Formation of Wear Resistant Steel Surfaces by Plasma
Immersion Ion Implantation
S. Mändl, B. Rauschenbach
Institute für Oberflächenmodifizierung, 04303 Leipzig, Germany
Abstract. Plasma immersion ion implantation (PIII) is a versatile and fast method for implanting energetic ions into
large and complex shaped three-dimensional objects where the ions are accelerated by applying negative high voltage
pulses to a substrate immersed in a plasma. As the line-of-sight restrictions of conventional implanters are circumvented,
it results in a fast and cost-effective technology. Implantation of nitrogen at 30 – 40 keV at moderate temperatures of
200 – 400 °C into steel circumvents the diminishing thermal nitrogen activation encountered, e.g., in plasma nitriding in
this temperature regime, thus enabling nitriding of additional steel grades. Nitride formation and improvement of the
mechanical properties after PIII are presented for several steel grades, including AISI 316Ti (food industry), AISI D2
(used for bending tools) and AISI 1095 (with applications in the textile industry).
and implanted at the whole surface simultaneously
(see Fig. 1). Chamber sizes up to 8 m3 and voltages of
more than 100 kV have been realised3. As the
extraction current density scales with the applied
voltage, a pulsed mode must be employed to limit the
thermal load of the sample and prevent melting within
seconds. During a 10 µs pulse, typical current
densities can exceed 10 mA/cm2 at a plasma density of
1010 cm-3, resulting in a total current of more than 50 A
for sufficiently large samples.
INTRODUCTION
Ion implantation is an indispensable technology in
semiconductor processing, due to its ability for
controlled insertion of specific atoms at a well defined
dose and a predetermined depth of a few micrometer
or less. However, several impediments exist for
widespread applications of ion implantation in
metallurgical and tribological problems.
Beside the already large footprint and cost of
ownership of commercial implanters, the transition
from flat wafers towards complex shaped 3-D objects,
necessitating a complicated target manipulation
system, is accompanied by the desire to treat large
parts, e.g. shafts or spindles, with dimensions of
several feet. Furthermore, case thicknesses of 100 µm
with inserted atom concentrations of up to 25% are
routinely used for wear resistant layers, thus requiring
implanted doses of more than 1018 ions/cm2.
Thus, a faster implantation, independent of the
sample size, coupled with a much lower cost can be
achieved. However, this can be achieved only by
sacrificing the mass separation and energy selection.
Any impurities present in the chamber will be coimplanted, together with all isotopes or molecule
combinations arising from the precursor, while a finite
rise time (≥ 0.5 µs), together with collisions during the
extraction, will lead to an energy spread and a profile
extended towards the surface4. Furthermore, a reliable
dose measurement is complicated by lateral dose
inhomogeneities and a very large, surface dependent,
secondary electron coefficient in the range between 3
and 55.
About 15 years ago, plasma immersion ion
implantation (PIII) was developed independently in
Australia and the U.S. as an alternative to conventional
beam-line implantation1,2, circumventing the above
mentioned restrictions. Here, the sample is directly
placed inside the plasma source, immersed in the
plasma. By applying negative high voltage pulses to
the sample, positive ions are extracted from the plasma
Despite the low range – 100 nm or less – of ion
with an energy of 100 keV or below, thick layers of 10
– 100 µm are achieveable, using implantation at
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
635
leads from AISI 1095 were directly used in real-life
wear tests. Ruby and stainless steel balls were used
for stainless steel and cold work steel, respectively.
elevated temperatures, thus facilitating diffusion. In
this processing mode, PIII is comparable to
thermochemical treatment like plasma nitriding or
carburizing6. However, due to the energetic ion
insertion, the low nitrogen activity at low temperatures
and surface oxide barriers present in some steels are
circumvented, so that PIII is a viable, complementing
technology. This will be shown subsequently by
several examples.
RESULTS
In the following subsections, the wear test results
are presented for the different steel grades, while a
short, general discussion is given in the next section.
gas supply
RF plasma source
(Ar, N2, O2, CH4, ...)
Stainless Steel – AISI 316Ti
Stainless steel exhibit a high corrosion resistance in
the presence of oxidizing agents, as present in
foodstuffs, hot petroleum gases or steam combustion
gases. Here, the grade 316Ti, due to the addition of
Mo and Ti, shows a better pitting resistance and high
temperature performance than the standard 304 grade.
sample
Nitrogen implantation at 400 °C leads to the
formation of expanded austenite8, characterized by a
lattice expansion of about 10% and a nitrogen content,
in solid solution, of some 10 – 20 at.%. At the same
time, a high hardness of more than 1200 HV is
obtained. The temperature is crucial to the successful
treatment, as beyond 420 °C a decay of the metastable
expanded austenite occurs into CrN and ferrite,
whereas the thermally activated diffusion leads to a
layer thickness below 1 µm at 350 °C even when the
sample is kept at this temperature for 10 hours.
high voltage
pulse generator
FIGURE 1. Schematic setup of a PIII experiment including
plasma source, high voltage pulse generator and sample.
2
10
Specific Wear Depth (µm/m)
EXPERIMENT
Nitrogen PIII treatments were performed in a HV
system using an ECR plasma source with a base
pressure of 2 × 10-6 mbar and a working pressure of 3
× 10-3 mbar. High voltage pulses of – 30 kV with a
rise time of less than 0.5 µs were used. After the
heating phase at 1 kHz, the pulse repetition rate was
adjusted between 100 and 500 Hz to maintain process
temperatures of 150 – 400°C. The total pulse numbers
ranged between 2 and 24 × 106, corresponding to doses
between 0.75 and 9 × 1018 cm-2. The dose per pulse
was obtained in a separate experiment7.
1
10
316Ti, Untreated
18
2
316Ti, 4 × 10 N/cm
18
2
316Ti, 9 × 10 N/cm
18
2
440B, 4 × 10 N/cm
18
2
321, 4 × 10 N/cm
0
10
-1
10
-2
10
-3
10
The investigated steel grades include stainless steel
AISI 316Ti, together with 321 and 440B for
comparison, cold work tool steel D2 and high carbon
spring steel 1095. Except for the latter grade, polished
flat samples were used for laboratory pin-on-disc wear
tests without lubrication, while needles and thread
FIGURE 2. Specific wear depth for untreated and PIII
treated 316 Ti, together with two other stainless steel grades,
440B and 321.
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In the present experiment, a thickness of 10 µm,
determined with glow discharge optical spectroscopy
(GDOS), was obtained after 2 h 40 min, corresponding
to an incident dose of 4 × 1018 atoms/cm2. The value
increased to 15 µm after 6 h (9 × 1018 atoms/cm2), due
to the diffusion limited layer growth.
PIII treatment at temperatures of up to 400 °C for a
time of 1 h.
The results of the wear tests, performed at contact
pressures of 0.7 GPa, as a function of the temperature
are shown in Fig. 3. A reduction of the specific wear
by up to 80% was observed.
At the lowest
temperature of 300 °C, a diffusion layer of 3 µm
thickness was observed. An increased diffusivity at
higher temperatures leads to thicker layers, which are
eroded slower and leads to lower effective wear at 350
°C. The behavior at 400 °C is different, correlated
with an increased compound layer at the surface,
which is known to adversely affect the wear.
As can be seen in Fig. 2, the wear resistance is
increased by 2.5 orders of magnitude. Even a layer
thickness of 10 µm is sufficient for industrial
applications as the wear is sufficiently retarded to stay
within the surface layer, so that no difference is seen
between the low dose and high dose implantation. For
comparison, results for grade 321, which is slightly
less corrosion resistant than 316Ti, and 440B, showing
high strength, are included. For these steels, even
better performances could be obtained.
An increase of the contact pressure to 1.4 GPa
resulted in a change of the wear mechanism from an
abrasive to an adhesive mode was observed. By
additionally implanting carbon from a methane
precursor, the friction coefficient, wear and diameter
of the wear track could be further reduced9.
Cold Work Steel – AISI D2
D2 is a high carbon, high chromium tool steel
giving a combination of excellent wear resistance, high
hardness and good toughness. It is suitable for many
cold working applications, including fine blanking
tools, punches, dies, and cold extrusion tools. Albeit it
can be conventionally nitrided, nitriding temperatures
beyond 500 °C necessitate an additional tempering
step and may lead to shape deviations for precision
tools.
Spring Steel – AISI 1095
The spring steel 1095 with a carbon content of 0.95
wt.% has the highest elastic limit and fatigue values of
the commonly used spring steels. It is particularly
suitable for high quality intricate shapes that can not
be formed from pre-tempered steel. Uses include
surgical blades, industrial knives and various
applications in the textile industry, including needles
and thread leads.
0.04
untreated
1 h nitrogen PIII
300°C
350°C
400°C
Amount of Thread Before Failure (g)
3
Specific Wear (µm /m)
0.06
0.02
0.00
FIGURE 3. Specific wear before and after PIII treatment at
different tempeatures.
For the present investigations, a special
pretreatment, namely tempering at a rather high
temperature of 450 °C was employed. This enabled a
8
6
4
2
0
DLC
CrN
Ti/C
PIII
Me:CH CrN ox. Hard Chrome
FIGURE 4. Lifetime of PIII treated and differently coated
needles.
637
coatings, no adhesion problems or a softening of the
bulk was observed.
Especially thread has been shown to be a very
abrasive medium, due to silica particles adhering to
natural cotton, but also to artificial polyester fibers.
Needles with a thickness of 0.3 mm have a typical
lifetime of 7 g thread in a controlled environment.
At the same time, PIII can be used for a fast and
cost-effective modification of large and complex
geometries, where the dose homogeneity can be
predicted and controlled by adjusting the process
parameters10. It is a modification of conventional ion
implantation which allows a significant broadening of
the industrial applications of ion implantation.
In the present experiment, several hard coatings
were compared to PIII treatments at 150 °C. Due to
the low diffusivity, the layer thickness was limited to 2
µm. Higher temperatures were not possible to avoid a
substrate softening. That kind of problem, together
with a mediocre or bad adhesion was observed for
some of the investigated coatings. For example, after
DLC coating, the bulk hardness decreased from 750 to
600 HV.
ACKNOWLEDGMENTS
Dr. J. Kothe from the institute for textile and
process technology, Denkendorf, is acknowledged for
organising the comparison of different methods for
improving tools in textile industry.
The results are presented in Fig. 4, showing a
thread throughput of 2 – 7 g before failure of the
surface layer. PIII treatment results in a slightly below
average improvement, albeit without any adhesion
problems or deterioration of the mechanical bulk
properties.
REFERENCES
1. Conrad, J. R., Radtke, J. L., Dodd, R. A., Worzala, F. J.,
and Tran, N. C., J. Appl. Phys. 62 4591-4596 (1987).
DISCUSSION & CONCLUSIONS
It has been shown for three examples, stainless
steel, cold work steel and spring steel, that it is
possible to form hard and wear resistant surface layers
using plasma immersion ion implantation. In each
case, a significant reduction of the wear rate was
obtained. For stainless steel, where the layer was
intact even after very long testing, thus resulting in a
wear reduction of several orders of magnitude. The
other two cases, treated at lower temperatures, resulted
in thinner layers, which were worn through by
abrasive wear. Thus, only a temporary delay of the
normal wear was observed, resulting in a reduction of
only 30 – 80%.
2. Tendys, J., Donnelly, I. J., Kenny, M. J., and Pollock, J.
T. A., Appl. Phys. Lett. 53 2143-2145 (1988).
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7. Manova, D., Mändl, S., and Rauschenbach, B., Plasma
Sources Sci. Technol. 10 423-429 (2001).
Hence, PIII is a feasible method to extend and
complement thermochemical surface treatment of
steels towards lower temperatures, facilitated by the
energetic ion bombardment. This results in an
increased nitrogen activation independent of the
substrate temperature, even at temperatures as low as
150 °C. Furthermore, surface contaminations and
oxides layers are sputtered by the incoming ions, while
thicker layers of more than a few nanometers are still
penetrated for ion energies near 20 – 50 keV.
8. Mändl, S., and Rauschenbach, B., Defect Diffus. Forum
188/190 125-136 (2001).
9. Thorwarth, G., Mändl, S., and Rauschenbach, B., Surf.
Coat. Technol. 125, 94–99 (2000).
10. Huber, P., Keller, G., Gerlach, J. W., Mändl, S.,
Assmann, W., and Rauschenbach, B., Nucl. Instrum.
Meth B 161/163 1085-1089 (2000).
Thermally activated diffusion of nitrogen and
carbon, however, is strongly suppressed in the lower
temperature range, thus limiting the layer thickness to
a few micrometer. In contrast to deposited hard
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