOPARTICLES ON THE TRIBOLOGICAL PROPERTIES OF STAINLESS STEEL
614 Materiali in tehnologije / Materials and technology 53 (2019) 4, 607–615
Figure 13: Wear rates as a function of: a) Al
2
O
3
particles size and
concentration and b) addition of CaSi dispersive media

As a result of this work, the major new findings con-
cerning the mechanistic role of additives on the wear
behaviour of the stainless steel were:
In general, the incorporation of the Al
2
O
3
nanoparti-
cles was found to increase the hardness of the austenitic
stainless steel. Furthermore, the addition of CaSi has a
significant effect on the distribution and de-agglome-
ration of the particles, thus modifying the hardness of the
material. Reinforcement particles concentration-size
dependent analysis shows that austenitic stainless steel
reinforced with 1.0 w/% Al
2
O
3
particles provides the
highest hardness increase, especially when using
50-nm-sized particles, followed by (0.5 and 2.5) w/%.
In terms of tribological properties the addition of
Al
2
O
3
nanoparticles reduces steady-state coefficient of
friction but leads to a prolonged running-in period.
Although the variation in coefficient of friction values is
not large, it shows reducing trend when the Al
2
O
3
part-
icles are added to the austenitic stainless steel. Moreover,
the coefficient of friction depends on the content and size
of Al
2
O
3
particles, with 500-nm Al
2
O
3
particles and
1.0 w/% concentration giving the best results, which can
be further improved by obtaining a more homogeneous
distribution of nanoparticles in the matrix.
Furthermore, by the incorporation of hard and wear-
resistant Al
2
O
3
ceramic nanoparticles the wear resistance
of the austenitic stainless steel has been improved by
more than an order of magnitude. Reinforcement parti-
cles concentration-size dependent analysis shows that the
lowest wear rate of the austenitic stainless steel is
obtained when reinforced with larger Al
2
O
3
particles
(500 nm) but in a lower concentration of 0.5 w/%. In the
case of 50-nm-sized Al
2
O
3
particles a similar level of
wear rate is obtained; however, the lowest wear rates are
obtained for larger particles concentrations of 2.5 w/%.
In terms of wear resistance more uniform distribution of
nanoparticles, obtained by adding the CaSi dispersive
medium, did not show any further improvement.
Acknowledgment
This work was done in the frame of the research
programs P2-0050, which is financed by the Slovenian
Research Agency. The authors would like to thank
Miroslav Pe~ar, in`. for the help with the AES analysis
and dr. Borut @u`ek for the hardness measurements.
5 REFERENCES
1
S. C. Tjong, K. C. Lau, Sliding wear of stainless steel matrix
composite reinforced with TiB2 particles, Mater. Lett., 41 (1999)4,
153–158, 1999, doi:10.1016/S0167-577X(99)00123-8
2
Z. Ni, Y. Sun, F. Xue, J. Bai, Y. Lu, Microstructure and properties of
austenitic stainless steel reinforced with in situ TiC particulate,
Mater. Des., 32 (2011) 3, 1462–1467, doi:10.1016/j.matdes.2010.
08.047
3
S. Chen, P. Seda, M. Krugla, A. Rijkenberg, High-modulus steels
reinforced with ceramic particles through ingot casting process,
Mater. Sci. Technol., 32 (2016) 10, 992–1003, doi:10.1080/
02670836.2015.1104082
4
F. Akhtar, Ceramic reinforced high modulus steel composites: pro-
cessing, microstructure and properties, Can. Metall. Q., 53 (2014)3,
253–263, doi:10.1179/1879139514Y.0000000135
5
B. N. Chawla, Y. Shen, Mechanical behavior of particle reinforced
metal matrix composites, Adv. Eng. Mater., 3 (2001) 6, 357–370,
doi:10.1002/1527-2648(200106)3:6<357:aid-adem357> 3.3.co;2-9
6
E. Pagounis, V. K. Lindroos, M. Talvitie, Influence of matrix
structure on the abrasion wear resistance and toughness of a hot
isostatic pressed white iron matrix composite, Metall. Mater. Trans.
A, 27 (1996), 4183–4191
7
P. L. Menezes, S. P. Ingole, M. Nosonovsky, S. V. Kailas, M. R.
Lovell, Tribology for scientists and engineers: From basics to
advanced concepts, Springer, 2013
8
S.-Y. Cho, J.-H. Lee, Anisotropy of wetting of molten Fe on Al2O3
single crystal, Korean J. Mater. Res., 18 (2018) 1, 18–21,
doi:10.3740/MRSK.2008.18.1.018
9
A. Kra~un, M. Torkar, J. Burja, B. Podgornik, Microscopic charac-
terization and particle distribution in a cast steel matrix composite,
Mater. Tehnol., 50 (2016) 3, 451–454, doi:10.17222/mit. 2015.310
A. KRA^UN et al.: EFFECT OF Al2O3 NANOPARTICLES ON THE TRIBOLOGICAL PROPERTIES OF STAINLESS STEEL
Materiali in tehnologije / Materials and technology 53 (2019) 4, 607–615 615