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Improvement of Wear Resistance of UHMWPE by Adding Solid Lubricating
Fillers
Article in Key Engineering Materials · September 2016
DOI: 10.4028/www.scientific.net/KEM.712.155
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Key Engineering Materials
ISSN: 1662-9795, Vol. 712, pp 155-160
doi:10.4028/www.scientific.net/KEM.712.155
© 2016 Trans Tech Publications, Switzerland
Submitted: 2016-05-01
Revised: 2016-05-31
Accepted: 2016-05-31
Online: 2016-09-27
Improvement of Wear Resistance of UHMWPE by Adding Solid
Lubricating Fillers
S.V. Panin1,2,a, L.A. Kornienko1,b, V.O. Alexenko1,2,c, L.R. Ivanova1,d
1
Institute of Strength Physics and Materials Sciences SB RAS, 34021, Tomsk, Russia
2
National Research Tomsk Polytechnic University, 634050, Tomsk, Russia
a
[email protected], [email protected], [email protected], [email protected]
Keywords: ultra-high molecular weight polyethylene, filler, graphite, molybdenum disulfide,
polytetrafluoroethylene, wear resistance, permolecular structure.
Abstract. For estimating effectiveness of adding solid fillers for composites with ultra-high
molecular weight polyethylene matrix tribotechnical characteristics of UHMWPE mixture with
graphite, molybdenum disulfide and polytetrafluoroethylene were investigated under dry friction,
boundary lubrication and abrasion. The optimum filler weight fraction was determined in terms of
increasing wear resistance. Permolecular structure and surface topography of wear tracks for
UHMWPE composites with different weight fraction of the fillers was studied. The mechanisms of
wear of polymeric composites “UHMWPE-graphite”, “UHMWPE-PTFE” and “UHMWPE-MoS2”
under dry sliding friction and abrasive wear are discussed.
Introduction
Ultra-high molecular weight polyethylene (UHMWPE) possesses performance characteristics quite
appropriate for polymers, while low friction coefficient, high wear and chemical resistance in
corrosive environments, high impact strength, low temperature of embrittlement provide its stability
and possibility for wide applications in various fields of engineering and operating conditions. The
use of composite materials with UHMWPE matrix ensures multiple increasing of wear resistance of
metal-polymer friction units. Recently, micro- and nanocomposites with UHMWPE matrix are
actively developed. Type and size of fillers are determined by the scope of application fields and the
environment where the composites are operated (vacuum, reactive and inert environment, cryogenic
or elevated temperatures) [1-4].
The solid lubricating filler (graphite, molybdenum disulfide, PTFE) are widely used in lubricants
(oil additive) and manufacturing of antifriction coatings (in case of PTFE) to be applied in a wide
temperature range (-45 to +400˚C). The choice of lubrication method depends on the structure
(design) and operating conditions of a friction pair. In the current paper a solid lubricant is added into
UHMWPE as a filler, as it can effectively operate at very low temperatures (e.g., cryogenic ones),
when the liquid or paste-type lubricants cannot withstand extreme conditions.
Under development of UHMWPE composite materials designers usually are guided towards the
preferential conditions of their operation: abrasion, dry sliding friction, friction in the boundary
lubrication, etc. In doing so, the minimum wear is characteristic for the case with the presence of
lubricating medium in the tribocontact area.
The aim of the study is a comparative analysis of the mechanical properties and tribotechnical
characteristics of UHMWPE composites with the solid lubricating fillers MoS 2, C and PTFE under
dry friction, boundary lubrication and abrasive wear.
Experimental
UHMWPE powder (GUR-2122 by Ticona, Germany) with a molecular weight of 4.0 million
carbon units and particle size of 5-15 microns, colloidal graphite, C-1 ( 1-4 µm), molybdenum
disulfide MVCh-1 ( 1-7 µm), F-4PN20 polytetrafluoroethylene ( 14 and 180 µm) were used.
Specimens of polymeric composites were prepared by hot pressing at specific pressure of 10 MPa
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Advanced Materials for Technical and Medical Purpose
and sintering temperature of 200 ºC with subsequent cooling rate of 5°C/min. Mixing of polymeric
blends of the UHMWPE and fillers (C, MoS2, PTFE) was carried out in a planetary ball mill
MR/0.5*4 with preliminary dispersing components in an ultrasonic bath.
Wear resistance of materials under dry friction was measured by "block-on-ring" scheme under
the load of 68.8 N and rotation speed of 100 rev/min (according to ASTM G99) with the use of
SMT-1 wear testing machine (sliding rate was 0.32 m/sec). The specimen size was equal to
7×7×10 mm. Diameter of 100Cr6 steel counterface was 62 mm. The friction surfaces of the
specimens were examined by optical profilometer Zygo New View 6200. Friction track area was
automatically calculated with its manual selection with the help of «Rhino Ceros 3.0» software.
Abrasion tests were performed with the use of MI-2 machine for rubber abrasion testing. Wear
resistance was estimated under loading of 0.15 MPa and sliding speed of 17.0 m/min relative to the
pair of specimens. Abrasive paper R 240 with a grain size of 58.5 µm was fixed on the counterface
(Russian Federation State Standard 426). Volume abrasion was determined by weighing the
specimens and further recalculation of weight loss in every 5 minutes. Method of testing meets the
requirements of ASTM G99 and DIN 50324. Tribotechnical characteristics were calculated by
averaging data over four specimens of each type.
Structural investigations were carried out with the use of scanning electron microscope LEO
EVO 50 under accelerating voltage of 20 kV onto cleavage surfaces of notched specimens
mechanically fractured after exposure into liquid nitrogen. Mechanical characteristics were
determined by tensile tests performed with the use of electromechanical testing machine
Instron 5582 for dog-bone shaped specimens with the data averaging over five samples of each type
(GOST 11262-80).
Results and discussion
Tribomechanical characteristics of UHMWPE as well as composites “UHMWPE + n wt. % C”,
“UHMWPE + n wt. % MoS2”, and “UHMWPE + n wt. % PTFE” are shown in Tables 1-3. It is
seen that mechanical properties (tensile strength, elongation at failure) change slightly, and the
friction coefficient of all tested composites is decreased. Tribotechnical characteristics of these
composites substantially depend on the filler weight fraction and type of wear testing (dry friction,
boundary lubrication, abrasive wear).
Table 1. Mechanical properties and friction coefficient of the composites “UHMWPE-С”
Content of
filler, C, wt.%
Density,
, g/cm3
0
3
5
10
0.936
0.953
0.967
0.989
Shore
Hardness,
D
56.7±0.6
57.5±0.5
57.5±0.4
57.6±0.6
Yield point
σ0,2, МPа
19.2±0.9
19.5±1.0
19.6±1.2
20.1±1.3
Ultimate
strength
σU, МPа
34.3±1.7
30.3±1.5
29.7±1.5
28.5±1.8
Elongation
at failure, ε, %
Friction
factor, ƒ
470±23.6
471±23.8
513±25.1
538±25.3
0.16
0.1
0.11
0.12
Table 2. Mechanical properties and friction coefficient of the composites “UHMWPE-MoS2”
Content of
filler, MoS2,
wt.%
0
3
5
10
Density,
, g/cm3
0.936
0.954
0.975
1.010
Shore
Hardness,
D
56.7±0.6
56.2±0.6
56.9±0.5
56.9±0.5
Yield point
σ0,2, МPа
19.2±0.9
18.4±0.8
18.6±0.7
18.7±0.9
Ultimate
strength
σU, МPа
34.3±1.7
26.2±1.9
26.9±1.6
26.7±1.9
Elongation
at failure, ε, %
Friction
factor, ƒ
470±23.6
494±24.6
515±25.3
535±24.1
0.16
0.095
0.1
0.11
Key Engineering Materials Vol. 712
157
Dry sliding friction
Diagrams of wear intensities at the steady-state wearing stage (I, mm2/min) of the above
mentioned composites are given in Fig. 1 (a, b). It seen that the lowest wear rate is observed in the
composites “UHMWPE + (3-5) wt. % C”, and “UHMWPE + 10 wt. % MoS2” (wear rate decreases
almost by 2 times as compared to pure UHMWPE). The data on wear track surface roughness in the
composites fully correlate with the data on wear intensity.
Table 3. Mechanical properties and friction coefficient of the composites “UHMWPE-PTFE”
Content of
filler,
PTFE, wt.%
0
5
10
20
40
Density,
, g/cm3
Shore
Hardness, D
0.926
0.97
1.00
1.06
1.22
59.5±0.6
59.8±0.5
59.6±0.6
59.7±0.6
59.8±0.6
Ultimate
strength
σU, МPа
32.3±0.9
29.2±1.0
27.0±1.2
24.7±1.3
20.2±1.0
Elongation
at failure, ε, %
485±23.6
465±23.6
428±25.1
406±24.3
217±23.2
Crystalli
nity
χ, %
44.8
39.5
35.8
35.0
26.0
Friction
factor, ƒ
0.12
0.067
0.067
0.068
0.075
a
b
Fig. 1. Wear intensity (I) and roughness of wear track surface (Ra) (b) for UHMWPE and
composites: а) pure UHMWPE (1), UHMWPE + 3 wt. % C (2), UHMWPE + 5 wt. % C (3),
UHMWPE + 10 wt. % C (4); b) pure UHMWPE (1), UHMWPE + 3 wt. % MoS2 (2), UHMWPE +
5 wt. % MoS2 (3), UHMWPE +10 wt.% MoS2 (4) at the steady-state wearing stage under dry
friction
For “UHMWPE-PTFE” composites the optimum filler weight fraction also makes
10 wt. % PTFE (Fig. 2, a, b). Moreover, the finer powder of the filler the more the effect is
pronounced (14 vs 180 µm). Thus, the optimum content of the solid lubricating fillers, providing
double increase in wear resistance under dry sliding friction was determined. Further increase in the
weight fraction of these fillers is not effective in terms of improving wear resistance of UHMWPE
composites under dry sliding friction.
Wear track surfaces (at the steady-state wearing stage) as well as permolecular structure of the
all above composites were investigated to clarify the relationship between the nature of the wear at
dry sliding friction, pattern of the formed structure and the type and weight fraction of fillers in
UHMWPE composites. It is shown that filling UHMWPE by particles of carbon and molybdenum
disulfide as well as polytetrafluoroethylene gives rise to a gradual change in permolecular structure:
the formation of spherulitic structure is suppressed, while permolecular structure of UHMWPE
becomes less uniform due to the fact that the filler particles prevent the spherulite growth during the
crystallization.
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Advanced Materials for Technical and Medical Purpose
The difference in the tribotechnical characteristics of UHMWPE with the three solid lubricating
fillers is related to their lubricity (adhesion to the metal counterface). As a matter of fact it is
determined by the specific lattice structure of graphite, molybdenum disulfide and
polytetrafluoroethylene [11].
Friction under boundary lubrication conditions. Results of wear tests of all composites in the
lubricating medium (distilled water) indicate that the investigated fillers in the distilled water
environment act as a solid lubricant, too. Firstly, under the boundary lubrication wear rate decreases
both for the pure (unfilled) UHMWPE, as well as for composites “UHMWPE + n wt. % MoS2”,
“UHMWPE + n wt. % C” and “UHMWPE + n wt. % PTFE”. Secondly, the wear rate of the
composites in the distilled water is lower than that for the pure UHMWPE in the same environment.
Wear track surface roughness (Ra) of the composites in the lubricating medium is lower than that of
pure UHMWPE.
a
b
Fig. 2. Wear intensity (I) and roughness of the wear track surface (Ra) of UHMWPE and
composites “UHMWPE-PTFE”: pure UHMWPE (1), UHMWPE + 5 wt. % PTFE (2),
UHMWPE+10 wt.% PTFE (3), UHMWPE+20 wt.% PTFE (4) and UHMWPE+40 wt.% PTFE (5)
at the steady-state wearing stage
Fig. 3. Wear intensity (I) and roughness of wear track surface (Ra) of UHMWPE and composites
“UHMWPE-С”, “UHMWPE-MoS2”: pure UHMWPE (1), UHMWPE + 3 wt. % С (2), UHMWPE
+5 wt.%С(3), UHMWPE + 10 wt.% С (4), UHMWPE+3 wt. % MoS2 (5), UHMWPE+5 wt. %
MoS2 (6), UHMWPE+10 wt.% MoS2 (7) at the steady-state stage of wear. Abrasive paper R 240.
Key Engineering Materials Vol. 712
159
These results testify for the fact that the fillers (molybdenum disulfide, graphite and
polytetrafluoroethylene) act as a solid lubricant under the wear of UHMWPE composites at dry
sliding friction (this is a similarity between liquid boundary lubrication and introducing solid
lubricant in the polymer).
Abrasive wear. The influence of fillers (C, MoS2 and PTFE) on the abrasion resistance of
UHMWPE microcomposites was also analyzed. The diagram of the abrasive wear intensity of the
composites “UHMWPE + n wt. % C” and “UHMWPE + n wt. % MoS2” with the abrasive grain
size of 240 (58.5 µm) is shown in Fig. 3. It might be seen that the abrasion resistance is increased
by 1.3-1.5 times when UHMWPE is filled with molybdenum disulfide and graphite. Moreover,
composites with molybdenum disulfide have somewhat higher wear resistance as compared to
composites “UHMWPE + n wt. % C”. Abrasion resistance depends on the graphite content in the
composite weakly (columns 2-4), and molybdenum disulfide is more effective when its content of
3-5 wt. %, even when the abrasive wear (columns 5-7) using P 240.
These above described results show that the molybdenum disulfide and graphite similar to the
case of dry sliding friction, facilitate the sliding of abrasive paper along specimen surface. However
the latter cannot "protect" the matrix from cutting because of incommensurable the sizes of the filler
and abrasive grains (1-4 µm vs. 58 µm). Abrasive wear intensity of UHMWPE filled with PTFE is
close to that of pure UHMWPE. The same dependence was revealed between the wear track surface
roughness (Ra) and the size of the filler. In this case fixed abrasive particles cut the matrix and soft
polytetrafluoroethylene cannot "protect" the matrix (grooves widths are comparable with that of
pure UHMWPE specimens). Apparently, soft polytetrafluoroethylene under abrasive wear only
facilitates sliding of abrasive paper along the specimen surface [5].
Summary
The content of graphite micropowder (3 wt. %), molybdenum disulfide (10 wt. %) and PTFE
(10 wt. %) to provide the higher wear resistance under the dry sliding friction (up to two times in
contrast with pure UHMWPE) was determined. This effect is accompanied by a decrease in the
friction coefficient down to 1.5 times.
Molybdenum disulfide, graphite and polytetrafluoroethylene play the role of a solid lubricant in
the wear of UHMWPE composites, as well as under friction in a lubricating environment.
In the case of abrasive wear the C and MoS2 do not perform effectively as a solid lubricant to
improve the wear resistance of polymer composites on UHMWPE because of the incommensurable
sizes of the filler and abrasive grains. In doing so molybdenum disulfide is more efficient filler from
the view point of abrasive wear resistance.
During abrasive wear the soft polytetrafluoroethylene filler is not able to "protect" the matrix
from cutting of the fixed abrasive particle. That is why “UHMWPE-PTFE” composites cannot be
effectively used under abrasive wear. Contrary, “UHMWPE-PTFE” composites with 10 wt. % of
the filler can be effectively used for friction units (particularly, in medicine) in the absence of the
lubricant.
Filling UHMWPE by the solid lubricating microparticles C and MoS 2 is effective for all three
kinds of wear. Therefore, the UHMWPE composites (UHMWPE + 10 wt. % MoS2 and UHMWPE
+ 3 wt. % C) can be effectively used for the friction units to be used without a lubricant under
extreme operating conditions.
Acknowledgments
The study was performed under support of fundamental Research Program of the State Academies
of Sciences for 2013–2020, RFBR project No. 14-01-00789 as well as grant of the President of the
Russian Federation for support of leading scientific schools No. NSh-2817.2014.1.
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Advanced Materials for Technical and Medical Purpose
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