Stainless steel surface modification induced by argon and krypton

Zbigniew W. KOWALSKI
Wrocław University of Technology, Wrocław, Poland
Stainless steel surface modification induced by argon
and krypton ion beam bombardment
Abstract. In many areas of science and technology various processes, such as: machining, chemical and ion etching, electron beam or photon flux
bombardment, etc. are used for surface modification of materials. Morphological properties of the surface depends on the type of process used. This
paper presents the results of studies of the basic aspects of the surface morphology of stainless steel modified in the process of ion etching, with the
use of perpendicular and oblique ( = 80 89) ion beam. In the experiments argon and krypton ion beams from GD ion source with hollow anode
(beam diameter of about 1 mm, energy up to 6 keV and ion beam density up to 0.5 mAcm-2) were utilized.
Streszczenie. W wielu dziedzinach nauki i techniki wykorzystywane są różne procesy technologiczne, takie jak: obróbka mechaniczna, trawienie
chemiczne i jonowe, bombardowanie wiązką elektronów lub strumieniem fotonów, itp. do modyfikacji powierzchni materiałów. Właściwości
morfologiczne powierzchni zależą od rodzaju stosowanego procesu. W niniejszej pracy przedstawiono wyniki badań podstawowych aspektów
morfologii powierzchni stali nierdzewnej modyfikowanej w procesie trawienia jonowego, przy użyciu prostopadłej i ukośnie padającej ( = 80 89)
wiązki jonów. W eksperymentach stosowano wiązkę jonów (o średnicy równej około 1 mm, energii dochodzącej do 6 keV i gęstości prądu – do
0,5 mAcm-2) z jarzeniowego źródła z wnękową anodą. (Modyfikacja powierzchni stali nierdzewnej wywołana bombardowaniem jonami argonu
i kryptonu).
Keywords: ion beam, surface topography, surface roughness, stainless steel.
Słowa kluczowe: wiązka jonów, topografia powierzchni, chropowatość powierzchni, stal nierdzewna.
Introduction
In many areas of science and technology different
processes are used for surface modification of various
materials [1-3], both at the micro- and nanoscopic scale.
Morphological properties of the surface depends on the
type of process used. Depending on defined application one
needs, for example, extremely rough surface and the other
– smooth and plain. A good tool that can be applied to
modify surface morphology in micro- and nanometre scale
is ion beam [4] – it enables to increase or decrease surface
roughness. Among the main aspects describing surface
morphological changes induced by any physical or chemical
process one should remember: (a) surface topography
visualized mainly by SEMs and near-field microscopes, and
(b) surface roughness defined by various parameters and
functions. Recently two additional aspects that seem to be
good complementary indicators of morphology changes
were proposed and called by the author [5, 6]: profile
variability and surface morphology arrangement – they stem
from fractal and harmonic analysis of surface roughness
profile, respectively. Results of ion beam induced surface
morphology depend not only on beam parameters but also
on surface features of modified material. The problem has
been studied in relation to surface morphologies of
1H18N9T stainless steel (made in Poland) modified by the
use of perpendicular and inclined argon and krypton ion
beams from GD ion source with hollow anode.
Experimental details
Ion source
Modification of stainless steel surfaces were performed
in apparatuses equipped with the GD (glow discharge) ion
guns applied for the first time to ion sputtering of a solid
surface in 1953 and described among others by Gillespie et
al [7] and Crockett [8] in early seventies of XX century. In
next years the sources in question were used [9] in
commercial apparatuses for ion milling of materials because
of simplicity of the gun and power supply design, ease of
operation, and facility of sputtering of non-conductive
materials without additional neutralizing systems.
GD ion gun consists of two circular parallel electrodes
(earthed cathode and anode at positive potential up to 6 kV,
yielding ion current of up to 0.1 mA and current density of
2
up to 5 A/mm ) and utilizes a cold cathode abnormal glow
352
discharge as the source of positive ions. Figure 1 presents
the source used in ion beam etching apparatuses.
Fig.1. Glow discharge GD gun (made in our university) utilized in
ion beam etching machines
The own design apparatus for ion beam etching of
various materials with GD ion gun, specimen holder
(enabling specimen rotation and tilting), working gas valve,
and optical microscope is shown in Fig. 2.
Fig.2. One of the apparatuses (made in our university) equipped
with GD ion guns and utilized in surface morphology modification of
various materials
Measurements and observations
Stainless steel surface roughness profiles together with
well known, described and normalized main roughness
parameters relating to: vertical properties (Ra – arithmetical
mean deviation of the surface profile), horizontal features
(Sm – mean spacing of profile irregularities), and one of the
statistical distributions (profile bearing length ratio tp) of the
profiles were measured by means of profilograph – high
PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 88 NR 11b/2012
40
Normal beam incidence
Inclined beam incidence
35
30
Sm [μm]
quality Rank Taylor Hobson’s “Talysurf” and calibrated
atomic force microscope made in our university [10].
Changes of surface roughness are directly connected
with alteration of surface topography that was examined by
the ex situ SEMs and AFM. Here, in the paper only selected
results of experiments are presented.
25
20
15
10
Specimens
Stainless steel is used in various areas of science and
technology. It is rather sputter resistant material. The steel
specimens delivered as preliminary ground were worked to
form disks: 15 mm  1.5 mm (diameter  thickness). A part
of them were additionally mechanically polished to obtain
relatively small values of roughness parameters.
Results and discussion
Stainless steel 1H18N9T surface topography and
roughness depend on GD ion beam bombardment process
parameters and on initial surface conditions (i.e. before ion
processing). Among them the angle  of beam incidence,
bombardment duration, and surface preparation (e.g. initial
surface roughness or an interplay of surface polishing vs.
ion beam irradiation lines) seem to be the more important.
Angle of beam incidence
The influence of argon ion irradiation on steel surface
roughness parameter Ra, for perpendicular ( = 0) and
grazing ( = 87  89) beam incidence is shown in Fig. 3
(the points in all presented figures are mean values of many
measurements and the smooth curves are polynomial or
exponential fits).
5
0
0
2
4
6
8
10
t [h]
Fig.4. Changes of Sm (t) induced on steel surface by argon ion
beam at incidence angles:  = 0 and  = 85  87
Those processes go faster for inclined bombardment
due to the influence of angle  on sputtering yield. As a
result, differences between Sm values for normal and
inclined beam incidence, small for the first two hours of
irradiation, noticeably grow with time t of the process
duration.
Figure 3 shows that besides widening of topographical
elements observed in Fig. 4, perpendicular bombardment
induces also deepening of the forms in question, that is
proved by ex situ SEM observations (see for example
micrographs presented in Fig. 5).
a)
b)
800
Inclined beam incidence
Normal beam incidence
700
500
400
300
Fig.5. Deepening of steel surface topography elements induced by
normal bombardment: a) low ion dose (peripheral part of the
-2
sample), b) maximal dose (up to 0.5 mAcm , 6 hrs, central part)
200
100
0
0
2
4
6
8
10
t [h]
Fig.3. Changes of Ra (t) induced on stainless steel surface by argon
ion beam from GD source at incidence angles:  = 0 (squares )
and  = 87  89 (rhombus). Positive anode potential Ua of up to 5
 6 kV
The surface roughness decreases substantially with the
bombardment duration for inclined beam incidence – it
means that ion polishing process occurred. However,
besides polishing, the gun can also be used as roughening
tool. This can be reached by normal ( = 0) irradiation.
The range of roughness changes induced by ion beam from
GD gun (from polished to rough surface in Fig. 3) increases
with bombardment duration – it is equal about 140 nm after
2 hours of bombardment (Ra varies from approximately 60
nm for  = 87  89 to about 200 nm for  = 0), and up to
about 630 nm after 10 hours of irradiation.
Together with parameter Ra alteration, the horizontal
parameters also change. Figure 4 shows the influence of
bombardment duration on mean spacing of profile
irregularities Sm for steel irradiated with the use of 6 keV
normal ( = 0) and very inclined ( = 85  87) argon ion
beam. Both curves increase with time t of beam
bombardment, independently of beam inclination. Results
presented in Fig. 4 mean widening of convex topographical
forms through eliminating of small dimension features by
more extensive elements, development of some
topographical forms “at the cost” of the others.
Besides vertical Ra and horizontal Sm parameters, the
changes of so-called profile bearing length ratio tp during
ten hours of perpendicular krypton ion beam bombardment
was studied. The parameter tp is important because it gives
information about probability of profile appearance for
approach less than selected c value. The parameter in
question was calculated for c = 50% (the middle of the
distance between the mean line [11] and the maximum
height of examined surface roughness profile). Figure 6
presents changes of tp50 vs. bombardment duration.
45
35
tp50 [%]
Ra [nm]
600
25
15
5
0
2
4
6
8
10
t [h]
Fig.6. Profile bearing length ratio tp50 giving information about
probability of profile appearance for approach less than selected
value c = 50% as a function of bombardment duration ( = 0)
The considered probability increases from about 10 %
for untreated steel surface to about 44 % for eight hours of
the surface irradiation.
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353
Kind of bombarding ions
The important factor that affects the surface
morphological properties is a kind of ions used in irradiation
process. Two kinds of noble gases were utilized to modify
stainless steel surface: argon and krypton. Argon ion beam
is a better roughening tool than krypton one – after ten
hours of 5 keV argon beam bombardment the relative
surface roughness Ra / Ra0 is two times greater than for 6
keV krypton irradiation as can be seen in Fig. 7.
10
5 keV Ar
8
Ra / Ra 0
6 keV Kr
6
4
2
0
0
2
4
6
8
10
t [h]
Fig.7. Relative vertical roughness parameter Ra / Ra0 vs. time of
steel surface irradiation ( = 0) for two kinds of ions (Ar and Kr).
Initial roughness: Ra0/5 keV = 84.5 nm, Ra0/6 keV = 123.3 nm
Sample features
In addition to ion beam parameters and bombardment
process conditions, the sample (especially sample surface
and subsurface) features are important and must be taken
into account, e.g. initial surface roughness or mutual
positioning of surface polishing and ion beam bombardment
directions.
The importance of initial roughness was proved in ion
beam polishing of titanium surface experiments [12]. Two
types of surfaces: initially rough with Ra0 = 1.63 m and
initially smooth with Ra0 = 0.23 m were examined. After ten
hours of argon ion beam bombardment (GD gun was used)
the final values of the parameter Ra was equal: 0.32 m and
0.08 m, respectively. That suggests different kinetics of
both ion polishing processes and indicates very oblique
argon ion beam as a more effective polishing tool for initially
rough surface. It is worth noting here, that expansion of
predominant topographical forms in horizontal direction was
more efficient for initially smooth surface.
Mutual positioning of surface polishing and ion beam
bombardment directions influences the resulting surface
morphology of stainless steel irradiated with the use of
argon beam – that can be observed in Fig. 8.
a)
b)
Fig.8. The influence of mutual positioning of surface polishing and
ion beam bombardment ( = 80  85, t = 10 hours) directions on
the resulting steel surface morphology: a) perpendicular, b) parallel
Both SEM photomicrographs show vertical traces of
oblique beam bombardment but with different directions of
surface polishing: a) horizontal in Fig. 8a (perpendicular to
the beam direction) and b) vertical in Fig. 8b (parallel to the
beam). Although the scratches resulting from polishing
354
process are more visible than ion sputtered traces due to
relatively small ion doses used in experiments (10 hours of
very oblique irradiation), the influence of ion beam
bombardment direction on surface morphology is clearly
seen and must be taken into account.
Conclusions
Surface morphology is usually characterized by well
known and very often investigated surface topography and
roughness. Comprehensive analysis of ion beam induced
morphology should also include its two additional aspects:
profile variability and surface morphology arrangement, that
seem to be good complementary indicators of morphology
changes. Such comprehensive analysis has been done and
results were published recently [6].
The paper concentrates on the first two main aspects:
topography and roughness of 1H18N9T stainless steel
modified by the use of perpendicular and inclined argon and
krypton ion beams from GD ion source with hollow anode.
It was shown that ion induced surface morphology
depends not only on beam parameters, such as: angle of
ion beam incidence, kind of ions or ion energy but also on
surface features of modified material, among others on
mutual positioning of surface polishing and ion beam
bombardment directions.
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_________________________
Author: prof. dr hab. inż. Zbigniew W. Kowalski, Politechnika
Wrocławska, Wydział Elektroniki Mikrosystemów i Fotoniki,
Wybrzeże Wyspiańskiego 27, 50-370 Wrocław,
E-mail: [email protected]
PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 88 NR 11b/2012