Tribofysische interpretatie van wrijving en slijtage voor kunststoffen op verschillende schaalgrootte

428 Pages·2007·27.93 MB·English

Tribofysische interpretatie van wrijving en slijtage
voor kunststoffen op verschillende schaalgrootte
Tribophysical Interpretation of Scaling Effects
in Friction and Wear for Polymers
Pieter Samyn
Promotoren: prof. dr. ir. P. De Baets, prof. dr. ir. G. Schoukens
Proefschrift ingediend tot het behalen van de graad van
Doctor in de Ingenieurswetenschappen: Materiaalkunde
Vakgroep Mechanische Constructie en Productie
Voorzitter: prof. dr. ir. J. Degrieck
Faculteit Ingenieurswetenschappen
Academiejaar 2006 - 2007
ISBN 978-90-8578-134-9
NUR 971, 978
Wettelijk depot: D/2007/10.500/8

De auteur geeft de toelating dit doctoraatswerk voor consultatie beschikbaar te stellen, en delen ervan
te kopiëren uitsluitend voor persoonlijk gebruik. Elk ander gebruik valt onder de beperkingen van het
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het aanhalen van de resultaten van dit werk.

The author gives the authorization to consult and to copy parts of this work for personal use only. Any
other use is limited by the Laws of Copyright. Permission to reproduce any material contained in this
work should be obtained from the author.

Copyright © Pieter Samyn
Gent, February 2007
Promotors:

prof. dr. ir. Patrick De Baets
Ghent University
Faculty of Engineering
Department of Mechanical Construction and Production (IR04)

prof. dr. ir. Gustaaf Schoukens
Ghent University
Faculty of Engineering
Department of Textiles (IR11)

Examination Committee:

prof. dr. Paul Kiekens (chairman) Ghent University
prof. dr. ir. Wim De Waele (secretary) Ghent University
prof. dr. ir. Patrick De Baets Ghent University
prof. dr. ir. Gustaaf Schoukens Ghent University
prof. dr. Nicholas D Spencer ETH Zürich (CH)
prof. dr. ir. Dick Schipper Twente University (NL)
prof. dr. ir. Geraldine Heynderickx Ghent University
prof. dr. Filip Du Prez Ghent University
prof. dr. Francis Verpoort Ghent University

Research Institute:

Ghent University
Department Mechanical Construction and Production
Laboratory Soete
Sint-Pietersnieuwstraat 41
B-9000 Gent
Belgium

Tel. [phone]
Fax. [phone]
Mail to [email protected]

It bubbles in my workshop,
searching is a lovely job
The patterns I draw,
are formulas of laws of nature
… Science is Art!

Dirk Brossé
Karel V Oratorium

(i)

Acknowledgements.

This work was performed under supervision of prof. dr. ir. Patrick De Baets and
prof. dr. ir. Gustaaf Schoukens. I like to express my gratitude to my promotors for the
freedom and opportunities they have given during my PhD study at Ghent University,
Laboratory Soete.

Tribology is a multidisciplinary science with many aspects. Therefore, I appreciate the
collaboration with other laboratories involved in polymer characterisation:

• Thank you drs. Jeroen Van Craenenbroeck and prof. dr. Francis Verpoort for
the many Raman spectra that you recorded (Ghent University, Laboratory of
Organometallics and Catalysis),
• Thank you prof. dr. Isabel Van Driessche for thermo-analytical measurements
(Ghent University, Department of Inorganic and Physical Chemistry),
• Thank you dr. ir. Karen De Clerck and Lieve Van Landuyt for DSC-
measurements (Ghent University, Department of Textiles),
• Thank you ing. Wim Van Daele for SEM microscopy (Ghent University, Cel
Corrosion),
• Thank you prof. dr. ir. Wim Van Paepegem and prof. dr. ir. Joris Degrieck for
your help with composite characterisation,
• Thank you ing. Tommy de Ghein for full-scale static tests of polymer elements
(Ghent University, Laboratory Magnel) under supervision of prof. dr. ir. Luc
Taerwe,
• Thank you ing. Michel De Waele, ing. Andries Vandevyver (Belgian Institute
for Welding) for optical microscopy,
• Thank you ir. Karen Vercammen at VITO (Flemisch Institute for Technological
Research) and dr. ir. Karel Van Acker for surface contact angle measurements.

Parts of my research were possible through co-operation with industrial partners. I
would like to acknowledge them sincerely for delivering test materials and for agreeing
to report test results:

• Mr. Freddy De Bruyne from Quadrant EPP (Tielt, Belgium) delivered
thermoplastic test materials,
• Mr. Koen Peeters and Mr. Philippe Cantillon from Dupont de Nemours
(Mechelen, Belgium) delivered different polyimide grades,
• The Nederlandse Rijkswaterstaat, especially ing. J.S. Leendertz and ing. E.
Suister, involved our laboratory in the redesign of the Maeslant storm surge
barrier. The experiences with practical design were an additional value for my
PhD research. I express also my appreciation to ir. Ludo Van Schepdael
(Solico BV, Oosterhout, The Netherlands) for results of numerical simulations.
(ii)

I am deeply grateful to prof. N.D. Spencer and prof. P. Smith offering me possibilities for
a short stay as a visiting student at ETH, Zürich in September 2006. It was an
unforgettable experience and real motivation for further scientific research.

The members of the examination committee spent a lot of time reading this work, but
their comments were very useful and they offered plenty of suggestions to be considered
for further research.

Parts of this research were financially sponsored by the Fund for Scientific Research of
the Flemish Community (FWO G.0295.01) and the Research Fund of Ghent University
(BOF 01109098).

But most of all, my parents and family supported and encouraged me throughout the
course of this PhD study. The interest of friends after weekly concert band rehearsals
and relaxing moments I found among them stimulated me to finish this work.

Thank you all once again,

Pieter
Gent, February 2007
(iii)

Summary.

Polymers are used in dry sliding applications such as bearings, due to self-lubricating
ability. Most experimental work from literature on friction and wear mechanisms focuses
on mechanical effects causing transfer and results from small-scale testing. Friction and
wear of polymers are mainly influenced by visco-elastic deformation and sliding
temperature while also depending on the test configuration. In present work,
tribophysical or tribochemical effects on worn polymer surfaces and polymer debris after
sliding against steel are investigated on micro-scale explaining transitions in tribological
performance. Extrapolations of friction and wear data from small-scale testing towards
large-scale testing are discussed in relation to polymer structures and wear debris
mobility depending on the contact area size. Sliding applications at either high-
temperature (100 to 260°C) or high-load (8 to 150 MPa) conditions are covered in this
work, using high-performance polymers such as sintered or thermoplastic polyimide, or
engineering polymers such as polyamide, polyoxymethylene, polyethylene terephthalate
and polyethylene.

Experimental techniques are presented in a first part of this work. Small-scale sliding
tests are done on a standard cylinder-on-plate configuration and large-scale sliding is
done on flat-on-flat configurations, showing acceptable statistical variation and inter-
laboratorial repeatability. An analytical model to calculate the maximum polymer sur-
face temperature T* during sliding is introduced and verified. A change from integrated
flash temperature concept to bulk temperature concept should be considered at high
loads, agreeing to a transition in visco-elastic polymer deformation. Raman spectro-
scopy and thermo-analytical polymer analysis are used to characterise wear products.
The importance of wear debris interactions forming transfer is demonstrated like this.

Micro-scale to small-scale relations in friction and wear for polyimides are investigated
in the second part of this work. Sintered polyimides show high friction and wear under
atmospheric conditions, which are mechanically controlled by brittleness at high loads.
A transition to low friction and stable wear manifests above 180°C, related to chemical
effects such as hydrolysis at low temperature and imidisation at high temperature.
Molecular orientation for semi-thermosetting polyimide is mainly concentrated on side
groups at low load and on the backbone structure at high load. Imidisation corresponds
to a transition from transverse orientation into axial orientation. Transfer of sintered
polyimides is often island-like and depends on either humidity or counterface type being
steel or diamond-like carbon coatings. Coefficients of friction on different counterfaces
are classified according to surface energy measurements, while the wear rates depend on
the production of abrasive third bodies during sliding. Thermoplastic polyimides show
thermo-mechanically controlled sliding and homogeneous transfer depending on normal
load and sliding velocity. Chemical modifications and degradation are noted at high
temperatures. Molecular orientation for thermoplastic polyimide is mainly located on the
backbone structure. The efficiency of graphite additives for sintered polyimide causing
smooth sliding is restricted to specific sliding conditions.
(iv)

Polytetrafluouroethylene additives for thermoplastic polyimide provide smooth sliding
over the entire testing interval.

Small-scale to large-scale testing correlations for engineering polymers are discussed in
the third part of this work. The most common parameter to characterise tribological data
for polymers is the contact pressure x sliding velocity (pv-value), but its use seems
restricted to a single testing scale. Four experimental models comparing small-scale and
large-scale tribological data are evaluated with a single or two mechanical parameters.
The most extensive is a macroscopic geometry model considering the influence of
thermal effects, sample geometry and contact conditions. Coefficients of friction for pure
polyacetals are in agreement for both testing scales. Correlating friction for pure
polyamide should consider the effect of crystallisation. Extrapolation of wear rates is not
possible due to transitions between mild wear, softening and melting and need specific
large-scale testing. Friction and wear data for solid lubricated or internally oil lubricated
polymers are more difficult to extrapolate due to interactions of lubricant supply with
softening, melting and deformation of the polymer bulk as presently investigated for
different polyamide grades. Differences depending on the testing scale are attributed to
limited debris mobility within large contact areas, promoting homogeneous film
formation on either the counterface or the polymer surface. High-temperature sliding
tests on small-scale are not representative for high-load tests on large-scale. Different
polymer structures are observed after high-load and high-temperature sliding: orientation
concentrates in either a crystalline phase (high temperature) or a rigid amorphous phase
(high load). Mainly post-polymerisation of debris improves the homogeneity of the
transfer film for polyesters, while crystallisation causes coherent but unfavourable
transfer. The influences of recoverable or permanent creep were illustrated from static
loading tests and agree to transitions in sliding performance.

Large-scale to full-scale extrapolation is illustrated in the fourth part of this work by a
case study on the redesign of a ball-joint incorporating polymer pads as bearing elements
(Dutch Ministry of Transport, Water Management and Public Works). Representative
large-scale test conditions are determined from a global analysis of the ball-joint
construction. The local stresses and deformation for a constrained polymer pad are
analysed from a numerical model (Solico BV). Functioning of polymers above yield
strength provides stable sliding but it requires dimensional stabilisation with an epoxy
composite reinforcing ring. Shear failure observed after full-scale static and dynamic
testing is avoided by optimisation of the geometry and processing parameters. A
representative failure criterion for strength of reinforcing composite rings can be scaled
from full-scale testing to small-scale short-beam-shear testing.

General conclusions are made in the fifth part of this work, relating the effect of
variations in polymer structure to sliding properties and giving an overview of the
demonstrated sliding mechanisms. Further research should be directed on either
fundamental knowledge on the effect of polymer structure on nano-scale or either
applied research on a quantitative classification of wear debris or in-situ measurements.