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SCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT DOCTORAL THESIS IN MACHINE DESIGN STOCKHOLM, SWEDEN 2018 Railway Open System Tribology
SCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT
DOCTORAL THESIS IN MACHINE DESIGN
STOCKHOLM, SWEDEN 2018
Railway Open System Tribology
Railway Open System Tribology
YEZHE LYU
KTH Royal Institute of Technology
System and Component Design
Department of Machine Design
SE-100 44 Stockholm, Sweden
ISBN: 978-91-7729-710-9
TRITA-ITM-AVL 2018:6
Academic thesis, which with the approval of KTH Royal Institute of
Technology, will be presented for public review in fulfilment of the
requirements for a Doctor of Engineering in Machine Design. Public
review: KTH Royal Institute of Technology, Brinellvägen 85, room
Gladan, at 10:00 on April, 27, 2018.
To my wife, Fangfei
and my daughter, Vilja
献给方飞和橙
Preface
Open system tribology involves the physical and chemical fundamentals
at the interacting surfaces in relative motion. For a long time, people used
to consider friction, wear and lubrication as the major topics of tribology
and neglected the interaction with the surrounding environment. Noise
and particle emissions, for example, strongly affect the health of human
beings and the environment, which should also be considered
simultaneously in tribology research. This thesis attempts to relate
frication, wear, noise and particle emissions from the railway system with
the open environment.
I started my PhD study from an unfavourable beginning within a topic
that I had no interest in. I had felt perplexed for months about whether to
continue the PhD study. I am deeply grateful to Prof. Ulf Olofsson, for it is
he who introduced the field of tribology to me at that difficult time and
encouraged me to continue my study under his supervision. Many thanks
are also given to my co-supervisor Dr Ellen Bergseth, for her consistent
patience, encouragement and support throughout my study. I was
fortunate and happy to learn a great deal from them.
I wish to give my appreciation to all of the people who helped with my
PhD research and preparation of the thesis, including the co-authors of
the appended papers: Assoc. Prof. Stefan Björklund (KTH), Dr Rickard
Nilsson (SLL), Dr Yi Zhu (Zhejiang University), Anders Lindgren
(Tyréns), Martin Höjer (Tyréns), Minghui Tu (KTH).
Thanks are extended to all of my colleagues at KTH: Assoc. Prof. Jens
Wahlström, Assoc. Prof. Anders Söderberg, Mr Peter Carlsson, Mr Staffan
Qvarnström, Dr Kenneth Duvefelt, Dr Mario Sosa, Dr Martin Andersson,
Dr Xuan Sun, Mrs Yingying Cha and Mr Edwin Bergstedt for their advice
and assistance during my studies. Many thanks are given to Prof. Martin
Törngren at KTH for his valuable comments on the thesis.
In addition, I want to acknowledge all of my friends: Ye Tian, Huijun
Wang, Xin Xu, Xuge Fan, Jiaqing Yin, Peng Liu, Dan Li, Linqin Wang,
Zhendong Liu and Yuyi Li for being with me during these years.
Finally, the deepest thanks go to my parents, my lovely wife Fangfei
and my cute daughter Vilja. Your love means everything to me.
Yezhe Lyu Stockholm, February 2018
Abstract
Tribology in the railway system is of increasing interest in the new railway era due to the demand for higher speed and load capacity. Since railway vehicles operate in an open environment, their performance depends greatly on temperature, humidity and natural and artificial contaminants. Meanwhile, the “feedback” of railway vehicles to the surroundings, such as noise and airborne particles, is of great importance to the human health and the environment. Therefore, this thesis aims to investigate the strong interaction between railway tribology and the open environment. The effects of temperatures from -35 °C to 20 °C, relative humidity from 40% to 85%, natural contaminants such as ice particles on friction, wear, noise and airborne particle emissions at the wheel–rail and wheel–block brake contacts have been investigated in both lab- and full-scale contexts.
Papers A and B investigated the effect of temperature, humidity and ice particles on the friction and wear at unoxidized and oxidized wheel–rail contacts. The results indicate that increasing humidity reduces the wear at unoxidized contacts. A decrease in temperature tends to intensify the wear until an ice layer has condensed on the wheel and rail surfaces at -25 °C. Ice particles encourage the generation of oxide flakes at the contacting path, largely inhibiting the wear process.
Paper C, which was a lab-scale test, studied the friction, wear and noise generation from pre-oxidized wheel–rail contact with varied surface features. Major results include that the wear regime transition from mild wear to severe wear is always accompanied by an increase in noise level of 10 dB and a broader bandwidth of noise.
Paper D was a validation of the major findings of paper C in a full-scale test, which also saw an increase in noise level as well as a broader bandwidth when the wheel–rail contact transformed from mild to severe wear.
Paper E studied the effect of humidity on the friction, wear and airborne particle emissions of three railway brake-block materials. The results show that cast iron generated the highest friction coefficient, wear and particle emission, and organic composite the lowest levels.
Paper F conducted a thorough literature review on the open system tribology at the wheel–rail contact. Commonly seen parameters such as temperature, humidity and natural and artificial contaminants on friction, wear, noise and particle emissions were investigated.
Keywords Wheel; Rail; Brake; Environmental conditions; Tribology; Noise; Particle
Sammanfattning
Järnvägsfordonen arbetar i en öppen miljö, vars prestanda och livslängd i form av friktion och nötning beror på temperatur, luftfuktighet samt naturliga och konstgjorda föroreningar. Samtidigt är återkopplingen, som järnvägsfordon ger till omgivningen i form av buller och luftburna partiklar, av stor betydelse för människors hälsa. Därför är målsättningen med denna avhandling att undersöka den starka samverkan som finns mellan järnvägsfordon och den öppna miljön. Effekten av, temperaturer från -35 °C till 20 °C, relativ fuktighet från 40 % till 85 % och naturliga föroreningar som snö, på friktion, slitage, buller och luftburna partikelutsläpp vid hjulräls- och hjulblockbromskontakter har undersökts, både i laboratorier och i full skala. Manuskript A och B undersökte effekten av temperatur, luftfuktighet och snö på friktionen och nötning vid oxiderade och icke-oxiderade hjulrälskontakter. Resultaten visar att en ökad luftfuktighet, minskar nötningen vid icke-oxiderade kontakter. Minskning av temperaturen tenderar att intensifiera nötningen, tills is kondenseras på hjul- och rälsytorna vid -25 °C. Iskristallerna, ökar hastigheten på genereringen av oxidflingor i kontakten, och förhindrar i stor utsträckning nötningsprocessen. Manuskript C, som är ett laboratorieprov, studerade friktionen, nötningen och ljudgenerering från föroxiderade hjulrälskontakter med varierande yttopografi. Viktiga resultat, är att övergången, från mild nötning till svår nötning, alltid åtföljs av en ökad ljudnivå och en ökning av ljudets bandbredd. Manuskript D är en fullskalevalidering av huvudresultatet från Manuskript C, vilket också uppvisade en ökning av ljudnivån om hjulrälskontakten gick från mild till svår nötning åtföljt av en ökning av ljudets bandbredd. Manuskript E studerade effekten av luftfuktighet på friktion, nötning och luftburna nötningspartiklar av tre olika blockbromsmaterial. Resultaten visar att gjutjärn genererade den högsta friktionskoefficienten, nötningsnivån och halten av partikelutsläpp. Blockbromsar tillverkade av organisk komposit, uppvisade den lägsta nivån för alla tre uppmätta parametrar. Manuskript F redovisar en litteraturgenomgång, hur det öppna systemet, påverkar tribologin i hjul-järnvägskontakten. Parametrar som temperatur, fuktighet, naturliga och konstgjorda föroreningar på friktion, slitage, buller och partikelutsläpp diskuterades i detalj.
Nyckelord Hjul; Räl; Broms; Miljöförhållanden; Tribologi; Ljud; Partiklar
Appended papers and the author’s contribution
Paper A
Yezhe Lyu, Yi Zhu, Ulf Olofsson. “Wear between wheel and rail: A pin-
on-disc study of environmental conditions and iron oxides”. Wear, 328–
329 (2015) 277–285. DOI: 10.1016/j.wear.2015.02.057.
Contributions: Lyu planned the experiments, did the literature survey,
the major part of the experiments, the data analysis and the major part of
the writing. Zhu performed part of the experiments and writing. Olofsson
supervised the work and wrote part of the paper.
Paper B
Yezhe Lyu, Ellen Bergseth, Ulf Olofsson. “Open system tribology and
influence of weather condition”. Nature Scientific Reports, 6: 32455,
DOI: 10.1038/srep32455.
Contributions: Lyu formulated the research questions and chose the
methodology to answer them. Lyu did the literature survey, the major
part of the experiments, the data analysis and the major part of the
writing. Bergseth did part of the experiments. Bergseth and Olofsson both
supervised the study and wrote part of the paper.
Paper C
Yezhe Lyu, Ellen Bergseth, Ulf Olofsson, Anders Lindgren, Martin
Höjer. “On the relationships among wheel–rail surface topography,
interface noise and tribological transitions”. Wear, 338–339 (2015) 36–
46. DOI: 10.1016/j.wear.2015.05.014.
Contributions: The experimental work was equally divided between Lyu,
Bergseth, Olofsson, Lindgren and Höjer. Lyu and Lindgren analysed the
data. All authors were involved in the writing of the text, of which Lyu did
the major part.
Paper D
Yezhe Lyu, Stefan Björklund, Ellen Bergseth, Ulf Olofsson, Rickard
Nilsson, Anders Lindgren, Martin Höjer. “Development of a noise related
track maintenance tool”. The 22nd International Congress on Sound and
Vibration, Florence, Italy, 12–16 July 2015.
Contributions: The experimental work was equally divided between all
the authors. Lyu did the literature review and analysed most of the data.
All authors were involved in writing and editing the text.
Paper E
Yezhe Lyu, Ellen Bergseth, Minghui Tu, Ulf Olofsson. “Effect of
humidity on the tribological behaviour and airborne particle emissions of
railway brake block materials”. Tribology International, 118 (2018) 360–
367. DOI: 10.1016/j.triboint.2017.10.011.
Contributions: Lyu formulated the research questions and chose the
methodology to answer them. Lyu also did the literature survey, data
analysis and the major part of the writing. Tu did part of the experiments
and writing. Bergseth and Olofsson supervised the work, discussed the
results and wrote part of the text.
Paper F
Ulf Olofsson, Yezhe Lyu. “Open system tribology in the wheel–rail
contact—A literature review”. Applied Mechanics Review, 69 (2017)
060802. DOI: 10.1115/1.4038229.
Contributions: Olofsson formulated the frame of the paper. Lyu did the
major part of the literature survey. Olofsson and Lyu were equally
involved in writing the paper.
Contributed papers not included in this thesis Yi Zhu, Yezhe Lyu, Ulf Olofsson. “Mapping the friction between railway
wheels and rails focusing on environmental conditions”. Wear, 324–325
(2015) 122–128.
Yezhe Lyu, Ellen Bergseth, Ulf Olofsson. “The effect of subzero
temperature and snow on the tribology of wheel–rail contact”.
Proceedings of the Third International Conference on Railway
Technology. Paper 153.
Benjamin White, Roger Lewis, Ulf Olofsson Yezhe Lyu. “The
contribution of iron oxides to the wet-rail phenomenon”. Proceedings of
the Third International Conference on Railway Technology. Paper 154.
Martin Höjer, Ellen Bergseth, Ulf Olofsson, Rickard Nilsson, Yezhe Lyu.
“A noise related track maintenance tool for severe wear detection of
wheel–rail contact”. Proceedings of the Third International Conference
on Railway Technology. Paper 146.
Yezhe Lyu, Jens Wahlström,Vlastimil Matejka, Anders Söderberg.
“Ranking of conventional and novel disc brake materials with respect to
airborne particle emissions”. Proceedings of the Eurobrake 2017.
EB2017-MDS-012.
Jens Wahlström, Yezhe Lyu, Vlastimil Matejka, Anders Söderberg. “A
pin-on-disc tribometer study of disc brake contact pairs with respect to
wear and airborne particle emissions”. Wear, 384–386 (2017) 124–130.
Jens Wahlström, Vlastimil Matejka, Yezhe Lyu, Anders Söderberg.
“Contact pressure and sliding velocity maps of the friction, wear and
emission from a low-metallic/cast iron disc brake contact pair”.
Tribology in Industry, 39 (2017) 460–470.
Contents
1 Introduction .......................................................................................... 1
1.1 Railway open system tribology ................................................................... 1
1.2 Oxide formation .......................................................................................... 6
1.3 The sliding part of the wheel–rail contact .................................................... 9
1.4 Noise and airborne particle emissions ...................................................... 11
1.5 Objectives ................................................................................................. 14
1.6 Summary of the methods .......................................................................... 15
2 Summary of appended papers .......................................................... 16
3 Discussion .......................................................................................... 19
3.1 Discussion on appended papers ............................................................... 19
3.2 Discussion on the validity of the results .................................................... 22
3.3 Contribution and impact of the thesis ........................................................ 23
4 Conclusions ........................................................................................ 25
5 Future work ......................................................................................... 26
6 References .......................................................................................... 27
Appended papers
A Wear between wheel and rail: A pin-on-disc study of environmental
conditions and iron oxides
B Open system tribology and influence of weather condition
C On the relationships among wheel–rail surface topography, interface noise
and tribological transitions
D Development of a noise related track maintenance tool
E Effect of humidity on the tribological behaviour and airborne particle
emissions of railway brake block materials
F Open system tribology in the wheel–rail contact—A literature review
INTRODUCTION
1
1 Introduction
This thesis deals with experimental methods at lab-scale and
full-scale levels for investigating the wheel–rail and wheel–
block-brake contacts in the railway open system with respect
to tribological phenomena. The first five appended papers
address the experimental methods for measuring the friction,
wear, sound and airborne particle emissions from the wheel–
rail and wheel–brake contacts. The last appended paper
presents a literature review of recently documented studies of
the open system tribology in wheel–rail contact.
An introduction to the railway open system tribology is
given in Section 1.1. Iron oxides [1], which are believed to be
essential in the tribological process of wheel–rail and wheel–
brake contacts, are introduced in Section 1.2. The importance
of the sliding part in the wheel–rail contact is demonstrated
in Section 1.3. Emission of sound and airborne particles is
discussed in Section 1.4. Section 1.5 presents the objectives of
this thesis. Finally, the methods used in the experimental
studies involved are summarized in Section 1.6.
1.1 Railway open system tribology
Tribology, the science of interacting surfaces in contact, is an
interdisciplinary subject that can be addressed from several
different viewpoints [2]. The interacting surfaces deliver
different characteristics in terms of friction and wear due to
the surface topography, hardness, plastic deformation, etc.
These parameters are often set in the manufacturing
processes and in the material selection of the surfaces and
bulk material. One possible division of tribological systems is
INTRODUCTION
2
by open and closed system tribology. If the system is sealed
away from the external environment, like bearings and seals,
one can call it a closed system; this enables a better control of
the friction, wear and applied lubrication due to the isolation
of external contaminants. In addition, such a system partly or
totally obstructs the emission of sound and particulates. In
contrast, an open system like tyre–road contact is exposed to
the external environment where artificial and natural
contaminants will exert an influence on friction and wear.
Sound and airborne particles from the friction and wear
process have no shield and will be emitted to the surrounding
air [3].
Railway vehicles contain several typical open tribology
systems – like wheel–rail contact and block brake–wheel
tread contact – that are exposed to natural contaminants
such as high humidity, rain, snow, sand and leaves.
Meanwhile, the radiated noise from the railway vehicles may
disturb passengers when a railway vehicle traverses sharp
curves. Particulates will also be generated from the wear
process at the wheel–rail contact and block brake–wheel
tread contact during acceleration and deceleration/braking of
the railway vehicles. Airborne particles can easily drift for
hundreds of metres in the open environment [4], which may
exert both short- and long-term adverse effects on lung
function [5].
In railway operations, a friction coefficient at a proper level
is demanded. Too high or too low a friction coefficient at the
wheel–rail contact will result in a series of adverse
consequences as shown in Figure 1. Too low a level leads to a
schedule delay due to poor traction and brake coefficients
[6]. A conspicuous trend of prolonged braking distances was
INTRODUCTION
3
recorded during the very rainy days in the railway traffic
statistics of the UK and Sweden [7]. Meanwhile, too high a
value causes rapid exhaustion of materials because of high
wear damage.
Temperature and humidity are among the most erratic
environmental parameters and have been found to affect the
friction and wear damage and friction coefficient at clean and
oxidized wheel–rail contact (Figure 2). If the temperature
dropped off to -15 °C, the wheel–rail contact yielded a wear
rate ten times higher compared with the one measured at 10
°C. If a further reduction in temperature to -35 °C was seen,
the wear rate at the wheel–rail contact fell off to the same
level as the one at 10 °C (Figure 3). This change is associated
with both physics and chemistry, in that at -15 °C the
ductility of wheel and rail steel largely decreases and thick
oxide layers that protect the materials flourish at -35 °C.
Figure 1. Typical friction coefficient levels at the wheel–rail
contact and their consequences [3].
INTRODUCTION
4
Figure 2. Friction and wear rate of clean and oxidized rail
material as a function of absolute humidity under varied
temperatures [8, 9].
Other parameters exist in the railway open system
including natural (water, leaves, etc.) and artificial (friction
modifiers and lubricants) contaminants. A large number of
studies have seen the decrease of friction and wear when
water is present at wheel–rail and brake–wheel contacts [10–
18]. Snow (water in solid form) has been proved to strongly
affect the friction coefficient at the wheel–rail contact under
different contact pressures (Figure 3). Although the
mechanism of reducing the friction coefficient using water
presence is still debated, most believe that water encourages
the generation of iron oxides at the contact, inhibiting
INTRODUCTION
5
friction [14, 19]. Further, water was found to have a better
lubrication effect than oil on extremely smooth surfaces Ra
(centreline average roughness) of 0.1 μm, showing a great
potential as an environmentally friendly lubricant [20].
Fallen leaves represent another commonly seen natural
contaminant for railway traffic. Once the leaves are dragged
into the wheel–rail and wheel–brake contacts, they will
chemically react with the steel materials and generate a
coherent black layer on the steel surfaces, reducing the
friction coefficient [21, 22].
Figure 3. Friction coefficient as a function of temperature,
which declined with snow (water in solid form) presenting at
the wheel–rail contact [19].
INTRODUCTION
6
Due to the complexity of the railway system, some artificial
contaminants are frequently added into the wheel–rail
contact to maintain a proper friction level. These include
positive friction modifiers that enhance the friction and
lubricants that reduce the friction. Positive friction modifiers
can be solid or liquid, and most are confidential commercial
products. They are usually added into the wheel head–rail
tread contact for maintaining the friction coefficient at a
proper level (normally 0.2–0.4) to eliminate noise and rail
corrugation [23–25]. A significant side effect of the positive
friction modifiers is that they introduce wheel–rail contact
insulation, resulting in signal interruption [26, 27].
Conversely, lubricants are usually applied to the wheel
flange–rail gauge contact on curving tracks to relieve the
wear and noise problem. A vast number of studies with
varied types of instruments have found that oil-based and
grease-based lubricants significantly downgrade the friction
coefficient at the wheel–rail contact [28–35]. Meanwhile, it is
important to appropriately manage lubrication at the rail
gauge. Over-dosed lubricants are apt to move to the rail head,
causing a loss of friction.
1.2 Oxide formation
This section introduces the iron oxides that are generated at
the wheel–rail contact and wheel–brake contact and
discusses the contribution of iron oxides to friction and wear.
Although the mechanisms of friction and wear at the wheel–
rail contact and wheel–brake contact are of great complexity,
the generation of iron oxide layers on material surfaces is
suggested to be responsible for the friction and wear process.
Mølgaard and Srivastava stated that oxidation clearly leads to
INTRODUCTION
7
the wear of dry surfaces of ferrous materials at moderate and
high sliding speeds, because the wear debris generated under
such conditions is mostly composed of oxides [36].
There are in total fifteen types of iron oxides, which are
different in chemical composition and crystal structure, but
only five kinds are usually found in the context of railway
traffic under different environmental conditions. Suzumura
et al. applied a portable X-ray diffractometer equipped with
X-ray fluorescence in situ and identified the iron oxides as
follows: hematite (Fe2O3) and magnetite (Fe3O4), which are
anhydrous, and goethite (α-Fe2O3·H2O), akaganeite (β-
Fe2O3·H2O) and lepidocrocite (γ–Fe2O3·H2O), which are
hydrated (also called rust) and only differ in crystalline
system and colour [37]. Iron oxides can naturally form on the
surfaces of wheel, rail and brake in an atmospheric
environment, but will generate more promptly during sliding
at the wheel–rail contact and wheel–brake contact where
temperature is high.
In an atmospheric environment, iron oxides will slowly
form through electrochemical corrosion, which involves the
oxidation of wheel and rail steels by oxygen as the oxidizing
agent [38]. This reaction usually occurs in wet weather (high
relative humidity), where the aqueous layer covering the
wheel and rail surfaces will act as an electrolyte (as shown in
Eq. 1). Under such conditions, the oxidation rate is controlled
by the amount of electrolyte (available water) and the
products are usually hydrated.
𝟒 𝑭𝒆(𝒔) + 𝟑 𝑶𝟐 (𝒈) → 𝟐𝑭𝒆𝟐𝑶𝟑 ∙ 𝒙𝑯𝟐𝑶 (1)
INTRODUCTION
8
When the weather is dry (low relative humidity),
electrochemical oxidation is unlikely to happen, or the rate of
reaction becomes quite low. However, oxidation of steel
would still happen through a thermal corrosion process.
Under such conditions, the water layer is not necessary and
the temperature becomes the controlling factor. With
increasing temperature, the diffusion of iron ions and oxygen
is expedited, speeding up the oxidation reaction. At room
temperature, the oxide layer is very thin, about several nm,
while a thick and coherent oxide layer will form at elevated
temperatures. Transformations between different types of
oxides may happen under alternating dry and wet cyclic
weather conditions. Generally, rusts (hydrated oxides) tend
to transform to anhydrous oxides under dry conditions and
the other way around under wet conditions [39]. The
dominating mechanism of the transformation is “topotaxy”,
which includes all chemical solid state reactions where one
crystal orientation changes to another crystal orientation
[40].
If the surface of the metal is smooth enough, a
homogeneous oxidation will occur, where a coherent and
compacted oxide layer will form on the surface. But this is
usually not the case for the wheel and rail surface. Since the
wheel and rail surfaces constantly experience rolling-sliding
contact, plastic deformation and tiny defects will occur on the
surfaces, encouraging pitting and localized corrosion. The
oxide will first form as a spot. If this spot has enough binding
force with the metal, it will stay and become an oxide layer
(oxide island), preventing the metals from experiencing
severe wear. On the contrary, if the binding force between the
oxide spot and the surface is too weak, it will be sheared off
INTRODUCTION
9
by the rolling-sliding contact and pulverized into tiny
particles, accelerating the abrasive wear as third bodies [41].
Great efforts have been made to study the influence of
oxides on the friction at the wheel–rail contact and wheel–
brake contact. A large drop of friction and wear was found at
the wheel–rail contact where an intact oxide layer formed
and thoroughly covered the surface [42, 43]. Iwabuchi et al.
demonstrated that the friction coefficient declines as the
oxide thickness increases, based on their own research in
combination with the work of Kragelʹskiĭ [44, 45]. Beagley
found that the friction coefficient becomes extremely low
when a paste containing iron oxide and a small amount of
water is formed [43]. An increase in the coverage of the oxide
layer leads to a decline in the wear coefficient [46]. The oxide
layers formed also contribute to a decrease in wear rate. Zhu
and Olofsson conducted a pin-on-disc study and found that
the oxidized samples at high humidity yielded a lower wear
rate compared with the unoxidized samples [47]. Stott
revealed that oxidation is beneficial for reducing wear during
sliding of metals by preventing metal–metal contact.
Thermal corrosion can be easily induced by frictional heat
[1].
1.3 The sliding part of the wheel–rail contact
Unlike the pure sliding wheel–brake contact, wheel–rail
contact can be divided into two types – the contact between
wheel tread and rail head, and the contact between wheel
flange and rail gauge (Figure 4). The wheel tread–rail head
contact is usually seen on straight tracks and wheel flange–
rail gauge contact always occurs on sharp curves. As with the
INTRODUCTION
10
wheel–brake contact, the wheel flange–wheel gauge contact
is also a pure sliding contact accompanied by very high
contact pressure and large plastic deformation. The wheel
flange–rail gauge contact usually experiences wear transition
from severe to catastrophic and wheel tread–rail head
contact from mild to severe [48]. The wheel tread–rail head
contact is a rolling-sliding contact that contains stick (no-
slip) and slip regions [49].
Figure 4. Schematic of two typical types of wheel–rail contact:
a) wheel tread–rail head contact and b) wheel flange–rail
gauge contact.
Zhu has described the stick-slip contact condition between
the wheel tread and rail head [50]. Briefly, there should be a
proper ratio between the rolling (stick) part and sliding (slip)
part at the wheel tread–rail head contact. The sliding (slip)
part always exists and results in the frictional heating of both
the wheel and rail [51]. As a consequence of occasionally
excessive temperature, phase transformation and materials
softening will happen, resulting in some troublesome damage
such as melting and cracks on the wheels and rails [52].
Besides, the slip (sliding) part dominates the friction
INTRODUCTION
11
coefficient at the wheel–rail contact, restricting the load
capacity, traction and brake coefficient. Therefore, the sliding
part of the wheel tread–rail head contact deserves detailed
investigation and a large number of studies have focused on
this topic [53–59].
1.4 Noise and airborne particle emissions
The railway system strongly interacts with the surroundings,
and is not only susceptible to environmental conditions but
also releases feedback such as noise and airborne particles.
The emission of noise and airborne particles works against
the promotion of the new era of the railway system with
higher speeds and loading capacities, and thus draws
considerable attention from railway engineers [60].
Noise from the railway system can be generated both on
straight tracks and curves. Rolling noise and impact noise are
usually heard on straight tracks and stem essentially from the
imperfect surface conditions of the wheels and rails. The
continuous noise caused by regular surface roughness is the
rolling noise and the intermittent noise due to imperfect rail
joints, wheel flats, switch and crossing gaps is called impact
noise [61]. Rolling noise is thought to be the main source of
noise from the railway system and can be reduced through
measures such as smoothing wheel and rail surfaces and
shielding bogies and rails [61, 62]. The same counter-
measures also work efficiently for impact noise.
Squeal noise is believed to be generated through a stick-slip
mechanism. When traversing a curve, the traction force
makes the wheel slip whilst the static friction makes the
wheel stick. These two forces alternatively dominate and
INTRODUCTION
12
motivate the resonance of the wheels (vibration around the
natural frequency) [63]. Squeal noise is much more
inadvertent than rolling noise and impact noise, which is
thought to be easier to eliminate. Since static friction is one
important parameter that activates squeal noise, friction
management at the wheel–rail contact becomes important
for its control, and will involve all parameters affecting the
friction coefficient at the wheel–rail contact, such as surface
finishing [64], friction modifiers [65], temperature, humidity
[66] and water presence [67]. The brake contact also radiates
squeal noise, though current research mainly focuses on the
automotive context [68–72]. Possible measures to reduce the
squeal noise from automotive brake systems include
modifications to the structure of the brake parts and
materials. Currently there is a knowledge gap on the effect of
environmental conditions on squeal noise.
Like noise, airborne particles will be emitted from both
wheel–rail contact and wheel–brake contact. The inhalable
particles are thought to be one of the most worrying
environmental problems, raising a series of public health
issues. Most studies on airborne particle emissions from the
railway system were conducted in the field, where the
contributions of wheel–rail contact and wheel–brake contact
are hard to distinguish [61]. Most of the emitted particles are
found to be deposited near railway tracks, but still some 25%
will drift for over 100 metres [61]. Figure 5 shows the
deposition of particles that originated from the railway
system on railway tracks. Nieuwenhuijsen et al. made a
comparison study of the abundant published measurements
of airborne particles from railway transport and saw a clear
indication that underground systems generate a higher
INTRODUCTION
13
concentration compared with above-ground environments
[73]. The composition of the emitted particles is complex.
More than fifteen elements were detected from the particles
collected from around the tracks [74, 75], among which Fe,
Cu, Zn and Mn are dominant [76]. One lab-scale test
dedicated to airborne particles from the wheel–rail contact
witnessed a vast number of nano-sized particles at elevated
temperatures, which usually results from high contact
pressure and sliding speed [55]. The only study found on the
effect of environmental conditions on particle emissions from
wheel–rail contact indicated that grease-lubricated wheel–
rail contact generated fewer particles than dry, water- and
oil-lubricated ones [77]. Investigations into the effect of open
systems on the particle emissions from railway brake systems
are also rare, and the only one found was conducted by
Olofsson [4]. Since the brake contacts should always be kept
clean to avoid losing braking efficiency, further research is
encouraged on environmental conditions (such as
temperature and humidity) rather than the additives that are
commonly found at wheel–rail contact.
Figure 5. Worn particles deposited at the rail foot during a field
test at Högdalen test depot, highlighted with arrows.
INTRODUCTION
14
1.5 Objectives
The open system tribology of the railway system is an open
field and one which is not studied sufficiently, especially the
area of wheel–block brake contact. The purpose of this thesis
is to enhance the scientific knowledge about the effect of
open environmental conditions on the friction, wear, noise
and particle emissions from wheel–rail contact and wheel–
block brake contact. This purpose is achieved by fulfilling the
following objectives:
• Develop experimental methods in terms of a test stand
for measuring the friction and wear from both the wheel–rail
contact and wheel–block brake contact under commonly
seen and well-controlled environmental conditions, e.g.
temperature above 0 °C and relative humidity.
• Apply the above test design to investigate the effect of
relative humidity and different contact pairs on the friction,
wear and airborne particle emissions from wheel–brake
contacts.
• Update and use the above test stand in more hostile
environments and investigate the effect of sub-zero
temperatures on the tribology at the wheel–rail contact.
• Investigate the relationships among rail surface
grinding features, generated noise and wear behaviours at
the wheel–rail contact in a lab-scale test. If they are related,
verify the validity in full-scale tests.
• Implement a thorough literature review of open
system tribology at the wheel–rail contact, summarize the
archived studies and suggest future work.
INTRODUCTION
15
1.6 Summary of the methods
This thesis comprises five different experimental studies
and one literature review on railway open system tribology.
In the appended papers, varied test methods are used based
on two different pin-on-disc tribometers (pin-on-disc I and
pin-on-disc II) and one full-scale equipped metro. In Table 1
the test apparatus, testing conditions and responses in all
appended papers are summarized.
Table 1. Summary of the test methods in the appended papers.
Appended papers A B C D E
Test
apparatus
Pin-on-Disc I × × ×
Pin-on-Disc II × ×
Full scale ×
Testing
conditions
Temperature
(°C) 3/10/20 -35/-25/-15/-5/3 20±2 20±2
Relative
humidity (%) 40/55/70/85 40±5 25/50/75
Contact
pressure (MPa) 900 900/1500 750 0.8
Sliding speed
(m·s-1) 0.01 0.01
0.055/
0.046 0.45
Responses
Friction × × × ×
Wear × × × × ×
Sound × ×
Particles ×
SUMMARY OF PAPERS
16
2 Summary of appended papers
This thesis comprises six appended papers (Appendices A to
F) that fulfilled the objectives of the thesis work. A summary
of the six appended papers is presented below.
Papers A and B describe a test stand for measuring the
friction and wear at the wheel–rail contact during varied
temperature and humidity levels. This test stand was put in a
well-controlled climate chamber isolated from its
surroundings. Paper C applied a professional acoustic
measuring technique in this climate chamber for excluding
background sound from the surroundings. Paper D
complemented this technique and reformulated it for a
running metro train so a full-scale field test could be
conducted. Paper E improved the test stand described in
Papers A and B and used it to measure the particle emissions
from wheel tread–brake block contact. Paper F conducted a
literature review of recently published studies on open
system tribology at the wheel–rail contact and suggested
possible directions for future research.
Paper A presents a study on the effect of commonly seen
environmental conditions (temperature and humidity) and
pre-treated oxide layers on the wear of wheel–rail contact.
Commercial railway wheels and rails are used as testing
materials in this pin-on-disc study. The results indicate that
wear mechanisms at the wheel–rail contact can be affected
by various environmental conditions and the presence of
oxide layers. Unoxidized wheel–rail contact in dry air was
dominated by adhesive wear. With a decrease in
temperature, abrasion was aggravated and increased the
wear rate. In humid air, wheel–rail contact underwent both
SUMMARY OF PAPERS
17
adhesive and oxidative wear and the self-generated oxide
layers prevented the materials from severe wear. Oxidized
wheel–rail contact is mainly subjected to abrasive wear.
Paper B applied the same test stand as used in Paper A,
which extended the testing temperature down to -35 °C and
added snow instead of humid air. The added snow particles
were found to melt into liquid-like layers due to pressure
melting, encouraging the formation of oxide flakes at the
contact path. With snow presence, the wear at the wheel–rail
contact is insensitive to change of temperature. When snow
was absent, the wheel–rail contact experienced extremely
high friction and wear around -15 °C, owing to the high
brittleness at this temperature. When the temperature
decreased further to -25 °C, a condensed ice layer at the
wheel–rail contact was prone to develop, which acted
similarly to the added snow particles, promoting the flake-
like oxide layers and relieving the wear process.
Paper C added professional acoustic measurement
instruments into similar test stands as used in Papers A and
B, but focused on noise generation. Five different surface
finishing features were tested against their influence on
friction, wear and sound generation. Surface features
perpendicular to the sliding direction generated the lowest
noise level for the whole duration of the test, and a smooth
surface emitted the highest noise level. Meanwhile, an
increase in noise level of 10 dB was always seen when the
wear transitioned from mild wear to severe wear and from
severe wear to catastrophic wear. A broader bandwidth of
sound amplitude was also accompanied with the wear
transition.
SUMMARY OF PAPERS
18
Paper D reformulated the test stand used in Paper C to be
used for the wheel–rail contact of a metro train. This
equipped train was run repeatedly in a 200 m radius test
depot to aggravate the wear transition from mild wear to
severe wear. The results presented similar trends to seen in
the previous lab-scale test (Paper C) in that mild to severe
transition always takes place, with an increase in sound
pressure and broadening of the sound pressure amplitude
distribution.
Paper E employed a pin-on-disc machine placed in a one-
way ventilated chamber to study the friction, wear and
airborne particle emission from three commercial railway
brake block materials within different levels of relative
humidity. The results suggested that cast iron yielded the
highest level of friction, wear and particle concentration, and
organic composite the lowest. Relative humidity also had a
notable effect on the friction, wear and particle emission of a
specific brake-block material.
Paper F reviewed most documented studies of open system
tribology at the wheel–rail contact, covering four important
tribological parameters in the railway system: friction, wear
and sound and particle emissions. In the friction part,
environmental conditions such as temperature, humidity,
natural and artificial contaminants are discussed, while
different wear mechanisms lead the wear part. Promising
trends for future research are suggested to investigate
friction, wear and sound and particle emissions
simultaneously, instead of separately, which is the case in
most studies.
DISCUSSION
19
3 Discussion
3.1 Discussion on appended papers
This thesis aimed to investigate tribological performance at
the wheel–rail contact and wheel–brake contact and reveal
the physical and chemical fundamentals responsible.
Further, the importance of the open system on investigations
of tribology was highlighted, something that is usually
neglected in most cases. In the thesis work, a well-controlled
test stand was designed and implemented for testing the
friction and wear at the wheel–rail contact and wheel–brake
contact, which was also updated for measuring noise and
particle emissions. In this section, the main results achieved
from the thesis work will be discussed according to the
sequence of the appended papers.
Papers A and B investigated the strong influence of
temperature, relative humidity and water on the wear rate of
oxidized and unoxidized wheel–rail contact. One of the key
findings is that an unoxidized wheel–rail contact in a dry
environment largely depends on the temperature, due to the
dominant adhesive wear mechanism. According to the SEM
observations, the adhesive characteristics become more
serious due to ductile-brittle transition in decreasing
temperatures from 20 °C to -15 °C, where micro adhesion
joints deteriorated to large area delamination (spalling of
bulk scraps). However, the condensed ice layer at
temperatures below -15 °C prevented further increase of the
wear rate due to the pressure melting phenomenon [78]. The
possibility of ice condensation at the wheel–rail contact was
confirmed by calculating the saturation vapour pressure at
DISCUSSION
20
low temperatures, which suggested high potential for an ice
condensation layer to form at temperatures below -15 °C [79].
The condensed ice layer encouraged the formation of a flake-
like oxide layer, protecting the surfaces from severe wear
[80]. When the wheel–rail contact is in a damp environment
or water droplets are present, a flake-like oxide layer also
forms in a similar way and prevents severe wear. The pre-
oxidized wheel–rail contact is found to be mainly dominated
by abrasive wear, where the oxide islands sheared off the
surface and then pulverized into tiny pieces of debris,
aggravating the wear process.
In paper C, two rail surface grinding features
(perpendicular and parallel to sliding direction) with two
surface roughness values (rough 0.9 μm Ra and medium 0.4
μm Ra) were compared with a polished surface 0.04 μm Ra,
with regards to the friction, wear and noise generation. This
test design was inspired by a previous study suggesting that
different surface manufacturing has an influence on rolling
noise [81]. Since the surfaces were pre-oxidized, as in the real
cases of operating tracks, the “initial stage”, where the pre-
oxidized layer was not worn off, was likely to be controlled by
ploughing and the featured surfaces (perpendicular and
parallel to sliding direction) yielded a higher friction
coefficient than the polished contact. After the pre-oxidized
layer was worn off, the polished surface started to lead the
friction and wear owing to the high level of adhesion, a result
that conforms to previous studies [82, 83]. Another
phenomenon observed in this study is the wear regime
transitions from mild to severe and from severe to
catastrophic were always accompanied by a sound level
increase of 10 dB and a broadening of sound amplitude
DISCUSSION
21
probability distribution. This test method was verified by
using two different test systems, and the achieved results
from each conformed to each other. Paper D complemented
the measuring technique described in paper C by applying it
to a metro train, which was run in a 200 m radius test curve
to provoke wear and noise generation. Similar results to
paper C were seen, in that the wear regime transition went
along with the increase of sound pressure, encouraging a
further validation of this sound-based technique for wear
conditions of wheel–rail contact in real railway traffic
systems.
Paper E studied the friction, wear and particle emissions
from three commercial railway brake-block materials under
different relative humidity levels, among which cast iron is
dominantly applied. Since the water vapour in the air also
can be detected by the particle counter, only an optical
particle sizer with a relatively large measuring range (0.3–10
μm) was used in this study. Cast iron suggested the highest
friction coefficient, wear rate and particle concentration at all
three relative humidity levels (25%, 50% and 75%), while
organic composite suggested the lowest. Vernersson et al.
also found that cast iron brake blocks yielded a wear rate ten
times higher than that of composite and sintered materials
[84]. A higher particle emission rate for cast iron compared
to composite brake block was observed in [4]. The high
content of Fe (96 wt. %) in cast iron is responsible for the
highest friction, wear loss and particle concentration due to
the high adhesion with its counterpart, the railway steel
wheel. The organic composite brake block is composed of
some 20 ingredients whose properties largely differ from
each other. Some ingredients are prone to adsorb moisture,
DISCUSSION
22
leading to a decline of friction, wear and particle emission. A
very low friction coefficient (less than 0.4) was observed on
organic composite brake block in humid air (50% and 75%
relative humidity), making it an unreliable material for the
complex working conditions of railway brake blocks. It
should also be noted that the current study used a relatively
low contact pressure and sliding speed, which is a reason for
the low particle emissions of organic composite material.
Further study with more hostile conditions is suggested to
check the performance of organic composite at evaluated
temperatures.
A comprehensive literature review on the topic of open
system tribology in the wheel–rail contact was carried out in
Paper F. No similar survey of the state-of-the-art on this
topic was found in the subject documentation. Commonly
seen environmental conditions, e.g. temperature, humidity,
natural and artificial contaminants were discussed in relation
to the friction, wear and sound and particle emissions at the
wheel–rail contact. Since Paper F is a review paper, more
detailed discussion can be found in the paper and will not be
repeated here.
3.2 Discussion on the validity of the results
This thesis comprises four lab-scale experimental studies
and one full-scale experimental study. Some of the observed
results in these studies are validated by reported phenomena
in the railway system. For example, paper B has found that
the wear rate at the wheel–rail contact generally increased
with a decreasing temperature from 3 to -15 °C; this is
confirmed by Green Cargo® (Swedish rail logistic operator),
DISCUSSION
23
who report that the wheel experiences a wear rate that is ten
times higher in winter in the north Sweden area compared
with that in summer. Bombardier® (a train manufacturer)
also report similar phenomena in their servicing of heavy
haul locomotives. Another finding in paper B is that at -35 °C,
an ice condensation layer is prone to form on wheel and rail
surfaces, which decreases the friction and wear. In a meeting
with the Swedish Welding Commission AG 60 Rail welding,
the members confirmed the observation that a thin ice
condensation layer often appears on the rail surface during
winter at temperatures below -15°C.
Paper D is a full-scale experiment for validating the
findings in paper C. In the paper, a wear transition from mild
to severe wear, and from severe to catastrophic wear, always
goes with an increase of generated sound pressure of 10 dB
and a broadening of the sound amplitude probability
distribution. This sound-based detecting technique shows
great potential for monitoring the mild to severe wear
transition in railway traffic and is now used in the Stockholm
metro system (on six trains) to determine maintenance
actions.
3.3 Contribution and impact of the thesis
In the past, people used to study the tribology at the wheel–
rail contact and wheel tread–brake block contact in an
ambient environment but the influences of temperature and
relative humidity were neglected. The research work
contained in this thesis found that different environmental
conditions (e.g. temperature, relative humidity, snow
particles) had a strong influence on the tribology at the
DISCUSSION
24
wheel–rail contact and wheel tread–brake block contact,
indicating that open system tribological contacts should be
tested in the conditions in which they are used.
Papers A and B have found that temperature and relative
humidity have a strong influence on the tribology at the
wheel-rail contact. This is of great importance to the railway
industry, which can choose suitable wheel grades for
operation in different seasons to reduce operation costs.
Papers C and D examined the sound generated and the wear
regime at the wheel–rail contact. A sound-based wear
detection technique has been developed and is now used on
six trains in the Stockholm metro system to determine
maintenance actions. Paper E has demonstrated a high level
of airborne particle emission from cast iron brake blocks,
indicating a need to develop alternative materials. Although
these findings are achieved in KTH and based on the railway
operation in Sweden, they can be beneficial to railway
operators in other areas where railway vehicles are working
in an open system. This thesis is strongly related to the
environment and one of its main focuses is airborne particle
emission. Therefore, it can bring some underlying research
projects from international, national or local foundations
whose aims include environmentally friendly development.
CONCLUSIONS
25
4 Conclusions
This thesis uses experimental methods to investigate two common
open system tribological contacts in the railway system with
regards to friction, wear and sound and particle emissions, both in
lab- and full-scale environments. A thorough literature review has
been carried out on the state of the art of open system tribology at
wheel–rail contact. Main conclusions have been drawn, as follows:
The pin-on-disc test stand placed in a well-controlled climate
chamber is a reliable and efficient method to study the tribology
of varied materials within different environmental conditions.
With this method, not only the temperature and relative
humidity can be controlled, but also plenty of contaminants
such as snow, water and friction additives can be applied.
Temperature and humidity are proven to strongly influence the
friction and wear at wheel–rail contact, by generating different
forms of oxide layers.
If this test stand is improved with professional acoustic
measuring equipment, it can be used to study the sound
generation from different tribological contacts.
Surface finishing features on the rail surface are found to affect
the sound level emitted from the wheel–rail contact; a surface
feature perpendicular to the sliding direction generated less
noise than a parallel feature and smooth surface.
Both lab- and full-scale tests demonstrated that a wear
transition from mild to severe wear is always accompanied by
an increase in noise level and a broadening of noise amplitude
distribution.
Promising research within open system tribology in the future
should take all tribological parameters (friction, wear, sound
and particle emissions) into account simultaneously.
Environmentally friendly additives should also be encouraged.
FUTURE WORK
26
5 Future work
This thesis focused on the commonly seen environmental
parameters (namely temperature, humidity, water) on the
tribology at the wheel–rail and wheel–brake contacts with regards
to friction, wear and noise and airborne particle emissions. The
railway system operates in a complex environment where some
other questions are still open. Therefore, some suggestions for
future research are given by the author:
One imperative trend for future study is to take all tribological
parameters (friction, wear, lubrication, sound and particle
emissions) into account simultaneously, rather than
independently, as is currently the case.
Environmentally friendly lubricants (e.g. those that are water-
based) should be investigated more, instead of traditional oil-
and grease-based ones.
More reliable and complete models for the open system
tribology at the wheel–rail and wheel–brake contacts should be
built to adapt to the complicated working environment of the
railway system.
Most documented studies on open system tribology are at lab
scale for accurately controlling environmental conditions. More
full-scale tests are suggested for validating the findings from
lab-scale studies.
This thesis only considered the micro-sized airborne particles
from wheel–brake contact. However, nano-sized particles are of
great interest as they have been found to form at higher contact
temperatures and adversely affect the health of human beings,
so more exhaustive research on this topic is encouraged.
REFERENCES
27
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