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Hardness and wear properties of boron-implanted poly(ether-ether-ketone) and poly-ether-imide
Surface and Comin.gs Te(:hnoh~y, .~i (1992) 267-272
267
Hardness and wear properties of boron-implanted poly(etherether-ketone) and poly-ether-imide Y o u n g c h u l Lee, E, H, Lee a n d L. K, M a n s u r Metals and Ceramics Division, Oak Ridge National Laborenory, Oak Ridge, TN 37831 (USA)
Abstraet The effects of boron beam irradiation on the hardness, friction, and wear of polymer surfaces were investigated. Typical high-
performance thermoplastics, poly(ether-e.ther-ketone)(PEEK) and a poly-ether-imide (Ultem) were studied after 200 key boron ion beam treatment at ambient temperature to doses of 2.3 × l0 t+, 6.8 x 10=+, and 2.2 × l0 ts ions cm -2, The hardnesses of pristine and boron-implanted materials were characterized by a conventional Knoop method and a load-depth sensing nanoindentation technique. Both measurements showed a significant increase in hardness with increasing dose. The increase in hardness was also found to depend on the penetration depth of the diamond indenter. Wear and friction properties were characterized by a reciprocating sliding friction tester with an SAE 52100 high-carbon, chrome steel ball at 0.5 and I N normal loads. Wear and frictional properties varied in a complex fashion with polymer type and dose. but not much with normal load. A substantial reduction in friction coefficient was observed for PEEK at the highest dose but no reduction was observed for Ultem. The wear damage was substantially redound at the highest dose for both Ultem and PEEK. For the system studied, the highest dose. 2.2 x l0 ts ions cm -~+ appears to be optimum in improving wear resistance for both PEEK and Ultem.
I. latmducttea Polymers have shown their importance in mechanical and structural applications since they are inherently light (typically 10%-20% the weight of metals) and easy to shape, Their main applications, however, have been limited to relatively mild surface contact conditions, Surface-sensitive mechanical properties such as surface durability and hardness must he improved to meet more demanding requirements, Polymer wear can he controlled by changing the bulk properties, by particulate reinforcement and by modifying the polymer surface [1]. Harder and tougher polymers are generally more resistant to abrasion and fatigue wear. A low surface energy is also preferred to reduce adhesive wear. Recently, it has been demonstrated in our laboratory that ion beam treatment can significantly improve surface hardness and wear resistance for various polymers [2]. In the present study, the effects of boron implantation dose on the surface hardness, friction, and wear properties of typical high-performance engineering plastics are reported. Two commercial thermoplastics, poly(cther-ether-ketone) (PEEK) and poly-ether-imide (Ultem) were investigated since they have good physical properties [3-6].
¢ther-imide (Ultem, registered by General Electric Company) films obtained from Wesflake Plastics Company (Lenni, PA) were investigated, The thickness of both films was 0,13 mm (5 rail), 2,2, Ion implantation
Two hundred key boron ions from the 0.4 MV Van de Graaff accelerator at Oak Ridge National Laboratory [7] were employed. Three doses, 2.3, 6.8. and 22 x l01+ionsem -2 were implanted into the specimen surfaces at ambient temperature. The specimen temperature was maintained near room temperature by limiting the beam current to less than 100hA cart-2 during irradiation. The depth of the surface depression produced by ion implantation was measured by using a Leitz Metalloplan optical microscope equipped with a Linnik interferometer.
2. Exlmrimeata4 ql+tails
2.3. Ion beam profiles Ion trajectories and damage profiles for the ion implantation were simulated by using TRIMg0 [8], a Monte Carlo program. The maximum of the boron distribution profile was estimated to be around 0.86 lam for PEEK and Ultem since they have similar densities and atomic compositions. Ionization by boron ions and their recoil atoms occurred within a depth range of 1.0 lain.
2,/, Materials
2.4. Hardness
Amorphous poly(ether-ethex-ketone) (PEEK, Victrex, registered by Imperial Chemical Industries) and poly-
Knoop tests were performed using a Leitz microhardhess tester. Loads used in this study were 0.02, 0.05, and
Elsevier Sequoia
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Boron-imldUnt rd P I- E K aml t :hem
0.1 N. An indentation dwcll of 15 s was used for all the sam~'flcs since the size of an indent was altered by cold flo~ of Ix)lymers. The hardness of the ion-implanted l',~lymer surface was also measured using a mechanical properties microprobe INanoindenter Ira) [9 13]. A loading rate of 10 nm s ~. an unloading rate of 7 nm s ~. and an indentation deplh of 500 nm were used as a standard testing condition. 2.5. Friction and wear tests
A tribomctcr with a back-and-forth sliding arrangement and a sphere-on-fiat geometry was used. A 9.5 mm I-!.~ inch) diameter SAE 5211X) high-carbon, chrome steel ball was cleaned before use. Tests without lubrication were performed with a normal force of 0.5 or I N on a lilm specimen bonded to an aluminum block. An average sliding velocity of 0.01 m s t and a stroke length of 3 mm were used. The friction force was measured by strain gauges interfaced to a personal computer. After each friction test. the sliding surfaces of the ball and tilms were examined using a Leitz microscope.
expected. A detailed investigation of the chemical structure change is in progress. As shown in Table I. the microhardncss increased with increasing boron dose for both PEEK and Ultem. especially at low applied loads. The penetration depth of the diamond Knoop indenter is shallower at a lower load. Assuming that the indenter shape was geometrically replicated in the lilms, the penetration depths h~r PEEK-C at 0,02, 0.05 and ILl N loads were 0.7, 1.6, and 2.1) IJm respectively anti those tor Ultem-(" were 0.~, 1,4. and 2,2 ttm respectively [16]. Only at 0,02 N. the penetration was stopped within the boron-imphmtcd layer. This explains why greater surhlce hardness values were measured at the lowest load by the Knoop indentation method. In nanoindentation, the load required to penetrate the sample surface was continuously measured as a function of penetration depth of the diamond indenter. The load cs. penetration depth curves are shown for PEF.K in Fig. I. The deeper the indent deptht the larger was the load needed. With increasing ion dose a higher load was required to penetrate the samc depth, owing to increased
3. Results =ud diseassiom
The chemical reactions caused by ion beam treatment were evidenced by a change in color. PEEK and Ultem became darker and more opaque with increasing boron dose as listed in Table I. Blackening of the ion-implanted polymer was attributed to carbonization and appeared to be common for most polymers at a sulticiently high dose (more than l0 ts ions cm -') [14]. Many polymers have been found to emit small molecules during ion implantation [14. 15] and. thus. the polymer surface is depressed. The step heights of the surface depression between the implanted and masked regions were measured by using interferograms and are listed in Table I. The step height increased with increasing ion dose as
P I { I ~ I IIq[
400
z
300
O d
100
0 O
20
40
60
~0
~?~U
Fig. I. I.oad measured as a fimction of dcplh h)r the PI':I'K samples during nanoindcntation.
TABLE I. Dose of horon 1200 keVk sample code. color, step height and K n o o p hardness at variou.~ loads h~r PI'.I-K and Ullem Polymer
PI'I'K
Boron done I× l0 ta ion.~ crn -')
1| 2.3 6.8 22
[ :Itet~)
0
2,3 6,S 22
Sample code
Pristine PEEK
('olor
PEEK-A PEEK-B I}EEK-( '
Transparem brown Yellow (.;olden brown Dark b r o ~ n
Pristine Ultem UIIcm-A Ultem-B (:Item-("
Transparent amtx'r Yclloxv Golden brown Dark brown
%top height was not large enough to measure.
~0
DEPTH (nm)
Step height (nnl)
K n o o p hardness I ( i P a l 0,02 N
I),I)5 N
~).I N
II
I).14
0,I(~
II. I ~,
' 51 1211
I).25 11.42 11.65
IL211 11.24 |1.31
(). I ? o.I 9 1.2 ¢,
[)
t),24
" 5()
q},30
120
1L49
U,25 (),32 ~},3S 11.3~
~),24 ~),2S q),32 1~,)2
11,43
269
Y, Lee et al, / Bor~m-impla.ted PEEK and Uhem
surface hardness. Ukern samples showed a similar behavior, At a depth of 100 am, 1,9, 3,6, and 5,5 times higher loads were required for PEEK-A, B, and C respectively, as compared with pristine PEEK, For Ultem, increases of 1.5, 2.5, and 3.9 times were observed for Ultem-A, B, and C respectively. Hardness was calculated using loading curves in Fig. I and a projected area of indentation according to the procedure desorihed in refs. 2 and 9. The hardness values at a depth of 100 nm (from nanoindenter tests) and those at 0,02 N load (from Knoop tests) for PEEK and Ultem are shown in Fig, 2, The hardness values of both polymers were increased as the ion implantation dose was increased. The nanoindenter hardness values increased more sensitively with boron dose than did the Knoop hardness values. The differences between the two hardness values are considered to be due to different indentation depth and different viscoelastic response of the samples to each test procedure. The hardness of URem was greater than PEEK in the pristine state, however, the hardness of PEEK increased more rapidly than Ultcm upon boron implantation, Figure 3 shows the friction coefficient bbetween PEEK and a chrome steel ball at I N load. At the beginning of the sliding test, the friction coeffi_-~.nt of pristine PEEK was lower than that of implanted PEEK bm increased rapidly during 300-2000 cycles, suggesting the onset of a fatigue wear process. The friction coefficient of pristine PEEK showed a relatively constant value during the 6000-10 000 cycle range. Generally, the friction coefficient of implanted PEEK did not vary much throughout the test and had equal or lower values than that of pristine PEEK at the end of the I0 000 cycles experiment. The lowest friction coefficient was observed for the highest dose implanted PEEK, The initial friction coefficient of boron-implanted PEEK decreased with increased boron dose. A similar behavior was observed in a lower load test with 0,5 N normal force, but the 4'-
~" P E E K , I O O h m
~'~" U L T E M , t O O I t m
A
PEEK,2g
'
'
ULTEM,:2g
-r-
. . . . . . . . . . . . . .1. . . . . . . . . . . . . . . . . . . . . . J . . . . .
o
10 15 20 OF 2 0 0 keV BORON (x 10"14 i o n / c m * 2 )
5 DO~
25
Fig, 2. Hardness values of PEEK and Ultem samples obtained from nanoindenter tests (at 100 nm depth) and from Knoop tests (at 0.02 N load).
~- 1.ei Z
B¢)rOh ~ O h l l C C l t ":2 ' " PRISTINE
m_ 1,s:
"'~\" :2,3 x 1 0 " 1 4
-'~'~" I~,{I X 1 0 " 1 4
.................................................
~ 1,4 A
0
1 ~"---~x 0,8 I
~0,6 ~ O 0.2 L
2
4
6
........
0
10
NUMBER OF SLIOING CYCLES (x 10"3) Fig. 3, Friction coefficients of PEEK samples with chrome steel balls at I N load,
friction coefficient of pristine PEEK showed a jump during the 2000-3000 cycle range. The friction tests were stopped at 10000 cycles (100 min) and the worn surfaces of the film and ball were observed by optical microscopy. Figure 4 shows PEEK films and their mating steel ball surfaces after 10 000 cycles of sliding at I N load, The ball used on pristine PEEK at I N was not damaged at all, but the polymer surface was noticeably abraded. The wear track of pristine PEEK showed scratches in the sliding direction indicating an abrasive process. Also, transverse cracks or bands, probably produced by a microfatigue process, were observed across the sliding direction. The major wear processes for pristine PEEK-Steel pair at the steady state are probably abrasive and fatigue wear. A relatively small amount of wear debris of PEEK was observed and it did not appear to act as abrasive particles. Both boron-implanted PEEK films and the corresponding steel balls produced a large amoum of small wear debris. The wear of both the film and the ball was reduced with increasing boron implantation. PEEK-A and B showed pitting damage, probably indicating the fatigue wear process caused by cyclic stress and deformation. The directional scratch marks on the wear tracks of all the boron-implanted PEEK indicated that abrasion was also a dominant wear process. Unlike abrasive wear for pristine PEEK, the wear process for boron-implamed PEEK is considered to be predominantly three-body abrasion. Since the test set-up allowed the wear debris to remain in the slider path, the debris acted as abrasive particles before it was pushed aside. The hard wear debris accelerated the damaging process and this caused PEEK-A and B to be damaged more severely than pristine PEEK. If the wear debris had been removed, the damage of boron-implanted PEEK would have be.ex= reduced. The debris found for boron-implanted PEEK wear might have been ground and agglomerated, and some agglomerated debris adhered on the ball surface.
Y, I.¢c cl al.
2711
Boro~-imphmled PEEK aml ( h e m
I"ig. 4, Worn surt'a~.'cx of chrome sled halls and I)EI!K samples al'ler I0 IX)I) c~,clen of ,,,liding a! I N load,
Besktes black dust, some birefringent tibrous debris associated with deforming and cutting was a l ~ observed for boron-implanted PEEK wear, The d e g r ~ of wear decreased with increasing I~,ron dose, probably owing to the increased surhlce hardness. It has been found for pure metals and ceramics that abrasive wear is linearly dependent on the relative hardness [17, 18], PEEK-C showed little damage and the width of its wear track was narrower than that of pristine PEEK, indicating that PEEK-C was more wear resistant than pristine PEEK. PEEK-C also showed a substantially lower friction coefficient than pristine PEEK. The damage on boron-implanted and pristine PEEK and their corresponding steel balls at 0,5 N load was less than the damage at I N load, but the damage behavior as a function of ba~ron dose at 0,5 and I N was similar. Figure5 shows the friction coellicient of Ultem chrome st~l pairs at I N load. The friction coellicicnt
2 I- 1.8 Z tu 1 . 6
Boron ionzlcm'2 ' '
PRISTINE
'~
2 , 3 X 10"14
""
6 , 8 X 10"14
i'
2,~ x 10"IS
;'7 1,4. t.z. k
Id,l 1 , 2 '
0 0
I : ,., ....
~ o.6,1,'
•
"..
--b
¢,
O,2 0~ 0
2:
4
6
8
10
NUMBER OF SLIDING CYCLES (x 10"3) Fig, 5, I,'rietion eoellicienis of t]ltcm samples tested using chrome steel ball~ at I N l o a d .
of pristine Ultem did nol vary much and it did not show a sudden jump. ;is was observed for pristine PEEK IFig, 3L The friction cocllicicnt of pristine Ultem at I N was around 0,75 ;it the steady state, which was a little lower than for pristine PEEK. The friction coefficient of the boron-implanted Ultcm was generally higher than that of pristine Uitem during the I0 (X)O cycles test. except for the initial stage, Like boron-implanted PI-EK in Fig, 3, the initial friction coefficient appeared to decrease with increasing boron dose. Figure 6 shows the worn surfaces of Ultem lilms and the chrome steel ball surfaces, The pristine Ultem tilm was severely damaged but the mating ball surhlce was only slightly scratched. Directional scratches aml severe plastic deformation, or galling, were observed on the wear track of the pristine Uitem lilm, The ~olume of wear debris from pristine Ultem was more than that from boron-implanted tJitem, The chrome steel balls used with boron-implanted Ultem were severely worn out, and the degrcc of the damage decreased with increasing boron dose, The damage on tire lilms also signiticantly decreased with boron dose, Like PEEK, the wear damage on boron-implanted and pristine Ultem films and their balls at I).5 N load was less than the damage at I N, Although the wear damage characteristics of pristine PEEK and Ultem were different, those of the two boronimplanted polymers a p ~ a r e d to be similar: the damage of the two I~ron-implanted polymers and their chrome steel balls were comparable and considerably decreased with increasing boron dose, Although no attempt was made to measure the volume of material removed during sliding wear, the wear track widths of boron-implanted PEEK and Ultem appeared to foUow the Archard wear law [17. It)I, which states that wear volume is propor-
Y. Lee et al, / Berlin-implanted PEEK and Ultem
271
Fig. 6, Worn surfaoes of chrome steel balls and Ultem samples after 10000 cycles of sliding at I N load,
tional to the normal load and inversely proportional to the hardness of the softer material surface. The increase in wear resistance of boron-implanted PEEK and Ultean as a function of boron dose may be better correlated with the nanoindenter hardness values than with tbe Knoop hardness values in Fig. 2.
ball accelerated the damaging process. For the system studied, the highest dose. 2.2 × l0 zs ions cm -', appeared to be the best to improve wear resistance for both PEEK and Ultem.
Ack~a~ 4. Coaclasioms The effects of boron ion ream irradiation on the hardness, friction and wear properties of PEEK and Ultem were investigated. The hardness values obtained from the conventional Knoop test and a load-de4~h sensing nanoindcntation technique increased with increasing boron dose for both polymers. The increase in the hardness values was found to depend on the penetration depth of the diamond indenter. This can be explained by the results of simulation using TRIMg0, which showed that 200 keV boron ions were implanted up to a d e ~ h of I gm in the surface of PEEK and Ultern. Wear and frictional properties varied in a complex fashion with polymer type and dose, but were not much affected by the normal load. A substamial reduction in friction coefficient was observed for PEEK at the highest dose, but no reduction was observed for Ultem. The wear damage of boron-implanted PEEK and Ultem greatly decreased with increased boron dose. which is considered to be better correlated with the nanoindenter hardness at 100 nm penetration depth than with the Knoop hardness at the lowest load (0.02 N). The wear resistance of low dose (2.3xl0 ~4 and 6.8 × 1014 ions o n -z) boron-implanted PEEK was not greater than that of pristine PEEK because hard wear debris from the boron-implanted layer and chrome steel
We would like to thank Dr P.J. l~lau and Mr G. R. Rao for suggestions and technical review. This research was sponsored by the US Delyartmcnt of Energy, Assistant Secretary of Conservation and Renewable Energy, Offw~ of Industrial Technologies. Advanced Industrial Concepts Division, and the Advanced Industrial Materials Program, under contract DE-AC05-84OR21400 with Martin Marietta Energy Systems. Inc., and also supported in part by an appointment to the US Department of Energy Postgraduate Research Program administered by Oak Ridge Associated Universities. The submitted manuscript has be.on authored by a contractor of the US Government under contract No. DE-AC05-84OR21400.
Refereaces I L.-H. Lee, in L.-H. Lee (e,d.), Polymer Wear and Its Control, American Chemical Society, 1985, Chapter 6. 2 E, H, Lee, M, B, Lewis, P, J, Blau and L. K, Mansur, J, Mater, R¢~s,, 6 (19911 610, 3 Y, Lee, Physical properties of poly(ether ether ketone), Ph,D. Dis~rtmion, University of Massachusetts at Amherst, 1988, 4 R. B, Rigby, Polym, New,~, 9 (1984) 325, 5 O, Ja¢ohs, K, Friedrich, G, Marom, K. Schulte and H. D, Wagner, Wear, 133 0990) 207, 6 J. gijwe, U, S. Tewafi and P, Vasudevan, Wear. 138 (1990) 61,
2"/2
Y. let,e~ al. / Boron-in~plamed PEEl( ~md Uhcm
7 M.B. Lewis. W. R. Allen. R.A. BuM. N. H. Packan. S. W. Cook and L. K. Mansur. Nucl. Inslrum..Methods BA3 {1989) 243. ,X .I.F. gicglcr. J. P. Biermlck and U. Littmark. The S~opping cmd Rm~ge o.t"Ions in SolMs, Pergamon, N e w York, 19R5. 9 M. F. Docrncr and W. D. Nix. J. Mater. Res.. I [1986)601. 10 W. ('. Oliver. C, J, McHargu~ and S.J. Zinkl¢. Thin SoIM Fibns. 153 11997) 185. II S..I. Zinklc. J. Ant. Ct,ram. Sot'.. 72 {1989) 1343, 12 M..I. Mayo and W. D. Nix, Acta MetMl.. 36 (1988) 2183. 13 A.K. Bhattacharya and W.D. Nix, int. J. Solids Structures. 24 (1988l 1287.
14 T, X,'enkatcsan, L. t.'alcagno, B.S. Elman and t.;, I:oti, in P, Mazzoldi and G, W, Arnold (,v,ls,k Ion Beam .,~h~l~ticmior o5 Insulmors, El,~vicr. New York, 1987, Chapter X, 15 M. B, Lewis and E. H. Lee. Nut/, Instrum, Me{h¢~ls,in the pre,~s, 16 Leil: Microh,rdness Tester Manual, Ernst Leite {Canadal Lid,, Midhmd, Ontario, 17 P, J, Blau. I"rictio~l ~md B~.ar l>mlsilhms o,f Mmerhd,~. Noycs, Park Ridge, N J, 1989. 18 R, D, C, Richardson, Pr.c, Ins*, Mcch, En,g,, I,~'2[1',~71 410, 19 J.F. Archard. in M. B. Petcrson and W.O. Wirier {cds.I. ASM/'. Wear ('omr,I Itandhook. ASME. Nesv York. 1980. p. 35.
267
Hardness and wear properties of boron-implanted poly(etherether-ketone) and poly-ether-imide Y o u n g c h u l Lee, E, H, Lee a n d L. K, M a n s u r Metals and Ceramics Division, Oak Ridge National Laborenory, Oak Ridge, TN 37831 (USA)
Abstraet The effects of boron beam irradiation on the hardness, friction, and wear of polymer surfaces were investigated. Typical high-
performance thermoplastics, poly(ether-e.ther-ketone)(PEEK) and a poly-ether-imide (Ultem) were studied after 200 key boron ion beam treatment at ambient temperature to doses of 2.3 × l0 t+, 6.8 x 10=+, and 2.2 × l0 ts ions cm -2, The hardnesses of pristine and boron-implanted materials were characterized by a conventional Knoop method and a load-depth sensing nanoindentation technique. Both measurements showed a significant increase in hardness with increasing dose. The increase in hardness was also found to depend on the penetration depth of the diamond indenter. Wear and friction properties were characterized by a reciprocating sliding friction tester with an SAE 52100 high-carbon, chrome steel ball at 0.5 and I N normal loads. Wear and frictional properties varied in a complex fashion with polymer type and dose. but not much with normal load. A substantial reduction in friction coefficient was observed for PEEK at the highest dose but no reduction was observed for Ultem. The wear damage was substantially redound at the highest dose for both Ultem and PEEK. For the system studied, the highest dose. 2.2 x l0 ts ions cm -~+ appears to be optimum in improving wear resistance for both PEEK and Ultem.
I. latmducttea Polymers have shown their importance in mechanical and structural applications since they are inherently light (typically 10%-20% the weight of metals) and easy to shape, Their main applications, however, have been limited to relatively mild surface contact conditions, Surface-sensitive mechanical properties such as surface durability and hardness must he improved to meet more demanding requirements, Polymer wear can he controlled by changing the bulk properties, by particulate reinforcement and by modifying the polymer surface [1]. Harder and tougher polymers are generally more resistant to abrasion and fatigue wear. A low surface energy is also preferred to reduce adhesive wear. Recently, it has been demonstrated in our laboratory that ion beam treatment can significantly improve surface hardness and wear resistance for various polymers [2]. In the present study, the effects of boron implantation dose on the surface hardness, friction, and wear properties of typical high-performance engineering plastics are reported. Two commercial thermoplastics, poly(cther-ether-ketone) (PEEK) and poly-ether-imide (Ultem) were investigated since they have good physical properties [3-6].
¢ther-imide (Ultem, registered by General Electric Company) films obtained from Wesflake Plastics Company (Lenni, PA) were investigated, The thickness of both films was 0,13 mm (5 rail), 2,2, Ion implantation
Two hundred key boron ions from the 0.4 MV Van de Graaff accelerator at Oak Ridge National Laboratory [7] were employed. Three doses, 2.3, 6.8. and 22 x l01+ionsem -2 were implanted into the specimen surfaces at ambient temperature. The specimen temperature was maintained near room temperature by limiting the beam current to less than 100hA cart-2 during irradiation. The depth of the surface depression produced by ion implantation was measured by using a Leitz Metalloplan optical microscope equipped with a Linnik interferometer.
2. Exlmrimeata4 ql+tails
2.3. Ion beam profiles Ion trajectories and damage profiles for the ion implantation were simulated by using TRIMg0 [8], a Monte Carlo program. The maximum of the boron distribution profile was estimated to be around 0.86 lam for PEEK and Ultem since they have similar densities and atomic compositions. Ionization by boron ions and their recoil atoms occurred within a depth range of 1.0 lain.
2,/, Materials
2.4. Hardness
Amorphous poly(ether-ethex-ketone) (PEEK, Victrex, registered by Imperial Chemical Industries) and poly-
Knoop tests were performed using a Leitz microhardhess tester. Loads used in this study were 0.02, 0.05, and
Elsevier Sequoia
26,~
Y.I.cc
~'I
al.
Boron-imldUnt rd P I- E K aml t :hem
0.1 N. An indentation dwcll of 15 s was used for all the sam~'flcs since the size of an indent was altered by cold flo~ of Ix)lymers. The hardness of the ion-implanted l',~lymer surface was also measured using a mechanical properties microprobe INanoindenter Ira) [9 13]. A loading rate of 10 nm s ~. an unloading rate of 7 nm s ~. and an indentation deplh of 500 nm were used as a standard testing condition. 2.5. Friction and wear tests
A tribomctcr with a back-and-forth sliding arrangement and a sphere-on-fiat geometry was used. A 9.5 mm I-!.~ inch) diameter SAE 5211X) high-carbon, chrome steel ball was cleaned before use. Tests without lubrication were performed with a normal force of 0.5 or I N on a lilm specimen bonded to an aluminum block. An average sliding velocity of 0.01 m s t and a stroke length of 3 mm were used. The friction force was measured by strain gauges interfaced to a personal computer. After each friction test. the sliding surfaces of the ball and tilms were examined using a Leitz microscope.
expected. A detailed investigation of the chemical structure change is in progress. As shown in Table I. the microhardncss increased with increasing boron dose for both PEEK and Ultem. especially at low applied loads. The penetration depth of the diamond Knoop indenter is shallower at a lower load. Assuming that the indenter shape was geometrically replicated in the lilms, the penetration depths h~r PEEK-C at 0,02, 0.05 and ILl N loads were 0.7, 1.6, and 2.1) IJm respectively anti those tor Ultem-(" were 0.~, 1,4. and 2,2 ttm respectively [16]. Only at 0,02 N. the penetration was stopped within the boron-imphmtcd layer. This explains why greater surhlce hardness values were measured at the lowest load by the Knoop indentation method. In nanoindentation, the load required to penetrate the sample surface was continuously measured as a function of penetration depth of the diamond indenter. The load cs. penetration depth curves are shown for PEF.K in Fig. I. The deeper the indent deptht the larger was the load needed. With increasing ion dose a higher load was required to penetrate the samc depth, owing to increased
3. Results =ud diseassiom
The chemical reactions caused by ion beam treatment were evidenced by a change in color. PEEK and Ultem became darker and more opaque with increasing boron dose as listed in Table I. Blackening of the ion-implanted polymer was attributed to carbonization and appeared to be common for most polymers at a sulticiently high dose (more than l0 ts ions cm -') [14]. Many polymers have been found to emit small molecules during ion implantation [14. 15] and. thus. the polymer surface is depressed. The step heights of the surface depression between the implanted and masked regions were measured by using interferograms and are listed in Table I. The step height increased with increasing ion dose as
P I { I ~ I IIq[
400
z
300
O d
100
0 O
20
40
60
~0
~?~U
Fig. I. I.oad measured as a fimction of dcplh h)r the PI':I'K samples during nanoindcntation.
TABLE I. Dose of horon 1200 keVk sample code. color, step height and K n o o p hardness at variou.~ loads h~r PI'.I-K and Ullem Polymer
PI'I'K
Boron done I× l0 ta ion.~ crn -')
1| 2.3 6.8 22
[ :Itet~)
0
2,3 6,S 22
Sample code
Pristine PEEK
('olor
PEEK-A PEEK-B I}EEK-( '
Transparem brown Yellow (.;olden brown Dark b r o ~ n
Pristine Ultem UIIcm-A Ultem-B (:Item-("
Transparent amtx'r Yclloxv Golden brown Dark brown
%top height was not large enough to measure.
~0
DEPTH (nm)
Step height (nnl)
K n o o p hardness I ( i P a l 0,02 N
I),I)5 N
~).I N
II
I).14
0,I(~
II. I ~,
' 51 1211
I).25 11.42 11.65
IL211 11.24 |1.31
(). I ? o.I 9 1.2 ¢,
[)
t),24
" 5()
q},30
120
1L49
U,25 (),32 ~},3S 11.3~
~),24 ~),2S q),32 1~,)2
11,43
269
Y, Lee et al, / Bor~m-impla.ted PEEK and Uhem
surface hardness. Ukern samples showed a similar behavior, At a depth of 100 am, 1,9, 3,6, and 5,5 times higher loads were required for PEEK-A, B, and C respectively, as compared with pristine PEEK, For Ultem, increases of 1.5, 2.5, and 3.9 times were observed for Ultem-A, B, and C respectively. Hardness was calculated using loading curves in Fig. I and a projected area of indentation according to the procedure desorihed in refs. 2 and 9. The hardness values at a depth of 100 nm (from nanoindenter tests) and those at 0,02 N load (from Knoop tests) for PEEK and Ultem are shown in Fig, 2, The hardness values of both polymers were increased as the ion implantation dose was increased. The nanoindenter hardness values increased more sensitively with boron dose than did the Knoop hardness values. The differences between the two hardness values are considered to be due to different indentation depth and different viscoelastic response of the samples to each test procedure. The hardness of URem was greater than PEEK in the pristine state, however, the hardness of PEEK increased more rapidly than Ultcm upon boron implantation, Figure 3 shows the friction coefficient bbetween PEEK and a chrome steel ball at I N load. At the beginning of the sliding test, the friction coeffi_-~.nt of pristine PEEK was lower than that of implanted PEEK bm increased rapidly during 300-2000 cycles, suggesting the onset of a fatigue wear process. The friction coefficient of pristine PEEK showed a relatively constant value during the 6000-10 000 cycle range. Generally, the friction coefficient of implanted PEEK did not vary much throughout the test and had equal or lower values than that of pristine PEEK at the end of the I0 000 cycles experiment. The lowest friction coefficient was observed for the highest dose implanted PEEK, The initial friction coefficient of boron-implanted PEEK decreased with increased boron dose. A similar behavior was observed in a lower load test with 0,5 N normal force, but the 4'-
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o
10 15 20 OF 2 0 0 keV BORON (x 10"14 i o n / c m * 2 )
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25
Fig, 2. Hardness values of PEEK and Ultem samples obtained from nanoindenter tests (at 100 nm depth) and from Knoop tests (at 0.02 N load).
~- 1.ei Z
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.................................................
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........
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NUMBER OF SLIOING CYCLES (x 10"3) Fig. 3, Friction coefficients of PEEK samples with chrome steel balls at I N load,
friction coefficient of pristine PEEK showed a jump during the 2000-3000 cycle range. The friction tests were stopped at 10000 cycles (100 min) and the worn surfaces of the film and ball were observed by optical microscopy. Figure 4 shows PEEK films and their mating steel ball surfaces after 10 000 cycles of sliding at I N load, The ball used on pristine PEEK at I N was not damaged at all, but the polymer surface was noticeably abraded. The wear track of pristine PEEK showed scratches in the sliding direction indicating an abrasive process. Also, transverse cracks or bands, probably produced by a microfatigue process, were observed across the sliding direction. The major wear processes for pristine PEEK-Steel pair at the steady state are probably abrasive and fatigue wear. A relatively small amount of wear debris of PEEK was observed and it did not appear to act as abrasive particles. Both boron-implanted PEEK films and the corresponding steel balls produced a large amoum of small wear debris. The wear of both the film and the ball was reduced with increasing boron implantation. PEEK-A and B showed pitting damage, probably indicating the fatigue wear process caused by cyclic stress and deformation. The directional scratch marks on the wear tracks of all the boron-implanted PEEK indicated that abrasion was also a dominant wear process. Unlike abrasive wear for pristine PEEK, the wear process for boron-implamed PEEK is considered to be predominantly three-body abrasion. Since the test set-up allowed the wear debris to remain in the slider path, the debris acted as abrasive particles before it was pushed aside. The hard wear debris accelerated the damaging process and this caused PEEK-A and B to be damaged more severely than pristine PEEK. If the wear debris had been removed, the damage of boron-implanted PEEK would have be.ex= reduced. The debris found for boron-implanted PEEK wear might have been ground and agglomerated, and some agglomerated debris adhered on the ball surface.
Y, I.¢c cl al.
2711
Boro~-imphmled PEEK aml ( h e m
I"ig. 4, Worn surt'a~.'cx of chrome sled halls and I)EI!K samples al'ler I0 IX)I) c~,clen of ,,,liding a! I N load,
Besktes black dust, some birefringent tibrous debris associated with deforming and cutting was a l ~ observed for boron-implanted PEEK wear, The d e g r ~ of wear decreased with increasing I~,ron dose, probably owing to the increased surhlce hardness. It has been found for pure metals and ceramics that abrasive wear is linearly dependent on the relative hardness [17, 18], PEEK-C showed little damage and the width of its wear track was narrower than that of pristine PEEK, indicating that PEEK-C was more wear resistant than pristine PEEK. PEEK-C also showed a substantially lower friction coefficient than pristine PEEK. The damage on boron-implanted and pristine PEEK and their corresponding steel balls at 0,5 N load was less than the damage at I N load, but the damage behavior as a function of ba~ron dose at 0,5 and I N was similar. Figure5 shows the friction coellicient of Ultem chrome st~l pairs at I N load. The friction coellicicnt
2 I- 1.8 Z tu 1 . 6
Boron ionzlcm'2 ' '
PRISTINE
'~
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of pristine Ultem did nol vary much and it did not show a sudden jump. ;is was observed for pristine PEEK IFig, 3L The friction cocllicicnt of pristine Ultem at I N was around 0,75 ;it the steady state, which was a little lower than for pristine PEEK. The friction coefficient of the boron-implanted Ultcm was generally higher than that of pristine Uitem during the I0 (X)O cycles test. except for the initial stage, Like boron-implanted PI-EK in Fig, 3, the initial friction coefficient appeared to decrease with increasing boron dose. Figure 6 shows the worn surfaces of Ultem lilms and the chrome steel ball surfaces, The pristine Ultem tilm was severely damaged but the mating ball surhlce was only slightly scratched. Directional scratches aml severe plastic deformation, or galling, were observed on the wear track of the pristine Uitem lilm, The ~olume of wear debris from pristine Ultem was more than that from boron-implanted tJitem, The chrome steel balls used with boron-implanted Ultem were severely worn out, and the degrcc of the damage decreased with increasing boron dose, The damage on tire lilms also signiticantly decreased with boron dose, Like PEEK, the wear damage on boron-implanted and pristine Ultem films and their balls at I).5 N load was less than the damage at I N, Although the wear damage characteristics of pristine PEEK and Ultem were different, those of the two boronimplanted polymers a p ~ a r e d to be similar: the damage of the two I~ron-implanted polymers and their chrome steel balls were comparable and considerably decreased with increasing boron dose, Although no attempt was made to measure the volume of material removed during sliding wear, the wear track widths of boron-implanted PEEK and Ultem appeared to foUow the Archard wear law [17. It)I, which states that wear volume is propor-
Y. Lee et al, / Berlin-implanted PEEK and Ultem
271
Fig. 6, Worn surfaoes of chrome steel balls and Ultem samples after 10000 cycles of sliding at I N load,
tional to the normal load and inversely proportional to the hardness of the softer material surface. The increase in wear resistance of boron-implanted PEEK and Ultean as a function of boron dose may be better correlated with the nanoindenter hardness values than with tbe Knoop hardness values in Fig. 2.
ball accelerated the damaging process. For the system studied, the highest dose. 2.2 × l0 zs ions cm -', appeared to be the best to improve wear resistance for both PEEK and Ultem.
Ack~a~ 4. Coaclasioms The effects of boron ion ream irradiation on the hardness, friction and wear properties of PEEK and Ultem were investigated. The hardness values obtained from the conventional Knoop test and a load-de4~h sensing nanoindcntation technique increased with increasing boron dose for both polymers. The increase in the hardness values was found to depend on the penetration depth of the diamond indenter. This can be explained by the results of simulation using TRIMg0, which showed that 200 keV boron ions were implanted up to a d e ~ h of I gm in the surface of PEEK and Ultern. Wear and frictional properties varied in a complex fashion with polymer type and dose, but were not much affected by the normal load. A substamial reduction in friction coefficient was observed for PEEK at the highest dose, but no reduction was observed for Ultem. The wear damage of boron-implanted PEEK and Ultem greatly decreased with increased boron dose. which is considered to be better correlated with the nanoindenter hardness at 100 nm penetration depth than with the Knoop hardness at the lowest load (0.02 N). The wear resistance of low dose (2.3xl0 ~4 and 6.8 × 1014 ions o n -z) boron-implanted PEEK was not greater than that of pristine PEEK because hard wear debris from the boron-implanted layer and chrome steel
We would like to thank Dr P.J. l~lau and Mr G. R. Rao for suggestions and technical review. This research was sponsored by the US Delyartmcnt of Energy, Assistant Secretary of Conservation and Renewable Energy, Offw~ of Industrial Technologies. Advanced Industrial Concepts Division, and the Advanced Industrial Materials Program, under contract DE-AC05-84OR21400 with Martin Marietta Energy Systems. Inc., and also supported in part by an appointment to the US Department of Energy Postgraduate Research Program administered by Oak Ridge Associated Universities. The submitted manuscript has be.on authored by a contractor of the US Government under contract No. DE-AC05-84OR21400.
Refereaces I L.-H. Lee, in L.-H. Lee (e,d.), Polymer Wear and Its Control, American Chemical Society, 1985, Chapter 6. 2 E, H, Lee, M, B, Lewis, P, J, Blau and L. K, Mansur, J, Mater, R¢~s,, 6 (19911 610, 3 Y, Lee, Physical properties of poly(ether ether ketone), Ph,D. Dis~rtmion, University of Massachusetts at Amherst, 1988, 4 R. B, Rigby, Polym, New,~, 9 (1984) 325, 5 O, Ja¢ohs, K, Friedrich, G, Marom, K. Schulte and H. D, Wagner, Wear, 133 0990) 207, 6 J. gijwe, U, S. Tewafi and P, Vasudevan, Wear. 138 (1990) 61,
2"/2
Y. let,e~ al. / Boron-in~plamed PEEl( ~md Uhcm
7 M.B. Lewis. W. R. Allen. R.A. BuM. N. H. Packan. S. W. Cook and L. K. Mansur. Nucl. Inslrum..Methods BA3 {1989) 243. ,X .I.F. gicglcr. J. P. Biermlck and U. Littmark. The S~opping cmd Rm~ge o.t"Ions in SolMs, Pergamon, N e w York, 19R5. 9 M. F. Docrncr and W. D. Nix. J. Mater. Res.. I [1986)601. 10 W. ('. Oliver. C, J, McHargu~ and S.J. Zinkl¢. Thin SoIM Fibns. 153 11997) 185. II S..I. Zinklc. J. Ant. Ct,ram. Sot'.. 72 {1989) 1343, 12 M..I. Mayo and W. D. Nix, Acta MetMl.. 36 (1988) 2183. 13 A.K. Bhattacharya and W.D. Nix, int. J. Solids Structures. 24 (1988l 1287.
14 T, X,'enkatcsan, L. t.'alcagno, B.S. Elman and t.;, I:oti, in P, Mazzoldi and G, W, Arnold (,v,ls,k Ion Beam .,~h~l~ticmior o5 Insulmors, El,~vicr. New York, 1987, Chapter X, 15 M. B, Lewis and E. H. Lee. Nut/, Instrum, Me{h¢~ls,in the pre,~s, 16 Leil: Microh,rdness Tester Manual, Ernst Leite {Canadal Lid,, Midhmd, Ontario, 17 P, J, Blau. I"rictio~l ~md B~.ar l>mlsilhms o,f Mmerhd,~. Noycs, Park Ridge, N J, 1989. 18 R, D, C, Richardson, Pr.c, Ins*, Mcch, En,g,, I,~'2[1',~71 410, 19 J.F. Archard. in M. B. Petcrson and W.O. Wirier {cds.I. ASM/'. Wear ('omr,I Itandhook. ASME. Nesv York. 1980. p. 35.