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Hardening of graphite surface by ion beam amorphization
Scripta METALLURGICA
Vol. 20, pp. 6 4 9 - 6 5 2 , 1986 P r i n t e d in the U.S.A.
P e r g a m o n P r e s s Ltd. All r i g h t s r e s e r v e d
HARDENING OF GRAPHITE SURFACE BY ION BEAM AMORPHIZATION J - P . Hirvonen (a) , D. S t o n e , M. N a s t a s i Ib) and S-P, Hannula (¢) Department of Materials Science and Engineering, Cornell University, Ithaca, N.Y. 14853 (Received (Revised
December February
ii, 19,
1985) 1986)
Introduction Ion implantation has been shown to possess potential as a practical method for improving the mechanical properties of surfaces. So far a number of tests have been performed (1,2). In these studies, metals - most often steels or other hard materials - have been used. However, the improvement of the mechanical properties at the surface is not only important for substances that already have considerable hardness, but also for materials vlth other unique properties. Graphite is useful in many applications. Mechanically, it is easily machined and has good dimensional stability. It has self lubricity and under certain conditions has excellent sliding properties. Chemically, it is generally impermeable, resistant to corrosion, and comparable wlth mating materials. It is a good conductor of both heat and electrlcal current. The ion implantation of graphite has been studied elsewhere (3) but without reference to the mechanical properties of the surface. It is expected, however, that heavy ion bombardment will cause structural changes at the graphite surface resulting in a modification of the mechanical properties. In this investigation we focus on these properties. Our main objectives have been to study how the surface mechanical properties are changed by an ion bombardment and to find the relationship between mechanical properties and microstructure. Experimental Highly oriented pyrolytic graphite samples with the c-axis perpendicular to the surface were bombarded at room temperature. A X e ion beam was chosen in order to prevent chemical doping effects. The energy and flux were 600 keV and 1.9xi013 ionslcm2s respectively. The range of the 600 keV Xe ions in graphite is, according to calculations, 180 nm (4). Consequently, the thickness of the modified layer is enough for micromechanical measurements. Three fluences, 1014, 10 Is and 1.8x10 x6 ions/cm 2 were used in micromechanical tests. The samples for these studies were lx20x20 mm 3 in size. Only the highest fluence was used in mlcrostucture studies performed utilizing transmission electron microscope. Implantations in this case were carried out directly into the already thinned sample . Indentation tests on irradiated and unirradiated specimens were carried out in a special apparatus designed for working at low ( I g) loads. Details of the experimental setup are given in reference (5). In each test the indenter was pushed into the surface at a constant rate of approximately 12 nm/s until the surface layer fractured; then the indenter was removed at the same rate. The indenter used was a commercially available Vickers pyramidal diamond that had a flat area at the tip of roughly l~m x l~m. The load and position were measured continuously vlth an apparatus having resolution capabilities of 3 mg and 2.5 nm, respectively. apermment address: D e p a r t m e n t of P h y s i c s , U n i v e r s i t y of H e l s i n k i , SF-O0170 H e l s i n k i , F i n l a n d bpresent address: C e n t e r f o r M a t e r i a l s S c i e n c e , Los Alamos N a t i o n a l L a b o r a t o r y , Los Alamos, New Mexico 87545, USA Cpresent address: Laboratory of Metallurgy, Technical Research Centre of Finland, SF-02150 Espoo, F i n l a n d
649 0 0 3 6 - 9 7 4 8 / 8 6 $3.00 + .00 C o p y r i g h t (c) 1986 P e r g a m o n P r e s s
Ltd.
650
HARDENING
OF G R A P H I T E
Vol.
20, No.
5
Results and Discussion In Fig. I are shown the results of TEM studies. The microstructure before the ion bombardment consists of large grains. The electron diffraction pattern is formed of single spots revealing a hexagonal six fold symmetry. In addition, some individual hexagonal layers have rotated around the c-axis. After the highest fluence ion bombardment, the diffraction pattern reveals two diffuse rings indicating that all material at the surface has become amorphous (6). The corresponding bright field picture also shows the uniform amorphous structure after the ion bombardment. Examples of data from the micromechanical tests are shown in Figs. 2 and 3. The fracture of the surface can be seen as the sudden shift in the load-lndentatlon depth curve. In the case of the unirradiated sample, the load remains relatively constant during the period of time shortly after rupture; on the ion bombarded samples the load decreases very sharply indicating the fracture of a significantly hardened surface compared to the bulk material. The average load-to-fracture, as a function of fluence, is given in Fig. 4. As can be seen, the strength of the surface layer approaches a maximum at the fluence of 1015 ionslcm 2 after which increasing the fluence has practically no effect. The examination of indentation traces also reveals remarkable changes in surface mechanical properties. As can be seen in Fig. 5 a, the deformation of the unbombarded surface takes place by the relative sliding between different hexagonal layers. Because ion bombardment destroys these individual crystal planes and replaces them with an amorphous, isotropic structure, the sliding mechanism is no longer possible in an irradiated surface region. This is clearly observed in Figs. 5 b and c. In these cases, when the load is increased to a sufficiently high value, the surface layer fractures. An increase in the Vickers hardness has been observed elsewhere for neutron irradiated graphite (7). These experiments were carried out at higher temperatures, and the structure of the graphite after irradiation was not studied. The increase in hardness was believed to result from a decrease in plasticity and a reduction of porosity. Our irradiation conditions differ remarkably from neutron irradiation. In heavy ion bombardment the damage production is restricted to a shallow area close to the surface. In this region the damage production rate and concentration is orders of magnitude higher than in neutron irradiation (8). In our case the lowest fluence is enough to cover the surface entirely with collision cascades and, by using higher fluences, the collision cascades overlap. As a result, an amorphous near-surface layer is formed with modified surface mechanical properties. We also find that the irradiated surface breaks up into small islands at all investigated fluences, Fig. 6. There are two explanations for the appearence of this damage. One is that, because the specific volume of amorphous carbon is greater than that of graphite (5.72 - 6.67 cm3/mole vs 5.33 cm31mole (9)), a tensile stress builds up at the surface, which probably becomes amorphous subsequent to the subsurface region where the damage production has the maximum value. The region closer to the surface is damaged, although it still remains crystalline. However, its plasticity is probably decreased, and instead of the mismatch by deforming plastically, the surface fractures. It has been observed that swelling caused by irradiation can be very anisotroplc (10). In the direction of the c-axis the crystal expands, whereas a contraction takes place parallel to the basal planes. In our samples the c-axis was perpendicular to the sample surface. Thus, this mechanism will produce a tensile stress at the surface region and might even create fracture before any of the material becomes amorphous. According to both of these explanations, the surface should break up into islands before the heavily damaged region reaches the surface. Other investigators who have seen similar surface damage (3) have suggested that it occurs between the heavily damaged subsurface region and less disturbed layer of graphite on top and only appears as the interface approaches the surface.
Vol.
20, No.
5
HARDENING
OF G P
6SI
HOPG UN IRRAOIATEO L f l O . 17g 0.2 c~
0.1
~
0.1
oi~
O2
'
o,
DEPTH IF, m)
FIG. 2. Load versus indentation e~e-~h-for unbombarded graphite.
3O
, HO~
IRRADIATED
- - 2.C
Lfl~'4
•
FRA
g
FIG. I. TEM micrographs of graphTte, a) unbombarded and b) bombarded at 600 keV 1.8x1016 Xe++/cm2. o0 ~
-0.5.
- " ~ oLO
'
,!s .5
'
~.0
OEPTH (F,m)
FIG. 3. Load versus indentation e~hfor graphite bombarded at 600 keV 1.8x1016 Xe+÷Icm ~.
4
0 I0 s
: tO ~
I0 s
I i0 ~
iOi7
FLU~NCE ( ions Icm | )
FIG. 4. u~tTon
FIG. 5. SEM pictures of In-'-~ntation traces on graphite, a) unbombarded, b) bombarded I0 1 4 , and c) bombarded 1.8x1016 Xe++Icm 2 at 600 keV.
Load-to-fracture of fluence.
as
a
652
HARDENING
10
OF G R A P H I T E
Vol.
20, No.
5
p,m FIG. 6. SEM picture of surface a~ge after bombardment a t 600 keV 10 *4 Xe ++ /em 2 .
Acknowledgements The authors acknowledge the support of the National Science Foundation in many facets of this research: D. Stone and S-P. Hannula through the Materials Science Center at Cornell, J-P. Hirvonen and M. Nastasi through contract (L. Toth), ion implantation through National Research and Resource Facility for Submicron Structures and electron microscopy through the Materials Science Center. We wish also to thank Dr. A.W. Moore from the Union Carbide Corporation for the providing of highly oriented pyrolytic graphite. References I. 2.
T. Varjoranta, J-P. Hirvonen and A. Anttila, Thin Solid Films 75, 241 (1981). J. K. Hirvonen in Ion Implantation and Ion Beam Processing of Materials (edited by G.K. Hubler, O.N. Holland, C.R. Clayton and C.W. White), p. 621. North-Holland, New York (1984). 3. B.S. Elman, M. Shayegan, M.S.Dresselhaus, H. Mazurek and G. Dresselhaus, Phys. Rev. B25, 4142 (1982). 4. J.P. Biersack and J.F. Ziegler in Ion Implantation Techniques (edited by H. Ryssel and H. Glaswischning), p. 157. Sringer Series in Electrophyslcs I0. Sprlnger-Verlag, Heidelberg (1982). 5. S-P. Hannula, D. Stone and C-Y. Li, Proceedings of Mat. Research Society, Vol. 40, p. 217. Noth-Holland, New York (1985). 6. J. Kakanoki, K. Katada, T. Hanawa and T. Ino, Acta Cryst. 13, 171 (1960). 7. R. Taylor, R.G. Brown, K. Gilchrist, E. Hall, A.T. Hodds, B.T. Kelly and F. Morris, Carbon 5, 519 (1967). 8. D. Kaletta, Rad. Eff. 47, 237 (1980). 9. CRC Handbook of Chemistry and Physics. The 51st edition 1970 - 1971. The Chemical Rubber Co, Ohio (1970). i0. B.T. Kelly, Physics of Graphite, p. 416. Applied Science Publishers, London (1981).
Vol. 20, pp. 6 4 9 - 6 5 2 , 1986 P r i n t e d in the U.S.A.
P e r g a m o n P r e s s Ltd. All r i g h t s r e s e r v e d
HARDENING OF GRAPHITE SURFACE BY ION BEAM AMORPHIZATION J - P . Hirvonen (a) , D. S t o n e , M. N a s t a s i Ib) and S-P, Hannula (¢) Department of Materials Science and Engineering, Cornell University, Ithaca, N.Y. 14853 (Received (Revised
December February
ii, 19,
1985) 1986)
Introduction Ion implantation has been shown to possess potential as a practical method for improving the mechanical properties of surfaces. So far a number of tests have been performed (1,2). In these studies, metals - most often steels or other hard materials - have been used. However, the improvement of the mechanical properties at the surface is not only important for substances that already have considerable hardness, but also for materials vlth other unique properties. Graphite is useful in many applications. Mechanically, it is easily machined and has good dimensional stability. It has self lubricity and under certain conditions has excellent sliding properties. Chemically, it is generally impermeable, resistant to corrosion, and comparable wlth mating materials. It is a good conductor of both heat and electrlcal current. The ion implantation of graphite has been studied elsewhere (3) but without reference to the mechanical properties of the surface. It is expected, however, that heavy ion bombardment will cause structural changes at the graphite surface resulting in a modification of the mechanical properties. In this investigation we focus on these properties. Our main objectives have been to study how the surface mechanical properties are changed by an ion bombardment and to find the relationship between mechanical properties and microstructure. Experimental Highly oriented pyrolytic graphite samples with the c-axis perpendicular to the surface were bombarded at room temperature. A X e ion beam was chosen in order to prevent chemical doping effects. The energy and flux were 600 keV and 1.9xi013 ionslcm2s respectively. The range of the 600 keV Xe ions in graphite is, according to calculations, 180 nm (4). Consequently, the thickness of the modified layer is enough for micromechanical measurements. Three fluences, 1014, 10 Is and 1.8x10 x6 ions/cm 2 were used in micromechanical tests. The samples for these studies were lx20x20 mm 3 in size. Only the highest fluence was used in mlcrostucture studies performed utilizing transmission electron microscope. Implantations in this case were carried out directly into the already thinned sample . Indentation tests on irradiated and unirradiated specimens were carried out in a special apparatus designed for working at low ( I g) loads. Details of the experimental setup are given in reference (5). In each test the indenter was pushed into the surface at a constant rate of approximately 12 nm/s until the surface layer fractured; then the indenter was removed at the same rate. The indenter used was a commercially available Vickers pyramidal diamond that had a flat area at the tip of roughly l~m x l~m. The load and position were measured continuously vlth an apparatus having resolution capabilities of 3 mg and 2.5 nm, respectively. apermment address: D e p a r t m e n t of P h y s i c s , U n i v e r s i t y of H e l s i n k i , SF-O0170 H e l s i n k i , F i n l a n d bpresent address: C e n t e r f o r M a t e r i a l s S c i e n c e , Los Alamos N a t i o n a l L a b o r a t o r y , Los Alamos, New Mexico 87545, USA Cpresent address: Laboratory of Metallurgy, Technical Research Centre of Finland, SF-02150 Espoo, F i n l a n d
649 0 0 3 6 - 9 7 4 8 / 8 6 $3.00 + .00 C o p y r i g h t (c) 1986 P e r g a m o n P r e s s
Ltd.
650
HARDENING
OF G R A P H I T E
Vol.
20, No.
5
Results and Discussion In Fig. I are shown the results of TEM studies. The microstructure before the ion bombardment consists of large grains. The electron diffraction pattern is formed of single spots revealing a hexagonal six fold symmetry. In addition, some individual hexagonal layers have rotated around the c-axis. After the highest fluence ion bombardment, the diffraction pattern reveals two diffuse rings indicating that all material at the surface has become amorphous (6). The corresponding bright field picture also shows the uniform amorphous structure after the ion bombardment. Examples of data from the micromechanical tests are shown in Figs. 2 and 3. The fracture of the surface can be seen as the sudden shift in the load-lndentatlon depth curve. In the case of the unirradiated sample, the load remains relatively constant during the period of time shortly after rupture; on the ion bombarded samples the load decreases very sharply indicating the fracture of a significantly hardened surface compared to the bulk material. The average load-to-fracture, as a function of fluence, is given in Fig. 4. As can be seen, the strength of the surface layer approaches a maximum at the fluence of 1015 ionslcm 2 after which increasing the fluence has practically no effect. The examination of indentation traces also reveals remarkable changes in surface mechanical properties. As can be seen in Fig. 5 a, the deformation of the unbombarded surface takes place by the relative sliding between different hexagonal layers. Because ion bombardment destroys these individual crystal planes and replaces them with an amorphous, isotropic structure, the sliding mechanism is no longer possible in an irradiated surface region. This is clearly observed in Figs. 5 b and c. In these cases, when the load is increased to a sufficiently high value, the surface layer fractures. An increase in the Vickers hardness has been observed elsewhere for neutron irradiated graphite (7). These experiments were carried out at higher temperatures, and the structure of the graphite after irradiation was not studied. The increase in hardness was believed to result from a decrease in plasticity and a reduction of porosity. Our irradiation conditions differ remarkably from neutron irradiation. In heavy ion bombardment the damage production is restricted to a shallow area close to the surface. In this region the damage production rate and concentration is orders of magnitude higher than in neutron irradiation (8). In our case the lowest fluence is enough to cover the surface entirely with collision cascades and, by using higher fluences, the collision cascades overlap. As a result, an amorphous near-surface layer is formed with modified surface mechanical properties. We also find that the irradiated surface breaks up into small islands at all investigated fluences, Fig. 6. There are two explanations for the appearence of this damage. One is that, because the specific volume of amorphous carbon is greater than that of graphite (5.72 - 6.67 cm3/mole vs 5.33 cm31mole (9)), a tensile stress builds up at the surface, which probably becomes amorphous subsequent to the subsurface region where the damage production has the maximum value. The region closer to the surface is damaged, although it still remains crystalline. However, its plasticity is probably decreased, and instead of the mismatch by deforming plastically, the surface fractures. It has been observed that swelling caused by irradiation can be very anisotroplc (10). In the direction of the c-axis the crystal expands, whereas a contraction takes place parallel to the basal planes. In our samples the c-axis was perpendicular to the sample surface. Thus, this mechanism will produce a tensile stress at the surface region and might even create fracture before any of the material becomes amorphous. According to both of these explanations, the surface should break up into islands before the heavily damaged region reaches the surface. Other investigators who have seen similar surface damage (3) have suggested that it occurs between the heavily damaged subsurface region and less disturbed layer of graphite on top and only appears as the interface approaches the surface.
Vol.
20, No.
5
HARDENING
OF G P
6SI
HOPG UN IRRAOIATEO L f l O . 17g 0.2 c~
0.1
~
0.1
oi~
O2
'
o,
DEPTH IF, m)
FIG. 2. Load versus indentation e~e-~h-for unbombarded graphite.
3O
, HO~
IRRADIATED
- - 2.C
Lfl~'4
•
FRA
g
FIG. I. TEM micrographs of graphTte, a) unbombarded and b) bombarded at 600 keV 1.8x1016 Xe++/cm2. o0 ~
-0.5.
- " ~ oLO
'
,!s .5
'
~.0
OEPTH (F,m)
FIG. 3. Load versus indentation e~hfor graphite bombarded at 600 keV 1.8x1016 Xe+÷Icm ~.
4
0 I0 s
: tO ~
I0 s
I i0 ~
iOi7
FLU~NCE ( ions Icm | )
FIG. 4. u~tTon
FIG. 5. SEM pictures of In-'-~ntation traces on graphite, a) unbombarded, b) bombarded I0 1 4 , and c) bombarded 1.8x1016 Xe++Icm 2 at 600 keV.
Load-to-fracture of fluence.
as
a
652
HARDENING
10
OF G R A P H I T E
Vol.
20, No.
5
p,m FIG. 6. SEM picture of surface a~ge after bombardment a t 600 keV 10 *4 Xe ++ /em 2 .
Acknowledgements The authors acknowledge the support of the National Science Foundation in many facets of this research: D. Stone and S-P. Hannula through the Materials Science Center at Cornell, J-P. Hirvonen and M. Nastasi through contract (L. Toth), ion implantation through National Research and Resource Facility for Submicron Structures and electron microscopy through the Materials Science Center. We wish also to thank Dr. A.W. Moore from the Union Carbide Corporation for the providing of highly oriented pyrolytic graphite. References I. 2.
T. Varjoranta, J-P. Hirvonen and A. Anttila, Thin Solid Films 75, 241 (1981). J. K. Hirvonen in Ion Implantation and Ion Beam Processing of Materials (edited by G.K. Hubler, O.N. Holland, C.R. Clayton and C.W. White), p. 621. North-Holland, New York (1984). 3. B.S. Elman, M. Shayegan, M.S.Dresselhaus, H. Mazurek and G. Dresselhaus, Phys. Rev. B25, 4142 (1982). 4. J.P. Biersack and J.F. Ziegler in Ion Implantation Techniques (edited by H. Ryssel and H. Glaswischning), p. 157. Sringer Series in Electrophyslcs I0. Sprlnger-Verlag, Heidelberg (1982). 5. S-P. Hannula, D. Stone and C-Y. Li, Proceedings of Mat. Research Society, Vol. 40, p. 217. Noth-Holland, New York (1985). 6. J. Kakanoki, K. Katada, T. Hanawa and T. Ino, Acta Cryst. 13, 171 (1960). 7. R. Taylor, R.G. Brown, K. Gilchrist, E. Hall, A.T. Hodds, B.T. Kelly and F. Morris, Carbon 5, 519 (1967). 8. D. Kaletta, Rad. Eff. 47, 237 (1980). 9. CRC Handbook of Chemistry and Physics. The 51st edition 1970 - 1971. The Chemical Rubber Co, Ohio (1970). i0. B.T. Kelly, Physics of Graphite, p. 416. Applied Science Publishers, London (1981).