NASA i 

NASA Technical Paper 1462 

Effect of Sterilization Irradiation 

on Friction and Wear of UltrahighMolecular-Weight Polyethylene 

William R. Jones, Jr., William F. Hady, 

and Aldo Crugnola 

https://ntrs.nasa.gov/search.jsp?R=19790015045 2018-05-14T12:43:51+00:00Z
I 

TECH LIBRARY KAFB, NM 

NASA Technical Paper 1462 

Effect of Sterilization Irradiation 

on Friction and Wear of UltrahighMolecular- Weight Polyethylene 

William R. Jones, Jr., and William F. Hady 

Lewis Reseurcb Center 

Clevelu nd, 0 hio 

Aldo Crugnola 

University of Lowell 

Lowell, Massachusetts 

National Aeronautics 

and Space Administration 

Scientific and Technical 

Information Office 

1979 
SUMMARY 

The effect of sterilization gamma irradiation on the friction and wear properties of 

ultrahigh-molecular-weight polyethylene (UHMWPE) sliding against 316L stainless 

steel in dry air at 23O C was determined. A sliding pin-on-disk apparatus was used. 

Experimental conditions, in most cases, included a 1-kilogram (9.8-N) load, a 0.061- 

to 0.27 -meter-per- second sliding velocity, and a 32 000- to 57 8 000-meter sliding 

distance. 

Although sterilization doses of 2.5 and 5.0 megarads greatly altered the average 

molecular weight and the molecular weight distribution of UHMWPE, the friction and 

wear properties of the polymer were not significantly changed. Average wear rates 

of 1.6, 1.3, and 1. 3~10-l~ cubic meter per newton-meter were obtained for the unirradiated and the 2.5- and 5.0-megarad-irradiated specimens, respectively. The corresponding average friction coefficients for these three irradiation levels were 0.44, 

0.44, and 0.38, respectively. 

INTRODUCTION 

The surgical replacement of diseased or damaged human joints with artificial devices has become commonplace. During 1976, there were 80 000 total-hip replacements and 30 000 knee replacements and in the United States (ref. 1). Most presently 

used prostheses consist of either a cobalt-chromium-molybdenum alloy (Vitallium) or 

surgical stainless steel (316L) articulating against ultrahigh-molecular-weight polyethylene (UHMWPE). 

The conventional sterilization technique of steam autoclaving cannot be used with 

polyethylene components because of their tendency to deform under those conditions. 

Normally, polyethylene must be sterilized by gamma irradiation (2.5-megarad dose). 

Gamma irradiation is known to cause crosslinking in polyethylene (ref. 2). Crosslinking, in turn, alters polymer molecular weight, crystallinity, elastic modulus, and 

other physical properties. 

Rostoker and Galante (ref. 3) show that changes in molecular weight (MW) due to 

variations in molding temperature can affect the wear properties of UHMWPE. Miller, 

et al. (ref. 4) also report lower wear rates with molded (higher MW) than with machined 

(lower MW) UHMWPE. Similar results were obtained by Seedhom, et al. (ref. 5). 

Matsubara and Watanabe (ref. 6) studied the effect of gamma irradiation on the 

friction and wear properties of high-density polyethylene. In general, wear increased 
with increasing radiation dose. Awatani and Minegaki (ref. 7) show a wide variation in 

the wear properties of gamma-irradiated polyethylene depending on the radiation atmosphere (air or vacuum). Recently, Shen and Dumbleton (ref. 8) studied the effect 

of high gamma doses (20 to 1000 megarads) on the friction and wear properties of 

UHMWPE. In general, both friction and wear increased with increasing dose. It was 

concluded that these high gamma doses would not produce a useful bearing material. 

However, there have been few studies of radiation effects at low doses (2.5 megarads), 

corresponding to sterilization conditions. Ungethum (ref. 9) reports some limited data 

on the effect of low radiation doses (0 to 15 megarads) on polyethylene wear in pin-ondisk tests. Wear increased with increasing dose, but no information is provided as to 

the test conditions other than load (68 and 98 N). 

Therefore, the objective of this investigation was to determine the effect of sterilization irradiation on the friction and wear properties of UHMWPE sliding against 316L 

stainless steel in dry air at 23' C. A pin-on-disk apparatus was used. Experimental 

conditions, in most cases, included a 1-kilogram load, a 0.061- to 0.077-meter-persecond sliding velocity, and a 32 000- to 57'8 000-meter sliding distance. Three irradiation dose levels were studied 0, 2.5, and 5.0 megarads. 

Mr. Casper Stark of Howmedica, Inc., irradiated the polymer specimens, and 

Mr. Gene Farling of Zimmer-USA provided the disk specimens. 

APPARATUS 

The pin-on-disk sliding friction apparatus is shown in figure 1. The test specimens were mounted inside a plastic chamber. This allowed the moisture content of the 

test atmosphere to be controlled. A stationary 0,476-centimeter-radius hemispherically tipped polymer rider was placed in sliding contact with a rotating 6.3-centimeterdiameter (1.22-cm-thick) metal disk. Constant sliding speed in the range 0.061 to 

0.27 meter per second was maintained. A normal load of 1 kilogram was applied with 

a deadweight. 

MATERIALS 

Rider specimens were made of ultrahigh-molecular-weight polyethylene. The 

riders were machined from a block of surgical-grade polymer (RCH1000) manufactured 

and supplied by Hoechst A. G. After machining, the riders were gamma irradiated in 

air at atmospheric pressure to doses of 2.5 and 5.0 megarads. A cobalt-60 source was 

used. 

2 
The polished metal disks were made from surgical-grade stainless steel 316L, 

supplied by Zimmer-USA, that had a Rockwell B hardness of 75 to 77. After original 

fabrication, the disks were rough ground with 180-grit silicon carbide paper and a water 

coolant. The specimens were then successively ground with finer grit papers of the 

same type until' a 600-grit finish was obtained. A polishing operation followed in which 

6-micrometer diamond paste on a nylon cloth was used with kerosene as the lubricant 

and coolant. The final polishing was done with a short-nap microcloth of 0.05- 

micrometer gamma alumina with distilled water as the lubricant and coolant. Final 

surface roughness was about 0.038 micrometer centerline average (CLA) (1.5 pin.) , 

which is comparable to that of commercial prosthesis components. 

TEST PROCEDURE 

Disk and rider specimens were scrubbed with a commercial (nonabrasive) detergent, rinsed thoroughly with distilled water, and then dried on clean filter paper. The 

specimens were assembled in the test chamber. The chamber was purged with the test 

atmosphere, dry air (<50-ppm H20), for a minimum of 10 minutes. The disk was set 

in motion and the rider loaded against it. Frictional force was measured by a straingage transducer. Rider wear was determined periodically by stopping the test and 

measuring the wear-scar diameter. No attempts were made to separate creep effects 

from wear. 

RESULTS 

Wear 

Effect of irradiation. - Table I summarizes the wear data and conditions for each 

test. Ten tests were performed with the unirradiated polymer, five with the 2.5- 

megarad-irradiated polymer, and seven with the 5.0-megarad-irradiated polymer. All 

tests except test 7 (0 megarad) were run with a 1-kilogram (9.8-N) load. Most tests 

were performed at 25 rpm, which yielded sliding velocities from 0.061 to 0.077 meter 

per second, depending on the disk wear-track circumference. Five tests, tests 8 to 10 

(0 megarad) and tests 6 and 7 (5 megarads) , were run at 100 rpm (0.24 to 0.27 m/sec). 

Tests are numbered sequentially at each irradiation level according to sliding distance, 

except for the high-load and high-speed tests. 

Table I1 further summarizes the friction and wear results for the three irradiation 

levels. Both the 2.5- and 5.0-megarad levels yielded the same average wear rate, 

1.3~10-~ cubic meter per newton-meter. This is about 20 percent less than the value 

3 
obtained for the unirradiated polymer, 1.6~10-l~ cubic meter per newton-meter. However, statistical analysis of these data using Student's t distribution indicates that this 

difference in wear rate is not significant at a 95-percent confidence level. Therefore, 

we can only conclude that sterilization irradiation to 5.0 megarads does not significantly change the wear properties of the polymer. 

Effect of load. - This investigation was not intended to be a complete parametric 

study. Therefore, only a few tests were performed at conditions other than the standard ones previously stated. One test, test 7 (0 megarad), was run at a 2-kilogram 

(19.6-N) load rather than the standard 1-kilogram (9.8-N) load. This test yielded a 

wear rate (in m /N-m) that was within the data scatter of the remaining unirradiatedspecimen tests. 

3 

Effect of slidigg velocity. - Five tests, tests 8 to 10 (0 megarad) and tests 6 to 7 

(5 megarads), were performed at 100 rpm (0.24 to 0.27 m/sec) rather than the standard 25 rpm (0.061 to 0.077 m/sec). Comparing the wear rates for these five tests 

with the lower speed results does not reveal any speed effect. 

~~ 

Effect of contact stress. - Because of the test geometry, the contact stress obviously decreased as the test progressed and the rider wear-scar area increased. Contact stress is shown as a function of wear-scar diameter for a constant l-kilogram load 

in figure 2. The stress is about 13 megapascals (1830 psi) for a 1-millimeter wearscar diameter. This is the wear-scar diameter that is normally observed after the 

first few hundred meters of sliding. Then the contact stress rapidly decreased as the 

wear-scar diameter increased, until it reached 0.79 megapascal (110 psi) at a wearscar diameter of 4 millimeters. This is the wear-scar diameter usually observed after 

several hundred thousand meters of sliding at test conclusion. 

Wear volume as a function of sliding distance (or decreasing contact stress) is 

shown in figure 3 for the unirradiated and the 2.5- and 5.0-megarad-irradiated specimens, respectively. In general, these log-log plots do not show any large changes in 

wear rate. However, if these data are plotted in the form of incremental wear rates 

(m /N-m) (i. e., wear rates between each wear-scar measurement) as a function of 

sliding distance, differences are more obvious. These data are shown in figure 4. The 

run-in effect is quite obvious below 3000 meters. The incremental wear rates for the 

unirradiated polymer (fig. 4(a)) appear to level out after run-in. Only test 10 yielded 

some late wear-rate increases. There appears to be a trend toward lower wear rates 

for the 2.5-megarad-irradiated polymers (fig. 4(b)) at longer sliding distances (or 

lower contact stresses). This trend is more obvious for the 5.0-megarad-irradiated 

specimens (fig. 4(c)). 

3 

4 
Friction 

Effect of irradiation. - Average friction coefficients for each test are given in 

table I. Average friction values for each irradiation level are given in table II. The 

5.0-megarad-irradiated polymers had an average friction coefficient of 0.38, compared with 0.44 for both the unirradiated and 2. &megarad-irradiated specimens. This 

lspercent difference is probably not significant when the standard deviations are considered. 

Effect of sliding distance. - During run-in the friction coefficients gradually increased for all tests until a fairly steady value was reached after 2000 to 3000 meters 

of sliding. For the unirradiated specimens, the friction reached a maximum and then 

declined during the latter part of the test (sliding distances less than 20 000 m) as 

shown in figure 5. For the 5.0-megarad-irradiated specimens, the friction coefficient 

was essentially level after run-in; this behavior is illustrated in figure 6. The 2.5- 

megarad-irradiated specimens yielded both types of behavior. 

Polymer Characterization 

The polymer riders from test 3 (0 megarad), test 3 (2. 5 megarads), and test 5 

(5 megarads) were studied by solution-viscosity techniques (ref. 10) as well as by differential scanning calorimetry. These data are given in table 111, along with microhardness data for these specimens. The characterization parameters are fully described in the appendix. The riders were cut into four quadrants, the immediate surface layer was discarded, and samples were shaved from the central-axis area of the 

quadrants. Each test was repeated until results were within &2 percent, except for the 

hardness measurements. 

Solution-viscosity measurements. - As shown in table 111, gamma irradiation 

caused a rather drastic change in the average molecular weight and in the molecular 

weight distribution. The amount of insoluble gel (i. e. , MW >6x10 ) was increased four 

times for the 2.5-megarad-irradiated specimens and over five times for the 5.0- 

megarad-irradiated specimens. The irradiation also decreased the weighb average 

molecular weight of the soluble polymer fraction by over an order of magnitude. In 

contrast, the amount of very low-molecular-weight polymer (<20 000) was increased by 

irradiation. Obviously, there are competing processes of chain scission and chain 

crosslinking. 

6 

Crystallinity measurements. - There was a slight increase in crystallinity with increasing irradiation, whether it was determined by density changes or from differential 

scanning calorimetry (DSC). Values ranged from 68 to 72 percent from the density data 

and 46 to 50 percent from DSC. 

5 
Differential scanning calorimetry. - The DSC trace for the unirradiated polymer is 

shown in figure 7. It exhibits a single melting peak and melting temperature range 

typical of UHMWPE. Unusual melting behavior was observed for both irradiated specimens. This behavior is illustrated for the 2. &megarad-irradiated specimens in figure 8. The melting range is quite similar to that for the unirradiated specimen, but 

now a double endotherm is present. 

Hardness measurements. - The hardness measurements in table I11 are averages 

for several specimens. There appears to be no difference in hardness for the three 

irradiation levels. 

DISCUSSION 

Wear 

Several investigators (refs. 11 to 13) have reported wear data for UHMWPE sliding 

against stainless steel under dry conditions. Tanaka and Uchiyama (ref. 11) measured 

the wear of superhigh-molecular-weight polyethylene with a pin-on-disk apparatus. A 

calculated wear rate of about 5x10-lCi cubic meter per newton-meter was obtained at a 

load of 1 kilogram and a sliding speed of 0.10 meter per second. Dowson, et al. 

(ref. 12) report a lower wear rate (1.3~10-l~ m3/N-m) with a tri-pin-on-disk apparatus. Data from the author's laboratory (ref. 14) yielded a much higher wear rate 

(1.5~10-l~ m3/N-m) for UHMWPE articulating against Vitallium in a total-hip simulator. This value is comparable to the unirradiated-specimen value from table 11 

(1.6~10-l~ m3/N-m). The data from this paper are several times greater than the 

literature results but an order of magnitude less than those for the total-hip simulator 

(ref. 14). The abnormally high wear rate from the simulator is no doubt related to the 

short duration of the test (8 hr). This time represents only 110 meters of sliding and 

therefore reflects run-in effects. There were also obvious geometry differences between the total-hip simulator and the pin-on-disk apparatus. 

The Tanaka work was performed on polyethylene from a source other than the one 

used in this study. This may explain the wear-rate differences. Dowson, however, 

used the same type of polyethylene (RCHlOOO), but there were differences in his test 

apparatus. He used truncated cones for the polymer specimens rather than hemispherical riders. The lower initial contact stresses obtained with the cones should yield 

lower run-in wear. In addition, he ran the three test riders on the same track, and 

this would probably affect the transfer-film characteristics. The difference in wear 

rate might be related to these factors. 

Another reason for higher wear rates is that polymer creep can cause apparent 

increases in the wear scars that are not related to the wear process. However, stati6 
cally loaded (19.6 N) riders with preformed wear scars showed no measurable change 

in scar diameter in tests as long as 2 weeks. Although other investigators (ref. 15) 

have shown creep to be important in wear measurements, it did not appear to be a 

major factor in this study. 

Irradiation Effects 

It is well known that thin, oriented polymer transfer films on a metal substrate can 

greatly affect friction and wear properties (refs. 16 to 20). It has been shown that a 

smooth molecular profile enhances this process and that bulky side groups or radiationinduced crosslinking hinders it. Indeed, Shen and Dumbleton (ref. 8) did not detect 

transfer films at all with irradiated polyethylene (20 to 1000 megarads). Because of the 

radiation-induced changes (table III) in the polymers of this study, we might expect differences in transfer-film characteristics. However, optical microscopy of these films 

did not show any obvious differences. The transfer films formed readily during run-in 

and did not appear to change during the remainder of the test. Transfer consists of 

small (<1 to >5 pm diam) discrete particles to much larger (10- to 70-pm major dimension) patches. However, patches were only occasionally observed. Examples of these 

types of transfer film are shown in figure 9. The thickness of the large patch can be 

estimated from interference colors to be between 0.4 and 0.8 micrometer. The small 

transferred particles are 0.4 micrometer thick or less. The surface roughness on the 

disk wear track attained a steady level of about 0.10 micrometer CLA (4 pin.) after 

run-in. 

Wear- Time Behavior 

Some investigators (refs. 3, 12, and 21) report wear-rate increases with UHMWPE 

during the latter stages of some wear tests. Rostoker and Galante (ref. 3) report a late 

wear-rate increase (after 10 m of sliding) with graphite-filled UHMWPE. Dowson 

(refs. 12 and 21) also observed wear-rate increases with UHMWPE. These increases 

were attributed to a fatigue mechanism. The onset of this higher wear-rate regime began after 100 000 to 500 000 meters of sliding. Others (refs. 22) observed phenomena 

that might be fatigue related. However, these late wear-rate increases were generally 

not observed in the longer term (100 000 m) test of this study. Although one or two 

tests yielded some incremental wear-rate increases (such as test 10) most yielded 

fairly constant wear rates (after run-in) or a gradually decreasing rate. The latter effect was particularly noticeable with the 5.0-megarad-irradiated specimens. 

6 

7 
PV Effects 

It is known (ref. 23) that polymer wear rates are accelerated when sliding conditions reach a certain severity. The severity is measured by the product of contact 

stress and sliding speed (PV) . Shen and Dumbleton (ref. 8) report a limiting PV for unirradiated UHMWPE of about %lo5 (Pa) (m/sec). They also found that this value was 

increased by gamma irradiation to 3.4~10 (Pa) (m/sec) for a 20-megarad dose. In the 5 

present study, test conditions were such that most tests would have continued above the 

limiting PV (i. e. , 2x10 (Pa) (m/sec)). However, at the latter stages of some longterm tests, the contact stress can decrease to such a level that the limiting PV is 

reached. This would show up as a decrease in wear rate as a function of time, as described in the previous section. This effect should be more pronounced at the 5.0- 

megarad level because of the increasing PV limit, as observed. 

5 

Differential Scanning Calorimetry 

As stated previously, the gamma-irradiated specimens exhibited unusual melting 

behavior, as illustrated in figures 7 and 8. The melting behavior of the unirradiated 

polymer (fig. 7) is typical of UHMWPE. However, the double endotherm present in the 

DSC trace for the 2.5-megarad-irradiated specimen (fig. 8) is unusual. This type of 

behavior is rarely seen with UHMWPE but is quite common in the fiber industry 

(refs. 24 to 26) , where it has been linked to the presence of extende&chain crystals 

that are produced during fiber orientation. Double endotherms are due to two types of 

crystals each exhibiting its own melting behavior. Therefore, the irradiated-UHMWPE 

data may be related to extended chain formation or, more likely, to some radiationinduced effect. At any rate, when a sample of the irradiated polymer was melted, recrystallized, and remelted, the double peak was absent. Nor was there a double peak 

in the DSC trace for material taken from the rider surface. Since these riders were 

machined, it is possible that the frictional heat from the machining process caused the 

polymer to recrystallize in the normal mode. 

SUMMARY OF RESULTS 

The results from the study of the effect of sterilization gamma irradiation on the 

friction and wear properties of ultrahigh-molecular-weight polyethylene (UHMWPE) 

sliding against 316L stainless steel in dry air are summarized as follows: 

1. The friction and wear properties of UHMWPE were not significantly changed by 

gamma irradiation doses of 2.5 and 5.0 megarads. 

8 
2. Characterization of UHMWPE by solution-viscosity techniques indicated that 

gamma irradiation increased the amount of insoluble gel (high-molecular-weight fraction) and also increased the amount of low-molecular-weight (<20 000) material. 

3. After run-in, wear rate either was steady or gradually decreased as a function 

of sliding distance. In general, no late wear-rate increases were observed. 

Lewis Research Center, 

National Aeronautics and Space Administration, 

Cleveland, Ohio, January 15, 1979, 

506-16. 

9 
APPENDIX - CHARACTERIZATION PARAMETERS 

The following parameters were used to characterize the polymers in this study: 

(1) Insoluble gel (high-molecular-weight fraction) - the percentage of the polymer 

that is not soluble in xylene at 135' C. This parameter represents crosslinked polymers and/or polymers with molecular weights of 6x10 or greater. 6 

(2) Soluble fraction - the percentage of polymer that is soluble in xylene at 135' C. 

This parameter represents polymers with molecular weights of less than 6x10 . 

6 

(3) Low molecular fraction - the percentage of the total sample that has a weightr 

average molecular weight of about 20 000 

at 135' C 

(4) Lntrinsic viscosity [77] - the intrinsic viscosity of the soluble 

(5) Weight-average molecular weight zw - the weight-average 

of the soluble polymer fraction according to the relation 

polymer in Decalin 

molecular weight 

[q] 1350 = 6.77~10- MW 4- 0.67 

(6) Crystallinity (from density) - the percentage of the polymer that is crystalline, 

as determined by the gradient-column technique (ASTM D1505) 

(7) Crystallinity (from DSC) - the percentage of the polymer that is crystalline, 

as determined by differential scanning calorimetry. Crystalline content was calculated 

for a heat of fusion of 284 joules per gram (68 cal/g) for polyethylene crystals 

The following measurements were included in the characterization: 

(1) Differential scanning calorimetry - A commercial DSC instrument was used at 

a heating rate of 10 degrees C per minute. 

(2) Hardness measurements - Microhardness measurements were made on the 

wear-scar surface, at test conclusion, with a diamond pyramid indenter. The hardness 

number was calculated from the formula 

DPH =l. 854 - 

P 

d2 

where P is 128 grams, d is the diagonal length of the square impression in millimeters, and DPH is the diamond pyramid hardness in kilograms per square millimeter. 

10 
REFERENCES 

1. Sonstegard, David A. ; Matthews, Larry S. ; and Kaufel-, Herbert: The Surgical 

Replacement of the Human Knee Joint. Scientific American, vol. 238, no. 1, 

Jan. 1978. 

2. Chapiro, A. , ed. : Radiation Chemistry of Polymeric Systems. Vol. XV, High 

Polymers. Interscience Publishers, 1962 

3. Rostoker, W. ; and Galante, J. 0. : Some New Studies of the Wear Behavior of 

Ultrahigh Molecular Weight Polyethylene. J. Biomed. Mater. Res., vol. 10, 

no. 2, Mar. 1976, pp. 303-310. 

4. Miller, D. A. ; et al. : A Comparative Evaluation of the Wear of Ultra-High Molecular Weight Polyethylene Abraded by Ti-6A1-4V. Wear, vol. 28, 1974, pp. 207- 

216. 

5. Seedhom, B. B.; Dowson, D. ; and Wright, V. : Wear of Solid Phase Formed High 

Density Polyethylene in Relation to the Life of Artificial Hips and Knees. Wear, 

vol. 24, 1973, pp. 35-51. 

6. Matsubara, K. ; and Watanabe, M. : The Wear Properties of High-Density Polyethylene Irradiated by Gamma Rays. Wear, vol. 10, 1967, pp. 214-222. 

7. Awatani, J.; and Minegaki, M.: Nippon Kikai Gakkai Rombunshu, vol. 29, July 

1963, pp. 1189-1196. 

8. Shen, C. ; and Dumbleton, J. H. : The Friction and Wear Behavior of Irradiated 

Very High Molecular Weight Polyethylene. Wear, vol. 30, 1974, pp. 349-364. 

9. Ungethum, M.: Requirements of Operational Tests and Test Results in Total Hip 

and Knee Arthroplasty. Advances in Artificial Hip and Knee Joint Technology. 

Engineering in Medicine, Vol. 2, M. Schaldach and D. Hohmann, eds., SpringerVerlag (Berlin), 1976, pp. 493-518. 

10. Crugnola, Aldo M. ; et al. : Ultrahigh Molecular Weight Polyethylene as Used in 

Articular Prostheses (A Molecular Weight Distribution Study). J. Appl. Poly. 

Sci., vol. 20, no. 3, Mar. 1976, pp. 809-812. 

11. Tanaka, Kyuichiro; and Uchiyama, Yoshitaka: Friction, Wear, and Surface Melting 

of Crystalline Polymers. Advances in Polymer Friction and Wear. Polymer 

Science and Technology, Vol. 5B, L.-H. Lee, ed. , Plenum Press, 1974, 

pp. 499- 53 1. 

12. Dowson, D. ; Atkinson, John R. ; and Brown, K. : The Wear of High Molecular 

Weight Polyethylene with Particular Reference to its Use in Artificial Human 

11 
Joints. Advances in Polymer Friction and Wear. Polymer Science and Technology, Vol. 5B, L.-H. Lee, ed., Plenum Press, 1974, pp. 533-551. 

13. Dumbleton, J. H. ; and Shen, C. : The Wear Behavior of Ultrahigh Molecular 

Weight Polyethylene, Wear, vol. 37, 1976, pp. 279-289. 

14. Swikert, Max A. ; and Johnson, Robert L. : Simulated Studies of Wear and Friction 

in Total Hip Prosthesis Components with Various Ball Sizes and Surface Finishes. 

NASA TN D-8174, 1976. 

15. McKellop, H. A. ; et al. : The Importance of Fluid Absorption, Creep and Lubricant in Wear Measurements of Polymers for Prosthetic Bearings. Presented at 

the 22nd Annual Meeting of the Orthopaedic Research Society, (New Orleans, La,) 

Jan. 28-30, 1976, paper 78. 

16. Fusaro, Robert L.: Effect of Atmosphere and Temperature on Wear, Friction and 

Transfer of Polyimide Films. ASLE Trans. vol. 21, no. 2, 1978, pp. 125-133. 

17. Pooley, C. M. ; and Tabor, D. : Friction and Molecular Structure: The Behavior 

of Some Thermoplastics. Proc. R. Soc. (London) Series A, vol. 329, no. 1578, 

Aug. 22, 1972, pp. 251-274. 

18. Rhee, S. H. ; and Ludema, K. C. : Mechanisms of Formation of Polymeric Transfer Films. Wear of Materials-1977. W. A. Glaeser, ed., Am. SOC. Mech. 

Eng. , 1977, pp. 482-486. 

19. Kar, M. K. ; and Bahadur, S. : Micromechanism of Wear at Polymer-Metal Sliding 

Interface. Wear of Materials-1977. W. A. Glaeser, ed., Am. Soc. Mech, Eng. , 

1977, pp. 501-509. 

20. Biscoe, B. J. ; Pooley, C. M. ; and Tabor, D. : Friction and Transfer of Some 

Polymers in Unlubricated Sliding. Advances in Polymer Friction and Wear. 

Polymer Science and Technology, Vol. 5A, L.-H. Lee, ed. , Plenum Press, 

1974, pp. 191-204. 

21. Atkinson, J. R. ; Brown, K. J. ; and Dowson, D. : The Wear of High Molecular 

Weight Polyethylene. Part I: The Wear of Isotropic Polyethylene Against Dry 

Stainless Steel in Unidirectional Motion. Trans. ASME, vol. 100, no. 2, 

Apr. 1978, pp. 208-221. 

22. Duff-Barclay, I.; and Spillman, D. T. : Total Human Hip Joint Prostheses - A 

Laboratory Study of Friction and Wear. Proc. Inst. Mech. Eng. (London), 

VO~. 181, pt. 35, 1966-67, pp. 90-103. 

23. Lancaster, J. K.: Estimation of the Limiting PV Relationships for Thermoplastic 

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12 
24. Bell, J. P. ; Slade, P. E. ; and Dumbleton, J. H. : Multiple Melting in Nylon 66. 

J. Polym. Sci., A-2, vol. 6, no. 10, Oct. 1968, pp. 1773-1781. 

25. Dumbleton, John €3. ; and Buchanan, D. R. : Chainfolding in Oriented Poly(ethy1- 

ene Terephthalate). J. Polym. Sci. , A-2, vol. 7, no. 4, Apr. 1969, pp. 667- 

674. 

26. Yubayashi, Tsuneo; Orita, Zenichi; and Yamado, Nohuo: Differential Thermal 

Analysis of Poly(ethy1ene Terephthalate) Fibers. Kogyo Kagaku Zasshi, vol. 69, 

no. 9, 1966, pp. 1798-1802. 

TABLE I. - SUMMARY OF TEST DATA 

[Load, 1 kilogram (9.8 N). J 

Radiation 

dose, 

negarads 

0 

2.5 

5.0 

Test 

1 

2 

3 

4 

5 

6 

a7 

8 

9 

10 

1 

2 

3 

4 

5 

1 

2 

3 

4 

5 

6 

7 

Total 

sliding 

distance, 

m ~. 

40. oX1o3 

22.2 

43.0 

45.0 

45. 6 

182.0 

41. 9 

130.0 

263.0 

440.0 

- 

32. oX1o3 

37.8 

42.2 

52.0 

185.0 

28.9~10~ 

36.0 

36.7 

93.5 

137.0 

133.0 

578.0 

Surf ace 

velocity, 

m/sec 

~ 

0.061 

.077 

.073 

.073 

.069 

.076 

.074 

.27 

.26 

* 27 

0.075 

.064 

.072 

.074 

.063 

~. ~ 

0.077 

.070 

.071 

-076 

.076 

.25 

.24 

L!. 

I 

~ ~~ 

Average 

coefficient 

of friction 

." ~~ 

0.55 

.51 

.48 

.34 

.45 

.40 

.37 

.47 

(b) 

.39 

0.36 

.52 

.42 

.44 

.45 

0.31 

.31 

.41 

.42 

-43 

.33 

.46 

~ 

Average 

wear rate, 

m /N-m 3 

2.5~10-l~ 

2.2 

1.7 

.90 

.88 

.75 

1.7 

1.7 

1.5 

2.6 

1.0~10-~~ 

2.3 

1.3 

1.1 

.76 

1.8~10-l~ 

2.0 

1.6 

.78 

1.1 

1.1 

.63 

a2-Kilogram (19.6-N) load. 

bNot available. 

13 

I 1- I I I 1111111 1111 II I I II 111 II 111 II II 1111 111111 111 I 
TABLE II. - SUMMARY OF FRICTION AND WEAR TEST 

DATA BY 

dose, 

O IlOI 

2-5 5.0 I 7” I 

‘ RADIATION LEVEL 

0.44&0.07 

0.44iO. 06 

0.38ztO. 06 

1.6iO. 66~10-l~ 

1.3ztO. 60 

1.3i0.52 

TABLE III. - CHARACTERIZATION OF POLYETHYLENE 

TEST SPECIMENS 

Insoluble gel (high-molecularweight fraction), percent 

Soluble fraction, percent 

Low-molecular-weight fraction, 

percent 

Intrinsic viscosity (soluble), 

decaliters per gram 

Weight- average molecular 

Density, kg/m3 

Crystallinity (from density), 

weight (soluble) 

percent 

Crystallinity (from differential 

scanning calorimetry), 

percent 

Differential scanning calorimetry 

Temperature at beginning 

of melt, OC 

melt, OC 

Temperature at peak of 

Temperature at end of 

melt, C 0 

Diamond pyramid hardness, 

kg/mm2 

T Irradiation dose, megarads 

0 

14 

86 

1 to 2 

13.41 

2 588 000 

0.937 

68 

46 

101 

-” 

134 

4.9*0.7 

2.5 

66 

34 

4 

2.03 

152 000 

0.941 

70 

49 

101 

127 (minor) 

133 (major) 

136 

5.2zt0.5 

1 

5.0 

78 

22 

5 

1.37 

86 000 

0.945 

72 

50 

101 

125 (minor) 

133 (major) 

137 

4.8*0.5 

14 
F CD-12247-26 

Figure 1. - Slow-speed friction rig. 

12 - 

10 - 

a 

f 8- 

h 

VI 

L 

I 

VI 

m 

u 

c 

0 

- 

2 6- c 

0) 

L 

0 1 2 3 4 

Wear-scar diameter, 

1 

mm 

10-11 10-10 10-9 10-8 

Wear volume, m3 

Figure 2. - Apparent contact stress as a function of wearscar diameter, Load, 1 kilogram (9.8 N). 
n 

Test nn 01 a 

10-10 

lO-1 

05 

A6 

V8 

na A 

0% 

0 

o 

10-11 (a) Irradiation level, 0 mqarad. 

D 

a 

D 

0 

A 

0 

0 

0 

10-11 (b) irradiation level, 2.5 megarads. 

Test 

01 

D2 

03 

04 

a5 

V6 

07 

0 

0 

A 

0 

00 

10-11 102 L"I 103 L 104 1 I I~ 1.1 105 I "1 L 1 1 106 I 

Sliding distance, m 

(c) Irradiation level, 5.0 megarads. 

Figure 3. - Wear volume as a function of sliding distance. 

16 
Test 

01 

D2 

03 

04 

05 

n 

A6 

v 4 

07 

0 

a 

a" 

II 

5~10-l~ 

4 

53 ic 

a0 

0 

8 

I Ill 

n 

I 

la) Irradiation 

1-1 II 

level, 0 megarad. 

I " 

Test 

01 

D2 

03 

a4 

05 

0 

D D 

0% 0 

D 

OD 

A0 A 

0 A 

om 0 

00 

1 

(b) Irradiation 

I OI loPo 

level, 2.5 megarads. 

1°01 u 

V Test 

01 

0 03 

D2 

04 

a5 

5- V6 

07 

6- 

4- 0 

DD 

a 0 

3- 

% Ao 

2- O v VOWD A 

vo "'B 

!LA 

1- 

V 0 

go 0 

0 OO 00 

0 0 

0 

102 103 Id' Id 106 

I I Ill I I Ill 1- 

Sliding distance, m 

IC) Irradiation level. 5.0 mqlarads. 

Figure4. - Incremental wear rate as a function of sliding distance. 

17 
2 

c 

n 

m 

L 

L 

m 

6- 

5- 

4- 

3- 

2- 

a102 

T 11 I1 

1- 'I I I 

I1 11 

103 104 

I 

2x105 

Sliding distance, m 

Figure 5. - Coefficient of friction as a function of sliding distance for test 6 (0 megarad). 

Load. 1 kilogram (9.8 N); sliding velocity, 0.076 meter per second. 

I 

.2- 

a102 

T 

Illlll 1 I ! I I1111 I I I II 

103 104 6x 104 

Sliding distance, m 

Figure 6. - Coefficient of friction as a function of sliding distance for 

test 2 (5.0 megarads). Load, 1 kilogram (9.8 N); sliding velocity. 

0.070 meter per second. 

I 

425 400 375 350 

Temperature, K 

Figure 7. - Differential scanning calorimetry curve for unirradiated polyethylene. 

325 

18 
I 

425 400 

Temperature, K 

Figure 8. - Differential scanning calorimetry curve for irradiated polyethylene (2.5 megarads). 

Figure 9. - Transfer film on 316L steel disk after 2500 meters of sliding. 

19 
~~. ~ ~ .. . " 

1. Report No. 1 2. Government Accession No. 

. "" ~" 

- 

- 

- 

- 

- 

1 

- 

1 

NASA TP-1462 1 " . . . . . " . 

4. Title and Subtitle EFFECTOF STERILIZATION IRRADIATION ONFRICTION AND WEAR OF ULTRAHIGH-MOLECULARWEIGHT POLYETHYLENE 

William R. Jones, Jr., William F. Hady, and Aldo Crugnola - " " 

9. Performing Organization Name and Address 

. ." - . ~ "" .. 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135 . - - " ".~. .. - 

12. Sponsoring Agency Name and Address 

. ~ ~ 

I3. Recipient's Catalog No. 

" " - . 

5. Report Date 

May 1979 

__ .. 

6. Performing Organization Code 

." . - . . . . - . .. 

8. Performing Organization Report No. 

E-9697 

10. Work Unit No. 

506-10 

-~ 

11. Contract or Grant No. 

~~~~ . "" 

~ ~.. .. 

13. Type of Report and Period-Covered 

Technical Paper 

14. Sponsoring Agency Code 

~ - 

7. Key Words (Suggested by Author(s)) 

~ ~ ___..~. . 

Polymer characterization 

Friction and wear of polymers 

Prosthesis materials 

Ultrahigh-molecular-weight polyethylene 

9. Security Classif. (of this report) 

~ " 

~ "" -~ 

15. Supplementary Notes William R. Jones, Jr., Lewis Research Center; William F. Hady, Lewis 

Research Center; Aldo Crugnola, University of Lowell, Lowell, Mass. Work done under 

NASA purchase order C-8756. 

The effect of sterilization gamma irradiation on the friction and wear properties of ultrahighmolecular-weight polyethylene (UHMWPE) sliding against 316L stainless steel in dry air at 

23' C was determined. A pin-on-disk apparatus was used. Experimental conditions included 

a 1-kilogram load, a 0.061- to 0.27-meter-per-second sliding velocity, and a 32 000- to 

578 000-meter sliding distance. Although sterilization doses of 2.5 and 5.0 megarads greatly 

altered the average molecular weight and the molecular weight distribution, the friction and 

wear properties of the polymer were not significantly changed. 

.. 

18. Distribution Statement 

~ ~- . 

Unclassified - unlimited 

STAR Category 27 

Unclassified ." . .- I Unclassified . 

' For sale by the National Technical lnforrnatlon Service. Sprlneileld. Vlrgrnla 22161 

NASA-Langley, 1979 
r 

National Aeronautics and 

Space Administration 

Washington, D.C. 

20546 

Official Business 

Penalty for Private Use, $300 

THIRDGLASS BULK RATE 

r. 

'~'' 

Postage and Fees Paid 

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