trobologia y desgaste

1. Tribology 101 – Introduction to
the Basics of Tribology
SJ Shaffer, Ph.D. – Bruker-TMT
Steven.shaffer@bruker-nano.com

2. Outline
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• Origin/Definition of “Tribology” (Term and Field)
• Encompassing Fields
• Fundamentals of Tribology:
• Surfaces in Contact
• Friction
• Lubrication
• Wear
• Concluding Words
• Upcoming Topics in Series

3. What is Tribology ?
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• Tribology comes from the Greek word, “tribos”,
meaning “rubbing” or “to rub”
• And from the suffix, “ology” means “the study
of”
• Therefore, Tribology is the study of rubbing,
or… “the study of things that rub”.
• This includes the fields of:
• Friction,
• Lubrication, and
• Wear.

4. “Tribology” is a new word…
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• Coined by Dr. H. Peter Jost in England in
1966
• “The Jost Report”, provided to the British Parliament –
Ministry for Education and Science, indicated… “Potential
savings of over £515 million per year ($800 million) for
industry by better application of tribological principles
and practices.”
But…
Tribology is not a new field!

5. The First Recorded Tribologist – 2400 B.C.
Transporting the statue of Ti – from a tomb at Saqqara, Egypt
Figure taken from
“History of
Tribology”, by
Duncan Dowson.
5
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6. The First Recorded Tribologist – 2400 B.C.
Transporting the statue of Ti – from a tomb at Saqqara, Egypt
6
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7. 7
The First Recorded Tribologist – 2400 B.C.
The first recorded tribologist – pouring lubricant
(water?) in front of the sledge in the transport of the
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8. A more famous Tribologist – 500 years ago
Leonardo Da
Vinci
Ball
Bearing
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4-Ball Test
Geometry
Sled Friction
Test
Geometry

9. A more famous Tribologist – 500 years ago
Leonardo Da
Vinci
Ball
Bearing
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4-Ball Test
Geometry
Sled Friction
Test
Geometry
ASTM D5183 - COF
ASTM D2266, D2596 – EP
ASTM D4172, D2783 -
Wear
ASTM D1894 – Static
and Kinetic COFs of
Plastic Film & Sheeting

10. A more famous Tribologist – 500 years ago
Leonardo Da
Vinci
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Ball
Bearing
4-Ball Test
Geometry
Sled Friction
Test
Geometry
Two
Observations:
1. The areas in contact have no effect
on friction.
2. If the load of an object is doubled,
its friction will also be doubled.
ASTM D5183 - COF
ASTM D2266, D2596 – EP
ASTM D4172, D2783 -
Wear
ASTM D1894 – Static
and Kinetic COFs of
Plastic Film & Sheeting

11. Tribology 101 - Basics
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Applications and Fields which
Encompass Modern Tribology

12. Tribology is All Around Us,
In Applications from Simple to Complex
and Scales from Small to Large
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• Individual Components
• Assemblies or Products
• Manufacturing Processes
• Construction/Exploration
• Natural Phenomena

13. Individual Components
Gears
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Bearing
s
Brake & Clutch
Pads

14. Assemblies or Products
Engine
s
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Curling
Stones
Rock
Climbing
Shoes
Pocket
Watch

15. Manufacturing Processes
Rollin
g
Stampin
g
Turnin
g
Grinding/
Polishing
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16. Construction/Exploration
Mine Slurry
Pumps
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Excavato
r
Chunnel Digging
Drill
Oil Drilling
Rig
Space
Shuttle

17. Natural Phenomena
Plate
Tectonics
Wind
Erosion
Wea
r
Water Erosion
Frictio
n
On/Off
Stiction:
Gecko Feet
Super-
hydrophobicit
y: Lotus Leaf
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18. Tribology 101 - Basics
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In Parallel to these different Scales,
There are Many Areas of
Engineering and Industry which
have a Need to
Use/Understand Tribology

19. Tribology is also in Virtually every Area of
Engineering and Industry
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• Aerospace
• Agriculture
• Automotive
• Engine: Piston ring/cylinder,
Bearings, valve seats, injectors
• Brakes/clutch
• Tooling/Machining/Sheet metal
forming
• Coatings Providers
• Low Friction
• Wear Resistant
• Thin Films or Hardfacings
• Cosmetics/Personal Care
• Dental Implants
• Energy
• Nuclear
• Wind
• Fossil
• Solar
• Fabric/Clothing
• Flooring
• Food Processing
• Highway/Transportation
Depts.
• Lubricant Manufacturers
• Medical Diagnostics
• Medical Implants
• Military
• Pharmaceutical
• Shoe Manufacturers
• Sports Equipment
Companies
• Universities/Educators
• Mechanical Engineering
• Materials Science
Engineering
• Physics

20. Commonality in Tribology
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What do All These Diverse Fields
and Applications have in Common?
What do we need to think about as
engineers and scientists when we
design products or friction/wear
experiments?

21. Commonality…
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Every Application has:
Surfaces in Contact, and
in Relative Motion
(e.g. sliding, rolling, impacting)

22. Tribology Basics - Surfaces in Contact
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So let’s begin by looking
closely at a surface…

23. The Surface is not Simple…
Lubricant
Adsorbed
Contaminant
s
Oxide
Surface
Properties
“Disturbed
Material”
Bulk
Material
Properties –
“Handbook
values”
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24. The Surface is not Simple…
Bulk
Material
Properties –
“Handbook
values”
Surface
Properties
“Disturbed
Material”
Adsorbed
Contaminant
s
Oxide
Lubrican
t
≈
mms -
cms
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nms -
µms

25. Nor is it Flat!
Bulk
Material
Properties
Adsorbe
d
Contaminant
s Oxide
Surface
Properties
Disturbed
Material
Lubrican
t
All engineering surfaces have a roughness, and
this roughness plays an important role in
tribology.
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26. Nor is it Flat!
Bulk
Material
Properties
Adsorbe
d
Contaminant
s Oxide
Surface
Properties
Disturbed
Material
Lubrican
t
All engineering surfaces have a roughness, and
this roughness plays an important role in
tribology.
Surface Roughness comes from all prior history of the
part: Manufacturing, handling and prior use in
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27. We need to think about…
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2 Aspects of a Surface:
• Physical - Surface Roughness
• Dictates Contact Area
• Dictates Contact Stresses
• Lubricant Paths or Reservoirs
• Chemical - Intervening Layers
• Chemical Compatibility
• Shear Strength
• Lubricant Properties, e.g.
Viscosity

28. We need to think about…
• Dictates Contact Area
• Dictates Contact Stresses
• Paths or Reservoirs
for Lubricants/debris
• Chemical -
Intervening Layers
• Chemical
Compatibility
• Shear Strength
2 Aspects of a Surface:
• Physical - Surface
Roughness
Ground
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Bead
Blasted

29. We need to think about…
2 Aspects of a Surface:
• Physical - Surface Roughness
• Dictates Contact Area
• Dictates Contact Stresses
• Lubricant Paths or Reservoirs
• Chemical - Intervening Layers
• Chemical Compatibility
• Shear Strength
• Lubricant Properties, e.g.
Viscosity, EP or boundary-
forming
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30. Surface Characterization
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Variety of Methods available, if needed
• Physical Characterization
• Roughness
• Macro – Waviness and Form (CMM)
• Micro – Surface Roughness
– Stylus Profilometers (contact)
– Optical Profilometers (non-contact)
– AFM (sub-micron)
• Hardness
• Indent, Scratch
• Chemical Characterization
• Infrared, XPS, Raman, Auger
• Lubricant Shear properties→Viscometry

31. Tribology 101-Basics
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Summary of Surfaces in Contact
• Tribo-Forces are Dictated by Interaction of
Asperities
• Asperities have Mechanical and Chemical
Properties
• Methods Exist to Characterize these
Properties
• Asperity Geometry and Distribution result
from Manufacturing Method, Handling and
Prior Rubbing History

32. Friction
Fundamental
s
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33. Friction Fundamentals
Conceptual Definition of Friction
Friction is the resistance
to relative motion
between two bodies in
contact.
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34. Where does the resistance come
from?
When objects touch – there are forces between
them.
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Microscopi
c forces of
molecular
Adhesion.
(includes electrostatic,
Van der Waals,
metallic bonds)
Microscopi
c forces of
mechanical
Abrasion.
(includes elastic
and plastic
deformation)

35. Where does friction come from?
Remember, there are also “contaminants” at the
interface
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Oxides,
Adsorbed films,
Adsorbed
gases,
Foreign or
“domestic”
particles

36. Friction Fundamentals – “The COF”
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• The Coefficient of Friction: A simple
constant of proportionality.

37. Friction Fundamentals – “The COF”
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• The Coefficient of Friction: A simple
constant of proportionality.
• Or is it?

38. Friction Fundamentals
Measuring Friction:
The Coefficient of Friction
Very Simple
Relation:
F=N
N
F
 = F/N =
“COF”
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39. Friction Fundamentals – “The COF”
• Suppose a colleague wants to know:
“What is the
COF of steel?”
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40. Friction Fundamentals – “The COF”
• Is that close enough for your needs?”
• A: “Well, dear colleague, you can use from
0.1 to 0.6. Take your pick.
“What is the
COF of steel?”
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41. Friction Fundamentals – “The COF”
Well not
really.
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?

42. Friction Fundamentals – “The COF”
• “Then I guess we’ll need a bit more
information.”
Well not
really.
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?

43. Friction Fundamentals – “The COF”
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What we need to know…
• “What steel?
• Stainless steel: 304, 316 , a 400-series or hardened 17-4PH or the like?
• Carbon steel: if so is it pearlitic or martensitic?
• Tool Steel?
• “Well I need to use it in water, so stainless steel, I guess.”
• “What is the function? “What is the mechanism?”
• “I’m designing a gear-driven mechanism, and I need to size the motor, assuming some
frictional loss in the gears, so I need the COF.”
• “Gears… Then, it needs to be hardened. How about the driven gear, what’s its
material?”
• “The same, I suppose.”
• “I’m not sure that’s a good idea, depending on the contact stress, sliding velocity
and surface finish. Do you know these parameters yet?”
• “Not yet, I’ll probably use standard values from my gear design handbook.”
• “OK, I gather you need low friction, how about lubricant or use of a lubricious coating,
are these permitted in the design?”
• ”A coating is OK, but I don’t think a liquid lubricant is permitted in this application.”
• “OK, a coating then. How long will it need to last?”
• “For the life of the mechanism. Can’t you just tell me the COF?”
• Really, I need more information, because I’ll likely need to run a test, depending on how
precisely you need the COF.”…

44. All things considered,
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The COF is Somewhat Complicated
• Surface roughness plays a role
• Lubricant plays a role
• Surface chemistry plays a role
• Contact Stress plays a role
• Contact geometry plays a role
• Environment plays a role
• Temperature plays a role
• Sliding speed plays a role
• …

45. All things considered
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It’s not so bad after all
Fortunately, while it appears
complicated, friction is relatively easy
to measure,
(Only two things: Normal Load and Friction
Force)
But, we have to measure it under the
right conditions.

46. Summary of Friction Fundamentals
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The equation is simple, but measuring it
correctly requires care:
When assessing a system’s tribology need,
we must consider:
Materials, Coating, Lubricant
Contact Area, Geometry,
Stress Surface Roughnesses
Sliding Speed
Sliding Mode (unidirectional, reciprocating,
multidirectional) Duty Cycle (continuous contact,
intermittent contact)
Environment
Temperature, Humidity,
Atmosphere (air, exhaust gases, vacuum)
Friction is NOT a Material
Property Friction is a “System”

47. Lubrication
Fundamental
s
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48. Lubrication Fundamentals
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• The role of a lubricant is to:
• Reduce Friction
• Prevent / Minimize Wear
• Transport Debris away from
Interface
• Provide Cooling

49. Lubrication Fundamentals:
Lubrication Regimes, with liquid present
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• In Liquid Lubrication, Regimes can be
based on: Fluid Film Thickness
• The Lambda Ratio is defined as the ratio of
the fluid film thickness to the composite
surface roughness*
•  > 3  full film (thick film)
lubrication, hydrodynamics
• 1.2 >  > 3  mixed or thin film
lubrication
•  < 1.2  boundary lubrication
* - composite surface roughness = (rq1
2 + rq2
2)1/2

50. Lubrication Regimes:
The Stribeck Curve
Journal
Bearing
Thick
Film
Speed*Viscosit
y Load
Thin
Film,
Mixed
Boundar
y
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51. Lubrication Regimes:
Boundary Lubrication – Solid Lubricants
• Solid Lubricants
• Compounds with Low Shear
Stress
• MoS2, Graphite, WS2, HBN
• Behave like a “deck of cards”
• Bonded Films
• DLC
• Resin-bonded PTFE
• Impregnated porous anodizing
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52. Summary of Lubrication
Fundamentals:
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• Key Factors in Lubricant Effectiveness
• Fluid Shear Properties
• Viscosity, Viscosity Index
• Pressure-Viscosity Index
• Chemistry
• Reactivity with the Surface
• Boundary Film-Forming Properties
• Extreme Pressure Constituents
• Shear strength of solid lubricant or coating
• Thermal Conductivity/Heat Capacity

53. Wear
Fundamental
s
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54. Wear Fundamentals
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Conceptual Definition of Wear
Removal (or displacement) of material
from one body when subjected to
contact and relative motion with
another body.

55. Wear Fundamentals - Wear Modes
6 Primary Wear Modes:
1. Abrasive Wear, Scratching
2. Adhesive Wear, Galling, Scuffing
3. Fretting/Fretting Corrosion
4. Erosive Wear, Cavitation, Impact, Electro-
arcing
5. Rolling Contact Fatigue, Spalling,
Delamination
6. Tribo-Corrosion
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56. Wear Fundamentals
• Abrasive Wear,
Scratching
“The harder
material scratches
the softer
material.”
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57. Wear Fundamentals
Galling of Stainless Steel
Samples
10
mm
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• Adhesive Wear, Galling, Scuffing
Begins as “local
welding”
Material “compatibility”
is important for
adhesive wear.
Stacking fault energy,
crystal structure,
natural oxide
formation all influence
adhesive wear.

58. Wear Fundamentals
• Fretting/Fretting Corrosion
Small
amplitude
displacemen
t (< 50 µm).
• Experiments generally have zones of no-
slip, and slip.
• Small adhesive pull-outs occur at the
boundary.
• Often these oxidize, so sometimes
1/29/2013 58

59. Wear Fundamentals
Cavitation
Damage
1 cm
Steam Control
Valve
“Fluting”
Damage
• Erosive Wear, Cavitation, Impact, Electro-arcing
Dependency on
particle size, shape,
composition, angle
of impingement, as
well as ductility of
“target”
Particle
Classification
1/29/2013 59

60. Wear Fundamentals
Spalled Bearing Inner
Race
• Rolling Contact Fatigue, Spalling, Delamination
• Reversing sub-surface shear
each
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Propagation to surface
of sub-surface-initiated
cracks
time the roller or ball passes
over the surface.
• Accumulation of these
stresses leads to subsurface
crack formation, usually at a
microstructural
inhomogeneity.
• Cracks grow toward surface
and particle spalls off.
• Debris typically gets rolled
over, creating additional

61. Wear Fundamentals
• Tribo-
Corrosion
Erosion-
Corrosion
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• Wear in the presence of
corrosion can have synergistic
effect.
• Can happen with erosion
or sliding wear.
• Bio-tribo-corrosion is
major area
• Down-hole drilling
environment is another
• ASTM Method G119 – Standard
Guide for Determining
Synergism between Wear and
Corrosion

62. Wear Assessment
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• The Wear Coefficient, k
• k  volume of material removed per unit load and sliding
distance
• Units of k are:
• mm3/Nm
• Please do NOT reduce the units of k to mm2/N or 1/kPa
• This has no physical meaning
• k can be used to predict component lifetimes, providing
the tribosystem does not change wear modes
• Duty cycle and directionality can influence wear
• Start-stop can be much more damaging than continuous
motion
• Unidirectional sliding is very different from reciprocating
sliding

63. Summary of Wear Fundamentals
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• Like Friction, Wear is a System Property, NOT a
Materials Property
• There are several distinct wear regimes, though
some can operate simultaneously, or sequentially
• Observed abrasive wear can results from initial
adhesive wear
• If you properly simulated the system and wear
mode, the wear coefficient, k, can be used to
predict lifetimes

64. Some
Final Words for
Today’s
Webinar
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65. Tribology Fundamentals
Key Concepts
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1.COF is not a material property, it is a system
property.
2.Wear Rate or wear resistance depends on the wear
mode, which is a function of the Tribosystem.
3.If we properly characterize and understand the
Tribosytem, the odds are better that we will
succeed, because we can make the right choice for
materials, contact geometry and chemistry, and
make the appropriate measurements to give us
the answer we seek for our design.

66. Means to
Assess Tribo-
systems

67. Tribology & Mechanical Testing (TMT)
• Universal platform for Tribology studies: Wear, Friction,.. when 2
surfaces meet.
• Large load range
• Wide variety of environments (corrosion, HT, liquid)
• Wide variety of configurations (rotating & translating motions)

68. Many different Tribology tests
Linear
Stage
Block-on-Ring
Drive
Reciprocating
Drive
Rotary
Drive

69. Indentation & Scratch Testing
• Large load range: nano & micro
• Wide variety of imaging options
• (AFM, profiler, optical)
• Indentation & Scratch Tester
Scratch test example
Indentation
example

70. www.bruker.com
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