Ion Beam Optimized Mechanical Characteristics of Glassy
Polymeric Carbon for Medical Applications
M. G. Rodrigues1, N. C. da Cruz2, E. C. Rangel2, R. L. Zimmerman3, D. Ila3, D. B.
Poker4 and D. K. Hensley4
1
Department of Physics and Mathematics – FFCLRP – USP, Ribeirão Preto, SP, Brazil
2
Department of Physics and Chemistry, UNESP, Guaratingueta, SP, Brazil
3
Center for Irradiation of Materials, Alabama A&M University, Normal, AL, USA
4
Oak Ridge National Laboratory, SMAC, Oak Ridge, TN, USA
Abstract. Glassy Polymeric Carbon (GPC) has medical applications owing to its inertness and biocompatible
characteristics. Commercial GPC prosthetics include mitral, aortic and hydrocephalic valves. Surface treatment of GPC
increases the adhesion of endothelic tissue on GPC and avoids the occurrence of thrombus in cardiac implant. In this
work, ion beam was used to improve the mechanical characteristics of GPC surface. Hardness was measured as a
function of depth in precursor and GPC samples heat treated from 300 to 2500 °C before and after bombardment with
energetic ions of silicon, carbon, oxygen and gold at energies of 5, 6, 8 and 10 MeV and fluences between 1.0x1013 and
1.0x1016 ions/cm2. Comparison shows that hardness increases of the bombarded samples depend on heat treatment
temperature. We verify that ion bombardment promotes carbonization due to an increased linkage between the chains of
the polymeric material in lateral groups that are more numerous for samples heat treated to 700 °C.
dependent on the specie, energy and fluence of the ion
beam. In this work, we show that ion bombardment
improves the hardness of GPC sample surfaces and
helps to avoid the propagation of micro cracks, another
problem involving GPC prosthetic cardiac valves.
INTRODUCTION
Glassy Polymeric Carbon (GPC) is made from
precursor resins by heat treatment in inert atmosphere
above 1000 °C. During the carbonization, chemical
changes take place eliminating non-carbon elements
by diffusion while aromatic rings condense to form
graphitic layers. These graphitic layers are in random
arrangement enclosing unconnected pores (1). GPC is
a pure impermeable isotropic carbon material. GPC is
a very hard material with a black, shiny and very
smooth surface. Owing to its chemical inertness and
biocompatibility, GPC is useful for medical
applications such heart valves and other prosthetic
devices (2).
EXPERIMENTAL
Samples of phenolic resin 1x1 cm2, with 1 mm
thickness, were cured at 90 °C on glass substrates. The
samples were cleaned with strong acid and distilled
water and then pyrolized in an argon atmosphere at
very slow heating rates. Before bombardment the
density and elemental composition of each sample
were determined. Samples prepared at 700, 1000 and
1500 °C were bombarded uniformly with ions of
silicon, carbon, oxygen and gold at energies and
fluencies shown in Table I. The analysis of the results
as a function of the heat treatment temperature (HTT)
is useful to study the effects of ion bombardment upon
materials with decreasing non-carbon elements until
the pure stable structure of GPC is formed for HTT
above about 1500 °C.
However, GPC cardiac valves can present
thromboembolic complications due the low adherence
of the endothelic tissue on its surface after the implant.
To prevent occurrence of thrombi and so improve the
biocompatibility of GPC cardiac valves, texturization
of its surface has been studied. In earlier work (3) it
was shown that bombardment with energetic ions of
silicon, carbon, oxygen and gold causes damage to the
GPC surface, exposing its pores immediately below
the surface, making them open. The roughness was
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A Hysitron Triboindenter was used to measure the
hardness of the samples to a depth less than the range
of each energetic ion. Table II shows the range of the
ion energies chosen.
Mechanical properties of non-bombarded samples
prepared at HTT of 300, 400, 700, 1000, 1500 and
2500 °C were compared with those of the ion
bombarded samples.
TABLE I: Ions, energies and fluencies used in the bombardment of GPC samples.
Ion
Energy (MeV)
Fluence (ions/cm2)
13
Silicon*
5
0.5 x 10
3.4 x 1013
Carbon*
6
3.0 x 1013
10 x 1013
13
Oxygen*
8
1.0 x 10
2.8 x 1013
Gold**
10
1.0 x 1015
* samples prepared at 700°C and 1500°C; ** samples prepared at 700°C and 1000°C
TABLE II. Ion range and electronic stopping power
for each ion bombardment.
Ion
Ion range
Electronic
HTT (°C)
stopping
(µm)
power (x103
eV/nm)
Silicon
700
4.20
2.137
1500
4.06
2.232
Carbon
700
6.89
1.041
1500
6.57
1.087
Oxygen
700
6.90
1.460
1500
6.56
1.525
700
2.90
2.951
Gold
1000
2.70
3.109
Samples
0
2500 C
0
1500 C
0
1000 C
0
700 C
0
400 C
0
300 C
3,5
3,0
Hardness (GPa)
10 x 1013
30 x 1013
10 x 1013
1.46 x 1016
2,5
2,0
1,5
1,0
0,5
0,0
100
1000
Depth (nm)
RESULTS
FIGURE 1. Hardness as a function of depth of phenolic
resin, the precursor of GPC, after heat treatment to 300°C,
400°C, 700°C, 1000°C, 1500°C and 2500°C.
The samples presented densities of 1.44, 1.52, 1.50
and 1.50 g/cm3 and Carbon-Oxygen-Hydrogen
percentages of 85-10-5, 90-8-2, 95-4-1, and 100-0-0
for heat treatment temperatures of 700, 1000, 1500 and
2500 °C, respectively.
Fig. 2 shows that there is no significant difference
between hardness of silicon ion bombarded samples
prepared at 700 and 1500 °C. By comparison with Fig.
1, this signifies that the bombarded samples prepared
at 700 °C have a greater relative increase in hardness
than for samples prepared at 1500 °C.
The results for the hardness of non-bombarded
GPC samples as a function of the depth are shown in
Fig. 1. A progressive increase of the hardness at a
depth of 100 nm reaching a maximum around 2.7 GPa
for the sample prepared at 1000 °C. The samples
prepared at 300 and 400 °C presented a relatively
constant hardness as a function of the depth as
expected for phenolic materials. The hardness of
samples prepared at 700, 1000 and 1500 °C decreases
with the depth and tends to stabilize around 100 nm of
depth. The hardness of the sample prepared at 2500 °C
increases with depth.
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0
12
2
0
700 C, 5.0x10 ions/cm
0
12
2
1500 C, 5.0x10 ions/cm
13
2
0
700 C, 3.4x10 ions/cm
0
13
2
1500 C, 3.4x10 ions/cm
14
2
700 C, 1.0x10 ions/cm
0
14
2
1500 C, 1.0x10 , ions/cm
0
2
0
13
2
0
700 C, 2.8x10 ions/cm
0
13
2
1500 C, 2.8x10 ions/cm
14
2
700 C, 1.0x10 ions/cm
0
14
2
1500 C, 1.0x10 ions/cm
4.0
6.0
3.5
4.8
3.0
Hardness (GPa)
Hardness (GPa)
7.2
3.6
2.4
Silicon
bombardment
1.2
0.0
0
30
60
90
120
150
180
13
2
0
14
1.5
1.0
Oxygen
bombardment
30
45
60
75
90
105
120
Depth (nm)
FIGURE 4. Hardness as a function of depth of GPC samples
prepared at 700 and 1500 °C, bombarded with 8 MeV
oxygen ion.
Fig. 3 shows that carbon ion bombarded samples
prepared at 700 °C presented greater relative hardness
increase than do samples prepared at 1500 °C, at the
same fluences. The figure does not show a significant
dependence of the depth where the hardness reaches an
approximately constant value (80 nm of depth) on the
HTT. The hardness increased with the fluence for the
sample prepared at 1500 °C, but not for the samples
prepared at 700 °C, which decreased.
0
2.0
15
FIGURE 2. Hardness as a function of depth of GPC samples
prepared at 700 and 1500 °C, bombarded with 5 MeV silicon
ion.
700 C, 3.0x10 ions/cm
0
13
2
1500 C, 3.0x10 ions/cm
2.5
0.5
210
Depth (nm)
2
700 C, 3.0x10 ions/cm
0
14
2
1500 C, 3.0x10 ions/cm
0
14
For the bombardment with gold ions, Fig. 5, the
sample prepared at 1000 °C, at a fluence of 1.0x1015
ions/cm2 has slightly greater hardness than the sample
prepared at 700 °C, at fluence of 1.46x1016 ions/cm2.
However, the increase of the hardness relative to the
non-bombarded samples was greater for the sample
prepared at 700 °C than for the sample prepared at
1000 °C. The depth for which the hardness reaches an
approximately constant value was smaller for the
sample prepared at the higher temperature (about 70
nm). Other samples bombarded with gold did not
present reproducible results due to their high
roughness presented after the bombardment. The error
bars in some results are consequence of the roughness
promoted by bombardment.
2
700 C, 1.0x10 ions/cm
5
Hardness (GPa)
13
700 C, 1.0x10 ions/cm
0
13
2
1500 C, 1.0x10 ions/cm
4
3
2
0
0
16
2
700 C, 1.46x10 ions/cm
0
15
2
1000 C, 1.0x10 ions/cm
Carbon bombardment
1
5.5
0
50
100
150
200
Gold bombardment
5.0
Depth (nm)
Hardness (GPa)
4.5
FIGURE 3. Hardness as a function of depth of GPC samples
prepared at 700 and 1500 °C, bombarded with 6 MeV carbon
ion.
For the bombardment with oxygen ions, Fig. 4, the
samples prepared at 700 °C showed a greater increase
in hardness than the samples prepared at 1500 °C, for
the same fluence. The graphs show increasing
hardness with the depth tending to be constant after 70
nm. The hardness of the samples prepared at 700 and
1500 °C increases with the fluence (until the fluence of
2.8x1013 ions/cm2) then decreases at a fluence of
1.0x1014 ions/cm2.
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0
20
40
60
80
100
120
140
160
180
Depth (nm)
FIGURE 5. Hardness as a function of depth of GPC samples
prepared at 700 and 1000 °C, bombarded with 10 MeV gold
ion.
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DISCUSSION
CONCLUSIONS
For the case of the non-bombarded samples the
increase of the hardness as a function of the HTT
observed is due to the increase of the number of crosslinks, accompanied by dehydrogenation, during the
formation of the material. The decrease observed in
the hardness for HTT above 1000 °C comes from the
rearrangement of the crystallite when the interwoven
ribbon cross links are broken, leading to a decrease in
the intermolecular forces. The increase in hardness
with depth observed in the low HTT non-bombarded
samples can be explained by the fact that the material
is more carbonized near the surface where the noncarbon species can diffuse faster than in the bulk of the
material.
The hardness of GPC depends on the HTT reaching
a maximum around 1000 °C. Bombardment with
silicon, carbon, oxygen and gold ions increases the
hardness of GPC. The relative increase in the hardness
is greater for low HTT samples that contain more noncarbon elements in lateral groups. The increase in
hardness is a result of the generation of free radicals
and the recombination of broken bonds. The
dependence of hardness increase in GPC on the
electronic stopping power of the projectile deserves
further analysis.
ACKNOWLEDGMENTS
For the case of the bombarded samples the energy
used was high enough to minimize the number of
nuclear
elastic
collisions,
which
produce
displacements of heavy nuclei from their original
positions generating disorder and degradation of the
material (4). The dominant process of energy
deposition in the samples results from electronic
interactions. Inelastic collisions between ions and
electrons produce excitement, ionization, breakup of
chemical bonds and free radical formation. The excited
atoms or molecules do not cause significant
modifications to properties of the bombarded
polymeric materials because the energy loss occurs
through phonons by vibrations of atoms of the
material. The breaking of bonds generally happens in
lateral groups liberating chemical species of the
material that in the case of the precursor GPC should
be H, CHx, HO or CO. The modified structures tend to
induce the formation either of unsaturated bonds (C=C
and C≡C) or the union of neighboring chains through
covalent bonds. This mechanism happens when there
is enough proximity for reaction among the free
radicals. The larger the number of reactive species
formed in the chains, the greater the probability of
linking the chains.
Fundação de Amparo à Pesquisa do Estado de São
Paulo-FAPESP (proc. 96/01386-2 and 96/04979-4)
and Alabama A&M University for financial support.
REFERENCES
1. G. M. Jenkins and K. Kawamura, Polymeric carbonscarbon fiber: glass and char, Cambrigde University
Press, London, 1976.
2. G. M. Jenkins, D. Ila, H. Maleki, Mater. Res. Soc. Symp.
Proc., 394 (1995) 181
3. M. G. Rodrigues, D. Ila, M. C. Rezende, A. Damião, R.
L. Zimmerman, Applications Of Accelerators In
Research And Industry: Proceedings Of The Fifteenth
International Conference, Jerome L. Duggan e I. Lon
Morgan, Denton, 1999, p.1237.
4. A. L. Evelyn, D. Ila, R. L. Zimmerman, K. Bhat, D. B.
Poker, D. K. Hensley, Mater. Res. Soc. Symp. Proc., 438
(1997) 499.
This may explain why samples treated at 700 °C,
which, a priori, are not GPC and still contain H, HO
and CO, had larger increases of hardness with
bombardment than those prepared at higher
temperature. For a high concentration of non-carbon
elements near the surface, the concentration of free
radicals is higher and the linking between the chains
higher than in the bulk of material. The profile of the
hardness near the surface correlates with the profile of
non-carbon species. Ion bombardment produces
greater relative increase in the hardness for samples
prepared at 700°C.
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