A. KRA^UN et al.: DISTRIBUTION OF Al2O3 REINFORCEMENT PARTICLES IN AUSTENITIC STAINLESS STEEL ...
973–980
DISTRIBUTION OF Al2O3 REINFORCEMENT PARTICLES IN
AUSTENITIC STAINLESS STEEL DEPENDING ON THEIR SIZE
AND CONCENTRATION
PORAZDELITEV DELCEV Al2O3 V AVSTENITNEM NERJAVNEM
JEKLU V ODVISNOSTI OD VELIKOSTI IN KONCENTRACIJE
Ana Kra~un1,2, Bojan Podgornik1, Franc Tehovnik1, Fevzi Kafexhiu1,
Darja Jenko1
1Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia
2Jo`ef Stefan International Postgraduate School, Jamova cesta 39, 1000 Ljubljana, Slovenia
ana.kracun@imt.si
Prejem rokopisa – received: 2017-04-13; sprejem za objavo – accepted for publication: 2017-09-22
doi:10.17222/mit.2017.042
Achieving a uniform distribution of reinforcement particles within a matrix is one of the challenges that impacts directly on the
properties and quality of a composite material. Therefore, the aim of the present work was to investigate the influence of the
reinforcing Al2O3 particles’ concentration and size on their distribution in reinforced austenite stainless steel. Austenitic
stainless steel reinforced with (0.5, 1.0 and 2.5) % of mass fractions of Al2O3 particles was produced by a conventional casting
route. In this study, an innovative pre-dispersion approach for the addition of particles into a steel melt was designed. The results
of this investigation indicate that the concentration and size of the Al2O3 particles has an impact on the distribution of the
reinforcement within the matrix. When the weight percent increased to 2.5 the concentration ratio of the particles’ distribution
decreases towards the bottom of the cast ingot. In this case also the size of particles starts to play a role, with the larger particle
size leading to an increased degree of incorporating particles into the steel matrix. The larger the particles the more particles are
found in the cast ingot.
Keywords: metal matrix composite, reinforced particles, distribution, conventional casting method
Doseganje enakomerne porazdelitve delcev v matrici je eden izmed izzivov, ki direktno vplivajo na lastnosti in kvaliteto
kompozitnih materialov. Namen raziskave je bil ugotoviti porazdelitev delcev v mikrostrukturi jekla glede na dodano
koncentracijo in velikost. Med procesom konvencionalnega litja avstenitnega nerjavnega jekla so bili dodani delci Al2O3 v
ute`nih procentih (0,5, 1,0 in 2,5). V tej {tudiji smo uporabili inovativni pristop dispergiranja delcev pred dodajanjem v jekleno
talino. Rezultati raziskave ka`ejo, da je koncentracija in velikost Al2O3 delcev vplivala na porazdelitev delcev v matrici. S
pove~anjem ute`nega procenta dodanih delcev na 2,5 se je koncentracijsko razmerje delcev zmanj{alo proti dnu ulitega ingota.
V tem primeru ima pomemben vpliv na porazdelitev tudi velikost delcev, ve~ji kot so delci ve~ jih je vklju~enih v matrico. Ve~ji
kot so delci, ve~ se jih nahaja v ulitem ingotu.
Klju~ne besede: kovinski kompoziti, utrjevalni delci, porazdelitev, metoda konvencionalnega litja
1 INTRODUCTION
The application of metal-matrix composites (MMCs)
as structural engineering materials has received in-
creasing attention in recent years.1–3 Ceramic particulates
such as borides, carbides, oxides and nitrides are added
to MMCs to improve their elastic modulus, wear
resistance, creep and strength.4–5
There are different routes by which MMCs can 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 regard 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
gravity between the ceramic particles and the metal
matrix.6–8
The current work aims at contributing to the knowl-
edge and understanding of the conventional casting route
and its possibility for ultrafine particle inoculation in a
steel matrix. This production route seems to show
potential and offers more cost efficiency in achieving the
dispersion of second-phase ultrafine range particles
compared to the typically used powder and metallurgical
techniques used until now. The aim of the present work
is to identify the distribution of particles in the steel
matrix that were introduced through a conventional
melting and casting method, and above all to determine
the influence of the different concentrations, sizes and
methods of adding Al2O3 particles on the distribution of
the latter in reinforced austenite stainless steel. In terms
of the methods the focus was on the influence of
dispersion medium CaSi (Ca-30 %, Si-70 %) on the
distribution homogeneity of the Al2O3 particles.
Materiali in tehnologije / Materials and technology 51 (2017) 6, 973–980 973
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 620.17:669.15-194.56:621.74 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(6)973(2017)
2 EXPERIMENTAL PART
2.1 Material
Austenitic stainless steel has been used for the work,
mainly due to its distinctive two-phase microstructure of
austenite and ferrite. It also belongs to the most used
group of stainless steels. They are paramagnetic, have a
face-centred cubic lattice and excel with a good combi-
nation of hot and cold workability, mechanical properties
and corrosion resistance. The chemical composition of
the base alloy is given in Table 1.
Table 1: Chemical composition of austenitic stainless steel in weight
percent (w/%)
Elements w/%
Si 0.33
Mn 1.24
Cr 17.4
Ni 10.1
Cu 0.36
Mo 1.29
V 0.08
C 0.02
As reinforcement particles, commercial Al2O3
powder from the company US Research Nanomaterials,
Inc. with a mean particle size of 500 nm (Figure 1) and
50 nm (Figure 2) was used. The Al2O3 particles were
selected due to their high chemical stability with respect
to Fe and high specific gravity. Particularly, it was
reported that the wetting angle  between Al2O3 and
molten iron alloy is less than 50°, even at high tempera-
tures and in many different types of atmospheres.9
As a dispersion medium CaSi (Ca-30 %, Si-70 %)
was used. Ca additions are made during steel making for
refining, deoxidation, desulphurization, and control of
the shape, size and distribution of oxide and sulphide in-
clusions. However, elemental Ca is difficult and dange-
rous to add to liquid steel because it has a high reacti-
vity.10 Therefore, Ca in the stabilized forms of calcium
silicon (CaSi), calcium manganese silicon (CaMnSi),
calcium silicon barium (CaSiBa) and calcium silicon
barium aluminium (CaSiBaAl) alloys or as calcium
carbide (CaC2) is used. In the present work CaSi was
used due to the fact that it is most commonly used as
deoxidant element in steelmaking, and does not cause
contamination of the steel melt. The aim of the CaSi
addition was to control the shape, size and distribution of
oxide particles added in the steel melt.
2.2 Specimens preparation
First a weighed quantity (14 kg) of austenitic stain-
less steel was melted in the open induction furnace (TYP
SF 70 sl) with a generator under normal atmospheric
conditions. The maximum temperature of the melt pro-
duction is up to 1750 °C. 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, 1.0 and 2.5) % of mass fractions of
Al2O3, as shown in Table 2. The Al2O3 particles powder
was wrapped into the aluminium foil, placed into the
mould and then the molten metal was poured over it into
the mould. During casting the aluminium foil melts and
dissolves in the metal.
Table 2: Addition scheme for the first set of experiments – Al2O3
particles size and concentration used.
Al2O3 particle size
500 nm 50 nm
Concentrations
(w/%)
0.5 0.5
1.0 1.0
2.5 2.5
The second set of experiments comprised four
batches where two different Al2O3 particle sizes (500 nm
and 50 nm) in weight percent of 1.0 were used. In two
cases the Al2O3 powder was mixed with the same amount
of dispersion media – CaSi (Ca 30 %, Si 70 %) and filled
into a cast-iron tube. For comparison two additional
batches were prepared where only Al2O3 powder sealed
into a cast iron tube was used, Table 3. The cast-iron
A. KRA^UN et al.: DISTRIBUTION OF Al2O3 REINFORCEMENT PARTICLES IN AUSTENITIC STAINLESS STEEL ...
974 Materiali in tehnologije / Materials and technology 51 (2017) 6, 973–980
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: SEI of Al2O3 nanopowder with a mean particle size of
50 nm
Figure 1: SEI of ultrafine Al2O3 powder with a mean particle size of
500 nm
tube had a length of 400 mm, outer diameter 12 mm and
wall thickness 2 mm, as shown in Figure 3. The ends of
the tube were sealed with pliers and the molten metal
was poured over the iron tube into the mould. The iron
tube melts and dissolves in the melt.
Table 3: Addition scheme for the second set of experiments – Al2O3
particles mixed with CaSi and dispersion medium.
Al2O3 particle size
500 nm (1.0 %) 50 nm (1.0 %)
Dispersion medium
(CaSi) (w/%)
1 1
0 0
2.3 Characterization
The microstructural changes and the dispersion of the
ceramic particles in the steel matrix were observed and
analysed by light microscopy (LM), scanning electron
microscopy (SEM), Auger electron spectroscopy (AES)
and transmission electron microscopy (TEM). Samples
for the microstructure analysis were taken at the bottom,
middle and top portions of the cast ingot, Figure 4.
Metallographic samples were prepared by grinding and
polishing, followed by chemical etching and analysed to
reveal the particle distribution. In the case of AES
ground and polished specimens were ion sputtered and
analysed in terms of the elemental composition in the
surface region. Samples for TEM were prepared by
slicing the specimens into thin 0.5–1.0-mm-thick plates
with a length of up to 3 mm. After polishing to a thick-
ness of 100±10 μm, final milling of the specimen was
carried out with an ion slicer.
For a representative analysis of the particles’ distri-
bution, three specimens located at different positions
within the ingot diameter were prepared for each posi-
tion (top-G, middle-S, bottom-N) and ten images taken
with a scanning electron microscope for each sample and
position. In order to do particle analysis efficiently, all
images were taken at the same magnification (1000×)
with similar contrast. Then ImageJ commercial software
was used to calculate and determine the particles distri-
bution and their volume fraction.
3 RESULTS AND DISCUSSION
3.1 Light microscopy (LM)
Figure 5 shows a LM image of the microstructure of
pure austenitic stainless steel with a distinctive two-
phase microstructure of austenite and -ferrites obtained.
LM micrographs of the microstructure with included
ultrafine Al2O3 particles; produced by the casting process
where the melted austenitic stainless steel was poured
over the Al2O3 particles wrapped into the Al foil is
shown in Figure 6 and 7.
It is clear that the distribution of Al2O3 particles with
a mean particle size of 500 nm is non-homogeneous and
concentrated in several isolated areas (Figure 6). How-
ever, the distribution of Al2O3 particles with a mean
particle size of 50 nm, shown in Figure 7, is more homo-
geneous with reduced clustering of the particles, as
compared to the 500-nm particles size case.
From the light micrographs in Figures 8 and 9 we
can see that the distribution of ultrafine Al2O3 particles
from the second set of experiments, Table 3. Distribution
A. KRA^UN et al.: DISTRIBUTION OF Al2O3 REINFORCEMENT PARTICLES IN AUSTENITIC STAINLESS STEEL ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 973–980 975
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 5: Cast microstructure of austenitic stainless steel with 6 % of
-ferrite
Figure 3: Schematic diagram of a cast-iron tube, filled with nano-
particles
Figure 4: Schematic diagram of subtraction and preparation of
metallographic steel samples
2 EXPERIMENTAL PART
2.1 Material
Austenitic stainless steel has been used for the work,
mainly due to its distinctive two-phase microstructure of
austenite and ferrite. It also belongs to the most used
group of stainless steels. They are paramagnetic, have a
face-centred cubic lattice and excel with a good combi-
nation of hot and cold workability, mechanical properties
and corrosion resistance. The chemical composition of
the base alloy is given in Table 1.
Table 1: Chemical composition of austenitic stainless steel in weight
percent (w/%)
Elements w/%
Si 0.33
Mn 1.24
Cr 17.4
Ni 10.1
Cu 0.36
Mo 1.29
V 0.08
C 0.02
As reinforcement particles, commercial Al2O3
powder from the company US Research Nanomaterials,
Inc. with a mean particle size of 500 nm (Figure 1) and
50 nm (Figure 2) was used. The Al2O3 particles were
selected due to their high chemical stability with respect
to Fe and high specific gravity. Particularly, it was
reported that the wetting angle  between Al2O3 and
molten iron alloy is less than 50°, even at high tempera-
tures and in many different types of atmospheres.9
As a dispersion medium CaSi (Ca-30 %, Si-70 %)
was used. Ca additions are made during steel making for
refining, deoxidation, desulphurization, and control of
the shape, size and distribution of oxide and sulphide in-
clusions. However, elemental Ca is difficult and dange-
rous to add to liquid steel because it has a high reacti-
vity.10 Therefore, Ca in the stabilized forms of calcium
silicon (CaSi), calcium manganese silicon (CaMnSi),
calcium silicon barium (CaSiBa) and calcium silicon
barium aluminium (CaSiBaAl) alloys or as calcium
carbide (CaC2) is used. In the present work CaSi was
used due to the fact that it is most commonly used as
deoxidant element in steelmaking, and does not cause
contamination of the steel melt. The aim of the CaSi
addition was to control the shape, size and distribution of
oxide particles added in the steel melt.
2.2 Specimens preparation
First a weighed quantity (14 kg) of austenitic stain-
less steel was melted in the open induction furnace (TYP
SF 70 sl) with a generator under normal atmospheric
conditions. The maximum temperature of the melt pro-
duction is up to 1750 °C. 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, 1.0 and 2.5) % of mass fractions of
Al2O3, as shown in Table 2. The Al2O3 particles powder
was wrapped into the aluminium foil, placed into the
mould and then the molten metal was poured over it into
the mould. During casting the aluminium foil melts and
dissolves in the metal.
Table 2: Addition scheme for the first set of experiments – Al2O3
particles size and concentration used.
Al2O3 particle size
500 nm 50 nm
Concentrations
(w/%)
0.5 0.5
1.0 1.0
2.5 2.5
The second set of experiments comprised four
batches where two different Al2O3 particle sizes (500 nm
and 50 nm) in weight percent of 1.0 were used. In two
cases the Al2O3 powder was mixed with the same amount
of dispersion media – CaSi (Ca 30 %, Si 70 %) and filled
into a cast-iron tube. For comparison two additional
batches were prepared where only Al2O3 powder sealed
into a cast iron tube was used, Table 3. The cast-iron
A. KRA^UN et al.: DISTRIBUTION OF Al2O3 REINFORCEMENT PARTICLES IN AUSTENITIC STAINLESS STEEL ...
974 Materiali in tehnologije / Materials and technology 51 (2017) 6, 973–980
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: SEI of Al2O3 nanopowder with a mean particle size of
50 nm
Figure 1: SEI of ultrafine Al2O3 powder with a mean particle size of
500 nm
tube had a length of 400 mm, outer diameter 12 mm and
wall thickness 2 mm, as shown in Figure 3. The ends of
the tube were sealed with pliers and the molten metal
was poured over the iron tube into the mould. The iron
tube melts and dissolves in the melt.
Table 3: Addition scheme for the second set of experiments – Al2O3
particles mixed with CaSi and dispersion medium.
Al2O3 particle size
500 nm (1.0 %) 50 nm (1.0 %)
Dispersion medium
(CaSi) (w/%)
1 1
0 0
2.3 Characterization
The microstructural changes and the dispersion of the
ceramic particles in the steel matrix were observed and
analysed by light microscopy (LM), scanning electron
microscopy (SEM), Auger electron spectroscopy (AES)
and transmission electron microscopy (TEM). Samples
for the microstructure analysis were taken at the bottom,
middle and top portions of the cast ingot, Figure 4.
Metallographic samples were prepared by grinding and
polishing, followed by chemical etching and analysed to
reveal the particle distribution. In the case of AES
ground and polished specimens were ion sputtered and
analysed in terms of the elemental composition in the
surface region. Samples for TEM were prepared by
slicing the specimens into thin 0.5–1.0-mm-thick plates
with a length of up to 3 mm. After polishing to a thick-
ness of 100±10 μm, final milling of the specimen was
carried out with an ion slicer.
For a representative analysis of the particles’ distri-
bution, three specimens located at different positions
within the ingot diameter were prepared for each posi-
tion (top-G, middle-S, bottom-N) and ten images taken
with a scanning electron microscope for each sample and
position. In order to do particle analysis efficiently, all
images were taken at the same magnification (1000×)
with similar contrast. Then ImageJ commercial software
was used to calculate and determine the particles distri-
bution and their volume fraction.
3 RESULTS AND DISCUSSION
3.1 Light microscopy (LM)
Figure 5 shows a LM image of the microstructure of
pure austenitic stainless steel with a distinctive two-
phase microstructure of austenite and -ferrites obtained.
LM micrographs of the microstructure with included
ultrafine Al2O3 particles; produced by the casting process
where the melted austenitic stainless steel was poured
over the Al2O3 particles wrapped into the Al foil is
shown in Figure 6 and 7.
It is clear that the distribution of Al2O3 particles with
a mean particle size of 500 nm is non-homogeneous and
concentrated in several isolated areas (Figure 6). How-
ever, the distribution of Al2O3 particles with a mean
particle size of 50 nm, shown in Figure 7, is more homo-
geneous with reduced clustering of the particles, as
compared to the 500-nm particles size case.
From the light micrographs in Figures 8 and 9 we
can see that the distribution of ultrafine Al2O3 particles
from the second set of experiments, Table 3. Distribution
A. KRA^UN et al.: DISTRIBUTION OF Al2O3 REINFORCEMENT PARTICLES IN AUSTENITIC STAINLESS STEEL ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 973–980 975
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 5: Cast microstructure of austenitic stainless steel with 6 % of
-ferrite
Figure 3: Schematic diagram of a cast-iron tube, filled with nano-
particles
Figure 4: Schematic diagram of subtraction and preparation of
metallographic steel samples
of Al2O3 particles (white arrows) is more homogeneous
and not concentrated in certain areas. Furthermore, clus-
tering of the particles is smaller than in the first set of
experiments without using the CaSi.
As shown in Figures 10 and 11, the microstructure of
austenitic stainless steel and the distribution of the in-
corporated nanosized particles is further modified when
using Al2O3 particles mixed with CaSi dispersion media,
from the second set of experiments, Table 3. In this case
the distribution of Al2O3 particles (white arrows) be-
comes much more homogeneous and almost equally
distributed within the metal matrix. Furthermore, the
A. KRA^UN et al.: DISTRIBUTION OF Al2O3 REINFORCEMENT PARTICLES IN AUSTENITIC STAINLESS STEEL ...
976 Materiali in tehnologije / Materials and technology 51 (2017) 6, 973–980
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 11: Cast microstructure of austenitic stainless steel with 6 %
of -ferrite and Al2O3 nanoparticles (white arrows); nanoparticles
powder (50 nm, 1.0 %) mixed with CaSi (Ca-30 %, Si-70 %)
Figure 8: Cast microstructure of austenitic stainless steel with 6 % of
-ferrite and Al2O3 (500 nm, 1.0 %) ultrafine particles (white arrows)
Figure 7: Cast microstructure of austenitic stainless steel with 6 % of
-ferrite and Al2O3 (50 nm, 1.0 %) nanoparticles (white arrows)
Figure 6: Cast microstructure of austenitic stainless steel with 6 % of
-ferrite and Al2O3 (500 nm, 1.0 %) ultrafine particles (white arrows)
Figure 10: Cast microstructure of austenitic stainless steel with 6 %
of -ferrite and Al2O3 (50 nm, 1.0 %) nanoparticles (white arrows)
Figure 9: Cast microstructure of austenitic stainless steel with 6 % of
-ferrite and Al2O3 particles (white arrows); ultrafine powder (500
nm, 1.0 %) mixed with CaSi (Ca-30 %, Si-70 %)
clustering of the particles reduced as compared to the
first set of experiments, without using the CaSi.
A size-dependent analysis show that Al2O3 powder
with a mean particle size 50 nm is more homogeneously
distributed than the 500 nm powder. In the cases when
we added the CaSi to Al2O3 the distribution of particles
was more homogeneous in all three sampling areas
(top-G, middle-S, bottom-N) of the cast ingot; from the
top to the bottom of the cast ingot.
3.2 Scanning electron microscopy (SEM)
After the metallographic examination the specimens
were subjected to SEM analysis in order to confirm the
incorporation/presence of Al2O3 ultrafine particles in the
microstructure and to analyse the particles clustering.
From the SEM/EDS elemental mapping analysis, shown
in the Figure 12, it was confirmed that the bright, small,
spot-like features represent the Al2O3 ultrafine particles,
A. KRA^UN et al.: DISTRIBUTION OF Al2O3 REINFORCEMENT PARTICLES IN AUSTENITIC STAINLESS STEEL ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 973–980 977
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 12: SEM/EDS elemental mapping of Al2O3 ultrafine particles
(500 nm, 1.0 nm %) in the cast microstructure of austenitic stainless
steel, from the first set of experiments, without using the CaSi
Figure 14: Cast microstructure of austenitic stainless steel with 6% of
-ferrite and Al2O3 particles (white arrows); ultrafine powder (500
nm, 1.0 %) mixed with CaSi (Ca-30%, Si-70%)
Figure 13: Cast microstructure of austenitic stainless steel with 6 %
of -ferrite and Al2O3 (500 nm, 1.0 %) ultrafine particles (white
arrows), without using the CaSi
Figure 15: AES spectrum of the Al2O3 ultrafine particles (500 nm,
1.0 %) in the cast microstructure of austenitic stainless steel, without
using the CaSi
of Al2O3 particles (white arrows) is more homogeneous
and not concentrated in certain areas. Furthermore, clus-
tering of the particles is smaller than in the first set of
experiments without using the CaSi.
As shown in Figures 10 and 11, the microstructure of
austenitic stainless steel and the distribution of the in-
corporated nanosized particles is further modified when
using Al2O3 particles mixed with CaSi dispersion media,
from the second set of experiments, Table 3. In this case
the distribution of Al2O3 particles (white arrows) be-
comes much more homogeneous and almost equally
distributed within the metal matrix. Furthermore, the
A. KRA^UN et al.: DISTRIBUTION OF Al2O3 REINFORCEMENT PARTICLES IN AUSTENITIC STAINLESS STEEL ...
976 Materiali in tehnologije / Materials and technology 51 (2017) 6, 973–980
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 11: Cast microstructure of austenitic stainless steel with 6 %
of -ferrite and Al2O3 nanoparticles (white arrows); nanoparticles
powder (50 nm, 1.0 %) mixed with CaSi (Ca-30 %, Si-70 %)
Figure 8: Cast microstructure of austenitic stainless steel with 6 % of
-ferrite and Al2O3 (500 nm, 1.0 %) ultrafine particles (white arrows)
Figure 7: Cast microstructure of austenitic stainless steel with 6 % of
-ferrite and Al2O3 (50 nm, 1.0 %) nanoparticles (white arrows)
Figure 6: Cast microstructure of austenitic stainless steel with 6 % of
-ferrite and Al2O3 (500 nm, 1.0 %) ultrafine particles (white arrows)
Figure 10: Cast microstructure of austenitic stainless steel with 6 %
of -ferrite and Al2O3 (50 nm, 1.0 %) nanoparticles (white arrows)
Figure 9: Cast microstructure of austenitic stainless steel with 6 % of
-ferrite and Al2O3 particles (white arrows); ultrafine powder (500
nm, 1.0 %) mixed with CaSi (Ca-30 %, Si-70 %)
clustering of the particles reduced as compared to the
first set of experiments, without using the CaSi.
A size-dependent analysis show that Al2O3 powder
with a mean particle size 50 nm is more homogeneously
distributed than the 500 nm powder. In the cases when
we added the CaSi to Al2O3 the distribution of particles
was more homogeneous in all three sampling areas
(top-G, middle-S, bottom-N) of the cast ingot; from the
top to the bottom of the cast ingot.
3.2 Scanning electron microscopy (SEM)
After the metallographic examination the specimens
were subjected to SEM analysis in order to confirm the
incorporation/presence of Al2O3 ultrafine particles in the
microstructure and to analyse the particles clustering.
From the SEM/EDS elemental mapping analysis, shown
in the Figure 12, it was confirmed that the bright, small,
spot-like features represent the Al2O3 ultrafine particles,
A. KRA^UN et al.: DISTRIBUTION OF Al2O3 REINFORCEMENT PARTICLES IN AUSTENITIC STAINLESS STEEL ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 973–980 977
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 12: SEM/EDS elemental mapping of Al2O3 ultrafine particles
(500 nm, 1.0 nm %) in the cast microstructure of austenitic stainless
steel, from the first set of experiments, without using the CaSi
Figure 14: Cast microstructure of austenitic stainless steel with 6% of
-ferrite and Al2O3 particles (white arrows); ultrafine powder (500
nm, 1.0 %) mixed with CaSi (Ca-30%, Si-70%)
Figure 13: Cast microstructure of austenitic stainless steel with 6 %
of -ferrite and Al2O3 (500 nm, 1.0 %) ultrafine particles (white
arrows), without using the CaSi
Figure 15: AES spectrum of the Al2O3 ultrafine particles (500 nm,
1.0 %) in the cast microstructure of austenitic stainless steel, without
using the CaSi
which are incorporated but non-uniformly distributed in
the steel matrix. Furthermore, a high degree of particles
clustering was observed.
Concentration-dependent analysis shows that at the
0.5 % to 1.0 % of mass fractions of the distribution of
particles is more homogeneous throughout the cast ingot
than in the case of 2.5 % of mass fractions. Size-depen-
dent analysis show that Al2O3 powder with a mean
particle size 50 nm is more homogeneously distributed
than 500 nm. The addition of CaSi plays an important
role in case of distribution and clustering of A2O3
particles, as shown in Figures 13 and 14 where Al2O3
particles were mixed with CaSi dispersion media, from
the second set of experiments, Table 3.
From Figures 13 and 14 we can see that distribution
of Al2O3 particles (white arrows) becomes much more
homogeneous and quite equally distributed within the
metal matrix when using CaSi dispersion media.
A. KRA^UN et al.: DISTRIBUTION OF Al2O3 REINFORCEMENT PARTICLES IN AUSTENITIC STAINLESS STEEL ...
978 Materiali in tehnologije / Materials and technology 51 (2017) 6, 973–980
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Figure 18: STEM – line profile of Al2O3 ultrafine particles (500 nm,
1.0 %) in the cast microstructure of austenitic stainless steel, without
using the CaSi
Figure 16: AES spectrum of the Al2O3 ultrafine particles (500 nm,
1.0 %) mixed with CaSi (Ca-30%, Si-70%) in the cast microstructure
of austenitic stainless steel
Figure 17: STEM/EDS elemental mapping of Al2O3 ultrafine parti-
cles (500 nm, 1.0 %) in the cast microstructure of austenitic stainless
steel, without using the CaSi
3.3 Auger electron spectroscopy (AES)
In the next step a surface analysis of the sample using
Auger electron spectroscopy technique was performed.
In Figure 15 the AES spectrum of the Al2O3 ultrafine
particles (500 nm, 1.0 %) in the cast microstructure of
austenitic stainless steel is shown. The AES spectra of
particles (P1 and P2) showing only O and Al peaks and
line scans over the particle/matrix interface confirm the
successful introduction of Al2O3 particles into the steel
matrix (P3) without any intermetallic reactions taking
place, which is true for all the Al2O3 particle sizes and
concentrations used. In Figure 16 the AES spectrum of
the Al2O3 ultrafine particles (500 nm, 1.0 %) mixed with
CaSi (Ca-30%, Si-70%) in the cast microstructure of the
austenitic stainless steel is shown. The AES spectrum of
particles (P1) shows only O and Al peaks. Also in cases
when CaSi was added to Al2O3 ultrafine and
nano-particles no intermetallic reactions or presence of
Ca and Si were observed. However, as already men-
tioned the distribution of particles was more homoge-
neous.
3.4 Transmission electron microscopy (TEM)
In the context of the microstructural changes a cha-
racterization and analysis of the ceramic particles incor-
poration in the steel matrix transmission electron mi-
croscopy (TEM) was also employed. With STEM/EDS
elemental mapping shown in the Figure 17 and STEM –
line profile analysis shown in the Figure 18 successful
incorporation and coherent bonding of the Al2O3 nano-
particles in the steel matrix was confirmed. No discon-
tinuities at the particle/matrix interface, modification of
metal matrix or formation of intermetallic phases could
be observed.
3.5 Particles distribution
The particle distribution analysis performed on the
specimens from the first set of experiments revealed that
Al2O3 particles of 0.5 % to 1.0 % of mass fractions,
regardless of the mean particle size of 500 nm or 50 nm,
results in a relatively homogeneous distribution of Al2O3
particles throughout the volume of the cast ingot. For
low particle weight percents of 0.5 % and 1.0 % the
distribution of Al2O3 particles in the cast ingot was found
to be more or less independent on the position, concen-
tration and size of particles, resulting in about 0.01
particles/μm2. However, as the weight percent increased
to 2.5 the particle concentration ratio starts to decrease
toward the bottom of the cast ingot (Figure 19). Further-
more, with the increased particle mass fraction, also their
concentration in cast ingot increased by up to 3 times,
with the size of particles starting to play a role. The
larger the particles the larger is the volume fraction of
the incorporated particles.
The particles distribution analysis from the second
set of experiments (Figure 20) shows that the use of dis-
persion agent reduces the influence of particles size (500
nm or 50 nm) on the particles’ distribution in the steel
matrix. It results in a difference in the particles’
concentration throughout the ingot being reduced from
about 0.01 particles/μm2 to less than 0.002 particles/μm2.
5 CONCLUSIONS
The results of our investigation indicate that steel
reinforced with ceramic ultrafine/nanosized particles can
be produced by using a conventional casting route. Ultra-
fine particles and nanoparticles where successfully incor-
porated into the steel matrix, being confirmed by diffe-
rent analysing techniques, including light microscopy
(LM), scanning electron microscopy (SEM), Auger elec-
tron spectroscopy (AES) techniques and transmission
electron microscopy (TEM).
By using CaSi as a dispersion media and introducing
Al2O3/CaSi mixture through a sealed iron tube reduced
A. KRA^UN et al.: DISTRIBUTION OF Al2O3 REINFORCEMENT PARTICLES IN AUSTENITIC STAINLESS STEEL ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 973–980 979
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 20: Influence of different sizes of Al2O3 particles and CaSi on
distribution of Al2O3 particles (1 %)
Figure 19: Influence of different concentrations and size of Al2O3
particles on their distribution in the cast ingot
which are incorporated but non-uniformly distributed in
the steel matrix. Furthermore, a high degree of particles
clustering was observed.
Concentration-dependent analysis shows that at the
0.5 % to 1.0 % of mass fractions of the distribution of
particles is more homogeneous throughout the cast ingot
than in the case of 2.5 % of mass fractions. Size-depen-
dent analysis show that Al2O3 powder with a mean
particle size 50 nm is more homogeneously distributed
than 500 nm. The addition of CaSi plays an important
role in case of distribution and clustering of A2O3
particles, as shown in Figures 13 and 14 where Al2O3
particles were mixed with CaSi dispersion media, from
the second set of experiments, Table 3.
From Figures 13 and 14 we can see that distribution
of Al2O3 particles (white arrows) becomes much more
homogeneous and quite equally distributed within the
metal matrix when using CaSi dispersion media.
A. KRA^UN et al.: DISTRIBUTION OF Al2O3 REINFORCEMENT PARTICLES IN AUSTENITIC STAINLESS STEEL ...
978 Materiali in tehnologije / Materials and technology 51 (2017) 6, 973–980
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 18: STEM – line profile of Al2O3 ultrafine particles (500 nm,
1.0 %) in the cast microstructure of austenitic stainless steel, without
using the CaSi
Figure 16: AES spectrum of the Al2O3 ultrafine particles (500 nm,
1.0 %) mixed with CaSi (Ca-30%, Si-70%) in the cast microstructure
of austenitic stainless steel
Figure 17: STEM/EDS elemental mapping of Al2O3 ultrafine parti-
cles (500 nm, 1.0 %) in the cast microstructure of austenitic stainless
steel, without using the CaSi
3.3 Auger electron spectroscopy (AES)
In the next step a surface analysis of the sample using
Auger electron spectroscopy technique was performed.
In Figure 15 the AES spectrum of the Al2O3 ultrafine
particles (500 nm, 1.0 %) in the cast microstructure of
austenitic stainless steel is shown. The AES spectra of
particles (P1 and P2) showing only O and Al peaks and
line scans over the particle/matrix interface confirm the
successful introduction of Al2O3 particles into the steel
matrix (P3) without any intermetallic reactions taking
place, which is true for all the Al2O3 particle sizes and
concentrations used. In Figure 16 the AES spectrum of
the Al2O3 ultrafine particles (500 nm, 1.0 %) mixed with
CaSi (Ca-30%, Si-70%) in the cast microstructure of the
austenitic stainless steel is shown. The AES spectrum of
particles (P1) shows only O and Al peaks. Also in cases
when CaSi was added to Al2O3 ultrafine and
nano-particles no intermetallic reactions or presence of
Ca and Si were observed. However, as already men-
tioned the distribution of particles was more homoge-
neous.
3.4 Transmission electron microscopy (TEM)
In the context of the microstructural changes a cha-
racterization and analysis of the ceramic particles incor-
poration in the steel matrix transmission electron mi-
croscopy (TEM) was also employed. With STEM/EDS
elemental mapping shown in the Figure 17 and STEM –
line profile analysis shown in the Figure 18 successful
incorporation and coherent bonding of the Al2O3 nano-
particles in the steel matrix was confirmed. No discon-
tinuities at the particle/matrix interface, modification of
metal matrix or formation of intermetallic phases could
be observed.
3.5 Particles distribution
The particle distribution analysis performed on the
specimens from the first set of experiments revealed that
Al2O3 particles of 0.5 % to 1.0 % of mass fractions,
regardless of the mean particle size of 500 nm or 50 nm,
results in a relatively homogeneous distribution of Al2O3
particles throughout the volume of the cast ingot. For
low particle weight percents of 0.5 % and 1.0 % the
distribution of Al2O3 particles in the cast ingot was found
to be more or less independent on the position, concen-
tration and size of particles, resulting in about 0.01
particles/μm2. However, as the weight percent increased
to 2.5 the particle concentration ratio starts to decrease
toward the bottom of the cast ingot (Figure 19). Further-
more, with the increased particle mass fraction, also their
concentration in cast ingot increased by up to 3 times,
with the size of particles starting to play a role. The
larger the particles the larger is the volume fraction of
the incorporated particles.
The particles distribution analysis from the second
set of experiments (Figure 20) shows that the use of dis-
persion agent reduces the influence of particles size (500
nm or 50 nm) on the particles’ distribution in the steel
matrix. It results in a difference in the particles’
concentration throughout the ingot being reduced from
about 0.01 particles/μm2 to less than 0.002 particles/μm2.
5 CONCLUSIONS
The results of our investigation indicate that steel
reinforced with ceramic ultrafine/nanosized particles can
be produced by using a conventional casting route. Ultra-
fine particles and nanoparticles where successfully incor-
porated into the steel matrix, being confirmed by diffe-
rent analysing techniques, including light microscopy
(LM), scanning electron microscopy (SEM), Auger elec-
tron spectroscopy (AES) techniques and transmission
electron microscopy (TEM).
By using CaSi as a dispersion media and introducing
Al2O3/CaSi mixture through a sealed iron tube reduced
A. KRA^UN et al.: DISTRIBUTION OF Al2O3 REINFORCEMENT PARTICLES IN AUSTENITIC STAINLESS STEEL ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 973–980 979
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 20: Influence of different sizes of Al2O3 particles and CaSi on
distribution of Al2O3 particles (1 %)
Figure 19: Influence of different concentrations and size of Al2O3
particles on their distribution in the cast ingot
particles clustering and more homogeneous distribution
of reinforcement nanoparticles in the steel matrix were
obtained. It was found that the concentration and the size
of particles have an impact on the distribution of the
reinforcement within the matrix. When the weight
percent is increased above 1.0 it starts to affect the
particles’ distribution, with the concentration ratio de-
creasing towards the bottom of the cast ingot. In this case
also the size of the particles plays a role: a larger particle
size leading to an increased degree of incorporated
particles in the steel matrix.
In this study, an innovative pre-dispersion approach
for more the effective addition of ultrafine particles and
nanoparticles into a steel melt through a conventional
casting route was designed. It is based on mixing ultra-
fine particles and nanoparticles powder with dispersion
media.
Acknowledgment
This work was done in the frame of the research
programs P2-0050, which are financed by the Slovenian
Research Agency. The authors would also like to
acknowledge help from Miroslav Pe~ar, in`., from Insti-
tute of Metals and Technology for the AES analysis.
6 REFERENCES
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Nano-Particles – A Review, Metals (Basel), 4 (2014) 1, 65–83,
doi:10.3390/met4010065
2 S. H. Lee, J. J. Park, S. M. Hong, B. S. Han, M. K. Lee, C. K. Rhee,
Fabrication of cast carbon steel with ultrafine TiC particles. Trans
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doi:10.1016/S1003-6326(11)61060-1
3 Y. Q. Liu, H. T. Cong, W. Wang, C. H. Sun, H. M. Cheng, AlN nano-
particle-reinforced nanocrystalline Al matrix composites: Fabrication
and mechanical properties. Met.Sic.Eng.A, 505 (2009), 151–156,
doi:10.1016/j.msea.2008.12.045
4 Z. Zhang, L. D. Chen, Consideration of Orowan strengthening effect
in particulate-reinforced metal matrix nanocomposites: A model for
predicting their yield strength. Scripta Mater., 54 (2006), 1321–1326,
doi:10.1016/j.scriptamat.2005.12.017
5 J. Llorca, Fatigue of particle-and whisker reinforced metal-matrix
composites. Prog Mater Sci., 47 (2002), 283–353, doi:10.1016/
S0079-6425(00)00006-2
6 B. N. Chawla, Y. Shen, Mechanical Behavior of Particle Reinforced
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doi:10.1002/1527-2648(200106)3:6<357::AID-ADEM357>3.3.CO;2-9
7 Z. Ni, Y. Sun, F. Xue, J. Bai, Y. Lu, Microstructure and properties of
austenitic stainless steel reinforced with in situ TiC particulate,Mater.
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8 F. Akhtar, Ceramic reinforced high modulus steel composites:
processing, microstructure and properties,Can. Metall. Q., 53 (2014)
3, 253–263, doi: 10.1179/1879139514Y.0000000135
9 S-Y.Cho, J-H. Lee, Anisotropy of wetting of molten Fe on Al2O3
single crystal. Korean J Mater Res., 18 (2008) 1, 18–21, doi:10.3740/
MRSK.2008.18.1.018
10 R. V. Väinölä, L. E. K. Holappa, P. H. J. Karvonen, Modern steel-
making technology for special steels, Journal of Materials Processing
Technology, 53 (1995), 453–465, doi:10.1016/0924-0136(95)
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980 Materiali in tehnologije / Materials and technology 51 (2017) 6, 973–980
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
A. MEHLE et al.: SURFACE CHARACTERIZATION OF PLATINUM STIMULATING ELECTRODES ...
981–988
SURFACE CHARACTERIZATION OF PLATINUM STIMULATING
ELECTRODES USING AN ELECTROCHEMICAL SCANNING
METHOD
KARAKTERIZACIJA POVR[INE PLATINASTIH STIMULACIJSKIH
ELEKTROD S POMO^JO ELEKTROKEMIJSKE VRSTI^NE
METODE
Andra` Mehle1, Janez Rozman2,3, Martin [ala4, Samo Ribari~3, Polona Pe~lin2
1Sensum d. o. o., Tehnolo{ki park 21, 1000 Ljubljana, Slovenia
2Centre for Implantable Technology and Sensors, ITIS d. o. o. Ljubljana, Lepi pot 11, 1000 Ljubljana, Slovenia
3Institute of Pathophysiology, Medical Faculty, University of Ljubljana, Zalo{ka 4, 1000 Ljubljana, Slovenia
4Analytical Chemistry Laboratory (L04), National Institute of Chemistry, Hajdrihova 19, Ljubljana, Slovenia
janez.rozman@guest.arnes.si
Prejem rokopisa – received: 2017-04-25; sprejem za objavo – accepted for publication: 2017-06-28
doi:10.17222/mit.2017.044
The purpose of this article is to investigate the electrochemical performance of platinum stimulating nerve electrodes (WE1 and
WE2) with different surface structures to define which one is able to produce a higher neural activation function during nerve
stimulation than that achieved by conventional electrodes. The purpose is also to present a method that enables the
electrochemical scanning of stimulating electrode surfaces. The surface of WE1 was modified using rough sand paper, while the
surface of WE2 was modified using fine sand paper. The potential at the different roughened surfaces in the sodium phosphate
mixture, when excited with specific current pulses, was measured against a Ag/AgCl reference electrode. Voltage transients
were recorded to determine the polarization across the electrode-electrolyte interface. The results indicate that the surface of
WE1 could deliver more current to the nerve tissue and more activation for a fixed input voltage than WE2. Namely, it is shown
that the mean |Zpol| of WE1 was lower than that for WE2 (237.1 
 vs. 251 
). Accordingly, the platinum electrode that was
superficially modified using rough sand paper is more suitable for safe and efficient nerve stimulation than the electrode that
was superficially modified using fine sand paper.
Keywords: platinum, polarization, interfaces, potential parameters
Namen ~lanka je preiskati elektrokemijske lastnosti platinastih elektrod WE1 in WE2 za stimulacijo `ivca, ki imata razli~ni
povr{inski strukturi, s ciljem dolo~iti katera od njiju je sposobna povzro~iti ve~jo aktivacijsko funkcijo vlaken med stimulacijo
glede na konvencionalno povr{ino. Namen je tudi predstaviti metodo, ki omogo~a elektrokemijsko vrsti~no preiskavo povr{in
stimulacijskih elektrod. Povr{ina WE1 je bila obdelana z grobim brusnim papirjem medtem, ko je bila povr{ina WE2 obdelana s
finim brusnim papirjem. Potencial razli~no grobih povr{in v fiziolo{ki raztopini pri vzdra`enju s specifi~nimi stimulacijskimi
impulzi je bil merjen glede na Ag/AgCl referen~no elektrodo. Napetostni prehodni pojavi so bili zajeti z namenom dolo~itve
polarizacije na prehodu med elektrodo in elektrolitom. Rezultati ka`ejo, da lahko WE1 dovede na `ivec ve~ toka in s tem dose`e
ve~jo aktivacijo pri konstantni napetosti kot WE2. Izkazuje se namre~, da je polarizacijska upornost |Zpol| pri WE1 manj{a kot
pri WE2 (237.1 
 proti 251 
). Potemtakem je elektroda, ki je bila bru{ena z grobim brusnim papirjem, bolj primerna za varno
in u~inkovito stimulacijo `ivca, kot elektroda, ki je bila bru{ena s finim brusnim papirjem.
Klju~ne besede: platina, polarizacija, prehodi, parametri potenciala
1 INTRODUCTION
In recent decades, considerable scientific and techno-
logical efforts have been devoted to understanding and
characterizing the interface between a stimulating elec-
trode and its surrounding medium. In this region, a trans-
duction of charge carriers occurs from electrons in the
metal electrode to ions in the tissue, which is exception-
ally important in determining how the electrodes respond
to charge injection. To be specific, characterizing the
electrode-tissue interface is crucial to determining safe
charge delivery to the nerve.1–3
In implantable prosthetic devices, electrodes are the
interfaces between the electronic circuitry and nerve
tissue and can be used for neural stimulation and/or
neural signal recording. In the past few years, implanted
electrodes have been used extensively for efficient stimu-
lation of peripheral nervous systems. Although much
effort has been made to find optimal anatomical targets
for different nerve-stimulation techniques, little work has
been done to improve the efficiency of nerve stimulation
using analytically driven designs and configurations of
the stimulating electrodes.
The electrode geometry itself plays a significant role
in controlling the activation of neuron populations.4 In
this connection, the electrode geometry can affect the
impedance, spatial distribution of the electric field in the
tissue, and consequently the pattern of neural excitation.
One approach to enhance the efficiency of neural stimu-
lation is to increase the irregularity of the surface current
profile, which can be quantified by defining a metric
known as the topological edginess.5
Materiali in tehnologije / Materials and technology 51 (2017) 6, 981–988 981
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 62-4-023.7:669.231.6:621.35 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(6)981(2017)