Thesis dissertation in order to obtain the degree in Doctor in Physics. Austenitici stainless steel modification by ion carburization and study of its stability under plasma focus irradiation. Developed at Universidad Nacional de Rosario (Argentina) in March 2012

1. Javier García Molleja
Director: Jorge Feugeas (UNR, Argentina)
Advisor: Mª Dolores Calzada (UCO, Spain)
Thesis presented in order to obtain the PhD degree in Physics
Thesis jury:
M. Magdalena Milanese (UNICEN, Argentina)
Roberto R. Koropecki (UNL, Argentina)
Oscar A. de Sanctis (UNR, Argentina)

2. 


Steels
Surface treatment
Plasma

3. 


Steels are alloys of iron and carbon (< 2 %)
with other elements in addition.
Fe-C diagram has an interesting zone when
the carbon content is low.
Under different temperatures there are some
stable crystalline phases, and there is an
eutectic point.

4. Carbon content develops carbide formation and steels properties are
changed.
Alloying elements affect final steel properties.
Main steel properties are the electric, magnetic, thermal and nuclear
ones.

5. 


Stainless steels have an 11 % of chrome as
minimum percentage. Passive film is formed
on the surface, avoiding the oxygen influence
in the structure.
They have high corrosion resistance under
high and low temperatures.
Molybdenum improves pitting corrosion
resistance.

6. 



Austenitic steels have a fcc structure (face
centered cubic).
This structure is stable under high temperatures
or when the steel itself has nickel (3.5-22 %) as
alloying.
Austenitic
steels
have
high
corrosion
resistance, they are weldable, good hygienecleanliness factor, with application under an
important temperature range, they are not
hardened by heat treatment and they have a lot
of uses.
One type of austenitic stainless steel very known
is the AISI 316L one, studied in this thesis.

7. 


Surface modification can be accomplished
with different techniques.
One of them is ion implantation with
energetic ions which penetrate in the surface
as projectiles.
Another one is atomic diffusion, where ions
diffuse in the bulk following the Fick law.

8. 

Surface modification of steels is obtained by
ion
carburization
and
ion
nitriding
techniques.
These processes are based on the generation
of C or N active species with plasmas.
Plasmas are generated by electric discharges
and the surface is under negative voltage.

9. 




Surface
modification
treatments are obtained
with plasmas.
This figure represents the
voltage-current curve of a
DC discharge under low
pressure conditions.
We work in the bright
region, called glow.
In this region there are
many species with different
temperatures.
Electron
temperature is high, so this
is important when we use
fragile
substrates,
thus
working with low gas
temperatures.
Neutral atoms are excited
by
collisions,
emitting
visible radiation.

10. Glow discharge has dark and bright zones.
The cathode region has the major voltage fall in the discharge. In
this region positive ions are accelerated to the cathode and electrons
gain energy and provoke ionizations and collisions in the negative
glow region. This region has high light intensity and high volumetric
electron density.

11. 

Reactors used
Instruments and physical principles used to
sample’s characterization

12. Alternatively, two reactors are used.
1. Reactor of 8.8 L of capacity with two windows.
Cathode has a surface of 164 cm2.
2. Reactor of 5.1 L of capacity with an upper
window. Cathode is 265 cm2 of surface, with
holes to sample insertion (2 cm diameter, 6 mm
thick).


13. 



Base pressure (rotatory pump) of ~10-2 mbar.
Surface cleaning: hydrogen plasma during 15
minutes at 1.484 mbar.
Working pressure was 5 mbar during
carburization. Temperature between 400 and
410 ºC.
Gas mixtures during carburization:
◦ C50-: 50 % Ar, 45 % H2, 5 % CH4.
◦ C80-: 80 % Ar, 15 % H2, 5 % CH4.

Process duration: 30, 60 or 120 minutes.

14. 
In order to compare carburization with
nitriding, some depositions with this
technique have been done. Gas mixture of:
◦ 80 % H2, 20 % N2.

Process duration: 80 minutes.

15. 


Optical Microscopy: OLYMPUS MG microscope
(property of Laboratorio de Metalurgia, IMAE).
Grazing Incidence X-Ray Diffraction (GIXRD):
PHILIPS X’Pert diffractometer. Cu K anode at
40 kV and 30 mA. Incident beam has a
parallel cross-section of 4x4 mm2.
Incidence angles used: 2 or 10º (0.7-3 m
depth). Scanning between 30 and 80º. Steps
of 0.03º and 1 s.

16. 

Vickers microindentation: SHIMAZDU HMV-2
(property of IMAE) microhardness tester.
Diamond tip with rectangular base, 136º
between opposite faces. Different loads
between 10-15 s.
Tribology: ball-on-disc method. TRM100
Dr.Ing. GEORG WAZAU tribometer. Alumina
ball of 5 mm radius, linear speed of 0.1
m/s, 8842 turns and scar located at 9 mm
from the sample center. Test reached 500 m
of length. Load of10 N. No lubrication.

17. Wear is known by measurement of
volume lost per unit of length:
S
1 2
r (
2
sen )
The angle is calculated with the
scar diameter, D, and the ball
radius r:
D2
arc cos 1
2r 2

18. 

Corrosion: During 60 days, sample immersion
(with a sample of austenitic steel) in one liter
of water with 5.85 wt. % NaCl.
Auger Electron Spectroscopy (AES): PERKIN
ELMER (property of INTEC, Santa Fe) device.
Ion sputtering with 4 keV Ar and a current of
15 mA. Electron gun was used under 2 keV of
energy and current of 2 A. Working pressure
was 5.33·10-8 mbar.

19. 
Focused Ion Beam (FIB): Ion gallium beam of
30 kV and 10 pA in a FEI Helios NanoLab™
600 - (Dualbeam) device, property of
Università de Roma Tre, Italy. Electrons and
secondary ions are detected. SEM (Scanning
Electron Microscopy) mode operation can be
chosen, too. Magnification of 350000X.

20. 


Fundamentals on expanded austenite
Study of base material
Surface analysis of treated steels
◦
◦
◦
◦

Auger analysis
Hardness analysis
Wear analysis
Corrosion analysis
Expanded austenite stability
irradiation and temperature
under
beam
◦ Expanded austenite stability under beam irradiation
 Expanded austenite stability
influence on hardness
under
beam
irradiation:
◦ Expanded austenite stability under high temperature

21. 

Experimental results show that C and N are
lodged in the lattice by fast diffusion,
reaching maximum percentages of 38 % for N
and 28 % for C without any precipitation.
This Colossal Supersaturation with these
elements in the austenite develops a new
phase called expanded austenite, which is
obtained with only 12 % of C or N in the
sample.

22. 

The austenitic stainless steel has a fcc crystal
structure with interstitial tetrahedral and
octahedral holes.
Considering the lattice composed solely with
Fe and C located in the holes, this lattice will
expand 33.6 pm if C accommodates in a
octahedral site and 86.8 pm if it is located in
a tetrahedral one.

23. 


Expanded austenite allows a higher carbon
concentration in the lattice, avoiding the
maximum solubility imposed by the Fe-C
diagram.
C is less soluble than N (less affinity with Cr), so
the concentration is lower, and the expansion
too. On the other hand, film thickness will be
high.
The structure of the expanded austenite is the
fcc one, but stressed and with defects. Some
authors think that the crystal structure is triclinic
or tetragonal.

24. 


Carbon ions created by the plasma are
impinged in the lattice and they will diffuse
following the Fick laws.
The formation of a stable solution or a
compound will depend on the Gibbs free
energies involved during the process.
Thus, diffusion is a thermal activated process.

25. 

Free mean path calculation for carbon ion
gives that this ion will imping in the lattice
with a depth of 5 Å and with an energy of 55
eV (considering the plasma properties, the
size of the cathode fall and the density of the
region).
Because of ion low energies, they will be
neutralized quickly in the lattice.

26. 

Lattice
parameter
is
known
by
GIXRD
measurements and the
use of Bragg’s law with
the three main peaks.
Using the Nelson-Riley
method (graph containing
lattice parameters versus
cotg( )·cos( )
product
and
obtaining
the
intercept value) a= 3.584
Å is obtained.

27. 


Under loads of 25 and 300 g, it is determined
that the hardness of the base material is 264
HV, or 2.589 GPa.
Under the same experimental conditions used
in tribology measurements, the reached value
was S= 2.23·10-8 m3/m.
Corrosion attack under a solution containing
chlorides, showed that there were neither
pitting nor external aggression, behavior
typical in AISI 316L steels.

28. Experimental data for C50-:
Sample
Time
(min)
Pressure
(mbar)
Voltage (V)
Current (A)
Temperature
(ºC)
C50-030
30
4.985
572
1.14
401
C50-060
60
5.001
545
1.08
406
C50-120
120
4.982
549
1.35
411
In the following table, experimental data for C80- were shown:
Sample
Time
(min)
Pressure
(mbar)
Voltage
(V)
Current (A)
Temperature
(ºC)
C80-030
30
4.993
415
1.43
409
C80-060
60
4.993
455
1.39
404
C80-120
120
4.984
520
1.36
406

29. 

For C50- samples the current density was 7.0
mA/cm2, and for C80- samples was 8.1
mA/cm2.
There was a graphitic film in the surface
composed by carbon atoms. They did not
enter in the steel structure. Soot is avoided
when methane proportion is lower than 5 %.

30. Cross-sections were analyzed under
Optical Microscopy.
Expanded austenite film is thicker when
the time treatment is higher.
Thickness of C50-030 was 14
m
(upper image), meanwhile the thickness
of C80-120 sample was 27 m (lower
image).
Interface is well sharp.
Expanded austenite film is resistant to
oxalic acid attack.

31. 


The upper region of the cross-section was analyzed by FIB. In
this expanded austenite zone there were no Hägg carbides.
a) showed the different crystallographic orientations in the base
material, with breadths of 2-5 m and b) showed a high amount
of defects and stacking faults in C50-030, with twins of 0.5-2
m thick.
The features in the upper region of the surface were provoked by
the polishing process.

32. 

Diffractograms obtained at
10º of incidence showed the
typical
austenitic
structure,
with
the
(111), (200) and (202)
planes shifted to lower
angular values, i.e. higher
lattice parameters.
A representation of the
expansion of C50- and C80samples using the NelsonRiley method showed that
the major expansion rate is
reached at the first 30
minutes
of
treatment,
obtaining
a
expansion about the 90 % of
the value reached at 120
minutes of treatment.

33. Sample
Lattice parameter (Å)
Expansion (%)
C50-030
3.660
2.12
C50-060
3.661
2.15
C50-120
3.666
2.29
Sample
Lattice parameter (Å)
Expansion (%)
C80-030
3.654
1.95
C80-060
3.669
2.37
C80-120
3.659
2.09

34. 

With AES, it is possible to measure the elemental
profile in the first 156 nm (65 minutes of sputtering).
In C50- the amount of Fe increases until reach a
stationary value (62 %). C is maximum on the surface
but it decreases quickly to a concentration of 15 %. O
is a surface contaminant and disappears in the first
layers. Cr concentration increases slowly.

35. 

For the C80- samples, Fe increases with depth until reach a value
of 60 %. C decreases with regard of its surface concentration and
carbon reach a stationary value which depends on the time
treatment: a) 15 %, b) 14 % and c) 28 %.
Cr concentration increases with depth, but with 5 minutes of
sputtering a maximum is reached (21 %), i.e. a dome-shaped
concentration profile is obtained. This is a consequence of the
preferential sputtering.

36. 



With AES is possible to see the carbon
chemical state inside the expanded austenite.
This technique is called Factor Analysis.
It is calculated the proportion of the carbon
bonded chemically (carbide type) and the
amount of carbon freely dissolved (graphitic
type).
Carbide type C is present in all samples and
the amount increases with depth. It is caused
by the high surface current density used (>
7.0 mA/cm2).

38. 


In the first 2000 nm Berkovich indentation was applied
using different loads on the surface.
Surface hardness in C50-030 was 11.8 GPa and in
C80-030 was 11.0 GPa. Both values were obtained at
200 nm depth.
In the maximum depth probed, hardness decreased in
both samples, until reach a value of 8 GPa.

39. 


For higher depths than 2 m
the last technique was not
longer valid, so Vickers
nanoindentations
in
the
samples’ cross-section were
made.
The maximum hardness was
reached
at
10
m
depth,
where
C50-030
reached 7.5 GPa and C80030 reached 8.8 GPa.
Hardness decreased until 15
m
depth,
obtaining
a
constant value of 5.2 GPa
from there to the maximum
depth probed.

40. 


For depths higher than 140
m it is mandatory to use
Vickers indentations with
loads of 300 g.
Analyses are obtained from
the expanded austenitebase material interface to 6
mm depth.
C50-030 hardness was
2.84 GPa, while for C80030 sample hardness was
2.70 GPa. At depths of 1
mm hardness decreased to
a value of 2.50 GPa, near
to the value of the base
material.

41. R
v

42. 

Expanded austenite is the responsible of the
wear resistance improvement.
More time treatment means higher wear
resistance and lower friction. The free
graphitic-type carbon acts like a solid
lubricant.
Sample
S (m3/m)
W%
C50-030
1.34·10-8
60
C50-060
0.15·10-8
7
C50-120
5.22·10-8
234
C80-030
0.36·10-8
16
C80-060
0.34·10-8
15
C80-120
0.16·10-8
7

43. 




At first sight, there is a graphitic film on surface.
For C50- there are gray spots of 80 m diameter.
They are pitting agglomeration of 5 m diameter,
each one. Far away from this agglomerations,
pitting have a diameter of 0.5 m.
The maximum density of pitting is located in the
grain borders and slip bands. They are high
energy centers and provoke the precipitation of
Cr, bonded with C.
The high corrosion resistance of the steels is
lost, but this effect is only located at the surface.
In C80- samples corrosion is general and the
penetration is estimated in 2 mm.

45. 
Carburized and nitrided AISI 316L steel
samples have been analyzed under beam
irradiation composed by light and energetic
ions. The behavior of this treated samples
under long heat treatments has been
analyzed, too. Their surfaces could be
covered by aluminum nitride, or not.

46. 



Samples used in this study were treated at
temperatures between 400-410 ºC during 80
minutes.
For carburization, the gas mixture was 50 %
Ar, 45 % H2 and 5 % CH4.
For nitriding, the gas mixture was 80 % H2
and 20 % N2.
In nitriding, current density was 1.55 mA/cm2
and in carburization current density was 2.08
mA/cm2.

47. 






Bombardment using light ions
was developed with a plasma
focus device, 2 kJ of energy
and Mather design. Anode had
a diameter of 40 mm.
Base
pressure
(cleaning
procedure): 0.013 mbar.
Working pressure: 1.6 mbar of
deuterium (D) or helium (He).
Loading voltage of capacitor
bank (4 F): 31 kV.
Samples were placed at 82 mm
from
the
anode,
power
developed was 10 MW/cm2.
Half of the sample was covered
with a steel foil.
Number of plasma focus
discharges: 1, 5 and 10.

48. Carburized, 1 pulse with D




Carburized, 10 pulses of He
Surface morphology is analyzed with Optical Microscopy at 500X.
In carburized samples there are no different effects with the change of
bombardment gas (D or He).
Higher number of shots, higher crater density and material sputtering.
High thermal gradients (in the first layers the temperature reached
1500 ºC during the first nanoseconds) provoke slip band crosslinking.

49. Nitrided, 1discharge of D




Nitrided, 10 discharges of He
Regarding nitrided samples, there are no differences using both types of
ions.
However, it is concluded that the resistance to the bombardment is
lower in nitrided samples than in the carburized ones.
There are craters and sputtered regions, combined with a peeling
provoked by the high thermal gradients involved during the process.
With the increasing number of ion discharges, the surface layers are
melted.

50. Carburized, 1 discharge of He



Carburized, 5 discharges of D
Nitrided, 5 discharges of D
FIB/SEM analyses using Ga+ ions determine with better
resolution the surface morphology, and their crosssection, too.
Both gases used to bombard the surface provoke craters.
In carburized samples there are crosslinking in slipping
bands. In the nitrided sample there are cracks and bulges
because of the first layers melting.

51. Carburized, 1 discharge of He




Carburized, 5 discharges of D
Nitrided, 5 discharges of D
Cross-section shows grains with strain due to the bombardment.
Steel samples with several discharges show melting and
amorphization located at the first layers.
At 3 m depth, thermal effects do not surpass the melting point.
Crystallite features can be observed, provoked by the nucleation
and short growth of stressed austenitic steel grains.

52. 



Crystallites are formed in high
extent at three micrometers of
depth.
Temperature rising provokes
the
restart
of
diffusive
processes, so this region lost
all their expansive elements (C
or N).
Quick temperature drop halts
the crystallite (of austenitic
stainless steel) coarsening, so
beads of 36 nm of diameter
and agglomerates of them are
formed.
The amorphous matrix induces
high residual stresses.

53. 

GIXRD with incidence of 2 and 10º show the
crystalline structure of bombarded samples and
with depths between 0.7 and 3 m, respectively.
The hidden region presents diffractograms
similar to the ones in the sample before the
plasma focus bombardment.
Carburized, with discharges of D
Nitrided, with discharges of D

54. 




Only the (111) peak is analyzed because there is a
similar behavior in (200) and (202) peaks.
Higher number of discharges provokes peak shifting
to higher angular values, that is, less lattice
parameter value.
There is another peak, located always in the same
angular position: 43.3º.
Combining with FIB/SEM, peak shifting is related to
the temperature rise and the N or C diffusion to inner
layers, so the fcc expansion is gradually reduced.
The peak anchored at 43.3º can be created by the
crystallites present in the first 3 m of material. This
value represents a lattice parameter of 3.6163 Å, with
a relative expansion of 0.90 %.

55. Carburized, with discharges of He



Nitrided, with discharges of He
In both cases there is a loss of austenitic expansion with a high
number of pulses.
In carburized samples there is no the peak located at 43.3º, and
there is an absence of crystallites when FIB/SEM analyses are
developed.
In nitrided samples the peak is present. Peak overlapping means
that the loss of expansion will finish when the lattice parameter
is the same than the lattice parameter related to the 43.3º peak.

56. Carburization treatment of AISI 316L steel
Discharges
Deuterium
(Å)
Expansion (%)
Helium (Å)
Expansion (%)
0
3.6696
2.39
3.6779
2.62
1
3.6545
1.97
3.6529
1.92
5
3.6491
1.82
3.6557
2.00
10
3.6429
1.64
3.6504
1.85
Nitriding treatment of AISI 316L steel
Discharges
Deuterium
(Å)
Expansion (%)
Helium (Å)
Expansion (%)
0
3.8073
6.23
3.7984
5.98
1
3.7491
4.61
3.7415
4.39
5
3.6691
2.37
3.6743
2.52
10
3.6468
1.75
3.6556
2.00

57. 


For light ions, the irradiation effect has no
effect due to the ion mass.
The effect depends on (by accumulation) the
number of discharges.
The highest expansion (of expanded
austenite) is obtained by nitriding, but the
degradation of this expansion is the same for
both treatments when the number of
discharges is high.

58. 
Using Vickers nanoindentations with loads of
25 g are useful in order of obtain the surface
hardness.
Nitrided AISI 316L steel
Discharges
Deuterium (HV)
Helium (HV)
0
999±51
965±61
1
827±34
815±34
10
---
---
Carburized AISI 316L steel
Discharges
Deuterium (HV)
Helium (HV)
0
461±25
443±34
1
364±34
293±6
10
366±16
367±14

59. 



Expanded austenite hardness was measured in
the zone covered by the foil. These values are the
same than the ones of treated samples before the
plasma focus irradiation.
Nitrided samples have higher hardness than the
carburized ones.
In nitriding, ten discharges provoked surface
melting and amorphization. Hardness could not
be measured. There are not difference changing
of bombarding gas.
In carburization, hardness values were similar
when D was used, but there were high hardness
variations when He was used.

60. 


Thermal shocks provoke the (111) peak
shifting to higher values, because of the
diffusion restart.
In order to study the mechanisms involved in
this process heat treatments of long duration
have been done.
Thin film aluminum nitride (AlN) deposition
was developed in order to study its
application as protective barrier against
oxidation.

61. 




Film coating is obtained by magnetron sputtering, in
reactive mode.
Magnetron has an Al target (high purity). DC voltage
applied is 260 V and 154 mA of current. Power
density is 5.01 W/cm2. Firstly, target cleaning with Ar
and H2 during 15 minutes and a shutter is used
during 20 minutes in order to obtain a stable
discharge.
The process is developed in a reactor of 94.03 L of
capacity with a base pressure of 9.60·10-5 mbar and
Ar purges.
Working atmosphere is 50 % Ar and 50 % N2 at
6.65·10–3 mbar and flow of 12.0 mL/min.
Target-substrate distance is 3 cm.

62. 


Under these conditions deposition rate is 11
nm/min, so the total thickness is 330 nm
after 30 minutes of deposition.
The film has no crystalline structure because
of the high substrate roughness (in nitrided
or carburized austenitic steel).
Nitrided or carburized samples with or
without AlN were submitted in a furnace at
225, 325, 405 y 504 ºC of temperature
during 20, 40 and 60 hours.

63. 


With GIXRD at 2º of incidence
carburized samples, with AlN
and without this film, were
analyzed. Both were heat
treated during 60 hours at 405
ºC.
It is observed that the AlN film
avoids the oxygen entrance
and there is a important
reduction in the α-Fe2O3
peaks.
The other peaks near to the
oxides and austenite ones are
provoked by the presence of
Cr3C2, because of the AlN had
no influence in the processes
developed in the expanded
austenite.

64. 





Analyses were done only in the (111)
peaks, because of the behavior is
similar in the (200) and (202) peaks.
This analysis is not dependent of the
presence or absence of AlN.
In carburized sample, analysis was
done by GIXRD at 10º.
For 225 and 325 ºC peak shifting to
higher angular values (than the peak
position of carburized sample not heat
treated) was not observed. It is
probable
the
ignition
of
reacommodation processes.
Time process did not have effect in the
lattice parameter reduction.
At 405 and 504 ºC there was high
peak
shifting
because
of
the
temperature surpassed a kind of
threshold in order to activate the
diffusion processes.

65. Austenitic AISI 316L stainless steel sample was carburized and
submitted to high temperature treatments during long times.
Previously, lattice parameter was calculated in 3.6880 Å, so the
equivalent relative expansion was 2.90 %.
Temperature
Time
Lattice parameter
Expansion
(ºC)
(h)
(Å)
(%)
225
20
3.6858
2.84
225
40
3.6841
2.79
325
20
3.6800
2.68
405
20
3.6594
2.10
405
40
3.6531
1.93
405
60
3.6521
1.90
504
20
3.6318
1.33

66. 


Heat treatments in nitrided
samples develop, according to
GIXRD at 10º analyses, a
similar behavior.
Diffusion
processes
are
activated after 300 ºC, but the
driving force is very low.
When the temperature reaches
the value at which nitriding
was
developed
(400
ºC)
diffusion processes are highly
triggered,
so
the
lattice
parameter reduction is greater
than at low temperatures.

67. Nitrided AISI 316L steel sample under high temperatures and long
times. Before the experiment, lattice parameter was 3.7082 Å, so
the relative expansion was 3.46 %.
Temperature
(ºC)
Time
(h)
Lattice parameter
(Å)
Expansion
(%)
225
20
3.7308
4.10
225
40
3.7294
4.06
325
20
3.7173
3.72
405
20
3.6801
2.68
405
40
3.6756
2.56
405
60
3.6710
2.43
504
20
3.6398
1.56

68. 

AISI 316L surface modification by nitriding
and carburization was studied. Low pressure
(5 mbar) plasma glow was used with DC
power. These processes developed expanded
austenite.
Expanded austenite stability was analyzed
under pulsed irradiation with light ion beams
and high energies. Stability under high
temperatures
and
long
times
was
analyzed, too.

69. 



The original austenite of steel is quickly
expanded during the first 30 minutes of
process.
The structure obtained was expanded
austenite, without macroscopic precipitates.
In
carburized
samples
(analyzed
by
AES), bonded C is formed in the fist half hour.
After that, C is added in a graphitic way.
High density currents provoked carbide
formation.

70. 


Hardness was higher than the base material
until 0.7 mm depth. The maximum value was
12 GPa.
Wear resistance was improved at higher time
treatments because of the role of the
graphitic C as solid lubricant.
Corrosion resistance was reduced on the
surface (156 nm depth).

71. 



Under D and He ion irradiation there was a higher
resistance
of
expanded
austenite
by
carburization than by nitriding.
For a particular treatment, damage effects were
the same without regarding the ion beam chosen.
Structure degradation was provoked by thermal
shocks. They triggered the diffusion processes
and the surface melting and amorphization.
Crystallites were formed by austenitic steel highly
stressed. This structure provoked a diffraction
peak located at 43.3º.

72. 



Crystalline structure was degraded at high
temperatures and long times.
AlN film coating avoided the surface
oxidation.
This degradation, at fixed temperature, did
not change by the duration process used.
After 405 ºC, which was the temperature
used in order to obtain expanded austenite,
the degradation process is highly enhanced.

73. 


Expanded austenite induces on the austenitic
stainless steels surfaces high hardness and
high wear resistance.
In ion carburization, the creation of a film
composed by C non chemically bonded
(graphitic) improves the wear resistance by
lubrication effects.
Corrosion resistance can be maintained if the
first 156 nm are removed, region where
carbides are located at the first stages, but
they are not present in deeper regions.

74. 









To my parents, my brother and my godmother, for their support.
To Geo, for her love and help.
To Jorge and Liliana for their affection and for being there every
moment.
To the Grupo Física del Plasma, for their friendship and
interesting conversations.
To electronic and mechanical workshop staffs, for their ideas
during experiments and the creation of devices.
To researchers and PhD-students from UNR, and all the people
with whom I have worked.
To my teachers and friends from Degree in Physics from UCO.
To my fellows of hBirra and others, for their friendship and
integration.
To my friends from Spain, for theirs anecdotes and encounters.
To Horacio for his excellent behavior with all the people and his
help during experiments. We will never forget you.

75. 
March 19th. ¡Viva la Pepa!