# Tribology of Lubricants

```Tribology
www.anton-paar.com
Outline
 Basics of Tribology: Introduction and definitions
 The different types of friction and wear
 Lubricants and surface treatments
 Rheometry and Tribometry: The Rheo-Tribometer
 Measurements on the Rheo-Tribometer
• Stribeck curve
• Static friction tests
• Stick slip measurements
 Rolling element bearing device
 Measurements
 Food tribology
2
Friction: Good or bad?
Problems related to friction
 A lot of energy is wasted by friction related heat production e.g in cars, wind
wheels etc. (No perpetum mobile)
 Friction reduces durability of material due to wear
 Functionality loss and annoyance due to noise
 Huge sums of money are lost because of friction and wear
Benefits related to friction
 Friction is required for functionality of material (screws, tires, brakes etc.)
 Friction enables music (bow instruments)
 Friction is required for a lot of todays applications
3
Different Types of Friction
Static Friction
Static frictional forces from the
interlocking of the irregularities of two
solid surfaces will increase
preventing any relative motion until
some limit where motion occurs.
Sliding friction
Sliding friction is when two solid
surfaces slide against each other.
Rolling friction
When a body rolls on a surface, the
force resisting the motion is termed
rolling friction.
Static Friction > Sliding Friction > Rolling Friction
4
Tribology: Friction-Coefficient
Friction Force FF
Sliding
Body 1
Body 2
Friction Coefficient:
µ = FF / FL
 µ is dimensionless,
 often f is used instead of µ
Amontons Laws:
 FF is independent on area
 FF ~ FL
Range of Friction Coefficients:
Dry sliding:
0.05 (PTFE under high loads) – 5.0 (gold sliding in vacuum). Steel: 0.3 - 0.6.
Lubricated sliding:
0.03 (hydrodynamic conditions) – 0.15 (boundary conditions)
Rolling friction:
0.002 (fully lubricated) – 0.05 (running dry)
5
Parameters influencing friction
Friction Partners
Lubricant
• Surface parameters
• Chemistry
• Chemical reactivity
• Viscosity
• Elasticity
• Stability
• Etc...
Environmental Conditions:
• Temperature
• Etc...
System Factors:
• Relative Speed
• Time
• Direction of movement :
• unidirectional
• bi-, multidirectional
• Etc...
• Humitidity
6
Tribological
System
Wear
Definition:
Wear is the erosion of a solid surface by the
action of another surface.
There are four principle of wear process:
When two solid surfaces slide over another the
asperities are plastically deformed and eventually
welded together. As sliding continues, these
bonds are broken, producing cavities on the
surface and abrasive particle which contribute to
future wear of surfaces
Abrasive wear
When material is removed by contact with hard
particles, abrasive wear occurs. The particles
either may be present at the surface of a second
material or may exist as loose particles between
two surfaces
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Wear
Corrosive wear
Corrosive wear is deterioration of useful properties in
a material due to reactions with its environment (e.g.
oxidation).
Surface fatigue
Surface fatigue is a process by which the surface of
a material is weakened by cyclic loading.
Parameters influencing wear:
 Wear partners: Material, hardness, surface roughness, shape, friction coefficient
 Particles: Chemistry, particle size
 Movement: Sliding, rolling pushing, uni- or bi-directional
 Environmental conditions: Air, inertgas, vacuum, humidity
 Temperature
 Sliding distance
8
Friction coefficient
Lubrication: The Stribeck curve / Speed ramp
1
5
2
4
Richard Stribeck
1861-1950
3
Sliding speed
9
1
Static friction, no movement, no wear
2
Boundery lubrication, very low sliding speed, solid contacts, high wear
3
Mixed lubrication, moderate sliding speed, partial solid contact, moderate wear
4
Elasto-hydrodynamic lubrication, intermediate sliding speed, thin lubrication film
5
Hydrodynamic librication, high sliding speed, developed lubrication film, no wear
Static Friction, Boundery Lubrication
1) Static Friction
 Direct solid contact of the friction
partners
 No movement
 No wear
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2) Boundery Lubrication
 Force is high enough to induce a very low speed
 Solid friction, Stick slip
 Load is carried by the contact points (asperities)
and the shear is taken by the absorbed
lubrication molecules.
 No hydrodynamic pressure build up
 No lubrication film present
 Asperities are protected by adsorbed lubricant
molecules and/or a thin oxide layer.
 High wear
Mixed Lubrication, Elasto-Hydrodynamic
Lubrication
3) Mixed Lubrication
 Low speed
 Low hydrodynamic pressure is build up in
the lubricant
 The loading is carried by a combination of
the hydrodynamic pressure and the
contact pressure between the asperities of
both surfaces.
 Lubricant film only in between contact
points
 Moderate wear
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4) Elastohydrodynamic Lubrication
 Intermediate speed
 Hydrodynamic pressure increases.
 Full, but still very thin lubrication film.
 Elastic deformation of the contact
points.
 Lubricant viscosity increase due to
increasing pressure.
 No wear
Hydrodynamic Pressure
Water ski:
1
3
4
Speed:
1<2<3<4
2
Water resistance
force
Normal force
Water pressure
Solid friction
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1)
Solid contact  no water flow under the skis
2)
Water pressure under the skis builds up as the drainage of water from the skis is lower
than the amount of incomming water
3)
Water pressure increases further acting against the normal force resulting in decrease of
friction
4)
Water resistance increases with speed until the skis cannot hold anymore (crash)
Hydrodynamic lubrication
5) Hydrodynamic lubrication
 High speeds
 The surface asperities are completely
separated by a lubricant film.
 The load and hydrodynamic pressure
are in equilibrium
 Thick lubrication film
 No wear
13
Types of Lubricants
Oils
• Friction Improver
• Dispersing Chemicals
• Viscosity Index Improver
• Foam Inhibiters
• Oxidation Inhibitors
• Pour point Improver
• Wear Protection
Base oil
Viscosity of the oil
depends on application:
Low Viscosity for low loads
and high sliding speeds
High viscosity for high
loads, low sliding speeds
and high temperature
Pour point has to be taken
into account at low
temperatures
14
Greases
Pastes
• Friction Improver
• Foam Inhibiters
• Oxidation Inhibitors
• Wear protection
• Friction Improver
• Foam Inhibiters
• Oxidation Inhibitors
• Wear protection
Thickener
• Metallsoaps
• Silicagel
• Bentonite
Thickener
• Metallsoaps
• Silicagel
• Bentonite
Base oil
Greases are used where oil is
not applicable due to construction or low temperatures.
A grease acts like a sponge as
the thinkener binds the oil and
releases it under pressure.
Solid Lubicants
• Molypdenum sulfite
• Graphite
• Coper
Base oil
Can only be used for mixed
lubrication, no hydrodynamic
lubrication possible. Useful for
Types of Lubricants
Solid Lubricants
Dry lubricants /Anti-friction
coatings
• Dispersing agents
• Corrosion inhibitors
• Wetting agents
Solid Lubricants
• Molypdenum sulfite
• Graphite
• Coper
• Synthetic resins
Solid lubricants produce sliding or seperation
films on metall surfaces due to their structure
and physical-chemical properties. They are used
as powders.
Examples: Molypdenum sulfite, PFTE, graphite,
coper
Solid libricants are oft used as primary film in
combination with lubricants.
15
Solvents / Water
Anti-friction coatings are
surface fixed solid lubricants
Tribological Problems
Product
Tribology
Theory
Model
Module
Product
Field
Analysis
tests
tests
tests
tests
costs
16
Tribometry, Modelling
The field test in the final device is the most accurate test for friction partners and lubricant. As
this is a very expensive model systems are normally used in the first stage of development.
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Tribology Device
Ball-on-3-plates
Based on a cooperation between
Werner Stehr
(Dr. Tillwich GmbH Werner Stehr
Murber Steige 26
72160 Horb-Ahldorf)
and
Anton Paar
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Tribology Device: Ball-on-3-Plates Principle
attached to a MCR Rheometer
Side View
Top View
M: Torque
FN
Normalforce
M: Torque
FN
Normalforce
19
Tribology Device: Ball-on-3-Plates Principle
Peltier Bottom Plate and Peltier Hood
20
Tribology Device: Ball-on-3-Plates Principle
attached to a MCR Rheometer
Specifications
Rheometer
Tribometer
Normal Force FN:
0.01N - 50N
2 N - 70 N
Torque M:
0.1µNm - 200mNm
Friction Force FF:
0.01 N - 44.5N
Rot. speed n:
10-6 - 3000 rpm
Sliding speed sv :
1  10-8 - 1.41m/s
Temperature:
-40°C - 200°C
21
Friction Coefficient for a Steel Ball on POM:
Dry, Penetrating Oil, Motor Oil
Reproducability: Good for dry and motor oil
For penetrating oil general behavoir is reproducable
1
dry
penetrating oil
0.1
0.01
motor oil
0
22
200
400
600
800
Sliding Speed vs
1,000
mm/s
1,400
Friction Coefficient for a Steel Ball on POM:
Dry, Penetrating Oil, Motor Oil
Rotational speed: 0.001 - 3000 rpm; NF= 10N, Normal load NL= 14N
Logarithmic scale for sliding speed
1
Steel/POM CR dry
Friction Factor
0.1
Steel/POM CR penetrating oil
Friction Factor
Steel/POM CR motor oil
Friction Factor
0.01
10
23
-4
-2
0
10
10
Sliding Speed vs
10
2
mm/s 10
4
Stick Slip
Static Friction > Pulling Force
Static Friction < Pulling Force  Sliding
Stick slip is usually an unwanted effect occuring at very low sliding speeds in the
boundery lubrication regime. It leads to vibrations and noise
Examples: Noise of train in a turn, sound of a violin, window cleaner in dry conditions, wet
finger on the rim of a crystall wine glass
Prevention of stick slip:
• Adequate materials and surfaces
• Use lubricants
• Choose higher speeds
• Dithering (Vibration with small amplitude and adequate frequency)
24
Measurements on a Steel/Rubber System
Measuring conditions: Steel/rubber dry and lubricated, Normal load 14 N, 10 rpm
1
0.9
0.8
0.7
Steel/rubber dry
run in effects
0.6
Friction Factor
0.5
Steel/rubber, 10mPas silicone oil
0.4
Friction Factor
0.3
Steel rubber, lubricant grease
0.2
Friction Factor
0.1
0
0
20
40
60
Sliding Distance ss
80
mm
120
Stick slip effects can be observed in dry and oil lubricated conditions but not
when lubricated with grease.
25
Tribology of Lubricants:
2 Different Greases at 25°C and - 40°C
Stribeck curves: Friction coefficient as function of sliding speed
0.4
0.35
CSR NGLI 0 25°C
0.3
0.25
CSR NGLI 2 25°C
0.2
0.15
CSR NLGI 0 -40°C
0.1
CSR NLGI 2 -40°C
0.05
0
0.1
26
1
10
Sliding Speed vs
mm/s
1,000
Tribology of Lubricants:
2 Different Greases at 25°C and -40°C
Static Friction: measured
0.4
0.35
0.3
CS NLGI 0 w/o run-in 25°C
Friction Factor
0.25
CS NGLI 0 with run-in 25°C
0.2
Friction Factor
0.15
CS NLGI 0 w/o run-in -40°C
Friction Factor
0.1
CS NLGI 0 with run-in -40°C
0.05
Friction Factor
0
0.0001
0.01
mm/s
Sliding Speed vs
27
100
Rolling-element Bearing measuring fixture
Measuring cones:
Min. inner bearing diameter:
Max. inner bearing diameter :
Min. outer bearing diameter:
Max. outer bearing diameter:
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3 mm
25 mm
10 mm
42 mm
Performance tests of rolling element bearings
Logarithmic speed ramp 0.1 to 3000 rpm
Load 10N, Speed ramp 0.1 to 3000 rpm, -40°C
40
mNm
35
30
CR NLGI 0 -40°C
25
M
Torque
CR NLGI 1 -40°C
20
M
M
15
Torque
CR NLGI 2 -40°C
M
10
Torque
5
0
1
10
100
1/min
1 000
Speed n
 Best high speed performance for grease of NLGI class 2
 Grease of NLGI class 0 with the highest friction at high speeds
 Torque decrease at high speeds due to friction heating
29
Example: Roll out Test
Load 10 N, Step to 3000 rpm for 10 seconds and then roll out
(only roll out intervall plotted)
10 000
1/min
1 000
Roll out NLGI 0 -40°C
n
Speed
Roll out NLGI 1 -40°C
100
n
Speed
Roll out NLGI 2 -40°C
n
n
10
Roll out NLGI 0 25°C
n
Speed
Roll out NLGI 1 25°C
1
- 40°C
25°C
60°C
0,1
0
5
10
15
Interval Time tint
20
s
25
 Roll out test proves the results of speed ramps
30
Speed
n
Speed
Roll out NLGI 2 25°C
n
Speed
Roll out NLGI 0 60°C
n
Speed
Roll out NLGI 1 60°C
n
Speed
Roll out NLGI 2 60°C
n
Speed
Wood Polymer Composites (WPC)
0.5
0.45
0.4
0.35
Stribeck PP 3
0.3
Friction Factor
0.25
Stribeck PP + 30% wood
0.2
Friction Factor
Stribeck PP + 50% wood
0.15
Friction Factor
0.1
0.05
0
0.00001
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0.0001
0.001
0.01
Sliding Speed vs
0.1
m/s
10
The higher the wood fraction the lower the friction coefficient
• Better humidity resistance
 Terrasses made of WPC tends to be more slippery the higher
the wood content. Effect is even more pronounced when
lubricated with water (rain).
• Improved rigidity
• Smaller expension coefficients
Food Tribology: Key Question
Is there any quantitative method to predict or
determine mouthfeel?
Today the mouthfeel is determined by a sensory panel having several
 Sensory panels are very expensive
 Trained people are required
 Time consuming
The human factor!
 Limited reproducibility
 Limited quantitative statement
It is impossible to avoid sensory panels for food design but if there
would be a prescreening methode to determine mouthfeel the number
of panels could be reduced which would save a lot of money.
32
From Rheology to Tribology:
Chewing, Swallowing, Drinking Mechanisms
In-mouth flow regimes during beverage consumption
Rheology
Tribology
Time course of drinking
Sensation
“creamy”,
“slippery”
Lubrication,
solution limited,
saliva limited,
“surfacedominated”
Controlling
elements
Mouthfeel is more than just flow
properties. It includes fracture and
failure (large strain rheology), but it
is also driven by friction and
lubrication properties
“dry”,
“powdery”,
“gritty”,
“burn”
Composite
behavior, related
to phase ratio,
“particledominated”
1
um
0
um
“thickness”, “mouth coating”
10
um
Viscosity,
diffusion,
“solutiondominated”
Yield
stress
related
100 – 800
um
polymer
Size
Tribology Zone
Rheology Zone
Food Physics Methods
Model to Human Sensory Perception
 In tribological experiments the soft texture of the mouth is represented by at least one
elastomer friction partner.
 Cargill discovered and patented some elastomers having specialized properties (elasticity,
surface etc.) for food tribological measurements. (Patents: WO 2008/148538 A1, WO 2008/148536 A1).
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Differentiation of Dairy Drinks
1.6
Applied Normal Force: 3 N
2% Reduced Fat Milk
T = 20°C
Friction Factor
1.2
Whole Milk
0.8
Half & Half
Fat Free Skim Milk
Heavy Cream
0.4
0.0
1
10
100
1000
Sliding Speed (mm/sec)
 Drinking is a fast process performed at high sliding speeds.
 Milk drinks having various fat contents can be tribologically differentiated
 Strong stick slip effects observed at low sliding speeds for low fat milks
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Conclusion
Features and Benefits
 Tribology measurements on a MCR rheometer, i.e. the Rheometer as
Tribometer
 Low as well as high sliding speeds can be set very accurately (wide range)
 Stribeck curves
 Stick slip measurements
 Measurements in the boundery and mixed regime
 Long time measurement at desired speed
 Possibility of force (torque) control
 Measurements of static friction coefficients
 Temperature control by Peltier system from -40°C up to 200°C; with the
additional Peltier hood uniform temperature distribution
 Flexible choice of friction partners (Steel, polymers, elastomers etc.)
 Rolling element bearing performance measurements
 Food tribology
35
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