1. Tarmo Korpela
Dissertations
Department of Chemistry
University of Eastern Finland
No. 124 (2014)
97/2008 TANSKANEN Jukka: One- and two-dimensional nanostructures of group 14 elemental hydrides
and group 13-15 binary hydrides
98/2009 JOKINIEMI Jonna: Structural studies on metal complexes of mixed amide esters and phenyl and
monoalkyl ester derivatives of dichloromethylene biphosphonic acid
99/2009 KALIMA Valtteri: Controlled replication of patterned polymer and nanocomposite surfaces for
micro-optical applications
100/2009 HYYRYLÄINEN Anna: Differentiation of diastereomeric and enantiomeric b-amino acids by
mass spectrometry
101/2010 KUNNAS-HILTUNEN Susan: Synthesis, X-ray diffraction study and characterisations of metal
complexes of clodronic acid and its symmetrical dianhydride derivatives
102/2010 NIEMI Merja: A molecular basis for antibody specificity – crystal structures of IgE-allergen and
IgG-hapten complexes
103/2010 RASILAINEN Tiina: Controlling water on polypropylene surfaces with micro- and micro/nano
structures
104/2011 SAARIKOSKI Inka: Tailoring of optical transmittance, reflectance, and hydrophobicity of
polymers by micro- and nanoscale structuring
105/2011 NISKANEN Mika: DFT Studies on ruthenium and rhodium chain complexes and a
one-dimensional iodine bridged ruthenium complex
106/2011 RÖNKKÖ Hanna-Leena: Studies on MgCl2
/alcohol adducts and a self-supported Ziegler-Natta
catalyst for propene polymerization
107/2011 KASANEN Jussi: Photocatalytic TiO2
-based multilayer coating on polymer substrate for use in
self-cleaning applications
108/2011 KALLIO Juha: Structural studies of Ascomycete laccases – Insights into the reaction pathways
109/2011 KINNUNEN Niko: Methane combustion activity of Al2
O3
-supported Pd, Pt, and Pd-Pt catalysts:
Experimental and theoretical studies
110/2011 TORVINEN Mika: Mass spectrometric studies of host-guest complexes of glucosylcalixarenes
111/2012 KONTKANEN Maija-Liisa: Catalyst carrier studies for 1-hexene hydroformulation: cross-linked
poly(4-vinylpyridine), nano zinc oxide and one-dimensional ruthenium polymer
112/2012 KORHONEN Tuulia: The wettability properties of nano- and micromodified paint surfaces
113/2012 JOKI-KORPELA Fatima: Functional polyurethane-based films and coatings
114/2012 LAURILA Elina: Non-covalent interactions in Rh, Ru, Os, and Ag complexes
115/2012 MAKSIMAINEN Mirko: Structural studies of Trichoderma reesei, Aspergillus oryzae and
Bacillus circulans sp. alkalophilus beta-galactosidases – Novel insights into a structure-function
relationship
116/2012 PÖLLÄNEN Maija: Morphological, thermal, mechanical, and tribological studies of polyethylene
composites reinforced with micro– and nanofillers
117/2013 LAINE Anniina: Elementary reactions in metallocene/methylaluminoxane catalyzed polyolefin
synthesis
118/2013 TIMONEN Juri: Synthesis, characterization and anti-inflammatory effects of substituted coumarin
derivatives
119/2013 TAKKUNEN Laura: Three-dimensional roughness analysis for multiscale textured surfaces:
Quantitative characterization and simulation of micro- and nanoscale structures
120/2014 STENBERG Henna: Studies of self-organizing layered coatings
121/2014 KEKÄLÄINEN Timo: Characterization of petroleum and bio-oil samples by ultrahigh-resolution
Fourier transform ion cyclotron resonance mass spectrometry
122/2014 BAZHENOV Andrey: Towards deeper atomic-level understanding of the structure of magnesium
dichloride and its performance as a support in the Ziegler-Natta catalytic system
123/2014 PIRINEN Sami: Studies on MgCl2
/ether supports in Ziegler–Natta catalysts for ethylene
polymerization
Friction and Wear of Micro-
Structured Polymer Surfaces
TarmoKorpela:FrictionandWearofMicrostructuredPolymerSurfaces 124
2.
3. Friction and wear of micro-structured polymer surfaces
Tarmo Korpela
Department of Chemistry
University of Eastern Finland
Finland
Joensuu 2014
5. 3
Abstract
Surface texturing is a common method to modify the surface properties of polymer, metallic
and ceramic materials. The most common goal for structuring a surface with controlled textures
is to provide the surface with a certain functionality, such as self-cleaning ability and modified
friction properties. In this study polyacetal, polypropylene and polypropylene/viscose fiber
composite surfaces were furnished with regular micro-pillar structures. As a widely used
industrial thermoplastic, polypropylene was used as a reference material to which polyacetal
and composite were compared. The manufactured micro-pillar structures were roughly 100 µm
in diameter and arranged in either square or hexagonal lattices.
The friction behavior induced by the micro-pattern and the durability of micro-structures were
evaluated under different sliding conditions. The durability of the micro-pillars was measured
by following the surface wear. The patterned polymer surfaces were slid against rough and
smooth steel surfaces with varying loads to study the wear behavior of the materials. Based
upon the wear behavior of patterned polypropylene, reinforcing polypropylene with viscose
fibers was studied as a mean to increase the durability of the micro-pillars.
The micro-structuring was found out to be an effective method in controlling the friction and
wear properties of studied surfaces. The density of the micro-pillar array allows a very fine
control of the contact area and hence the effective surface pressure, which is the key to control
the adhesion of the polymer surface.
The surface texturing is often used to mimic the self-cleaning ability of the lotus plant. As
texturing of polypropylene surfaces with sub-micron structures has been shown to successfully
mimic this property, the durability of the larger micro-pillars was utilized to design new, wear-
resistant superhydrophobic surfaces.
6. 4
List of original publications
This dissertation is a summary of publications I - III and a joint research publication IV.
I Tarmo Korpela, Mika Suvanto and Tuula T. Pakkanen, Friction and wear of
periodically micro-patterned polypropylene in dry sliding, WEAR 289 (2012), 1.
II Tarmo Korpela, Janne Salstela, Mika Suvanto and Tuula T. Pakkanen, Periodically
micro-patterned viscose fiber-reinforced polypropylene composites with low surface
friction, WEAR 310 (2014), 20.
III Tarmo Korpela, Mika Suvanto and Tuula T. Pakkanen, The influence of micro-scale
surface structuring on the sliding of polypropylene and polyacetal, submitted for
publication.
IV Eero Huovinen, Laura Takkunen, Tarmo Korpela, Mika Suvanto, Tuula T. Pakkanen,
and Tapani A. Pakkanen, Mechanically Robust Superhydrophobic Polymer Surfaces
Based on Protective Micropillars, Langmuir 30 (2014), 1435.
The author’s contribution to aforementioned publications:
The key ideas for the topics in Publications I - IV are based on conversations between
supervisors and the author. The author played an essential role in design of the surface
structures, developing the measurement set-ups and performing the wear and friction analysis in
Publications I – IV. He has performed the surface fabrication, tribological measurements and
result analysis in Publications I and III. The author has performed the friction/wear tests and
designed the surface structures used in Publication II. In Publication IV, the author had an
integral part in the development of the wear testing methods and in the design of the studied
surface structures.
7. 5
Contents
Abstract.................................................................................................................................3
List of original publications ..................................................................................................4
Contents.................................................................................................................................5
Abbreviations ........................................................................................................................6
1. Introduction.......................................................................................................................7
1.1. Surface contact.............................................................................................................8
1.2. Friction, wear and polymers..........................................................................................9
1.3. Measuring the friction ................................................................................................10
1.4. Cellulose fiber composites..........................................................................................11
1.5. Aims of the study .......................................................................................................12
2. Experimental ...................................................................................................................13
2.1. Materials....................................................................................................................13
2.2. Fabrication of the surface structures............................................................................13
2.3. Tribological measurements.........................................................................................14
2.4. Characterization methods ...........................................................................................15
3. Polymer surfaces with controlled contact area...............................................................17
4. Micro-structured polypropylene surfaces.......................................................................20
4.1. Micro-structures and sliding friction ...........................................................................20
4.2. Wear of the micro-pillars............................................................................................20
4.3. Fiber reinforced micro-structures................................................................................21
4.4. Friction and wear of the patterned composites.............................................................24
4.5. Summary....................................................................................................................26
5. Micro-structured polyacetal surfaces..............................................................................27
5.1. Polyacetal surfaces and wear ......................................................................................27
5.2. Friction behavior........................................................................................................28
6. Protective structuring......................................................................................................30
6.1. Surface design............................................................................................................30
6.2. Wear resistance of protected surfaces..........................................................................32
6.3. Applicability of the method ........................................................................................35
7. Conclusions......................................................................................................................36
Acknowledgements..............................................................................................................37
References ...........................................................................................................................38
Appendix: Supplementary information
8. 6
Abbreviations
General:
Ap Contact area
ALD Atomic layer deposition
COF Coefficient of dynamic friction
CVD Chemical vapor deposition
ε Elongation at break
E Young’s modulus (Elastic modulus)
MM Micro-micro double structure
MN Micro-nano double structure
MS Multiscale double structure
Ra Arithmetic roughness
Rm Ultimate tensile strength
SEM Scanning electron microscopy/microscope
SC Surface coverage of micro structure; also: solid surface fraction
TGA Thermogravimetric analysis
WS Specific wear rate
Materials:
MAPP Maleic anhydride grafted polypropylene
POM Polyacetal (Polyoxymethylene)
PP Polypropylene homopolymer
VF Viscose fiber
9. 7
1. Introduction
Friction is a universally present, well-known phenomenon. The friction force is best described
as a physical force that resists motion; an English dictionary1
defines the friction as “the force
that makes it difficult for things to move freely when they are touching each other”.
This definition is the form of friction most of us are familiar with. The amplitude and strength
with which two surfaces interact with each other are the basis for many applications, both
natural and man-made. In nature, these attributes are often driven by fine-scale surface
structures, such as the dry adhesive bonding of the gecko feet2-5
or the mechanical bonding of
the thistle burrs6
(Figure 1). In man-made surfaces, the surface patterning by roughening,
specific pattern processing or mold casting are often used to mimic the behavior of natural
surfaces7
, such as increasing traction between car tire and asphalt (Figure 1).
In addition to the physical properties of material, the surface topography affects its surface
properties. The properties drawn from the material composition are, for example, mechanical
strength, corrosion resistance and electrical/heat conductivity. The surface structuring allows
fine-tuning of the surface properties; the feats accomplished by surface structuring include
modified friction8-10
, adhesion11,12,13
, hydrophobicity14
and optical properties15
as well as
specific wetting of surfaces13
– abilities that the material in question would not possess without
surface structuring.
Figure 1. Natural and man-made surfaces, whose main function is to affect adhesion, traction
and friction of the surface via surface structures: car tires16
(A), gecko feet17
(B), Velcro ®18
(C), thistle burr hooks6
(D) and cockroach climbing pads19
(E).
Polymers and their composites (plastics) are a material group that is common in industrial and
everyday applications. The most significant advantages of using plastics in machine design
either as mechanical parts or coatings are their lightness, often good chemical resistance, easier
shaping and lower cost when compared to metals. 20-22
10. 8
1.1. Surface contact
Tribology encompasses the science and related technology of interacting surfaces in relative
motion. The term tribology itself is derived from Greek word tribos (τριβος), meaning rubbing
and abrasion. Tribology is an interdisciplinary science that connects physics, mechanics,
chemistry and material science and that focuses on examining the interconnected relations
between friction, wear and lubrication. 23
Chemical and physical properties of a solid surface are characterized by its material
composition, preparation method, imminent surface environment and surface roughness
(topology). There are multitudes of ways to affect the chemical and physical surface properties
of materials, such as plasma treatment24,25
, atomic layer deposition (ALD)26-28
, chemical vapor
deposition (CVD)26,29,30
, surface texturing31-33
and etching. Out of these, surface texturing via
various means (mold techniques34,35
, laser ablation36-38
, embossing39
, machining40,41,42
etc.) is an
option that does not change the bulk properties of a given material but can substantially affect
the desired surface properties.
When considering a physical contact between two solid surfaces, the topologies of the
contacting surfaces play an important role. 43-46
This is especially true when they are in sliding
motion relative to each other and the asperities come into contact with the opposite surface
(Figure 2). 45,47,48
Figure 2. Representation of a surface with asperities in contact (contact points circled with red
dotted line).
Because the real contact area between contacting surfaces is directly linked to the surface
profile, measuring and controlling the surface profile has an important role in friction and wear
studies. An un-processed surface is often labelled by its surface roughness42
, but with controlled
surface textures the real contact area can be estimated with a good precision.49
The portion of
the surface that is in contact with another surface (Figure 2 red spots) is usually called surface
fraction or surface coverage (SC). For a given surface with known surface area A and SC
value, the real contact area Ap can be defined as:
Ap = SC * A (1)
The true contact area Ap in turn directly affects the surface pressure of the surface (Pressure =
Load / Area), as only the contacting asperities face the compressing stress (Fig. 2).45
This aspect
of surface contact is important as the micro- and nanoscale structures are subject to collapse.
14,50,51
Controlled surface coverage has also been studied as a method to keep external lubricant
on the sliding surface, in practice using the non-bearing surface portions as a lubricant
depositary. 49,52,53
11. 9
1.2. Friction, wear and polymers
Sliding mechanisms possessing self-lubricating polymers are widely employed in several fields
of engineering for their ability to provide low friction without the need of external lubrication.54-
56
In such bearings the sliding surface of the polymer element usually matches a partner surface
made of steel or other hard metals, since polymers are known to be more effective in terms of
friction/wear performances against a metal surface than when sliding against themselves. 57-59
This disparity stems from the ability of the polymer chains of the mating surfaces to entangle
and intermingle.60
The friction of a solid polymer object against a counter surface can be divided into two types:
static friction which hinders the onset of movement and dynamic friction which impedes with
the sliding movement. In order to initiate the movement or to continue the movement, the acting
force F must overcome the corresponding friction resistance. Furthermore, there are two main
mechanisms that define the friction force between a polymer surface and a hard counter surface:
the adhesive friction that forms from the adhesion between the contacting asperities of the
surfaces (Figure 2)61
, and abrasive friction in which the energy is dissipated due to plastic
deformation and abrasion. 62,63
The adhesive friction is the major friction mechanism of
industrial engineering polymers64
, as the abrasive environment for moving parts is avoided for
obvious reasons.60
The static friction force of a surface that resists mutual sliding is directly
proportional to its real contact area: 65,66
FT = τs * Ap, (2)
where A is the real contact area of the plastic surface and τs is the shear stress [Pa] required to
initiate the sliding between the surfaces. The ratio between the required friction force and the
normal force FN acting upon the contact (pressing the surfaces together) is called the friction
coefficient µ:
µ = FT / FN (3)
With hard, inelastic materials the friction coefficient µ is dependent on the materials
hardness.67,68
However, with plastic materials the real contact area Ap is dependent on the
normal force FN acting upon the contact, as the elastic deformation of plastic under stress
increases junction growth.69
The friction between a polymer surface and a hard counter surface
is directly linked to the wear of the polymer surface. As a softer material (relative to the counter
surface), polymer surfaces wear and form a transfer layer, composed of polymer wear debris,
between the contacting surfaces as shown in Figure 3.70
Figure 3. Formation of a transfer layer: initial cycle (A), accumulation of wear debris between
the surfaces (B) and ejection of wear debris to outside of the contact zone (C).70
12. 10
If the adhesive shear between the polymer and counter surface is high enough, the wear is
increasingly severe.64
Otherwise, the transfer layer will act as a solid lubricant between the
surfaces.71,72
The polymer’s ability to create a uniform transfer layer is thus an important aspect
regarding its friction and wear properties when sliding against the given counter surface. As the
wear debris originates mainly from the polymer face, the composition of the polymer surface
will affect the transfer layer behavior – especially in the case of composite materials. 72,73
1.3. Measuring the friction
Friction and wear measurements are carried out with a tribometer. Tribometers are devices
designed to measure friction and wear in a specific set-up that matches the tribological system
of interest as closely as possible.49,74-76
A carefully selected measurement system provides a
simplified environment which can simulate the critical aspects of the studied friction/wear
problem, whose parameters in the actual equipment are often complex or unknown. Conversely,
a poorly selected or designed tribometer system can provide entirely unusable results. The main
purpose of a tribometer is the simulation of friction and/or wear under controlled conditions.
Measuring these properties in an actual system, for example the wear between pistons and
cylinders in a combustion engine, would be both time- and resource-consuming.77
Another
reason for using simplified test machines is that they can be designed to allow very precise
measurement of all the related parameters (e.g. friction, temperature and lubrication).78
In laboratory scale, the three tribometer types most often used are pin-on-disc, block-on-ring
and pin-on-plate. Simplified schemes of these configurations are presented in Figure 4. The
friction in these systems is measured with sensitive force sensors that tell either how much force
the sliding exerts on the sample or how much the contact decelerates the movement. Friction
coefficient is gained by dividing the measured counteracting force with the applied normal load.
Figure 4. Illustrations of a pin-on-disk (A), block-on-ring (B) and pin-on-flat (C) tribometer
configurations. Green arrow: movement of the counter surface, red arrow: direction of the
measured friction force.
The wear of the sample can be measured in several ways. Assuming that the wear of the hard
counter surface is diminishingly small, the geometry of the softer sample is known and the
sample wears off in a predictable manner, the wear can be expressed as a lost volume. The
wear can also be expressed as a lost mass by weighing the sample before and after the wear
measurement. The wear is usually expressed as a wear rate (lost volume per distance), to which
the applied load can be incorporated to gain the specific wear rate WS [mm³/Nm].
WS = Lost volume [mm³] / (Load [N] * Distance [m]) (4)
13. 11
The final results from the tribological measurement are thus often expressed as the stable
friction coefficient (COF, or µ) and the specific wear rate. Even though these values stand true
for the given system, the single values tell very little of the system itself. In tribosystems all
variables such as humidity, atmosphere and temperature play a role; for this reason it is
important to report not only the friction coefficients and wear rates, but all of the variables
affecting the sliding process. 44,49,53,58,79
1.4. Cellulose fiber composites
A composite is a material that consists of two or more distinguishable phases. When discussing
about composites, one of the material fractions (usually the most abundant and uniform phase)
forms the matrix and the others are either fillers or reinforcement materials. In cellulose fiber-
polymer composites, the polymer acts as the matrix fraction and cellulose fibers as the
reinforcing filler material.80-83
Usually the matrix material is also compounded with suitable
bonding agent to improve the adhesion between the cellulose fiber and the polymer
matrix.61,80,84
As a filler material, cellulose fibers have been used as early as 1908 to reduce raw material
costs and to improve the physical properties of the matrix material.85
Natural, plant-based
cellulose fibers used as composite fillers include hair-like natural fibers84,86,87
(cotton, kapok),
fiber bundles88,89
(hemp, jute, flax) and hard fibers89-91
(sisal, coir). The cellulose can also be
processed after purification to functionalize it (e.g. nitrocellulose92
) and to improve its physical
properties or processability (e.g. viscose fiber93
). The chemical structure of cellulose is
portrayed in Figure 5A.
There are both economic and environmental advantages in using natural fibers as reinforcing
material in polymer composites.94
The decrease in the raw material cost and enhances in the
physical properties (such as durability) of the material are the main targets.95
The other benefits
of using cellulose-based composites include a low density, high toughness, biodegradability and
increased abrasive wear resistance.
Figure 5. Chemical structure of cellulose chain (A) and the esterification reaction of its
hydroxyl group with maleic anhydride grafted polymer (B). 98
14. 12
Despite of the advantages of using the cellulose fibers as a filler material, there are challenges
when the fibers are to be compounded with chemically inert polymers such as polyethylene or
polypropylene.96
Because of the hydrophilic nature of the polyolefin materials, strong adhesion
between the polymer and alcohol groups of the cellulose chain might be difficult to achieve.
Because of this, coupling agents are added as a third phase in the composite. The coupling
agents are designed to include functional groups that can chemically bind to the alcohol groups
of cellulose chain and a backbone chain that is compatible with the matrix polymer. Examples
of functional modifications for improved adhesion are maleated97,98
coupling agents (Figure 5B)
and silylation99,100
or acetylation101
of the cellulose chain.
1.5. Aims of the study
The aim of the present study was to gain insight into the surface characteristics of micron-scale
surface textures, mainly the durability of the micro-structures and their effect on the sliding
behavior of thermoplastic polymers. With adequate knowledge on these subjects, it is possible
to design and fabricate functional surfaces, based on surface structuring, that possess good
durability in everyday wear processes.
The task was challenging as normally the durability of micro/nano-structures are studied with
low-force procedures. It was thus necessary to first seek such measurement conditions where
the studied materials would face sufficient wear and a surface pattern would affect the behavior
of materials. The next step was to produce micro-patterns of adequate dimensions to withstand
the inflicted stress. The specific objectives of this study are as follows:
1. To study the durability and wear mechanisms of micro-scale pillar structures against
rough and smooth steel surfaces, making use of commercially available thermoplastics.
2. As the surface pressure is directly linked with the solid surface fraction, to find out the
relation between wear and effective surface pressure i.e. the pressure that a single
micro-pillar faces when a load is applied on the patterned area.
3. To find out whether the micro-pillar structuring can be used as a tool to produce
polymer surfaces with either high or low dynamic friction.
4. To find out if the micro-pillar structures can be reinforced by composite blending with
fibers to further affect the surface properties.
5. To apply the gained knowledge of micro-pillar durability to design functional surfaces
that can withstand surface damage, using carefully positioned micro-structured
patterns.
15. 13
2. Experimental
The materials, micro-structuring fabrication method and characterization methods used in this
study are described in a scope encompassing all studied surfaces. Individual, exact parameters
such as the shape and micro-machining parameters of patterned areas are described in detail in
publications I – IV.
2.1. Materials
In this study, examination of friction and wear behaviors of polymer and composite materials
were based on isotactic polypropylene (PP, HD 120 MO). Polypropylene acted as the bedrock
material to which the behaviors of polyacetal (POM, Hostaform C9021SW) and
polypropylene/viscose fiber (PP/VF) composites were compared.
Composites of polypropylene and viscose fiber (VF, Danufil® KS), 10 µm – 20 µm in
diameter, were prepared with PP acting as the matrix material and VF as the filler material.
Maleic anhydride grafted polypropylene (MAPP, Polybond 3200) was used as a coupling agent.
The amount of added viscose fiber ranged from 10 w-% to 40 w-%.
2.2. Fabrication of the surface structures
The surfaces were manufactured with injection molding using micro-structured mold inserts.
The inserts were fabricated from aluminum sheets with a micro-working robot (Figure 6A). An
aluminum foil was cut into suitable pieces and micro-structured with hard metal needle, after
which the aluminum foil was fixed onto a steel prop with two-component glue (Figure 6B).
Figure 6. The micro working robot (A), an aluminum insert (B), injection molding (C) and the
final mold insert (D).
All sample surfaces were manufactured via melt mixing and injection molding using the
produced aluminum inserts (Figure 6C). The injection molding was done with a laboratory-
scale machine, having a separate twin-screw melt-mixing unit.
16. 14
2.3. Tribological measurements
The friction and wear measurements were carried out with a pin-on-disk type tribometer. Two
different set-ups were used with measurements: flat pin specimen against rotating disc (pin-on-
disk) and flat pin against linearly reciprocating counter face (pin-on-flat). With pin-on-disk
measurements, linearly ground stainless steel discs were used as counter surfaces. A smooth,
stainless steel rod was used as counter face with pin-on-flat measurements where the movement
of the linear counter face is driven by a rotating pulley block. In both configurations, the
patterned side of the sample faced the counter surface. These set-ups are clarified in Figure 7.
The wear of the sample was measured by monitoring the vertical position of the sample holder
pin. As the steel counter surfaces face negligible wear during the measurements, the deviation
in vertical position of the pin gives the absolute wear of the sample in vertical dimension which
can be used to calculate the wear rate for the sample as shown in Figure 8.
Figure 7. The two set-ups used with friction/wear studies: pin-on-disk (A) and pin-on-flat (B),
with schematic presentations of the round (C) and linear (D) counter surfaces. Trajectory of the
sample is traced with green arrows.
The round counter surfaces of steel were used to study the abrasive durability of the micro-
patterned PP and PP/VF composite samples. The discs were ground to the desired roughness
levels in one direction, resulting in the sample passing the grooves and ridges on the counter
surface in both perpendicular and parallel direction (see Figure 7C). To promote abrasive wear,
the steel discs were ground to relatively high arithmetic roughness Ra values, ranging from 0.1
µm to 1.5 µm. A constant load of 2 N was used with the disc counter surfaces, keeping the
pattern density of polymer surface and the roughness of steel surface as the changing variables.
17. 15
This method was selected for wear testing because it allows measuring the wear rates and
friction in continuous, uninterrupted sliding.
The linear counter surface of steel was used with PP and POM measurements to study the effect
of surface pressure on the sliding friction and wear. With this type of counter surface, the
roughness of the steel bar was kept low (Ra value below 0.1 µm in the sliding direction) to
minimize the wear of the patterned surfaces. Unlike with the round counter surfaces, multiple
loads ranging from 2N to 10 N were used with the patterned sample specimens.
With both configurations, the friction and wear data were analyzed after the stable sliding
conditions had been reached. The obtained data was filtered so that the friction and wear could
be inspected from specific stages within a measurement cycle. With pin-on-disk measurements,
the sampling points were the locations where the sample passed the surface grooves in the
parallel and perpendicular directions (Figure 7C). With the pin-on-flat measurements, friction
and wear depth were only monitored from the middle part of the counter surface where the
sliding speed matches the input speed (Figure 7D).
A typical friction/wear curve pair obtained from a measurement is presented in Figure 8, with
the stable friction level and wear rate emphasized. As the micro structure wears off during the
sliding, the height sensor reading of the tribometer equals the height loss of the micro-pillars.
The slope of the height loss curve h [10-3
mm/m] is defined as the wear rate coefficient for the
surface. This coefficient h and the contact area Ap [mm²] of the surface can be used to calculate
the specific wear rate WS [mm³/Nm] for the measured sample surface:
WS = h * Ap * Load -1
(5)
Figure 8. Friction (red) and height loss curves (blue) from a typical measurement of patterned
PP against a round counter surface of steel. An illustration of the micro-pillar height loss in
different stages of sliding is presented below the height curve.
2.4. Characterization methods
18. 16
Scanning electron microscopy (SEM) and optical microscopy were used to monitor the quality
of the manufactured aluminum inserts and the replication accuracy of the injection molded
polymer specimens. SEM was used to measure the surface dimensions, the extent of wear and
wear mechanism of the micro-structures. To prevent the charging of the samples in the SEM
measurement, all samples were sputtered with a thin layer of gold. Even though SEM was used
to analyze the morphology of the composite samples and dispersion of the viscose fibers within
the PP matrix, the optical microscopy was used to scrutinize the worn surfaces of patterned
composite samples as the viscose fibers embedded in the PP matrix were more visible in the
optical microscope.
The wear mechanisms of the micro-pillars were studied mainly with SEM by analyzing the
morphology of the wear debris accumulated on both the counter and sample surfaces. The
actual level of the surface wear was determined through the height loss curves (Figure 8)
obtained with the tribometer.
When using the SEM to measure the height of the micro-structures, the structured samples were
cut into 200 µm thick slices with a microtome. The same slice thickness was used with the
composite samples when studying the morphology of the different composite series.
The composition of the composite samples was studied with thermogravimetric analysis (TGA)
and the mechanical properties of the composites were determined via tensile testing.
Determining the fiber content of the composites using TGA was possible owing to the different
decomposition temperature ranges of the viscose fiber and polypropylene. Cross-sections of the
patterned composites were evaluated with SEM to study the fiber orientation and density in the
micro-pillars, beneath the micro-pattern and within the bulk material.
19. 17
3. Polymer surfaces with controlled contact area
Friction and wear measurements of the patterned polymer surfaces formed the basis of this
study. However, to link up the friction and the wear behavior with the surface topology for
sliding surfaces, the true contact area of the surface must be known in sufficient precision. For
micro-patterned polymers, the size of the micro-pillars, their spatial positioning and shape must
be well-defined before any results can be drawn between the pattern and its frictional/wear
effect on the patterned surface.45
On this regard, the positioning of the micro-pillars was directly
controlled by the contact area that the patterned surface should possess.
Two surface structure types composing of ordered micron-scale pillar arrays were manufactured
in order to study the effect of structuring. The first structure has the pillars in a square lattice
(Figure 9A, B) and the second in hexagonal lattice (Figure 9C, D) which allows denser pillar
arrangement.102
Both structure types are characterized by three pattern parameters: the spacing
W, the top diameter T and the height H which are represented graphically in Figure 9.
Figure 9. Schematic presentation of the studied square (A, B) and hexagonal (C, D) micro pillar
structures. W = distance between two pillars in grid, T = top diameter of the pillars and H =
height of the pillars.
The primary property of the patterned surfaces is the area fraction i.e. the fraction of the surface
covered by the micro pillars. This surface coverage (SC) value can be calculated with equations
5 and 6 for both surface types.
2
4 W
T
SCSquare
2
32 W
T
SCHex (6, 7)
Likewise, to fabricate a surface with well-defined SC value the spacing parameter W can be
solved from eq. 5 and 6 as follows:
SC
T
WSquare
2 SC
T
WHex
32
(8, 9)
In total, eight different micro-patterns were designed: four with the pillars in square lattice
(target SC values 15 %, 25 %, 35 % and 45 %) and four with the pillars in hexagonal lattice
(target SC values 10 %, 30 %, 50 % and 70 %). The spacing parameter W values for these
structures were calculated with equations 8 and 9 and are displayed in Table 1. The height of
20. 18
the structures was set to 50 µm. The patterns were fabricated with the micro-working robot
using a tungsten carbide needle with a top diameter of circa 100 µm. After injection molding,
the surface dimensions of the structures were measured with SEM, which are listed in Table 1.
The height of the micro-pillar structures was measured to be 49 ± 3 µm in all of the structures.
SEM images of the molded PP surfaces are presented in Figure 11.
The patterns with square lattice (P15 to P45) were manufactured on round 15 mm mold
inserts with a single pattern on each mold (Figure 10A). The patterns with hexagonal lattice
(P10 to P70) were fabricated on 25 mm mold inserts, fitting all patterns on a single mold
(Figure 10B). The dimensions and set-ups of the patterned areas are presented in Figure 10.
Figure 10. The two different mold insert types used with patterned samples: a 1.5 cm coin (A)
and a 2.5 cm coin (B).
Table 1. Set and measured surface parameters for the fabricated surfaces.
Pattern Lattice Set parameters Measured parameters
SC [%] a
W [µm] T [µm] SC [%] AP [mm²] b
P15 Square 15 229 106 ± 2 16.8 ± 0.5 13.2 ± 0.4
P25 Square 25 177 104 ± 4 26.9 ± 2.1 21.1 ± 1.6
P35 Square 35 150 104 ± 4 37.8 ± 2.6 29.7 ± 2.0
P45 Square 45 132 105 ± 6 49.2 ± 5.3 38.7 ± 4.1
P10 Hexagonal 10 301 100 ± 3 10.0 ± 0.5 2.5 ± 0.1
P30 Hexagonal 30 174 100 ± 2 29.9 ± 1.5 7.5 ± 0.4
P50 Hexagonal 50 135 99 ± 3 48.9 ± 2.5 12.2 ± 0.6
P70 Hexagonal 70 114 - c
66.5 ± 1.2 16.6 ± 0.9
a
Target SC of the structure
b
Contact area of the pattern; SC * patterned area
c
Deformed pillars; see Figure 11
Bending of the walls of the aluminum mold having the highest surface coverage, P70, was
found to affect strongly the true contact area as the micro-pillars were slightly deformed (Fig.
3B). The true area of a single micro-pillar on the P70 surface was determined numerically to
calculate the true contact area (Figure 11E) by tracing the outlines of micro-pillar tops in an
image manipulation program.
21. 19
Figure 11. SEM images of the square (A, B: P15, P45) and hexagonal (C, D: P10, P70) lattice
patterns. Determination of the pillar surface area with the P70 surface is shown in (E).
The square lattice structures were studied using polypropylene and PP/VF composites. These
surfaces were slid exclusively against the round, abrasive counter surfaces. The hexagonal
lattice structures were fabricated on PP and POM surfaces, and used with the linear set-up. The
experimental set-ups are shown in Figure 12.
Counter surface Round Linear
Sample shape Round, 15mm Square, 5 mm side
Surface Square lattice Hexagonal lattice
Materials PP, PP/VF PP, POM
Figure 12. The patterned surfaces and materials used with the round (A) and linear (B) counter
surfaces.
22. 20
4. Micro-structured polypropylene surfaces I,II
In sliding contact, the friction of PP is heavily dependent on the interaction between the sliding
surface and the transfer layer between surfaces. The micro-scale pattern on the PP surface was
found to affect the formation of the transfer layer by allowing removal of the wear debris from
the contact zone. With patterned surfaces the wear debris can accumulate between the micro-
pillars, affecting the growth of the transfer layer.
4.1. Micro-structures and sliding friction I
The main target of testing the square patterns against abrasive surfaces was to determine the
durability of the PP micro-pillars. The PP surfaces behaved near-identically against all the
rougher counter surfaces (from Ra05 to Ra15), so the friction values against the Ra01 counter
surface are compared only against those of the Ra15 counter surface. The friction curves for
these counter surfaces are shown in Figure 13A, which clearly shows that the micro-pattern has
a distinct effect on the dynamic friction of PP. Regardless of the counter surface roughness, the
friction values ascended with increasing SC value. Increase in the roughness of the counter
surface expectedly increased the friction levels. It should be noted that the friction of the more
densely patterned surfaces rose over that of the un-patterned surface, which indicates that the
surface structure enables a better contact between the PP surface and the steel counter surface
(increased adhesion). As seen from Figure 13B, the friction was only slightly affected by the
structuring when comparing to the un-patterned surfaces.
Figure 13. Friction levels of the PP surfaces as functions of the surface SC value (A) and
counter surface roughness (B) with apparent trends highlighted. In (A), the friction levels of un-
patterned surfaces are shown with dotted lines.
4.2. Wear of the micro-pillars I
The calculated specific wear rates were inspected as functions of both the SC values and the
counter surface roughness, as shown in Figure 14. When comparing the sample surfaces as a
function of counter surface roughness, the un-patterned surface stands out with the highest
23. 21
specific wear rates (Figure 14B). Like in the friction analysis, the rougher counter surfaces
(Ra05, Ra10 and Ra15) gave similar wear rate values while the measurements with the Ra01
counter surface resulted in visibly lower wear rates. As shown in Figure 14A, the wear rates for
the rougher counter surfaces show a distinct similarity to each other while the smoothest
counter surface shows marked deviation. This can also be seen in the apparent wear mechanism
of the surfaces; surfaces slid against the Ra01 counter surface exhibited creeping wear103
(Figure 14C) while the samples slid against rougher counter surfaces wore mostly with flaking
and micro cutting (Figure 14D).
Figure 14. Wear rates of the patterned surfaces as functions of the surface SC value (A) and
counter surface roughness (B). In (A), the wear rate levels of the un-patterned surfaces are
shown with dotted lines. SEM images of worn P15 surfaces slid against Ra01 (C) and Ra15 (D)
counter surface demonstrate the difference in wear mechanisms.
4.3. Fiber reinforced micro-structures II
The abrasive testing of the micro-scale pillars in square lattice showed that micro-patterning
had a stabilizing effect on the sliding friction of polypropylene, with the roughness of the steel
counter surface dominating the friction and wear levels.I
As composites are known to often
have better mechanical properties than the pure matrix material, an experiment was carried out
to study the combined effect of micro-patterning and composite blending. The research was
carried out by furnishing a series of viscose fiber (Figure 15A) reinforced polypropylene
(PP/VF) surfaces with micro-pillar patterns. To allow comparison between the behaviors of PP
24. 22
and composites, the composites surfaces were patterned with the same square lattice pattern
than the PP surfaces.
The size of the viscose fiber cords in relation to the micro-pillars and possible fiber placements
within the micro-pillars are depicted in Figure 15. In addition to patterned surface, tensile test
specimens were produced for each composite series to determine the mechanical properties of
the composites. The true fiber content of the composite samples was determined with
thermogravimetric analysis (TGA) for the whole patterned sample and the bulk area just
beneath the micro-pillar structure (Figure 16). According to the earlier study98
using this
method, the fiber content of a TGA sample can be obtained as a relative weight loss in the
decomposition range of viscose fiber (from 200 °C to 400 °C). The weighed PP, VF and MAPP
mass portions and the measured fiber contents of the composite series are displayed in Table 2.
Figure 15. Size of the studied viscose fibers (A) and two most probable intrusions of a fiber
into the micro-bumps: penetration (B) and looping (C).
The higher concentration of fibers in the patterned samples (Figure 16, Table 2) is due to the
fibers entangling near the mouths of the micro-depressions during the molding process, as the
diameter of a single fiber (5 µm to 20 µm) is quite large when compared to the diameter of the
micro-depressions of the mold (around 100 µm).
Figure 16. Portrayal of the TGA samples for the composites: bulk (A) and pattern (B).
Table 2. Weighed and measured compositions for the composite series.
Matrix composition Weighed Measured VF portions
Series PP/MAPP a
[%] VF [%] Bulk b
[%] Pattern b
[%] Fibers c
VF10 89.4 / 0.06 10 10.0 14.6 0 – 2
VF20 78.8 / 1.2 20 18.9 24.5 0 – 3
VF30 68.2 / 1.8 30 29.6 33.0 1 – 5
VF40 57.6 / 2.4 40 36.1 37.3 2 – 7
a
PP and MAPP formed the matrix fraction
b
See Figure 16
c
Average number of exposed fibers per micro-pillar, determined through optical microscopy.
25. 23
As the three rougher counter surfaces (Ra05, Ra10 and Ra15) were noted to have insignificant
differences in terms of the friction levels and the wear severity for the PP surfaces, the patterned
composites were tested only against the roughest (Ra15) and the smoothest (Ra01) counter
surfaces. The recorded friction and wear results were compared to those of identically patterned
PP surfaces.
Optical microscopy of the worn composite surfaces was an effective method to study the fiber
concentration of the micro-pillars, as shown in Figure 17A and B. A schematic presentation of
the fiber intrusion to the micro-cavities of the mold is depicted in Figure 17C. The mechanical
properties of the composite series were determined by tensile testing and are listed in Table 3.
Table 3. Measured mechanical properties of the studied composite series.
Series E [GPa] Rm [MPa] ε [%]
PP 2.9 ± 0.2 40.7 ± 1.5 45.1 ± 8.3
VF10 3.2 ± 0.5 40.9 ± 1.7 14.6 ± 2.6
VF20 3.2 ± 0.2 47.8 ± 1.2 7.7 ± 0.7
VF30 3.9 ± 0.4 55.5 ± 1.5 6.4 ± 0.4
VF40 4.7 ± 0.5 71.1 ± 3.3 5.7 ± 0.8
E: Young’s modulus Rm: Ultimate tensile strength ε: Elongation at break
Figure 17. Optical microscopy images of VF10 (A) and VF40 (B) P15 surface after
measurement illustrates the presence of fibers within micro-pillars. Alleged fiber arrangement
in micro-pillars and fiber revelation with increasing wear is depicted in (C).
26. 24
4.4. Friction and wear of the patterned composites
Examples of friction behavior for patterned surfaces are shown in Figure 18. The two most
notable effects of the surface patterning are readily visible: the patterned surface has shorter
run-in period and its sliding friction coefficient has less fluctuation when comparing to the un-
patterned surface. SEM images of the worn composite surfaces are shown in Figure 18C–F
illustrating the differences in wear behavior caused by different counter surface roughness and
the different fiber loadings. The cavities between the micro-pillars works as “litter bins”,
allowing accumulation of the excess wear debris in the formation of the transfer layer.
The recorded stable friction values against both the smooth and the rough counter surfaces are
presented in Figure 19 for PP (reference), VF10 and VF40 series. The friction and wear rate
values of composite series VF20 and VF30 fell between those of VF10 and VF40 and are thus
omitted from the curve set for clarity.
Figure 18. Friction behavior of patterned and un-patterned surfaces sliding against Ra01 (A,B)
and Ra15 (D,E) counter surfaces. Close-up images of the worn composite surfaces are shown in
(C) and (F) for the corresponding measurements.
27. 25
It is readily visible that the addition of a small amount of fiber (the VF10 series) has a minor
impact on the physical properties of PP. There is only a slight improvement in the mechanical
properties (Table 3), and with low SC values, the friction values and wear rates of VF10 are
higher or at the same level with those of pure PP (Figure 19). As with the PP surfaces, the
surfaces having low SC values were found out to possess both the lowest friction values and
wear rates (with the exception of VF10 series). Increase in the fiber content improved both the
mechanical properties and the friction/wear properties, with the VF40 series resulting in the
lowest measured friction and wear rates. This relation indicates strong adhesion between the
viscose fiber and PP matrix.
The addition of fiber affects the durability of PP micro-pillars slightly when slid against the
smooth Ra01 counter surface, but more noticeable results are seen with the rough Ra15 counter
surface (Figure 19C and D). As with the PP surfaces, the decreasing SC value reduces the wear
rate when comparing to un-patterned surfaces. With the fiber-richest composite, the patterned
surfaces with the highest SC value have higher wear rate than the corresponding un-patterned
composite (Figure 19C). This result most likely originates from the denser pillar arrangement:
the wear debris has less space to pack into, and the hard bits of viscose fiber within the wear
debris promote abrasive wear.
Figure 19. Measured stable friction levels (A, B) and wear rates (C, D) for the series PP, VF10
and VF40 against smooth (A, C) and rough (B, D) counter surfaces.
28. 26
4.5. Summary
The real contact area between the PP and steel surfaces was found to directly affect the
adhesion between the surfaces. As the surface structuring method allows very precise control of
the contact area, the surface pressure and adhesion between the contacting surfaces can be fine-
tuned with the micro-structuring method. The micro-pillar structuring method also allows the
wear debris to accumulate between the structures instead of piling up within the contact area,
thus decreasing the adhesive wear of the PP surface. The fluctuation in the dynamic friction
coefficient likewise decreased because of the wear debris removal.
The sliding friction of the patterned polypropylene-viscose fiber composite surfaces was noted
to behave similarly with the patterned PP: the micro-structuring improved the stability of
sliding friction as well as providing a link between the material hardness and the friction levels.
Furthermore, combining the high fiber loading with the scarce micro-scale patterns resulted in
surfaces with a high mechanical strength and a very low dynamic friction when slid against
smooth steel surface. The wear and friction results of un-patterned PP/VF surfaces were used
for comparison purposes when drawing conclusions of the synergetic effects of the micro-
patterning and fiber reinforcement as both modifications also have their own, distinct effects on
the sliding process. This synergetic effect is further enhanced by the ability of the micro-pattern
to remove wear debris from the contact area, providing a more stable sliding environment.
29. 27
5. Micro-structured polyacetal surfaces III
Polyacetal is an important, widely used engineering thermoplastic due to its high stiffness,
excellent dimensional stability, high abrasion resistance and low friction levels. Micro-
structuring of polyacetal has been studied with micro-pit arrays, but an alternative approach of
texturing POM with micron-sized pillars to influence its friction has not been examined before.
As polyacetal has a reputation of a durable low-friction polymer, the friction measurement of
micro-patterned POM surfaces was carried out using the hexagonal pattern against smooth steel
surface. The behavior of the polyacetal surfaces was compared to that of identically patterned
PP surfaces.
Unlike with the measurements in Publications I and II, the surfaces were handled through
effective surface pressure p instead of surface coverage SC value. The effective surface pressure
for patterned surfaces was calculated from the used load and contact area Ap (listed in Table 1
on page 18):
p = Load / Ap (10)
The literature value for the compressive strength for PP is roughly 40 MPa and 110 MPa for
POM. The highest surface pressure that the patterned area faced was met with the lowest SC
value (P10) and highest load (10 N), being roughly equal to 3.8 MPa. As this pressure is only
one tenth of the PP’s compressive strength and even less in the case of POM, the possibility of
material collapse under pressure was not taken into account.
5.1. Polyacetal surfaces and wear
The specific wear rate curves for patterned POM and PP were determined like with the
aforementioned PP surfaces, using the measured height losses with eq. (5) and are presented in
Figure 20A and B. Under surface pressures of 0.5 MPa the wear rates of POM and PP are
within the same range (20 … 100 * 10-6
mm³/Nm), while at higher surface pressures POM has
lower wear rates than PP. The wear behavior of the flat surface portions (un-patterned surfaces
and the micro-pillar tops) were extremely similar, showing signs of fatigue wear. The dissimilar
appearance of the wear debris peeling off the micro-pillars (Figure 20C, D) clearly indicates the
difference in material ductility. The morphology of both POM and PP debris is flat and ribbon-
like in nature, but the PP debris has a visibly broken appearance while POM forms continuous
ribbon-like extrusions.
The wear mechanisms of the POM and PP micro-pillars have elements of superficial creeping70
, fatigue wear104
and extrusion103
. The wear debris coming off the micro-pillars consisted of
long and flat flake-like particles. As the typical wear marks tied to abrasive wear types were
absent on the contact area (tops of the micro-pillars), the cutting and other abrasive mechanisms
can be ruled out. According to these observations, fatigue and adhesive wear models can best
describe the wear mechanism of the micro-pillars when sliding against a smooth steel surface.
30. 28
Figure 20. Specific wear rate curves of POM (A) and PP (B) surfaces as functions of effective
surface pressure. Close-ups of worn micro-pillars (P10) are shown to illustrate the difference in
wear types between POM (C) and PP (D).
5.2. Friction behavior
Friction curves for patterned POM surfaces are presented in Figure 21, alongside friction curves
for the PP surfaces with matching patterns. The friction levels of POM are lower than those of
PP, and the friction behavior of POM follows closely that of PP when the surface pressure is
above 0.3 MPa: the friction first decreases with increasing pressure, comes across a minimum
and finally starts to increase with the increasing pressure.
Under surface pressure of 0.3 MPa, however, the POM surfaces behave differently and the
friction levels decrease with decreasing surface pressure. This was deduced to stem from the
stiffness of POM: too low stress prevents the POM surface to conform to the counter surface
31. 29
and decreases the amount of micro-pillars in contact with the counter surface. This was
confirmed by SEM studies as seen in Figure 22.
Figure 21. Dynamic friction levels of POM (A) and PP (B) surfaces as functions of effective
surface pressure. Apparent trends are shown with dotted lines.
The low heat deflection temperature of PP helps explaining the increasing friction at the higher
surface pressures.105
The deflection temperature of PP drops from 100 °C down to 70 °C when
the compressive stress increases from 0.5 MPa to 1.8 MPa.106
As the highest surface pressure
used in the present study equals nearly 4 MPa, it is plausible that the deflection temperature
drops low enough that the frictional heat can soften the micro-pillar tops. This increases the
adhesion between PP, transfer layer and steel surface that explains the rise in the sliding
friction. The heat deflection temperatures affects POM less, as the heat deflection temperatures
for POM are 160 °C at 0.5 MPa pressure and 110 °C at 1.8 MPa pressure. However, the low
thermal conductivity of polymer materials, including POM, can result in a very high
temperature at the interface. Coupled with the high contact pressure, it is very probable that the
frictional heat induced by continuous sliding softens the POM transfer layer enough to increase
the adhesion, leading to elevated friction levels.59
Figure 22. SEM image of POM P30-2N surface, depicting partial surface contact due to
insufficient surface pressure (green highlight).
32. 30
6. Protective structuring IV
During the aforementioned studies with patterned polypropylene, the durability of the surface
structures in the used load and micro-pillar density ranges was noted to be adequate.I-III
The
results from the PP durability tests (page 28) showed that the 100 µm wide micro-pillar
structures withstood surface pressures in the scale of megapascals without collapsing. As the
scarcest studied pattern had a solid surface fraction of only 10 %, it raised a question about the
free, un-patterned surface area. As the area free of micro-pillars does not come into contact with
the counter surface, it can contain structures with functional properties as shown in Figure 23.
Figure 23. Schematic presentation of the protective structuring.
The possibility of sheltering a functional surface with durable micro-scale structures was
studied with superhydrophobic structures, fabricated via injection molding. Three different
types of hydrophobic, micro-scale structures were studied as the functional surface: two kinds
of hierarchical double-scale pillar structures and one micro-poached surface. The protective
structuring was designed to be weaved within the repeating lattice of the functional structure
having hierarchical patterns.
Superhydrophobic surfaces are extremely hydrophobic i.e. extremely difficult to wet. By
definition107
, the contact angle between water droplet and wetted surface exceeds 150 ° and the
sliding angle hysteresis is lower than 10 ° (the lotus effect). Hydrophobic surfaces have contact
angles in excess of 90 °.
Albeit hydrophobic structures were used as the sheltered functional surface, it is not obligatory
to restrict the protective pillar approach to them. The functional surface can be nearly any
surface whose effective function depends on its surface area. Other possible targets for this type
of approach are, for example, catalytic surfaces, solar cells and optical grids. The height of the
functional surface is the only restricting factor as the protective pillars have to be taller than the
sheltered surface in order to work.
6.1. Surface design
Areas of 5 mm x 5 mm were patterned with the micro-structures. The designed structures were
a double micro-pillar structure MM (micro-micro), a nano-topped micro-pillar structure MN
(micro-nano) and a variably spaced micro-pillar structure MS (multiscale). The structure types
33. 31
and repeating lattices of the used superhydrophobic patterns are shown in Figure 24A and B.
For the hierarchical structures, the protective structuring was generated by substituting a known
portion of the double-scale base pillars with protective pillars. The substitution ratios were
chosen so that the protective pillars would contribute approximately 5, 15 or 25 % for the solid
surface fraction. The same replacement grids were used to place the protective pillars on the
micro-poached surface.
As the main function of the protected surface is superhydrophobicity, the contact angle of the
surfaces was used as a measure of functionality for the surfaces. The height of the structuring
and initial contact angles of fabricated surfaces are listed in Table 4. Illustrative SEM images of
the protected surfaces are shown in Figure 24C and D for hierarchical and micro-poached
surfaces.
Figure 24. Schematic presentations of the studied hierarchical (A) and multi-scale (B)
structures. SEM images of the fabricated MM (C), MN (D) and MS (E) structures demonstrate
the surface morphology.
Table 4. Surface data and contact angles for the protected surfaces.
Height [µm] of the Contact angle [°]
Surface structure protection Base 5% a
15% a
25% a
PP b
- - 103 - - -
Micro-micro (MM) 40 + 20 65 158 156 151 149
Micro-nano (MN) 40 + 11 65 157 154 150 150
Multiscale (MS) 20…30 65 159 152 150 146
a
Contact angles for the protected surfaces, prior wear testing
b
Un-patterned, smooth PP (for reference)
34. 32
6.2. Wear resistance of protected surfaces
The wear resistance of the surfaces was tested with two methods: static pressure and abrasive
scratching. For the static pressure testing, the sample surface was positioned between two
smooth steel plates and a static weigh ranging from 10 kg to 100 kg was applied on the upper
steel plate for one (1) minute. With abrasive testing the samples were slid against very rough
counter surface with the compressing load ranging from 1 N to 5 N. Schematic illustrations of
the wear procedures are presented in Figure 25, alongside the optical microscopy images
depicting the structural damage the surfaces sustained during the aforementioned wear
measurements.
The measured contact angles of the worn surfaces were plotted against the apparent surface
pressure, i.e. the pressure that the used load would produce on an area of 5 mm x 5 mm (the
patterned area). In compression tests the surface pressure ranged roughly from 4 MPa to 40
MPa and from 2 kPa to 20 kPa in the scratching tests.
Figure 25. Schematic presentations of the set-ups for the static pressure (A) and scratch (B)
tests. Structures of the 15%-protected and un-protected MS surfaces are presented for before
(C) and after (D - G) the wear tests.
35. 33
As can be seen from the contact angles in Table 4, increasing the portion of protective features
decreased the contact angle of the surfaces, with the 25 % protected surfaces losing their
superhydrophobicity (CA less than 150 °). However, the surfaces with higher portion of
protective features also withstood more severe wear deformation. An example of the behavior
of functional surfaces under compression are presented in Figure 26A, contrasting the
difference between protected and un-protected surface. The general shape of the curve is very
clear and can be divided into three regions (marked in Figure 26):
1. Resistant region. When the pressure is within this range, the structure does not lose its
functionality.
2. Yield point (Failure range). The pressure on the protective pillars has reached the
compression strength of the used material. As the functional surface now participates
in the contact, functionality decreases. In the used case, a rapid drop of contact angle is
observed.
3. Collapse region. After the initial failure, the functional surface is further compressed.
Functionality continues to decrease.
Similar behavior can be detected with the abrasive sliding tests, presented in Figure 26B.
Although the pressure is far lower than in compression testing, the abrading movement wears
the surface very fast. Unlike with the compression test, the protective pillars face material loss
in scratch test. This causes the third step from the three regions presented above to disappear:
after the protective pillars are scraped away, the surface behaves like un-protected surface. This
is visible in Figure 26B: after the yield point has been reached, the contact angle drops to the
level of an un-protected surface.
Figure 26. Contact angle of a functional surfaces MS (blue) and protected MS 15% (green) as a
function of surface pressure for pressure testing (A) and abrasive scratching (B). The
compression-resistant and failure pressure ranges have been highlighted. The black dotted line
represents the superhydrophobic CA limit.
The yield point values for the studied surfaces are listed in Table 5. It can be seen that the
higher portion of protective features offers higher resistance against the tested wear conditions.
A special case is the hierarchical MN series which itself is slightly wear-resistant, withstanding
36. 34
a load of 10 kg (circa 4 MPa) without failing. This most likely explains the higher yield points
recorded for this series when comparing to MM and MS surfaces, which behave very similarly.
It should be noted that just prior to the yield point only the protective features are contacting the
counter surface. The true stress the protective surfaces face prior to the failure point is equal to
the calculated apparent surface pressure divided by the surface fraction of protective features.
This stress value just before the yield pressure tells about maximum pressure the protective
features can withstand. When taking the next data point of the curve into account i.e. the
pressure at which the surface has already failed, a failure range can be obtained. These failure
ranges presented in Table 5 are very similar between the surfaces with different sheltering
portions. These values indicate that an individual protective pillar can withstand 1 MPa surface
pressure during an abrasive scratch and a static surface pressure of approximately 100 MPa.
Table 5. Measured yield pressure values of the protected surfaces.
Series Protection Yield a
[MPa] CA [°] b
Failure range [MPa] c
Static compression measurements
MM
- 0 158 (139) -
5 % 3.9 156 (142) 78 – 157
15 % 15.7 149 (130) 105 – 131
25 % 23.5 144 (134) 94 – 110
MN
- 3.9 157 (134) 0 – 78
5 % 7.8 149 (142) 157 – 235
15 % 23.5 150 (135) 157 – 183
25 % 27.5 145 (132) 110 – 126
MS
- 0 159 (145) -
5 % 3.9 150 (145) 78 – 157
15 % 15.7 150 (135) 105 – 131
25 % 23.5 144 (128) 94 – 110
Abrasive scratch measurements
MM
- 0 158 (130) -
5 % 0.04 155 (130) 0.8 – 1.6
15 % 0.12 150 (127) 0.8 – 1.1
25 % 0.16 144 (128) 0.6 – 0.8
MN
- 0 157 (130) -
5 % 0.08 150 (130) 1.6 – 2.4
15 % 0.12 148 (130) 0.8 – 1.1
25 % 0.16 148 (130) 0.6 – 0.8
MS
- 0 159 (130) -
5 % 0.08 148 (129) 1.6 – 2.4
15 % 0.08 147 (125) 1.6 – 2.4
25 % 0.16 140 (120) 0.6 – 0.8
a
The maximum load the structure withstood before failing
b
Contact angle at yield point and (after yield point)
c
Surface pressure per single protective pillar at failure range
37. 35
The more durable behavior of the MN-surfaces was concluded to stem from its superimposed
structure. Unlike with MM and MS surfaces, the rug-like structure of MN-surfaces does not
have as much space for structural collapse. This allows the functional structure to participate in
the contact area while still maintaining the general shape of the sub-structure and thus its
functionality. However, this is only applicable when the surface structures do not face a too
high stress. This explains the higher endurance of the un-protected MN surface and elevated
robustness of the protected MN surfaces under static pressure when comparing to MM and MS
surfaces. This property of the MN surfaces does not help with the scratch tests, as any surface
area in contact with the abrading counter surface is ripped off. This explains why the MN
surface does not display higher wear resistance than MM or MS surfaces against abrasive
scratch.
6.3. Applicability of the method
The wetting experiments proved that functional surfaces with shielded features have superior
durability against mechanical compression and abrasive wear as compared to the unprotected
surfaces. Even a small fraction (5 %) of sacrificial surface is sufficient to protect the functional
content while maintaining the functional surface properties.
In this study it was also proven that protected functional surfaces can be manufactured via one-
step injection molding method which is very suitable for mass production. The micro-working
robot technique provides a fast and flexible tool to produce complex multi-scale molds. The
method allows production of structures scaling from a few microns up to hundreds of microns
in a single run.
38. 36
7. Conclusions
The basic function of surface patterning is usually related to the manipulation of the physical
characteristics of the surface. Many studies have been directed towards manufacturing surfaces
with specific functional abilities such as superhydrophobicity, increased adhesion or optical
properties with cost-effective methods. The aim of this dissertation was to produce and study
micro-patterned polymer surfaces with controlled friction levels and to advance the field of
applications available for micro-structured surfaces.
Surface patterns consisting of micron-scale pillar arrays were fabricated on polypropylene and
polyacetal surfaces. The effect of surface patterns on sliding friction was studied in addition to
the durability of the produced patterns. In addition to pure polymer, composites of
polypropylene and viscose fiber were studied to find out if the micro-pillars of the structure can
be reinforced with fibrous material.
The patterned surfaces were found to best affect the sliding friction when slid against very
smooth surfaces. It was noted that the both wear and friction are dependent on the surface
pressure, which in turn can be directly controlled by adjusting the surface pattern parameters.
The insight gained from these studies was utilized to design protective structuring where more
durable micro-pillars act as bumper against physical damage to the surface. The method was
proved highly effective as the 5x5 mm² sample area withstood a load as high as 50 kg before
losing its functional property – which is roughly equal to a mass of 10 metric tons resting on a
surface area of an average milk carton (7x7 cm²).
As a summary, it can be concluded that a patterning plastic materials with micron-scale surface
structures is an attractive and effective approach to modify the properties of the polymer
surface. Controlled structuring allows very fine control of the solid surface fraction and
effective surface pressure, which in turn control the contact behavior of the surface such as
friction, wear and hydrophobicity.
39. 37
Acknowledgements
This study, in all of its length, has been carried out during years 2008 – 2014 at the Department
of Chemistry, University of Eastern Finland (until 2010 University of Joensuu) within the
Inorganic Materials Chemistry Graduate Program (EMTKO) and the Doctoral Program of
Chemistry of UEF. The research has been carried out in the BNN (Biomimetic Nanomaterials
and Nanocomposites) and PRI (Polymer Research Initiative) projects, financially supported by
the ERDF and TEKES. Financial support from the Doctoral Program of Chemistry of UEF is
also gratefully acknowledged.
I am most grateful to my supervisors, Prof. Tuula Pakkanen and Prof. Mika Suvanto for their
guidance, patience and for making it possible for me to explore the interesting, intertwined
worlds that is polymer science. I highly appreciate the professional advices and the
encouragement I have been given.
I would like to thank Dr. Laura Takkunen, Janne Salstela and Dr. Eero Huovinen for their
valuable collaboration work and scientific discussions. In particular, I want to thank my office
mates: Markus Erola, Dr. Henna Stenberg and Dr. Hanna-Leena Rönkkö. The years I spent in
the office with you have been very entertaining.
As a whole, I also thank the research groups of Materials and Physical chemistry. Yu, Sami,
Maija, Adil, Mika, Inka, Tiina, Janne - it has been a pleasure to discuss with you and share
work and funny moments with you all.
A special thank goes to my children Toni, Kai, Hanna, Ville, Aapo, Irina, Lidiia and Paavo. The
summers we spent together were very special to me – it always warms me to see the hunger for
knowledge that shines from young students. Keep up the good work, I really wish to see you
aiming high and succeeding with your ambitions.
Also, heartwarming thanks to the dice-rolling groups I have had privilege to take part in – Jussi,
Atte, Jere, Niko, Mika, Kai², Jarno and Pasi. The cabin trips and overindulgence in conjunction
with the board games have made it easier to plough through the years.
Even more special thanks go to the chemistry student organization, Bunsen ry and its active
members. The official and unofficial activities I have participated with you have, perhaps, been
the most mind shaping/warping experiences yet. Don’t burn the faculty down.
Lastly, I’m very grateful for my family. I want to express my loving thanks to my family –
Sirkka, Esko, Merja, Teemu, Tero, Tuomo. You are the ones to whom I can always trust with
my everything.
Joensuu, October 2014 Tarmo Korpela
40. 38
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46. Tarmo Korpela
Dissertations
Department of Chemistry
University of Eastern Finland
No. 124 (2014)
97/2008 TANSKANEN Jukka: One- and two-dimensional nanostructures of group 14 elemental hydrides
and group 13-15 binary hydrides
98/2009 JOKINIEMI Jonna: Structural studies on metal complexes of mixed amide esters and phenyl and
monoalkyl ester derivatives of dichloromethylene biphosphonic acid
99/2009 KALIMA Valtteri: Controlled replication of patterned polymer and nanocomposite surfaces for
micro-optical applications
100/2009 HYYRYLÄINEN Anna: Differentiation of diastereomeric and enantiomeric b-amino acids by
mass spectrometry
101/2010 KUNNAS-HILTUNEN Susan: Synthesis, X-ray diffraction study and characterisations of metal
complexes of clodronic acid and its symmetrical dianhydride derivatives
102/2010 NIEMI Merja: A molecular basis for antibody specificity – crystal structures of IgE-allergen and
IgG-hapten complexes
103/2010 RASILAINEN Tiina: Controlling water on polypropylene surfaces with micro- and micro/nano
structures
104/2011 SAARIKOSKI Inka: Tailoring of optical transmittance, reflectance, and hydrophobicity of
polymers by micro- and nanoscale structuring
105/2011 NISKANEN Mika: DFT Studies on ruthenium and rhodium chain complexes and a
one-dimensional iodine bridged ruthenium complex
106/2011 RÖNKKÖ Hanna-Leena: Studies on MgCl2
/alcohol adducts and a self-supported Ziegler-Natta
catalyst for propene polymerization
107/2011 KASANEN Jussi: Photocatalytic TiO2
-based multilayer coating on polymer substrate for use in
self-cleaning applications
108/2011 KALLIO Juha: Structural studies of Ascomycete laccases – Insights into the reaction pathways
109/2011 KINNUNEN Niko: Methane combustion activity of Al2
O3
-supported Pd, Pt, and Pd-Pt catalysts:
Experimental and theoretical studies
110/2011 TORVINEN Mika: Mass spectrometric studies of host-guest complexes of glucosylcalixarenes
111/2012 KONTKANEN Maija-Liisa: Catalyst carrier studies for 1-hexene hydroformulation: cross-linked
poly(4-vinylpyridine), nano zinc oxide and one-dimensional ruthenium polymer
112/2012 KORHONEN Tuulia: The wettability properties of nano- and micromodified paint surfaces
113/2012 JOKI-KORPELA Fatima: Functional polyurethane-based films and coatings
114/2012 LAURILA Elina: Non-covalent interactions in Rh, Ru, Os, and Ag complexes
115/2012 MAKSIMAINEN Mirko: Structural studies of Trichoderma reesei, Aspergillus oryzae and
Bacillus circulans sp. alkalophilus beta-galactosidases – Novel insights into a structure-function
relationship
116/2012 PÖLLÄNEN Maija: Morphological, thermal, mechanical, and tribological studies of polyethylene
composites reinforced with micro– and nanofillers
117/2013 LAINE Anniina: Elementary reactions in metallocene/methylaluminoxane catalyzed polyolefin
synthesis
118/2013 TIMONEN Juri: Synthesis, characterization and anti-inflammatory effects of substituted coumarin
derivatives
119/2013 TAKKUNEN Laura: Three-dimensional roughness analysis for multiscale textured surfaces:
Quantitative characterization and simulation of micro- and nanoscale structures
120/2014 STENBERG Henna: Studies of self-organizing layered coatings
121/2014 KEKÄLÄINEN Timo: Characterization of petroleum and bio-oil samples by ultrahigh-resolution
Fourier transform ion cyclotron resonance mass spectrometry
122/2014 BAZHENOV Andrey: Towards deeper atomic-level understanding of the structure of magnesium
dichloride and its performance as a support in the Ziegler-Natta catalytic system
123/2014 PIRINEN Sami: Studies on MgCl2
/ether supports in Ziegler–Natta catalysts for ethylene
polymerization
Friction and Wear of Micro-
Structured Polymer Surfaces
TarmoKorpela:FrictionandWearofMicrostructuredPolymerSurfaces 124
