Download 738 influence of the oxide layer in the erosion of metals caused by

JORNADAS SAM/ CONAMET/ SIMPOSIO MATERIA 2003
08-17
INFLUENCE OF THE OXIDE LAYER IN THE EROSION OF METALS CAUSED
BY ELECTRICAL PLASMA DISCHARGING
I. Barrientos Cabrera a, A. Lasagnia, F. Soldera a, J. Vivas Hohlb , F. Mücklicha
a
Functional Materials, Saarland University, PO Box 15 11 50 D-66041, Saarbrücken, Germany
[email protected]
b
GEMAT, Universidad Tecnológica Nacional – Unidad Académica Confluencia, Plaza Huincul, Argentina
Single-discharge erosion experiments were done in pure metals samples so as in pre-oxidized samples of RuAl
intermetallic compounds. The discharges were produced in air at 9 bar and room temperature using the samples
as the cathode. The volume of the craters produced by the discharge were measured using White Light
Interferometry and correlated with several properties of materials, in view of different models of material
erosion. It was found that the quotient between the eroded volume and the spark energy correlates well with a
function of the energy necessary to heat the material up to the melting point and to melt it. Moreover, it was
found that the crater shape is related to the surface tension of the molten material. When the sample had an oxide
layer, this was destroyed allowing the metal to contribute to the electron emission, necessary for sustaining the
discharge. The form, continuity ,and thickness of the oxide layer have a strong influence on the crater shape as
well as the crater dimensions. The close relationship between processing, phase properties, geometrical design
and functional application requirements could be very clearly demonstrated. These investigations may be
relevant for the development of high performance materials for electrode and contact applications.
Keywords: oxidation, plasma erosion, electrical discharge, electrode materials
1. INTRODUCTION
A spark produces an energy input into the material
which increases very locally the material temperature
even over the boiling point. Thus, a part of the
electrode material is melted and sometimes
evaporated. Three erosion mechanisms have been
suggested to describe the spark erosion: the particle
ejection model [1], the theory of sputtering [2] and the
Llewellyn Jones equation [3]. These models are
related with (1) the melting enthalpy (defined as the
energy required to heat up the material from the initial
temperature to the melting point plus the latent heat of
melting), (2) the sublimation energy, and (3) the
vaporization enthalpy (defined as the melting enthalpy
plus the energy required to heat the material up to the
boiling point and the latent heat of vaporization)
respectively.
Previous investigations [4] have shown that there is a
strong relation between the melting enthalpy and the
quotient “eroded volume / spark energy”. This relation
is given by Eq. 1:
EV
1
= α Tm
SE


ρ ∫ cps ( T ) dT + Lm 
 To

(1)
being cps : specific heat of solid; Lm : latent heat of
melting; ρ: density; Tm : melting temperature; T0 :
initial temperature (298K); EV: eroded volume; SE:
spark energy; α.: constant of proportionality.
This together with the presence of particles of metal in
the cathode surface suggested that the electrode
erosion occurs following the particle ejection model
proposed by Gray et al.[1]. In this erosion mechanism,
first the spark produces a pool of molten material with
a high plasma pressure acting on it. After the abrupt
cessation of the discharge, molten metal is ejected
from the pool due to the unbalanced recoil force from
the material against the electrode gap. Thus a crater
forms on the electrode.
In applications at high temperature, like in spark
plugs, the oxidation of the material contributes to the
general material erosion of the electrode. Furthermore,
the formation of an oxide layer in the electrode surface
may influence the crater formation and the material
erosion. This influence will be investigated in this
paper using intermetallics compounds of RuAl with
different microstructures, which present oxide layers
with different morphologies [5, 6]. This will be
compared with the crater formation in pure metals.
2. EXPERIMENTAL
Individual sparks were done using cylindrical samples
(3 mm diameter) as cathode and a spark plug with a Pt
electrode as the anode. The electrode gap was 1 mm
and the gap medium was air at 9 bar absolute. A
description of the electrical system, so as the
oscilloscope and the probes used to register the
voltage and current of the spark can be found in [7].
The eroded surfaces were measured using a “Zygo
New View 200 3D Imaging Surface Structure
Analyzer” [8] and examined using scanning electron
microscopy (SEM).
The pure metals samples (W, Ir, Ru, Pt, Ni, Au, Ag,
Cu, Al, Pb, and Sn) were polished with SiC paper and
diamond particles down to a grain size of 0.5 µm.
The RuAl samples were produced by two methods:
a) casting (RuAlS): Ru and Al powders with nominal
composition Ru 50 Al50 were melted under argon
atmosphere in an arc oven with a cold copper plate.
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Despite the stoichiometric starting composition, the
arc-melting technique does not produce single phase
materials, but a two-phase microstructure with
primary RuAl dendrites surrounded by an
intergranular α-Ru phase.
b) powder metallurgy + casting (RuAlP): Ru and
Al powder (average particle size of 14 µm and 22 µm
respectively) were mixed together in the relation 47,6
at. % Ru and 52,4 at. % Al. The samples were cold
pressed at 990 MPa and axially hot pressed in high
vacuum (10-5 mbar) in two steps: firstly at 500 °C and
264 MPa for 0,5 h and then at 700 °C and 264 MPa
for 1 h. Finally they were annealed at 1600 °C for 12h
in a vacuum atmosphere (10-5 mbar), which produced
a single-phase structure, with a residual porosity of
1.9 %. After that they were arc-melted like in case (a)
to reduce the porosity (~ 0.6 %).
The RuAl intermetallics samples were pre-oxidized at
1000°C for different times (1 to 20 h).
3. RESULTS AND DISCUSSION
3.1 Crater formation in pure metals
Three different types of craters can be observed from
the experimental results. Because of its special
topography, we have called them “normal craters”,
“flat craters” and “undulated surface craters”.
Normal craters (Fig. 1c and d) present a depression in
the middle and an external rim. These craters are
typical in Al, Sn, Pb, Cu, and some times in Pt and
Ni. This structure is caused by the displacement of
molten material to the outside zone. These craters also
present the higher material lost. The topography of
Undulated surface craters (Fig. 1a) is very different
in comparison with normal craters. They present an
undulated surface resulting in the lower eroded
volume. They are found in W, Ir, Ru, and some times
in Pt and Ni. The shape of Flat craters (Fig. 1b) is
between normal and undulated craters. Its eroded
volume is also between these last ones. These craters
were observed in Ag, Au and in some cases in Pt and
Ni.
Comparing the topography of the craters with the
surface tension of the metal evaluated at the melting
point, it is possible to see that when the surface
tension is large, the shape of the craters corresponds to
the undulated surface craters. For medium values the
craters are flatter (flat craters), and its eroded volume
lies between “normal craters” and “undulated surface
craters”. Normal craters and undulated surface
craters can be also found at medium values of the
surface tension. The “normal craters” are obtained
for smaller values of the surface tension resulting in
high eroded volume.
This result is in accordance with “the particle ejection
model” since it is more difficult to eject a molten
particle from the molten pool if the surface tension of
the metal at its melting point is considerably high.
Then, the molten pool oscillates without loosing
material, freezes the shape during rapid cooling, and
after solidification remains as a flat undulated surface.
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Figure 1. Topography of the three kinds of craters
(WLI): (a) undulated surface crater in W, (b) flat
crater in Ag, (c) normal crater in Cu, (d) SEM image
of a crater in Cu correspondent to Fig. c.
In materials with small melting enthalpy (Sn, Pb and
Al), a different erosion morphology can be observed.
These metals present a main big crater (like normal
craters) or a large succession of overlapped craters,
and surrounding them a large eroded zone with small
craters [4]. This brings evidence of the movement of
the spark over the material surface producing a big
eroded zone.
3.2 Crater formation in RuAl
The crater shape in single phase RuAl corresponds to
the “normal craters”. The eroded volume was
between those for Al and Ru (see Fig. 2). Since the
melting enthalpy of RuAl is not known, it was not
possible to evaluate it with equation (1). Also the
surface tension is not know, which does not allow to
correlate its value with the shape of the crater.
Figure 2. Eroded volume (EV) divided by the spark
energy (SE) for the different materials. Each bar
represent the average of 10 sparks.
3.3 Influence of the oxide layer in the crater
formation
After oxidation at 1000 °C, the single phase RuAl
samples (RuAlP) show the formation of a dense and
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compact oxide scale and an Al-depleted sub-layer
(Fig. 3). The oxide scale is very regular and covers all
the sample-surfaces. The formation of the Al-depleted
layer, which is a common phenomenon also for other
aluminides [9,10], is caused by the outward diffusion
of Al. This more reactive element migrates outwards,
driven by the large affinity for oxygen. In NiAl, the
Al-depleted layer is still the β-NiAl phase.
Conversely, in RuAl, this region forms a new phase,
rich in Ru.
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about 18 µm compared with only 2,5 µm in RuAlP
[5].
The craters in pre-oxidized single phase RuAl samples
show a rounded form (Fig. 5). Their diameter is about
10 µm and the depth changes with the thickness of the
oxide scale. In the case of the figure, the crater is
about 1,8 µm deep. Around the crater there is an
elevation caused by the melting and re-solidification
of the oxide. The SEM image in back scatter electrons
(BS) mode shows a strong contrast between the bright
crater and the dark oxide layer outside the crater. This
means that the oxide layer was removed by the
discharge until the metal appeared. EDX analysis
showed a large enrichment in Ru, which is due to the
presence of the Al-depleted layer (see Fig. 3).
Figure 3. Cross section of a single phase RuAl sample,
which was oxidized in air for 20 h. A dense protective
Al2 O3 -scale and an Al-depleted layer are formed.
Figure 4. Cross section (FIB) of a two phase RuAl
sample produced by casting and oxidized at 1000 °C
for 3 h. In the intergranular phase there is a strong
oxidation.
Samples with an intergranular α-Ru phase (RuAlS)
show a strong intergranular oxidation, that penetrates
deep into the material (Fig. 4). The external alumina
scale covers only the RuAl-grains and remains open
where the intergranular phase was present. After the
oxygen penetrates in the intergranular phase through
the interface, Ru oxidize forming RuO2 . At the
existing high temperature and in further contact with
oxygen volatile ruthenium oxides RuO3 and RuO4 are
formed from RuO2 and evaporate owing to their
appreciable high vapor pressure [11,12]. The Al2 O3 scale is less dense and less protective than in RuAlP
samples, and therefore their growth velocity is much
higher and do not show any passivation. For the same
oxidation times, this scales are much thicker than in
single phase samples. For example, for 100 h they are
Figure 5. Crater in a preoxidized single phase RuAl
sample. The oxidation time was 4 h. a) SEM
secondary electrons image and SEM back scatter
electrons image (small square). b) Profile along a line
crossing in the middle of the crater (WLI).
In the same figure it is possible to see some droplets,
which are white in the BS image. This means that
these droplets are made of Ru, which were melted
during the discharge, ejected from the molten pool and
then re-solidified. This indicates that despite the
different shape of this craters compared with craters in
pure metals, the crater formation mechanism is similar
and follows the particle ejection model [1].
In RuAlS samples, the craters are always found in the
intergranular phase (F ig. 6) and they have a very
irregular form. Many small craters can be seen
distributed in a large eroded region, of about 60 µm.
With Finite Element Methods (FEM), the electrical
field near the surface was simulated, and it was found
that near the oxide peaks (see Fig. 4) there is an
increase of 2,5 times in the electrical field. This
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should attract the discharge and therefore the craters
are found in this regions.
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material loss shows a strong increase, particularly for
two phase samples, where the oxide thickness was
particularly large.
4. CONCLUSIONS
Figure 6. SEM image of a crater in a pre-oxidized two
phase RuAl sample. The discharge impacts in the
intergranular phase forming a crater with an irregular
shape.
The crater shape in different metals and its
corresponding material loss show a strong connection
with the melting enthalpy and surface tension at the
melting point. This permits a classification of the
crater shape in “normal craters”, “flat craters” and
”undulated surface craters”. Craters in RuAl
corresponds to the normal craters, and their material
loss lies between those of Al and Ru. The structure
and thickness of the oxide scale in RuAl depends
strongly on the previous microstructure. Single phase
RuAl have a continuous Al2 O3 -scale. In two phase
RuAl there is a strong intergranular oxidation, which
interrupts the oxide scale. Moreover, the scale is much
thicker than in single phase material. The sparks
destroy in each case the oxide scale, since they need
the contribution of the metallic substrate for the
emission of new electrons. In this way, for thicker
scales like in two phase RuAl, the craters and
consequently the material loss is much larger than for
thiner scales.
5. REFERENCES
Figure 7. Crater depth as a function of the oxide scale
thickness in pre-oxidized RuAl samples. The scales in
two phase samples (produced by casting) are much
thicker than in single phase samples (reactive hot
press + casting). In each case, the depth of the crater is
larger than the thickness of the oxide scale.
The crater depth is always larger than the oxide
thickness in each sample (Fig. 7). That means, the
discharge destroys the oxide layer until the plasma
reaches the metallic substrate. This is independent of
the thickness of the layer, since it is necessary for the
sustaining of the discharge, that electrons from the
cathode are delivered to the plasma, contributing with
the charge transport. This electrons cannot be
provided by the ceramic and must be provided by the
metal. This have a great consequence in the erosion
resistance of the material, showing that when the
thickness of the oxide scale increases, the crater size
also increases. Thus the material lost will be greater
for thicker oxide scales, as it is shown in Fig. 2
compared to the material lost obtained in Al, Ru and
not oxidized RuAl. For short oxidation times, the
material loss is comparable to that for no oxidized
samples. However, for large oxidation times the
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