Tribology: The Science of Interacting Surfaces
MET377
Surface Engineering
Tribology
What is
tribology?
•
Tribology is defined as the science and technology
of interacting surfaces in relative motion, having its
origin in the Greek word
tribos
meaning rubbing.
•
It is a study of the friction, lubrication, and wear of
engineering surfaces
with a view to
understanding
surface interactions in detail and then
prescribing
improvements in given applications.
What is
tribology?
•
Economically very
important
-- 6%
GDP
(Jost)
•
Probably more failures are caused by tribological
problems than fracture, fatigue, plastic
deformation,
etc.
•
Tribological problems are often related to systems
issues.
Four Elements of
Tribology
•
Surface interactions with its environment, including
lubrication and
lubricants
•
Generation and transmission of forces at the
interface
•
Response of materials to the force generated at the
interface
•
Design of tribological
systems
Some of the Basic
Questions
•
What is friction?
•
How is the friction force
generated?
•
What is the coefficient of
friction?
•
How do materials
wear?
•
What is the effect of the applied load on friction and
wear?
•
What is the role of
lubricant?
•
How do you
lower
friction?
•
How should we reduce the wear rate of
materials?
What
is
friction?
•
Friction is
a
result of energy dissipation at the (sliding)
interface.
Friction is
affected by the
following:
1.
2.
3.
4.
5.
6.
7.
Presence of
wear
particles and externally
introduced
particles at the sliding
interface
Relative hardness of
the materials
in
contact
Externally applied
load
and/or
displacement
Environmental conditions such
as
temperature and
lubricants
Surface
topography
Microstructure
or
morphology
of
materials
Apparent contact
area
Atomic Arrangement
of the Solid
Surface
(to
lower
the free
energy
of
formation)
(a)
reconstruction,
(b) segregation, (c)
chemisorption,
(d) formation of compounds, (e)
physisorption,
(f)
nucleation
of atoms on the
surface
Figure by
MIT OCW. After Estrup,
1975.
(a)
(b)
(c)
(d)
Chemical
and Physical State
of the Solid
Surface
General Characteristics of
Metals
1.
Metallic
bonding
2.
Structure -- fcc, bcc,
hcp
3.
Defects control mechanical
properties.
•
Dislocations
•
Vacancies
4.
Higher energy state at the surface -- Surface energy
5.
Importance of
microstructure
6.
Alloys
--
substitutional,
interstitial,
multiphase, dispersion
strengthened,
7.
Reactive -- oxides, carbides,
intermetallics
General Characteristics of
Polymers
1.
Covalently bonded
long chain
molecules
2.
Thermoplastics, thermosets, elastomers,
3.
Linear polymers -- HDPE,
PTFE
4.
Semicrystalline vs amorphous
polymers
5.
Low melting point, glass transition
temperature
6.
Difference in molecular weight at the surface
and the
bulk
= f(nucleation
conditions)
General Characteristics of
Ceramics
1.
Mostly ionic
bonding, some have
covalent
bonding
2.
Combination
of metallic and
nonmetallic
elements
3.
Oxides and nitrides are very stable -- Low
free energy of
formation
4.
Carbides are very hard and have very high
temperature
5.
Brittle
6.
Electrically non-conducting
7.
Abrasives
General Characteristics of
Composites
1.
Typically resin
+
fibers or filler or
both
2.
Fiber orientation
important
3.
May be designed to achieve specific
properties
4.
Used without
lubricants
General
Characteristics
of a Solid
Surface
1.
Surface
energy
2.
Surface
may
be different
from the
bulk
in
atomic structure, mechanical properties,
chemical
state
3.
Wear resistance
4.
Corrosion resistance
5.
Hardness
Role of
lubricants
1.
Change surface energy
(monolayer)
2.
Reduce
metal to metal
contact
through wetting
3.
Prevent particle
agglomeration
through wetting
Lubrication
Lubrication
is the process or technique employed to reduce
wear of one or both surfaces in close proximity, and moving
relative to each another, by interposing a substance called
lubricant between the surfaces to carry or to help carry the load
(pressure generated) between the opposing surfaces.
Mechanical Properties of the
Surface
1.
Is the surface harder or
softer
than the
bulk?
2.
Does it matter?
Oxide
Me
tal
What is friction coefficient?
varies as
a
function of the sliding distance.
S
c
a
l
e
i
s
s
u
e
s
i
n
t
r
i
b
o
l
o
g
y
Scales
in
Tribology
and
Typical
Values
Scale
Range
of
friction
coefficient
(
)
Applications
& wear
coefficient
(k)
10
-10
m
=
0.001~0.6
k ~
0
AFM
lithography
Friction at
Dry Sliding
Interface
•
Plowing
Mechanism
•
Particle Agglomeration
•
Height of Agglomerated
Particles
•
Friction
Coefficient
and the
Number
of
Agglomerated
Particles
•
Reduction
of Friction
by
Elimination
of
Particles
Effect of Boundary
Lubrication
~ 0.1
•
Cause?
–
Plowing
•
What is the role of a lubricant?
–
Lower
shear
stress
–
Transport
particles
–
Prevent
particle
agglomeration
–
Prevent adhesion
Introduction to
Wear
•
Plastic deformation at the interface often leads to
wear, i.e., deformation induced
wear.
•
Wear can also be caused by chemical
processes.
•
There are many different kinds of wear
mechanisms
The process leading to loss of material is known as
"wear“
Types of wear
Adhesive wear
Abrasive wear
Surface fatigue
Fretting wear
Adhesive wear
Adhesive wear are caused by relative
motion, "direct contact" and plastic deformation
which create wear debris and material transfer
from one surface to another.
Example of Adhesive Wear:
•
Chalk on board-while writing
Abrasive Wear
Abrasive wear occurs when a hard rough
surface slides across a softer surface. ASTM
International (formerly American Society for Testing
and Materials) defines it as the loss of material due
to hard particles or hard protuberances that are
forced against and move along a solid surface.
Types of Abrasive wear
Abrasive wear is commonly classified according to the type of contact and the
contact environment
The two modes of abrasive wear are known as two-body and three-body
abrasive wear
Two-body wear occurs when the grits or hard particles remove material from the
opposite surface.
Three-body wear occurs when the particles are not constrained, and are free to
roll and slide down a surface.
Two-body wear Three-body wear
Erosive Wear
Erosive wear can be described as an extremely short sliding motion
and is executed within a short time interval. Erosive wear is caused
by the impact of particles of solid or liquid against the surface of an
object.
Fretting wear
Fretting is the repeated cyclical rubbing between
two surfaces, which is known as fretting, over a period of
time which will remove material from one or both
surfaces in contact
Wear
Mechanisms
Friction and Wear of Polymers and
Composites
•
Why do we use polymeric
bearings?
Low
friction
No need
to
lubricate
Bio-compatible
Ease of manufacturing
Low
noise
Low
cost
Applications
of Polymeric
Bearings
•
Industrial
applications
Gears
Ball bearing
cages
Journal bearings
Sliders
Cams
Seals for shafts,
etc.
Tribological
Applications
of Polymeric
“Bearings” in Medicine and Related
Areas
•
Bio-medical
applications
Valves
Hip
joints
Knee
joints
Pump
components
Common
Plastics Used in
Tribology
•
Thermoplastics (with and
without
fibers)
polyethylene
(PE)
Ultra-high molecular weight
PE
Polyoxymethylene (POM, acetal) -- “Delrin
Polytetrafluoroethylene
(PTFE)
Polyamide
(nylon)
Polycarbonate
Common
Plastics Used in
Tribology
•
Thermosetting plastics (with and without
fibers)
polyurethane
phenolics
polyester
phenolics
polyimide
Common
Plastics Used in
Tribology
•
Elastomers
(reinforced with
carbon
or
fibers)
silicone rubber -- medical applications
natural
rubber
polybutadiene
rubber
--
tires
nitrile rubber -- good resistance to
oil
PTFE
•
Highly linear
•
Relatively weak inter-molecular
force
•
Easy transfer of molecules to the counter
face
•
Consequently -- low
=0.09
Highly Linear
Polymers
Friction and Wear Mechanisms of
PTFE
•
Process
1.
Deformation of
molecules
near
the
surface due
to the
applied
force
at the
surface
2.
Stretching of molecules, orienting them along the sliding
direction
3.
Transfer
of
thin
films of 50
to
200
A
thick
4.
Sliding of PTFE on
PTFE
Friction and Wear Mechanisms of Other
Ductile Thermoplastics
(LDPE, PP,
PMMA)
•
LDPE
(Low-density polyethylene)
and
PP
(ductile)
–
Wear particles are thick and
lumpy
debris
–
Less
elongation
•
PMMA
(Poly(methyl methacrylate))
(brittle)
–
Cracks
can
develop at the surface
Basic Mechanism of Friction in
Polymers
•
Viscoelastic-plastic deformation at the sliding
interface
•
Plowing
•
Asperity
deformation
•
Wear particle deformation
Conclusions
1.
Friction
is
a manifestation
of
the
energy
consumed
when
two surfaces
in
contact slide relative to
each
other --
with
and
without
the normal
load.
2.
F
riction
is
caused by plowing, adhesion and
asperity
removal.
3.
In a majority of engineering applications that
involve
a
metal surface sliding
against another,
the friction force
is generated by plowing of the
surfaces by wear
particles.
The friction force
is also generated by
the
work done to shear asperities and in some rare
cases,
by
the
adhesion between
the two contacting
surfaces.
Conclusions
4.
Friction
is not an inherent
material
property.
It
depends on the relative hardness
of
materials that
are
sliding
against each other.
The friction
is
the
highest
when
the two surfaces
have exactly
the
same
hardness.
5.
Removal of wear particles by
the
use of undulated
surface reduces the coefficient
of
friction to a
level of
boundary lubricated cases with boundary
lubricants.
6.
Boundary lubricants lower the friction coefficient
by
preventing wear particle agglomeration and plowing,
but
still there
is
a
metal-to-metal
contact,
which leads
to plowing and the observed
coefficient of friction of
about
0.1.
Conclusions
7.
Polymers are used extensively
in
diverse applications
because of their unique tribological properties. For
instance, highly linear polymers have
low
coefficients
of
friction.
8.
Composites can be made with polymeric materials
and
fibers
or
fillers to satisfy a specific set
of
functional
requirements.
Questions for self-assessment
1. How can performance of an engineering component
be related with service
life?
2. Describe common causes of deterioration in
performance of material.
3. What are the common types of wear experienced by
metals?
4. Explain the mechanisms of adhesive wear.
5. Describe factors affecting adhesive wear.
6. How do service conditions affect the adhesive wear?
7. Explain classical law of adhesive wear.
8. What is abrasive wear? Describe mechanisms of
abrasive wear.
9. Explain the factors affecting abrasive wear of metals.
10. How do material properties affect the erosive wear?
11. Explain mechanism of erosive wear.
12. Describe the factors affecting erosive wear of
metals.
Chapter 2
Tribology is the study of friction, lubrication, and wear of surfaces in relative motion. It plays a crucial role in various industrial applications, with economic significance and implications on failure rates in engineering systems. The discipline focuses on surface interactions, forces transmission, material response, and system design, addressing essential questions about friction, wear, lubrication, and surface conditions that influence these phenomena.
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Presentation Transcript
MET377 Surface Engineering Tribology
What is tribology? Tribology is defined as the science and technology of interacting surfaces in relative motion, having its origin in the Greek word tribos meaning rubbing. It is a study of the friction, lubrication, and wear of engineering surfaces with a view to understanding surface interactions in detail and then prescribing improvements in given applications.
What is tribology? Economically very important -- 6% GDP(Jost) Probably more failures are caused by tribological problems than fracture, fatigue, plastic deformation, etc. Tribological problems are often related to systems issues.
Four Elements of Tribology Surface interactions with its environment, including lubrication and lubricants Generation and transmission of forces at the interface Response of materials to the force generated at the interface Design of tribological systems
Some of the Basic Questions What is friction? How is the friction force generated? What is the coefficient of friction? How do materials wear? What is the effect of the applied load on friction and wear? What is the role of lubricant? How do you lower friction? How should we reduce the wear rate of materials?
What is friction? Friction is a result of energy dissipation at the (sliding) interface. Friction is affected by the following: 1. Presence of wear particles and externally introduced particles at the sliding interface Relative hardness of the materials in contact Externally applied load and/or displacement Environmental conditions such as temperature and lubricants Surface topography Microstructure or morphology of materials Apparent contact area 2. 3. 4. 5. 6. 7.
Chemical and Physical State of the Solid Surface Atomic Arrangement of the Solid Surface (to lower the free energy of formation) (a) reconstruction, (b) segregation, (c) chemisorption, (d) formation of compounds, (e) physisorption, (f) nucleation of atoms on the surface (a) (b) (c) (d) Figure by MIT OCW. After Estrup, 1975.
General Characteristics of Metals 1. Metallic bonding 2. Structure -- fcc, bcc, hcp 3. Defects control mechanical properties. Dislocations Vacancies 4. Higher energy state at the surface -- Surface energy 5. Importance of microstructure 6. Alloys -- substitutional, interstitial, multiphase, dispersion strengthened, 7. Reactive -- oxides, carbides,intermetallics
General Characteristics of Polymers 1. 2. 3. 4. 5. Covalently bonded long chain molecules Thermoplastics, thermosets, elastomers, Linear polymers -- HDPE, PTFE Semicrystalline vs amorphous polymers Low melting point, glass transition temperature Difference in molecular weight at the surface and the bulk = f(nucleation conditions) 6.
General Characteristics of Ceramics 1. Mostly ionic bonding, some have covalent bonding Combination of metallic and nonmetallic elements Oxides and nitrides are very stable -- Low free energy of formation Carbides are very hard and have very high temperature Brittle Electrically non-conducting Abrasives 2. 3. 4. 5. 6. 7.
General Characteristics of Composites 1. 2. 3. 4. Typically resin + fibers or filler or both Fiber orientation important May be designed to achieve specific properties Used without lubricants
General Characteristics of a Solid Surface 1. 2. Surface energy Surface may be different from the bulk in atomic structure, mechanical properties, chemical state Wear resistance Corrosion resistance Hardness 3. 4. 5.
Lubrication Lubrication is the process or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each another, by interposing a substance called lubricant between the surfaces to carry or to help carry the load (pressure generated) between the opposing surfaces. Role of lubricants 1. Change surface energy (monolayer) 2. Reduce metal to metal contact through wetting 3. Prevent particle agglomeration through wetting
Mechanical Properties of the Surface 1. Is the surface harder or softer than the bulk? 2. Does it matter? Oxide Metal
What is friction coefficient? 1 varies as a function of the sliding distance. 0.6 0.2 0 20 40 60 80 0 Distance slid( m)
Scale issues in tribology Scales in Tribology and TypicalValues Range of friction coefficient( ) & wear coefficient(k) Scale Applications =0.1~1 k =10-5~10-2 10-4m machinery brake,tools =0.001~0.2 k=10-7~10-5 10-6m lubrication roller bearing =0.01~0.6 k=10-8~10-5 10-8m head/ disk MEMS =0.001~0.6 k ~0 10-10m AFM lithography
Friction at Dry Sliding Interface Plowing Mechanism Particle Agglomeration Height of Agglomerated Particles Friction Coefficient and the Number of Agglomerated Particles Reduction of Friction by Elimination of Particles
Effect of Boundary Lubrication ~ 0.1 Cause? Plowing What is the role of a lubricant? Lower shear stress Transportparticles Prevent particle agglomeration Prevent adhesion
Introduction to Wear Plastic deformation at the interface often leads to wear, i.e., deformation induced wear. Wear can also be caused by chemical processes. There are many different kinds of wear mechanisms The process leading to loss of material is known as "wear Types of wear Adhesive wear Abrasive wear Surface fatigue Fretting wear
Adhesive wear Adhesive wear are caused by relative motion, "direct contact" and plastic deformation which create wear debris and material transfer from one surface to another. Example of Adhesive Wear: Chalk on board-while writing
Abrasive Wear Abrasive wear occurs when a hard rough surface slides across a softer surface. ASTM International (formerly American Society for Testing and Materials) defines it as the loss of material due to hard particles or hard protuberances that are forced against and move along a solid surface.
Types of Abrasive wear Abrasive wear is commonly classified according to the type of contact and the contact environment The two modes of abrasive wear are known as two-body and three-body abrasive wear Two-body wear occurs when the grits or hard particles remove material from the opposite surface. Three-body wear occurs when the particles are not constrained, and are free to roll and slide down a surface. Two-body wear Three-body wear
Erosive Wear Erosive wear can be described as an extremely short sliding motion and is executed within a short time interval. Erosive wear is caused by the impact of particles of solid or liquid against the surface of an object.
Fretting wear Fretting is the repeated cyclical rubbing between two surfaces, which is known as fretting, over a period of time which will remove material from one or both surfaces in contact
Wear Mechanisms Classification WearMechanisms Wear coefficient K(range) 10-4 10-4 10-4 10-4 10-2to10-1 10 -6 to10-4 Wear dominated by mechanical Behavior ofMaterials 1.Asperity deformation and removal 2.Wearcausedby plowing 3.Delaminationwear 4.Adhesive wear 5.Abrasivewear 6.Frettingwear 7.Wear by solid pa rticleimpingement 1.Solut ionwear 2.Oxidat ionwear 3. Diffusionwear 4.Wearbymelting ofthesurfacelayer 5.Adhesive wear at high temperatures Wear dominated by chemical behavior of materials
Friction and Wear of Polymers and Composites Why do we use polymeric bearings? Low friction No need to lubricate Bio-compatible Ease of manufacturing Low noise Low cost
Applications of Polymeric Bearings Industrial applications Gears Ball bearing cages Journal bearings Sliders Cams Seals for shafts, etc. Tribological Applications of Polymeric Bearings in Medicine and Related Areas Bio-medical applications Valves Hip joints Knee joints Pump components
Common Plastics Used in Tribology Thermoplastics (with and without fibers) polyethylene (PE) Ultra-high molecular weight PE Polyoxymethylene (POM, acetal) -- Delrin Polytetrafluoroethylene (PTFE) Polyamide(nylon) Polycarbonate
Common Plastics Used in Tribology Thermosetting plastics (with and without fibers) polyurethane phenolics polyester phenolics polyimide
Common Plastics Used in Tribology Elastomers (reinforced with carbon or fibers) silicone rubber -- medical applications natural rubber polybutadiene rubber -- tires nitrile rubber -- good resistance to oil
Highly Linear Polymers PTFE Highly linear Relatively weak inter-molecular force Easy transfer of molecules to the counter face Consequently -- low =0.09
Friction and Wear Mechanisms of PTFE Process 1. Deformation of molecules near the surface due to the applied force at the surface 2. Stretching of molecules, orienting them along the sliding direction 3. Transfer of thin films of 50 to 200 Athick 4. Sliding of PTFE on PTFE
Friction and Wear Mechanisms of Other Ductile Thermoplastics (LDPE, PP, PMMA) LDPE (Low-density polyethylene) and PP (ductile) Wear particles are thick and lumpy debris Less elongation PMMA(Poly(methyl methacrylate)) (brittle) Cracks can develop at the surface
Basic Mechanism of Friction in Polymers Viscoelastic-plastic deformation at the sliding interface Plowing Asperity deformation Wear particle deformation
Conclusions 1. Friction is a manifestation of the energy consumed when two surfaces in contact slide relative to each other -- with and without the normal load. 2. Friction is caused by plowing, adhesion and asperity removal. 3. In a majority of engineering applications that involve a metal surface sliding against another, the friction force is generated by plowing of the surfaces by wear particles. The friction force is also generated by the work done to shear asperities and in some rare cases, by the adhesion between the two contacting surfaces.
Conclusions 4. Friction is not an inherent material property. It depends on the relative hardness of materials that are sliding against each other. The friction is the highest when the two surfaces have exactly the same hardness. 5. Removal of wear particles by the use of undulated surface reduces the coefficient of friction to a level of boundary lubricated cases with boundary lubricants. 6. Boundary lubricants lower the friction coefficient by preventing wear particle agglomeration and plowing, but still there is a metal-to-metal contact, which leads to plowing and the observed coefficient of friction of about 0.1.
Conclusions 7. Polymers are used extensively in diverse applications because of their unique tribological properties. For instance, highly linear polymers have low coefficients of friction. 8. Composites can be made with polymeric materials and fibers or fillers to satisfy a specific set of functional requirements.
Questions for self-assessment Chapter 2 1. How can performance of an engineering component be related with service life? 2. Describe common causes of deterioration in performance of material. 3. What are the common types of wear experienced by metals? 4. Explain the mechanisms of adhesive wear. 5. Describe factors affecting adhesive wear. 6. How do service conditions affect the adhesive wear? 7. Explain classical law of adhesive wear. 8. What is abrasive wear? Describe mechanisms of abrasive wear. 9. Explain the factors affecting abrasive wear of metals. 10. How do material properties affect the erosive wear? 11. Explain mechanism of erosive wear. 12. Describe the factors affecting erosive wear of metals.
