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MARTES / TUESDAY
20
by the machining tool. The main feature are
the scratches that have been caused by the
action of the disc sliding on the pin surface,
leaving a typical unidirectional lay structure,
lined up with the sliding direction of the disc
on the pin surface, see Figure 3c.The wear
zone has a polished appearance to the naked
eye. On the other hand, in the wear zone
(Figure 3c) the continuous sliding of the disc
on the pin surface has caused, besides the
scratches already mentioned, a microstructure
in the form of a ripple-like microstructure
or lay of the fi bres forming the UHMWPE.
The ripples are perpendicular to the sliding
direction. As conclusion, this microstructure
demonstrates an evident alignment of the
UHMWPE fi bres depending on the sliding
direction.
Optical micrographs of the worn surfaces
revealed the ripple-like structure for all the
pins studied (Figure 3d). However, the size of
the ripples was different if the pin was from a
XLPE material (Figure 3e and 3f). The ripplelike
structure is clearly smaller compared to
the microstructure founded for the unirradiated
or irradiated UHMWPE. The ripples on
XLPEs are smaller because their fi bres are
smaller when compared to noncrosslinked
UHMWPEs. This is a consequence of the
heat treatment that XLPEs undergo after
irradiation, for these materials performed at
155°C, which acts as a remelting process
for the UHMWPE fi bres. Other feature on
the worn surfaces are the higher number of
scars for irradiated and XLPEs than for the
unirradiated UHMWPE, and at the scars are
shallower. Both, features, the morphology, the
morphology of the ripples and the scratches,
indicate a lower grade of deformation of the
irradiated and XLPEs, because the crosslinking
induced by irradiation.
The observation by means of SEM was focused
in the formation of UHMWPE particles
that detach from the pin surface producing
wear debris. In the following, the scanning
electron micrographs of the unirradiated
(Figure 3g), irradiated (Figure 3h), XLPE I
(Figure 3i) and XLPE II (Figure 3j) materials
are shown.
From SEM observations, Figures 3g to
3j show that the UHMWPE pins exhibit a
cracked surface texture. This texture is
evidenced as microcracks present in every
direction but preferentially occurring
between ripples, highlighting the ripple-like
microstructure of the UHMWPE seen under
optical microscopy. These microcracks are,
however, an artefact produced bay gold sputtering,
necessary to render conductive the
UHMWPE surface. They do not represent
microcracks in the UHMWPE surface. This
artefact could not be avoided, even with very
short sputtering periods.
Besides the ripple-like microstructure, other
features can be observed, as for example,
the particle formation in the form of fi brils on
the worn surfaces. The study of the particle
formation is an essential point to understand
the wear processes occurring, because it has
been hypothesised that wear particles will be
liberated from the articulating surface after
the cyclic accumulation of a critical amount
of plastic strain.
The grade of fi bril formation is higher for the
unirradiated than for the irradiated material
and then less for the XLPEs. For non-XLPEs,
fi brils are oriented parallel to the sliding direction
and can extend over several ripples.
For XLPEs, more than fi bril formation there
is formation of rounded particles smaller than
those formed for the non-XLPEs. The particle
formation of The XLPEs corroborates again
the lower deformation capacity of the XLPEs
in comparison with irradiated and unirradiated
UHMWPEs. In Figure 3j, the concentration
of plastic strain on the polyethylene surface
on the XLPEs can be observed.
Considering weight loss results, the particle
formation in XLPEs should have been higher
than for the non-XLPEs, since the UHMWPE