RBS and PIXE Ion Beam Methods for Characterizing Ni-Co
Alloys
Iulia Muntele*, Claudiu Muntele*, Ruth Jones**, Robert L. Zimmerman*,
Daryush Ila*
*Center for Irradiation of Materials, Alabama A&M University – Research Institute, PO Box 1447,
Normal AL-35762
**NASA/Marshall Space Flight Center, Huntsville, AL 35812
Abstract. Various electroplating/electroforming processes have been used for years to produce nickel-cobalt
components for applications from space industry to tool refurbishing. The mechanical properties (hardness, strength,
etc.) are drastically affected by the nickel to cobalt ratio of the alloy, as well as the amount of organic additives and trace
contaminants present in the plating tank. Traditionally, chemical or optical methods were used for characterizing the
constituents of the resulting deposit. In this paper we present the usefulness of nuclear methods of analysis based on
accelerated ion beams for performing both qualitative and quantitative compositional characterization of such alloys.
Samples of electroformed materials were prepared in a nickel sulfamate bath with nickel-cobalt ratios ranging between
70:30 (or 2.33:1) and 80:20 (or 4:1) atomic percent. The samples were analyzed using PIXE (Proton Induced X-Ray
Emission) with proton beam of 1 MeV and RBS (Rutherford Backscattering Spectrometry) with 6 MeV nitrogen beam.
cathode raised some problems related with the
hydrogen formation at the reducing electrode - the
cathode, but it is supposed to minimize the
temperature gradient, the gradient of concentration and
facilitate a uniform flow of the plating solution across
the surface of the mandrel.
One of the main parameters to be correlated and
controlled is the proper concentration of ions in the
electroforming solution, which provides the proper
composition of the resulting deposit. For the ion ratio
in solution we used optical absorption measurements,
and for the nickel to cobalt ratio in the deposit we used
PIXE [3] and RBS [2] measurements. We know from
literature [1] that the cobalt ion is deposited
preferentially ahead of the nickel ion. For this reason,
to obtain a 70:30 percent ratio of cobalt to nickel
content into deposit, a concentration of ~3 g/l of cobalt
and 80 g/l of nickel has to be maintained in the plating
bath. Small variations of the ion concentrations in
solution were observed during a plating cycle. These
small variations are attributed to the intrinsic
variations of the pH and temperature, and they don’t
have a significant contribution to the final composition
of the deposit.
INTRODUCTION
Alabama A&M University Research Institute
allocated a laboratory space in the Center for
Irradiation of Materials – H. J. Foster for assembling
an electroforming facility. This space was specially
remodeled to meet the requirements of a laboratory
with office space. The electroplating facility built is
dedicated to electroform Ni-Co alloys from nickel
sulfamate baths. Based on previous experience the
proposed geometry featured a square tank with the
mandrel and the anode baskets hanging over the side.
This classical plating geometry appeared to have
several inconveniences among which were the
gradient of the temperature over the surface of the
mandrel, the concentration gradient from the top to the
bottom of the tank, non-uniform flow of the plating
fluid due to stirring of the plating solution. To simplify
the control of the temperature gradient, minimize the
concentration gradient, and even the distribution of the
electric field lines, the proposed geometry was
abandoned and replaced instead by a cylindrical
geometry with an appropriate placement of the cathode
and anodes. The new approach in the placement of the
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EXPERIMENT
We used the NEC Pelletron tandem accelerator at
the Center for Irradiation of Materials of Alabama
A&M University to perform the RBS and PIXE
analysis. Electroformed deposits from the plating bath
described above were first analyzed using RBS. One
of the reasons to use RBS in determination of the
Ni:Co ratio in the deposit is the ability of this
technique to give information on the elemental
composition of the samples to be analyzed. This
analysis method exploits the backscattering of charged
particles on the electrostatic potential of the nuclei in
the target material. The two-body collision kinematics
shows that the energy of the scattered projectile
depends on the mass of the scatterer (the target
nucleus), revealing its identity. From here, one can
determine with great accuracy parameters such as
stoichiometry, elemental area density, thickness, and
impurity distribution of solid targets. From simulated
data using the computer code RUMP, a beam of 10
MeV nitrogen was proposed (Figure 1) as suitable for
providing enough isotope separation for analyzing
these samples. Due to practical considerations (beam
current and duration of analysis) a lower energy
nitrogen beam (6 MeV) was used instead. The
challenge presented by these particular samples was
that natural Ni has several isotopes, 58Ni (68.27%),
60
Ni (26.1%), 61Ni (1.13%), 62Ni (3.59%), and 64Ni
(0.91%), while natural Co has only one, 59Co, placed
right in between the most abundant isotopes of Ni.
PIXE analysis was performed to complement the
RBS measurements. While the RBS method gives
direct information on the stoichiometry of the sample,
PIXE requires additional measurements on standard
samples. Great care must be taken in preparing the
samples as well as in arranging the detection
geometry. As far as the detection geometry is
concerned, it is important to have a reliable method to
record the total integrated charge on the sample. For
this reason a collimator was placed within a 5 cm
distance from the sample and a 250 V potential was
applied between the collimator and the sample’s
holder to retain the secondary electrons scattered from
the target. The charge integrator was placed in series
with the 250 V battery. The 1 MeV proton beam was
obtained from a KOH cathode using a sputtering ion
source. The X-ray detector used is silicon-PIN with
beryllium window of 180 eV resolution at the 5.9 keV
X-ray line of 55Fe (55Mn) source, and
thermoelectrically cooled to –30oC. The energetic
calibration of the acquisition electronics was done
using sodium and nickel standard samples containing
99.5% respectively 99.9% metal.
FIGURE 1. RUMP simulations for 10 MeV 14N3+ used as
projectile with FWHM of 20 keV (solid line) and 60 keV
(dotted line).
RESULTS
The computer code RUMP was used for the
analysis and simulation of RBS data. The code has
embedded stopping power tables for hydrogen,
deuterium, and helium for energies up to 3 MeV.
Outside these energies and particle ranges, stopping
powers are generated as needed using the Ziegler
Universal Stopping Power Model. In the case of our
experiment the parameters chosen were: geometry
“GENERAL”, beam energy 6 MeV, beam current 20
nA, total integrated charge 35 µC, resolution of the
detector plus the electronics 75 keV. The resulting
spectrum is shown in the Figure 1. One can see that at
this energy of the nitrogen projectile and resolution of
the detection electronics was not possible to resolve
the Co and Ni isotopes into individual peaks or
plateaus. However, RUMP simulations were carried
out and the resulting fit on the slope of the shoulder
(the continuous line overlapped over the raw data in
Figure 2) gave a Ni:Co ratio of 7:2 (or 3.5:1).
Approximately 1% of other impurities were identified
in the deposit as chromium and copper. While the
source for chromium could be identified in the thin
passivation layer applied on the surface of the mandrel
before plating, the copper, which was included in the
simulation just to give the best fit, could not be
justified. Further investigations using PIXE were
required to confirm the Ni:Co ratio in the deposit and
to confirm or rule out the copper contaminant.
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20 mA/cm2. Also it can be observed that the ratio of
Ni:Co is retained by all the samples analyzed, in this
case not depending on the current density at which the
electroforming was carried out.
FIGURE 2. RBS spectrum of an electrodeposit obtained at
20 mA/cm2 current density. The vertical scale represents a
relative yield, normalized to the total integrated charge.
FIGURE 4. PIXE spectra from three samples obtained at
consecutive current densities.
The ratio of Ni:Co obtained from the PIXE analysis
is 18:7 (or 2.57:1). In all the spectra (see Figure 3) it
can be seen an iron peak at channel 686. This peak is
due to the scattering of protons from beam line parts.
CONCLUSIONS
Electroformed deposits were obtained from nickel
sulfamate solution baths, with the resulting deposit
being analyzed by two different ion beam based
methods: RBS and PIXE. With an adequate calibration
and charge collection, PIXE proves to have a better
resolution despite the fact that is an indirect method
for quantitative analysis. In order to efficiently employ
RBS for this particular compound, the resolution of the
RBS system should be improved and other incident
ions and energies should be tried. Further work is
necessary in this direction. Also, plans of extending
the ion beam analytical techniques to include absorbed
hydrogen profiling using the nuclear reaction
1
H(15N,αγ)12C are considered for the near future.
FIGURE 3. PIXE spectrum of an electrodeposit obtained at
20 mA/cm2 current density along with spectra of Ni and Co
standards, and the sum of Co and Ni in their respective
proportions.
Also, the deposit obtained at 20 mA/cm2 displays a
higher level of sodium than the other two samples.
This can be either due to higher contamination on this
particular sample, or to a larger amount of
incorporated additives containing sodium from the
plating solution.
REFERENCES
1. D. Golodnitsky, N. V. Gudin, G. A. Volyanuk,
“Plating and Surface Finishing”, Feb. 1998.
2. Joseph R. Tesmer, Michael Nastasi, “Handbook of
Modern Ion Beam Materials Analysis”, Materials
Research Society, 1995.
Successive spectra of samples obtained at three
different current densities (20, 27 and 30 mA/cm2)
from the electroforming bath are shown in Figure 4.
Their acquisition was taken for the same amount of
charge integrated. The graph shows the same
anomalous sodium content in the sample obtained at
3. Steven A. E. Johansson, John L. Campbell, Klass
G. Malmqvist, “Particle Induced X-Ray Emission
Spectrometry (PIXE)”, John Wiley and Sons Inc.,
1995.
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