Materials Science and Engineering A291 (2000) 27 – 36

www.elsevier.com/locate/msea

Tribological behaviour and microscopic wear mechanisms of

UHMWPE sliding against thermal oxidation-treated Ti6Al4V
W. Shi *, H. Dong, T. Bell
School of Metallurgy and Materials, The Uni6ersity of Birmingham, Birmingham B15 2TT, UK

Received 14 February 2000; received in revised form 2 May 2000

Abstract

Tribological behaviour of ultra-high molecular weight polyethylene (UHMWPE) pins sliding against thermal oxidation
(TO)-treated Ti6Al4V alloy discs with different levels of average surface roughness was investigated under water lubrication
conditions. When rubbing against a smooth counterface (Ra B 0.030 – 0.035 mm), UHMWPE was found to be worn predominantly
via a micro-fatigue mechanism. To advance the scientific understanding of the microscopic wear mechanisms of UHMWPE, a
technique involving permanganic etching coupled with high resolution SEM analyses of wear surfaces and cross-sections was
adopted to yield new insight into the micro-fatigue mechanism. It was found that stress-induced preferential orientation of the
crystalline lamellae in the UHMWPE led to the origin of ripples containing micro-cracks at their valleys. The cyclic loading
promoted lateral propagation and inter-connection of these micro-cracks, thus giving rise to eventual spallation of the surface
material as wear debris. Based on the experimental results, a micro-fatigue wear mode is proposed. © 2000 Elsevier Science S.A.
All rights reserved.

Keywords: UHMWPE; Sliding; Wear mechanism; Orientation

1. Introduction adhesive wear and fatigue wear have been identified as

three basic mechanisms although many wear patterns
Ultra-high molecular weight polyethylene (UHM- or surface damage features were reported, and different
WPE) has been the principal material of choice for classification schemes were adopted by various re-
total joint replacement (TJR) prosthesis over the last searchers [7–9]. Surface fatigue wear or micro-fatigue
three decades [1]. However, it has gradually been recog- wear was observed on retrieved and simulator tested
nised that fine wear debris generated from articulating UHMWPE acetabular cups [7] as well as unidirection-
surfaces (which are mostly UHMWPE) in body tissue ally tested specimens [10]. It has been found that such a
can cause adverse cell response and bone resorption, wear process is usually characterised by regular surface
thereby leading to aseptic loosening and thus premature ripples and associated with very low wear rates [7,10].
failure of TJR prostheses [2]. Therefore, the life span of However, the micro-fatigue mechanism may be the least
a TJR prosthesis is closely related to the tribological understood or the most misunderstood. Great efforts
behaviour of the bio-materials (especially UHMWPE) have been made by Wang et al. [11,12] in the context of
employed. Clearly, understanding the wear mechanism mechanistic and morphological origin of wear debris
and thus controlling the wear processes of such bio-ma- and the orientation-induced property anisotropy, which
terials are of vital importance for further extending the advances the understanding of the mechanism to some
lifetime of TJR prostheses. extent. However, direct microscopic evidences for the
Wear behaviour and mechanisms of UHMWPE have micro-fatigue wear process in terms of microstructure
been the subject of many papers [3 – 6]. Abrasive wear, evolution, formation and propagation of cracks, and
final spallation of material as debris are still missing.
* Corresponding author. Tel.: +44-121-4145197; fax: + 44-121-
Furthermore, although to date, there are many com-
4147373. binations of materials being employed in TJR prosthe-
E-mail address: w.shi@bham.ac.uk (W. Shi). sis, the UHMWPE/Ti6Al4V couple still remains one of

0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 0 9 7 2 - 2
28 W. Shi et al. / Materials Science and Engineering A291 (2000) 27–36

the most important bio-materials systems because of SiC abrasive paper to an average surface roughness
its proven clinical performance [13]. Our previous (Ra) value of 0.23–0.24 mm.
work has demonstrated that the tribological behaviour The counterface material in the pin on disc sliding
of UHMWPE can be significantly improved by sur- wear test was commercial Ti6Al4V titanium alloy.
face engineering the Ti6Al4V alloy counterface using Discs of approximately 100 mm diameter and 10 mm
the recently developed thermal oxidation (TO) treat- thickness were ground with 1200 c grit SiC abrasive
ment [10]. For example, the wear factor of paper. Prior to surface treatment, specimens were pol-
UHMWPE was about 2.5 times lower when sliding ished with diamond paste to an Ra value of 0.02 mm.
against a TO-treated Ti6Al4V disc than against a ni- Specimens were surface engineered using the newly
trogen implanted one. Therefore, thermo-chemical developed thermal oxidation (TO) process [15]. Repol-
treatment with oxygen has been viewed as a potential ishing prior to wear tests was carried out with dia-
surface engineering technique for titanium orthopaedic mond paste owing to the subsequent increase in
implant bearing surfaces [10,14]. It is of interest that surface roughness caused by the surface oxidation
the worn surface of the UHMWPE pin was character- treatment. The final surface roughness was controlled
ised by typical surface features of regular arrays of in the range from 0.020 to 0.065 mm.
ripples. Clearly, advancing the understanding of the
micro-fatigue wear mechanism in UHMWPE against 2.2. Wear testing
the TO-treated Ti6Al4V disc is of fundamental impor-
tance from both a scientific and clinical point of view. A conventional pin-on-disc tribometer equipped
However, no attention has been paid to the wear with a computer was employed to measure wear rates
mechanism in UHMWPE articulating to thermally and coefficients of friction of UHMWPE sliding
oxidised titanium alloys. The full potential of the op- against TO-treated Ti6Al4V discs under water lubri-
timum combination of UHMWPE/TO-treated cated sliding conditions. A schematic of the test ma-
Ti6Al4V alloy as sliding wear pairs will not be re- chine is shown in Fig. 1. The TO-treated Ti6Al4V
alised until the wear performance is fully investigated discs were located on a turntable driven at a constant
and the fundamental wear mechanisms are under- speed of 0.25 m s − 1 and UHMWPE pins were loaded
stood. Thus, the present study was directed towards at a contact stress of 5 MPa by means of static
investigating the wear mechanisms and microstruc- weights. These parameters are representative of condi-
tural response of UHMWPE sliding against TO- tions taken in human body for larger, heavier loaded
treated Ti6Al4V alloy with a view to exploring the joints [16]. The computer collected friction force and
possibility of further increasing the life span of TJR. converted it into friction coefficient data. All wear
In addition, the effect of initial surface roughness of tests were carried out in a dust-free environment.
counterfaces on the tribological properties of Truncated cylindrical pins were introduced to offer
UHMWPE was also studied. Finally, based on the a substantial foundation of the wear surface, which
experimental results and the wear mechanisms re- permits wear to take place over a relatively small
vealed, a micro-fatigue wear model is proposed and area. Before wear testing, the discs were degreased
the possibility of increasing the wear resistance of with acetone. The polymeric pins were ultrasonically
UHMWPE via surface engineering techniques is dis- cleaned to remove any loose particles, and were then
cussed. immersed in distilled water for 2 days to pre-soak
them. After each test period of about 21 km, the pins
were removed and any wear debris attached to the
2. Experimental details trailing edge of the wear surfaces was removed. Sub-
sequently, the specimens were ultrasonically rinsed,
2.1. Materials dried and weighed using a balance with accuracy of
0.01 mg. In view of the water absorption by the poly-
The medical grade UHMWPE (GUR 4150 HP) mer pins, a control pin of the same material and size
used in this study was supplied by Poly Hi Solidur was immersed in distilled water to monitor the rate of
(UK) Ltd. in the form of 12.5-mm diameter, extruded water absorption during the test period, which was
and annealed rod. The crystallinity of as-received accounted for in the process of calculating the weight
UHMWPE was estimated to be about 50% using X- loss of the UHMWPE pins. The experiments were
ray diffraction (X Pert Pw 3040, Philips). The density terminated after reaching a total sliding distance of
of homogeneous material and average molecular about 105 km. Wear volume loss was calculated from
weight, as quoted by the manufacturer are 0.932 g the weight loss by assuming the density of UHMWPE
cm − 3 and 3.6 × 106 g mol − 1, respectively. The sam- to be 0.932 g cm − 3. The average specific wear rate of
ples were machined from bar stock material, and then UHMWPE was calculated using the following for-
the surfaces to be tested were ground with 1200 c grit mula:
 W. Shi et al. / Materials Science and Engineering A291 (2000) 27–36 29

Fig. 1. Schematics of pin-on-disc tribometer, test configuration and test pin.

Wear factor (mm3 N − 1 m − 1) 3. Experimental results

Volume loss (mm3)
= 3.1. Wear and friction
Sliding distance (m)× Load (N)
The variation in weight loss of UHMWPE as a
function of sliding distance and average surface rough-
2.3. Post-test microanalysis ness of TO-treated Ti6Al4V counterfaces is presented in
Fig. 2. It is apparent from these curves that the wear
The wear morphologies of UHMWPE and Ti6Al4V curves for UHMWPE against TO-treated Ti6Al4V
alloy surfaces were examined using both optical mi- counterfaces are approximately linear. The wear rates,
croscopy and scanning electron microscopy (SEM, which were obtained from linear regression of the data
JEOL 6300). To detect any transferred polymer on the shown in Fig. 2, are given in Table 1, together with the
TO-treated Ti6Al4V counterface, a FT-IR spectroscope wear factors or special wear rates. It can be seen from
(Nicolet Magna-IR 760) was employed with a reflecting Table 1 that when the counterface Ra increased from
mode. The cross-sections of the wear pins were also 0.020–0.025 to 0.060–0.065 mm, the average wear fac-
examined using SEM in order to reveal the details of tor of UHMWPE after 105 km sliding distance in-
micro-fatigue cracking. Prior to SEM observation, the creased by about an order of magnitude, from 1×10 − 8
samples were coated with gold to prevent them from to 8.45× 10 − 8 mm3 N − 1 m − 1.
charging. Wear debris was collected from the distilled The evolution of friction coefficients of UHMWPE
water using 0.2-mm filter paper. sliding against TO-treated Ti6Al4V counterfaces with
Microstructural analysis was carried out using a high
resolution Hitachi S-4000 field emission gun (FEG)-
SEM. The worn surfaces needed to be etched in order
to reveal representative lamellar reconstruction in
UHMWPE during water lubricated sliding wear. Etch-
ing was carried out at room temperature for about 17
h, under constant agitation using a permanganic
reagent consisting of 0.5 wt.% potassium permanganate
in a mixture of 50 vol.% concentrated sulphuric acid
and 50 vol.% orthophosphoric acid. After being etched,
samples were washed successively with two parts by
volume of concentrated sulphuric acid to seven parts
water solution cooled to −20°C with dry ice, 30 vol.%
hydrogen peroxide (to reduce any manganese dioxide
or permanganate present), distilled water and acetone.
This technique of permanganic etching preferentially Fig. 2. Variations in weight loss of UHMWPE with sliding distance
etched away the amorphous regions, leaving the crys- against TO-treated Ti6Al4V counterfaces with different average sur-
talline regions relatively intact [17]. face roughness.
30 W. Shi et al. / Materials Science and Engineering A291 (2000) 27–36

Table 1 craters (Fig. 4a). The formation of such grooves along

Average wear rates (mm3 m−1) and wear factors (mm3 N−1 m−1) the direction of the sliding implies that abrasive wear by
asperities on the disc surface may be the dominant wear
Average surface Wear rate Wear factor
roughness, Ra (mm) (mm3 m−1) (mm3 N−1 m−1) mechanism. Under a very high magnification, micro-
crack arrays perpendicular to the sliding direction were
0.020–0.035 1.07×10−4 1.03×10−8 also found (Fig. 4b). These microcracks can be attributed
0.040–0.045 4.29×10−4 5.19×10−8 to the excessive traction arising from the effect of the
0.060–0.065 7.51×10−4 8.45×10−8 high friction associated with the rough counterface.
Fig. 5 shows typical morphologies of polymeric de-
bris, which were generated during the wear of
UHMWPE sliding against rougher counterfaces. De-
tailed observations revealed that two surfaces of this
plate-like particle, as marked A and B in Fig. 5a,
possessed different morphologies. As shown in Fig. 5b,
side A was characterized by many fine slightly arc-
shaped grooves. It is thus deduced that side A was the
surface of the pin, which was articulated to the counter-
face. These grooves were probably formed under the

Fig. 3. Coefficients of friction of UHMWPE as a function of sliding

distance against TO-treated Ti6Al4V counterfaces with different sur-
face roughness.

different Ra values (in the range of 0.020 – 0.065 mm) is

shown in Fig. 3. The data points represent the mean
values of friction coefficient data obtained from at least
two different tests. Some features in Fig. 3 merit specific
attention. Firstly, the friction of coefficient of
UHMWPE is closely related to the surface roughness of
the TO-treated counterfaces. In general, the rougher the
counterface, the higher the friction coefficient. Secondly,
for a given counterface roughness, the coefficient of
friction evolved with time. As shown in Fig. 3, when
UHMWPE slides against a smooth counterface (Ra =
0.020–0.025 mm), the tribo-pair exhibit an initial value
of 0.035, followed by a rapid decrease to reach a constant
value of 0.025. A similar trend was observed for counter-
faces with Ra values of 0.040 – 0.045 and 0.060 – 0.065 mm.

3.2. Microscopic characterisation

It was found that the wear morphologies of

UHMWPE pin surfaces varied with the surface rough-
ness of the TO-treated titanium counterface. Two dis-
tinct wear morphologies were identified on the
UHMWPE pin surface after sliding against a smooth
(Ra =0.020–0.025 mm) and a rough (Ra =0.060–0.065
Fig. 4. The morphologies of the worn surface for UHMWPE sliding
mm) counterface. The worn surface of UHMWPE pins
against a rough (Ra =0.060 – 0.065 mm) counterface: (a) a SEM image
sliding against a rough counterface (Ra =0.060–0.065 (the counterface sliding direction was diagonal from bottom left to
mm) looked dull and was characterised by grooves top right) and (b) a high resolution SEM image (the counterface
parallel to the sliding direction with some cracks and sliding direction was horizontal).
 W. Shi et al. / Materials Science and Engineering A291 (2000) 27–36 31

against a smooth counterface (Ra = 0.020–0.025 mm)

looked shiny and featureless under the optical micro-
scope (Fig. 6a). Close observations under the SEM
revealed fine regular ripples (Fig. 6b), which were trans-
verse to the sliding direction with a wavelength of
approximately 2 mm. A similar feature was also ob-
served in high molecular weight polyethylene sliding
against stainless steel [18]. The details of these ripples
were examined using a high resolution SEM and typical
features are presented in Fig. 7a. It can be seen that
these ripples consist of alternate sub-micron cracks and
ridges. A representative micrograph (as shown in Fig.
7b) from the cross-section along with the sliding direc-
tion further revealed that the wear surface was micro-
scopically zigzagged. It is found by comparing Figs. 6b
and 7b that the distance between these peaks on the
cross-section (Fig. 7b) corresponds to the wavelength of
the ripples on the surface (Fig. 6b).
Close examination of the wear surface under high
magnification also revealed that in some areas, ripples

Fig. 5. SEM photographs showing the typical morphologies of

UHMWPE debris generated during wear of UHMWPE sliding
against rougher counterfaces.

action of asperities on the Ti6Al4V counterfaces. Side B

exhibited the typical morphology of shear fracture, as
Fig. 6. Wear morphologies of UHMWPE sliding against a smooth
evidenced by the projection (peak-like) morphology on
(Ra =0.020 – 0.025 mm) counterface: (a) optical micrograph and (b)
the surface (Fig. 5c). SEM micrograph. The counterface sliding directions were vertical
However, the wear surface of UHMWPE pins and horizontal for (a) and (b), respectively.
32 W. Shi et al. / Materials Science and Engineering A291 (2000) 27–36

were replaced by a series of very fine wavy lines (Fig.

8). It should be pointed out that these very fine lines
looked very similar in appearance to the ripples, how-
ever, the interspace between these regular features was
different from one another. The interspace of the for-
mer was about one order of magnitude smaller than
that for the latter. It is also of great interest to note that
these wavy lines very much resembled typical striations
observed on metal fracture surfaces involving fatigue
processes. Some very small debris, as shown in Fig. 9,
was collected from the water lubricant, which is about
one to two orders of magnitude smaller than that
shown in Fig. 5.
The wear tracks on TO-treated Ti6Al4V discs were
found to be slightly polished, but neither abrasive
grooves nor transferred polymeric material can be ob-
served on the counterfaces under microscopes (optical
and SEM) regardless of their original surface rough-
ness. FT-IR analysis further confirmed that the amount
of transferred polymer, if any, was not detectable in the
accuracy of the facility. It is frequently reported that
there is a strong tendency for UHMWPE to be trans-
ferred onto metallic counterfaces owing to its low cohe-
sive strength and the high adhesive strength between
UHMWPE and most metallic surfaces [19]. It is thus
deduced that when water lubricated, TO-treated
Ti6Al4V possesses excellent resistance to polymer trans-
fer. This may be ascribed to the low materials compat-
ibility between TO-treated Ti6Al4V and UHMWPE,
and to the significantly improved wettability to water
conferred by the surface oxide layer formed during TO
treatment [10].

Fig. 7. High resolution SEM images of (a) the typical feature of 3.3. Microstructure e6olution
ripples consisting of alternate submicron cracks and ridges and (b)
cross-section along with the sliding direction showing a zigzagged
worn surface. The counterface sliding directions were from left to In order to correlate the worn morphologies of the
right and from right to left for (a) and (b), respectively. UHMWPE pin to the reconstruction of molecular

Fig. 8. High resolution SEM image showing a series of very fine wavy Fig. 9. High resolution SEM image of UHMWPE wear debris
lines parallel to ripples after the flakes break off. The counterface collected from water lubricant after a UHMWPE slide against a
sliding directions were from bottom left to top right. smooth counterface.
 W. Shi et al. / Materials Science and Engineering A291 (2000) 27–36 33

crostructure of the as-received material, showing ran-

domly distributed microstructural features comprising
crystalline lamellae and inter-lamellar amorphous re-
gions. However, after sliding against TO-treated
Ti6Al4V alloy, the crystalline lamellae of UHMWPE
surface were not random but preferentially oriented to
the direction of sliding, as reflected in Fig. 10b,c.
Clearly, accompanying the wearing process, reconstruc-
tion of the molecular chain structure of the linear
polymer occurred, thus forming aligned lamellae. Com-
paring Fig. 10b,c reveals that the larger the surface
roughness of the counterface, the higher the extent of
preferred lamellae developed largely due to higher sur-
face traction. Therefore, it follows that preferentially
oriented structures were generated by plastic flow of
such large structural units as spherulite into macrofibril
under the combined effects of the applied normal stress
and tangential stress arising from the frictional force.

4. Discussion

4.1. Microstructure e6olution

It is known that UHMWPE is a linear semi-crys-

talline polymer consisting of lamellar crystals and inter-
lamellar amorphous regions. After permanganic
etching, the amorphous regions will preferentially etch
away and the crystalline regions remain relatively in-
tact, thus allowing the microstructure to be revealed
[17]. As shown in Fig. 10a, the crystalline lamellae are
randomly distributed in the as-received material. This is
presumably because the pin was machined from an-
nealed rod and the wear surface is the transverse sec-
tion, and thus no appreciable preferred orientation
would be expected.
On the other hand, as evidenced in Fig. 10b,c, the
crystalline lamellae are aligned in the direction of slid-
ing. It is believed that the inelastic response of the
microstructure of UHMWPE may begin with molecular
plastic flow in the amorphous regions via inter-lamellar
shear, inter-lamellar separation and lamellar-stack rota-
tion [20]. Either tie or entanglemental molecular chains
would be heavily strained consequently. Once plastic
deformation in the amorphous regions is exhausted, the
crystalline lamellae are deformed predominantly
through crystallographic slip [21]. As a result, the crys-
Fig. 10. High resolution SEM photographs showing the microstruc-
talline lamellae are preferentially oriented to the sliding
ture of (a) as-received UHMWPE, (b) UHMWPE sliding against a direction, and reconstruction of the microstructure in
smooth counterface and (c) UHMWPE sliding against a rough the surface of UHMWPE pins should occur upon
counterface. The arrows show the sliding direction. wearing. Direct evidence of lamellae alignment in the
wear surface has also been found by Klapperich et al.
chain structure of linear polymer during sliding wear [4] by means of chlorosulphonic acid staining and
tests, the pin surfaces were etched and examined in the TEM, which supports the finding of the present study
high resolution SEM. Fig. 10a represents the mi- using permanganic etching and high resolution SEM.
34 W. Shi et al. / Materials Science and Engineering A291 (2000) 27–36

Mechanically, such preferred orientation of the mi- would make the near-surface susceptible to damage
crostructure of UHMWPE to the sliding direction caused by shear stress arising from surface traction.
would cause property anisotropy. It is expected that Therefore, lamellae alignment should play an important
crack propagation may, to some extent, be retarded by role in the wear of UHMWPE.
the crystalline regions in a randomly distributed mi-
crostructure of UHMWPE. However, crystalline lamel- 4.2. Micro-fatigue mechanism
lae alignment to the sliding direction upon wearing
Fig. 6b shows regular arrays of surface ripples on the
worn surface after sliding against the smooth counter-
face associated with the lowest wear factor. Similar
wear morphologies were also observed by Wang et al.
[7] on retrieved or simulator tested UHMWPE acetabu-
lar cups, and they found that such a wear morphology
always corresponds to very low wear rates. Clearly,
advancing the understanding of the wear mechanism is
of fundamental importance from both a scientific and
clinical point of view. However, this wear process is the
least understood.
As demonstrated earlier, the new approach
combining permanganic etching with high resolution
SEM analysis of the worn surfaces and cross-section,
directly revealed important information on the nature
and evolution of the microstructure and the spatial
wear morphologies of UHMWPE. This makes it possi-
ble to gain new insight into the origin and propagation
of micro-cracks during micro-fatigue process under the
action of repeated loading. Based on the experimental
results, a model of the micro-fatigue process of
UHMWPE is introduced schematically in Fig. 11.
During the unidirectional sliding, every asperity on
the UHMWPE surface was cyclically traversed by pass-
ing asperities on the TO-treated Ti6Al4V counterfaces.
Under the action of repeated loading, the contact areas
of asperities on wear surfaces of UHMWPE would be
plastically strained possibly via, as discussed in the
previous section, plastic deformation in the amorphous
regions and crystallographic slip in the lamellae. Subse-
quently, the accumulation of plastic strain associated
with the incubation period of micro-fatigue wear pro-
moted the development of parallel arrays of tongue-like
features (Fig. 11a). As wear proceeds and thus the
strain accumulates gradually, these tongues grow later-
ally and become thick, thus forming characteristic rip-
ples (Fig. 11b). At a critical strain level, breakage of the
tie molecular chains and amorphous disentanglement
may occur. The broken molecular chain with chain-end
radicals may be transformed into stable end groups by
hydrogen abstraction. This would trigger a cascade of
additional chain ruptures, hence leading to the nucle-
ation of a flaw or a sub-micron crack [22]. Conse-
quently, some sub-micron cracks perpendicular to the
sliding direction initiate in localised positions of the
ripples where ductility of the polymer is exhausted (Fig.
Fig. 11. A schematic model of the micro-fatigue process of
11b). Eventually, this promotes the formation of micro-
UHMWPE unidirectionally sliding against a TO-treated Ti6Al4V cracks and their propagation along the ridge of these
counterface in water. ripples, as evidenced in Fig. 7a, under the repetitive
 W. Shi et al. / Materials Science and Engineering A291 (2000) 27–36 35

loading. Micro-cracks along ridges of the ripples may be predominated by an abrasive mechanism, micro-fa-
progress towards the core at an angle of about 45° tigue mechanism prevails when UHMWPE slides
presumably under the action of liquid press pressure. against a smooth counterface.
Consequently, this leads to the formation of stretched The technique combining permanganic etching with
tongues or lips on the worn surfaces. Indeed, cracks at high resolution SEM observation can be used to di-
the valleys of the ripples downwards to the core were rectly reveal the wear-induced preferred orientation of
clearly observed on the cross-section along the sliding lamellar crystals in the UHMWPE microstructure.
direction (Fig. 7b). Finally, the formation of well devel- On wearing, the lamellar crystals in the surface of
oped surface lips would cause the transfer of stress into semi-crystalline linear UHMWPE were aligned in the
the subsurface shear plane. As indicated earlier, crys- direction of sliding. This stress-induced preferential ori-
talline lamellae are preferentially oriented to the direc- entation would lead to mechanical property anisotropy.
tion of sliding, so it is believed that cracks propagate Based on the experimental evidence of the evolution
mainly through the amorphous regions but bypassing of the microstructure morphology and the formation
lamellar crystals under the action of continuous cyclic and propagation of micro-cracks, in conjunction with
stress [4]. As expected, cracks would develop following rational interpretation, a micro-fatigue wear model for
a wavy route, as schematically illustrated in Fig. 11c, semi-crystalline linear UHMWPE is proposed.
until resultant parts of the flakes undermined by the
dynamic fatigue fracture break off as small wear debris
(shown in Fig. 9). Actually, a series of wavy lines
parallel to the surface ripples and normal to the sliding Acknowledgements
direction were observed on the worn surface by high
resolution SEM (Fig. 8). These parallel wavy lines The project was supported by the European Commis-
could be interpreted as micro-fatigue striations repre- sion under IC15-CT96-0705. One of the authors (W.
senting successive periods of the propagation of micro- Shi) acknowledges the financial support of an Overseas
cracks. Research Studentship (ORS) from the Committee of
It is clear from the experimental evidence and above Vice-Chancellors and Principals (CVCP). In addition,
discussion that crystalline lamellae alignment will in- the authors would like to thank Dr R.H. Olley at the
evitably occur in the surface of uncross-linked J.J. Thomson Physical Laboratory (Reading, UK) for
UHMWPE. This microstructural reconstruction will his help in providing technical information on etching
impart some unfavourable effect on the mechanical UHMWPE.
properties of UHMWPE in terms of property an-
isotropy and weakened resistance to micro-fatigue
wear. Consequently, from the wear mechanism pro- References
posed in this study it can be deduced that any surface
modification techniques which can convert the smooth [1] J. Charnley, J. Bone Jt. Surg. Ser. B (Br.) 54 (1972) 61.
molecular microstructure into a three-dimensionally [2] W.H. Harris, Clin. Orthopaed. Relat. Res. 311 (1995) 46.
cross-linked molecular structure can be used to effec- [3] J. Fisher, D. Dowson, Proc. Inst. Tech. Eng. Met. 205 (1991) 73.
[4] C. Klapperich, K. Komvopoulos, L. Pruitt, J. Tribol. 121 (1999)
tively enhance the wear resistance of UHMWPE. In-
394.
deed, it has been reported elsewhere [23] that nitrogen [5] B. Berbyshire, J. Fisher, D. Dowson, C.S. Hardaker, K. Brum-
plasma immersion ion implantation can significantly mitt, Wear 181-183 (1995) 258.
improve the wear resistance of UHMWPE by cross- [6] Y.Q. Wang, J. Li, Mater. Sci. Eng. A266 (1999) 155.
linking the molecular structure. [7] A. Wang, A. Essner, V.K. Polineni, D.C. Sun, C. Stark, J.H.
Dumbleton, in: I.M. Hutchings (Ed.), New Directions in Tribol-
ogy, Mechanical Engineering Publications, London, 1997, p.
443.
5. Conclusions [8] D. Dowson, Wear 190 (1995) 171.
[9] J.R. Cooper, D. Dowson, J. Fisher, Wear 162-164 (1993) 378.
The wear factor of UHMWPE sliding against a [10] H. Dong, W. Shi, T. Bell, Wear 225-229 (1998) 146.
[11] A. Wang, D.C. Sun, C. Stark, J.H. Dumbleton, Wear 181-183
TO-treated Ti6Al4V disc under water lubricated condi-
(1995) 241.
tions is closely related to the surface roughness of the [12] A. Wang, D.C. Sun, S.S. Yau, B. Edwards, M. Sokol, A. Essner,
counterface, varying from 1×10 − 8 to 8.5× 10 − 8 mm3 V.K. Polineni, J.H. Dumbleton, Wear 203-204 (1997) 230.
N − 1 m − 1. Low wear factor values correspond to the [13] D. Dowson, in: P.J. Blau (Ed.), ASM Handbook 18, Friction,
lower surface roughness levels of the counterface. Lubrication, and Wear Technology, ASM International, Materi-
als Park, OH, 1992, p. 656.
Two distinct wear mechanisms operated in
[14] R.M. Streicher, H. Weber, R. Schon, M. Semlitsch, Biomaterials
UHMWPE depending on the roughness of the counter- 12 (1991) 125.
face. Whereas the wear of UHMWPE sliding against a [15] H. Dong, A. Bloyce, P.M. Morton, T. Bell, TO Surface Treat-
rough counterface (Ra \0.035 – 0.040 mm) is found to ment Process, Patent pending.
36 W. Shi et al. / Materials Science and Engineering A291 (2000) 27–36

[16] D. Dowson, in: D.T. Clark, W.J. Feast (Eds.), Polymer Surfaces, Metallic Materials, Proceedings of the 3rd Leeds-Lyon Sympo-
Wiley, Chichester, UK, 1978, p. 399. sium On Tribology, MEP, London, 1978, p. 99.
[17] R.H. Olley, D.C. Bassett, Polymer 23 (1982) 1707. [20] L. Lin, A.S. Argon, J. Mater. Sci. 29 (1994) 294.
[18] J.R. Atkison, K.J. Brown, D. Dowson, J. Lubrication Technol. [21] P.B. Bowden, R.J. Young, J. Mater. Sci. 9 (1974) 2034.
100 (1978) 208. [22] H.H. Kausch, Polymer Fracture, Springe, Berlin, 1978, p. 194.
[19] D. Dowson, J.M. Challen, K. Holmes, J.R. Atkinson, in: D. [23] H. Dong, T. Bell et al., First Annual Progress Report, Inco-
Dowson, M. Godet, C.M. Taylor (Eds.), The Wear of Non- Coper-nicus IC15-CT96-0705, 1998.