introduction to tribology dr. amaya igartua contact ......tekniker-wisco-mecauto simulation tests...
TRANSCRIPT
Introduction to Tribology
Dr. Amaya Igartua
Contact: [email protected]
Head of Tribology Group
Tel: 34 680656085
www.tekniker.es
Bilbao, 5th June 2012
2
Tribology Concepts
Topics
• Friction and Wear Resistance Concepts
• Main Friction and Wear Mechanisms
• Lubrication
• Surface Treatments
• Selection of Surface Treatments
3
Friction and Wear Concepts. Tribology
The knowledge and control of friction has worried all civilizations, who, leave
without complete understanding the fundamentals of this phenomenon, but
have developed strategies for controling it and use it.
Tribology is a part of the Enginering and of the Materials Sience which studies
the friction, lubrication and wear phenomena.
The origin of this speciality was in 40’s, but it was in 1966, in the Cavendish
Laboratory of the University of Cambridge, when it got the oficial name of
Tribology, derived from Greek triboz = friction.
Tribology has increased the interesest of engineers in the last 30 years, because
among other things, the elevated economical and environmental expenses
caused by friction and wear.
4
Friction and Wear Concepts . Impact
An adequate understanding of the tribological problems and of the technologies
for solving them can provide huge savings. In one of the first reports where it
was analized (Jost Report, 1966) the next estimation was presented:
Savings in energy by means of a lower friction................................... 5%
Savings in labor...................................................................................... 2%
Savings in lubricants.............................................................................. 2%
Savings in maintenance and spare parts............................................. 45%
Savings in less breakdowns................................................................... 22%
Savings in energy in investments............................................................ 4%
Savings by mean of enlarging the life of plants................................... 20%
TOTAL.................................................................................................. 100%
5
Friction and Wear Concepts. Multidisciplinar
Mathematics
FRICTION
WEAR LUBRICATION
Nanotechnologies Mechanics of Solids
Surface chemistry Mechanics of Fluids
Organic Chemistry
Maintenance techniques
Materials Science
Physics
6
Friction and Wear Concepts : FRICTION
The modern formulation of the “Friction laws” was born in S XVIII
(Amontons- Coulomb) establishing that...
1. There is a Friction Force, opposed to movement, which is proportional to
the normal force between the contacting surfaces
2. The friction force is independent of the (apparent) contact area
3. The friction force is independent of the sliding velocity
7
Friction and Wear Concepts : FRICTION
The paradoxical second law is explained taking into consideration that the
REAL contact area increases with the contact pressure.
In fact, it can be
demostrated that, for
most of the materials, this
increase is proporcional
to the contact force,
which also explains the
first law and alows to
define a COEFFICIENT
OF FRICTION
m=F/W
8
Friction measurement
Ball on Disc
SRV test
machine
upper holder
Piston
ringDisc cut directly from the
cylinder liner,
maintaining the curvature
Piston ring holder,
cut directly from
the piston
Adapter for
piston ring
holder
SRV test
machine
upper holder
Piston
ringDisc cut directly from the
cylinder liner,
maintaining the curvature
Piston ring holder,
cut directly from
the piston
Adapter for
piston ring
holder
Diseño de Materiales
- Composición Química
- Proceso de fabricación
Ensayos básicos
Ensayos de
simulación
Ensayos en Motor
Buenas Propiedades
Buenas Propiedades
Inadecuado
Inadecuado
Basic Tests
Simulation tests
Engine tests
9
Piston Ring/Cylinder Liner Configuration
0,1
0,15
0,2
0,25
0,3
0 1000 2000 3000 4000 5000 6000
Time (s)
COF
CrN/TiN NiPCO+Si3N4
WC-C Nano-CrC75-NiCr25 (-10um)
WC CoCr//CrC NiCr Chrome Plated
CrC75-NiCr25 (32um)
Configuration cylinder/piston rings
-45+20 μm
Deposition process Coating composition Friction faverage Piston Ring Mass loss
(mg)
Electrodeposition Hard chrome 0.16 0.2
Electrolytic NiPCO+Si3N4 0.15 0.1
PVD TiN-Ti-CrN-CrN 0.16 0.0
PVD WC-C 0.13 0.0
HP-HVOF Nano-75Cr3C2-25NiCr Powder size –10 µm
0.15 0.1
HP-HVOF WC-CoCr /CrC-NiCr
Powder size -30+10µm 0.21 0.0
HP-HVOF 75Cr3C2-25NiCr Powder size
-45 +20µm 0.15 0.2
10
Wear results after the duration test
1) Increase of the comprised aperture
2) Increase of the internal diameter of the cylinder in the top dead
center of the compresion piston ring
Wear in the
piston ring
(µm/100h)
Wear in the
cylinder liner
(µm/100h)
Selecting coatings engine tests
11
• The problem of friction is very complex since it takes part simultaneously diverse mechanisms depending on the type of material and contact in each particular situation.
• All the mechanism that can be proposed to explain the friction has to be disipative, that means that consume energy.
Some of them implies:
• Formation and rupture of chemical bonds between the surfaces.
• Plastic deformation in one of the surfaces.
• Cracks and Microfractures in one of the surfaces
• Cycles of inelastic deformation in one of the surfaces.
Main Friction and Wear Mechanisms
12
• The friction coefficients between metals, in the air, is usually relatively high µ = 0,5 - 1. In the case of ductil metal can reach values of µ = 2.
• The alloys have lower friction coefficients that pure metals due to segregation effects of some components in the surface.
• The tendency to oxidation of the surface (due to temperature, high sliding velocity,..) help to maintain low friction coefficients.
• If the oxide layers are inestable or fragil, it can be produced metal-metal contact producing microweldings.
• The temperature also plays their role through changes in the present phases, in the mechanical properties (more ductility) or simply, melting the first superficial layer.
• In the case of metals, the friction is mainly due to bond formations generated by contact of asperities submitted to an intense plastic deformation.
• If there not were oxide layers, absorbed molecules or contamination, the friction coefficient will increase seriously generating weldings. This occurs mainly in vacuum.
Mechanisms of Friction in Metals
13
Friction in ceramic materials
• The friction coefficient are usually lower than in case of metals, µ = 0,2 - 0,8.
• The adhesion mechanism are less active due to the difficulty to form bonds, that in ceramics are ionic or covalents.
• The deformation mechanism are not too active due that the materials have less displacement possibilities, except when are due to chemical alteration.
• The crack formation is normally a relevant mechanism of friction.
• The oxide layers, their hydratation or absorption of molecules reduces their friction coefficient.
• Coatings can be applied to ceramics reducing friction coefficient and wear.
0.416----0.56Opaque vitreousglased ceramic
bulk
5.83x10-33.26x10-30.15DLC PVD Coatingon opaque glazed
tile
Volume loss ofthe pin (mm3)
Volume lossof the plate
(mm3)
µmeanSample
0.416----0.56Opaque vitreousglased ceramic
bulk
5.83x10-33.26x10-30.15DLC PVD Coatingon opaque glazed
tile
Volume loss ofthe pin (mm3)
Volume lossof the plate
(mm3)
µmeanSample
14
Friction in polymers
• The friction coefficients are normally lower than in case of metals or ceramics, µ = 0,1 - 0,5.
• There is a higher dependence than in other materials on the load, sliding speed or temperature.
• Adhesion and deformation (due to hysteresis cycles) are the main wear mechanism.
• Due to their low ratio E/H, the contact between the polymers are mainly elastic. In the low rough surfaces, this represent a decrease of the µ when increasing the load.
• The transference of polymer films to the countermaterial, give solid lubrication mechanism, reducing the friction also bellow 0,1.
-3525
80150
100
80,00
0,05
0,10
0,15
0,20
0,25
0,30
0,35
0,40
COF ()
T (ºC)
v (mm/s)
F = 50N
Rod
15
High friction applications
1.- Model tests: PIN ON DISC
2.- Dinamometer Inertia tests: Disc brakes
Not always the friction is a problem: in many case, we need a certain friction coefficients sometimes high (tires against road) or simply constant (materials for disc brakes)
Real disc brake tests
Simulation tests
16
Strategy to reduce the friction
• When the friction is a problem, it is possible to reduce it by means a change in the materials, due to a work performed in conditions that favor a reduction of the friction or by means of the use of the lubricants.
• The lubrication is the use of an interlayer material (normally a fluid, but can be a solid) able to separate the surfaces in contact.
17
Liquid Lubrication
Is the most typical lubrication case that consist in the interposition of a fluid between the surfaces in movement. Depending on the thickness of the fluid, there are different situations
• Hydrodynamic lubrication: The film is enough thick (1 -100 µ m) to facilitate the fluid to support all the contact pressure. It present very low friction coefficients depending on the fluid viscosity and the sliding speed.
• Elasto-hidrodynamic lubrication: The film is thinner (0,01 -10 µ m), this will generate deformations in the zones near the surface and changes in the viscosity of the lubricant film.
• Boundary lubrication: There are points in the surfaces that enter in contact and only can be protected by means of molecular films that are adhered to the surface
The Stribeck curve allows to distinguish
the three mentioned regions, studying
the dependence of the µ with the ratio
ηV/W (viscosity x speed /load).
18
Liquid lubrication
In boundary lubrication, the roughness of the surface has an important role, as lower is the roughness, the protection of the layer will be better.
In the cases of limit lubrication, the disolution in the lubricant of polar molecules, will give a protective layer in the contact
19
Solid Lubrication
In some cases, it is not possible to have liquid lubrication:
• Components that works in vacuum or space conditions
• When there is risk of inflamation or chemical reaction
• Risk of food contamination due to the presence of lubricant
• Need to avoid the use of lubricant due to wrong effects in the environment
In these cases, it can be used solid lubricants, with adhered solid films that reduce the friction coefficient but not so much as for hydrodynamic lubrication. TEKNIKER-WISCO-Mecauto
Simulation tests
Test in
components
20
Solid lubrication
The main strategies in solid lubrication are:
• The use of graphite films, in certain humidity conditions.
• Use of othe lamelar solid lubricants, type MoS2, that works better in vacuum conditions.
• Use of fine films of ductil metals (Silver, Lead).
• Use of polymeric films as polyethylene or PTFE.
21
• We understand by wear, the loss of material as result of the contact
between 2 surfaces in relative movement.
• There are many different mechanisms that drive to wear and that in some occasions operate simultaneously.
• Not always is simple to perform the diagnosis about the reasons of a particular wear case.
• The quantification of the wear can be performed by mass loss, but it is usual to measure the geometrical volume loss, shape loss, increase of roughness,…
• Metallic alloys can suffer different type of wear: adhesive, abrasive, erosion, tribocorrosion, cavitation, fretting …
AFM Confocal Microscopy Profilometry
Type of wear
22
Wear measurement
• The most common tests uses universal tribometers to measure the friction in more aggressive conditions to accelerate the wear
• In a tribometer type ball on disc, it will be possible to produce a worn circunferece, which aspect or lost in volume will give us an idea of the suffered wear.
23
Wear Coefficient
• In order to have an universal parameter that allows the comparison of the wear resistance of different materials, a wear coefficient is defined following Archard equation: Loss volume/ Length= K* Load/ Hardness
• This K coefficient is adimensional and indicates the tax of material loss in function of the load in contact and the surface hardness of the material.
• In some occasions, is better to work with a dimensional coefficient defined as: k = K/H, that takes into account the hardness of the material. Their unit will be m2N-1
• Another Coefficient is:
where V, is the volume loss,
P, the normal load and L, is the length of the tests, dimensions m2/N
24
Type of wear
There are a wide variety of wear phenomena:
• Adhesive wear
• Abrasive wear
• Fatigue wear
• Erosion
• Fretting.
• Tribocorrosión.
• Cavitation
• • ...
The most common are the first two.
25
Adhesive wear (boundary region)
• Consist of the loss of material in the surface by means of the material scratch or microwelding in the surface that can adhere to the counter material, due that the joints to the coutermaterial are stronger than the with the own worn material.
• In the case of metallic surface can be distinguised two different regimes: smooth (normal) or severe (catastrophic).
• The adhesive wear is controlled by the characteristic of the metal (facility to oxidation, lubrication additives,..) and for the following parameters:
– Contact pressure
– Sliding speed
• In function of these parameters can be established tribological maps to indicate the regions of smooth and severe wear.
AT182 / Chromated
0
0,05
0,1
0,15
0,2
0,25
0,3
0 500 1000 1500 2000 2500
Time (sg)
Fri
cti
on
co
efi
cie
nt
0
500
1000
1500
2000
2500
3000
3500
friction Load (N) Stroke (mm)
26
Soft versus severe wear
In the case the metallic surface suffer soft
or severe wear is due to the competence
between the mechanism of elimination
of material and the oxidation speed.
When the elimination of the material is quicker
than the oxidation rate, there is not time to create
an oxide protection layer and the wear is accelerated
with further elimination of material.
Soft wear: Fine particles 0,01-1 µ m, oxidized particles,
small increase of roughness, and smooth wear
Severe wear: Thick particles 20-200 µm, metallic particles,
the roughness increase and the wear is 100-1000 times higher
27
Abrasive Wear
Consist of the elimination of the material from one
of the surface in contact due to the strong plastic
deformation produced by the presence of
hard particles in the counter surface. The hardness
of these particles are higher than the worn surface:
y = 0.0001x + 0.0013
R2 = 0.9989
y = 0.0002x + 0.0285
R2 = 0.9998
y = 0.0003x + 0.0452
R2 = 0.9975
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 500 1000 1500 2000 2500 3000 3500
number of cicles
Ma
ss
lo
ss
(g
)
Zirconium ceramic glazed transparent ceramic glazed
matt ceramic glazed
Abrasion machine
Test results
28
FALEX MULTI-SPECIMEN.Ball on Disc Test (point contact)
Upper Ball : Steel, Si4N3 , 12.7 mm
Disc: Steel
Load: 2-700lb; 0.8-5.8 GPa
Speed: 40-7000rpm
Sliding Speed: 0.05-15m/s
Time: Variable (5min- 3hours)
Temperature: 25-250ºC
Output: Coefficient of friction
Increase of Temperature
Wear
Abrasion Tests
Si4N3 ball
Adhesion Tests
Steel ball
29
The abrasive wear is influenced by:
-The roughness of the surface
-The particle size
-The angle of the particles
-The thermal treatment
The erosion is produced due to the
impact of particles and depends on the
impact angle and the hardness of the
material.
Ductile material are affected by angle <
30º and fragil material at 90º
Abrasive and Erosion Wear
Erosion machine
30
Fatigue wear, pitting
• It is a phenomena of subsuperficial fatigue that can be produced in ball or roller bearings and gears.
• The maximum contact is produced in the subsurface, and it is produced the weakness of failure in the subsurface that goes to the surface generating a hole or pitting.
Evaluación del Pitting
Máquina “Ball on Rod”:
Fatiga contacto punto
Máquina “Cylinder on
Cylinder”: Fatiga contacto
línea
Máquina “Rolling/Sliding”: Fatiga
contacto línea
31
Fretting
• Wear bellow low stroke vibrations (1 - 100 µm), combined with oxidative process.
• The wear is increased when decreasing the stroke and the presence of oxygen and oxidant agents.
Ensayos de FRETTING
Coeficiente de Fricción Medio
0
0,5
1
1,5
0 200 400 600 800 1000 1200 1400
Tiempo (s)
COF
Amplitud: 150µm (2798) Amplitud: 150µm (2801)Amplitud: 150µm (2802) Amplitud: 150µm (2803)Amplitud: 150µm (2804)
0
0,25
0,5
0,75
1
50 100 150 200
Stroke (micras)
COF
COF
partial-slip
gross-slip
32
Tribocorrosión
It is produced when corrosion
and wear phenomena are
combined, producing synergic
phenomena.
Inconel Inco+Cr
Inco+CrN Inco+TiN/CrN
33
Cavitation wear
Cavitation wear is a form of wear where the erosive medium or counter-body is a
fluid.
Occurs when metal is removed from parts by the impact of collapsing cavitation
bubbles on the surfaces. Cavitation itself is associated with partial vacuums
formed in a liquid by sudden changes in pressure. It may be caused by vibration
and reduced or uneven liquid flow conditions.
It is a type of failure very common in airplane and boat propellers, and pumps.
34
Typical surface treatments
Electrodeposition
35
PVD coatings
PVD coatings in Cutting Tools, >80% penetration
TiN, TiCN, TiAlN, CrAlN…..
36
Broaching
0
0,02
0,04
0,06
0,08
0,1
0,12
0,14
Fla
nk w
ear
VB
Unbeschichtet 0,026 0,047 0,074 0,09 0,091 0,092
TEE003/ CrTiAlN 0,029 0,042 0,055 0,068 0,091 0,093
TEE009/ CrMoTiAlN 0,037 0,039 0,041 0,044 0,048 0,05
100 200 300 400 500 600
mm
High corner wear (0,079 - 0,14 mm) on coated broaching tools after 200
strokes.
PVD coatings
37
PVD Nanocomposites
Ensayo de Carga de Soldadura
Sin Lubricante
00,10,20,30,40,50,60,70,80,9
11,11,2
0 100 200 300 400
Tiempo (sg)
Co
ef.
Fri
cció
n
0
200
400
600
800
1000
1200
Stro
ke
(µm
)
TiN TiN+Ag(1) TiN+Ag(2) Stroke-TiN Stroke-TiN+Ag(1) Stroke-TiN+Ag(2) Carga
Carga (N)
AFM: SEM:
Nanocomposite coatings:
Clusters Ag (40nm)
Matriz TiN
38
Protection of aluminium alloys
Anodic oxides
Widely used to protect aluminum from corrosion.
•The metal and many of its alloys are anodized in such acids as boric
(B2O3·3H2O), oxalic, phosphoric (H3PO4), and sulphuric (H2SO4) under
conditions in which an oxide is formed on the surface.
•The anodizing of aluminum is widely utilized to protect aluminum
components on automobiles, buildings, storm doors, and windows.
•Organic coatings
•They are widely used barrier coating for protecting aluminum, steel, and
zinc against atmospheric corrosion.
•The main function of the coating is to serve as a barrier to water, oxygen,
and ions and thus prevent the cathodic reaction H2O + 1 /2 O2 + 2e-
2OH- from occurring beneath the coating.
•Organic coatings are effective only if they protect the entire metal
surface.
Anodized
aluminium
plunger
39
Electroplating
Like other of surfaces used in tribological applications,
electroplated coatings have two primary categories:
•Hard coatings are normally used to resist many
forms of wear, such as those that involve abrasive,
adhesive, and erosion process. Some degree of
toughness in these coatings is often desirable, in
order to resist cracking.
•Soft coatings are sometimes used on bearing
surfaces to provide low shear strength. They are
typically used at ambient temperatures and low loads.
Electroplated materials:
•Chromium, Nickel (Cr coatings are under concern)
•Precious metal (Gold, Paladium, Paladium-nickel)
•Soft metals (tin, lead-tin, and silver alloys, …lead is under concern)
40
Plasma electrolytic oxidation (PEO)
•This process transforms the surface of Aluminium, Titanium or
Magnesium into a complex ceramic matrix with outstanding
resistance to corrosion and wear.
•The process consists on applying a plasma discharge in the
interface metal electrolyte which transforms the substrate
surface into a dense and hard ceramic oxide without damaging
the substrate by thermal expansion.
Some of the main applications:
•Automotion: For wear and corrosion protection of piston
heads, cylinder liners, Piston top ring grooves,.
•Aerospace: use on satelites.
•Paint Industries.
Low material loss
41
líquido sólido
aire q
Wettability characterization
PEO Coatings (120 mm and 20 mm) on Al-7%Si and Al-12%Si,
Wetting properties of the
coated/uncoated aluminiums
0
10
20
30
40
50
60
Aluminium LM6 Aluminium LM 13 Aluminium TEKSID
Init
ial
Co
nta
ct
An
gle
(º)
Uncoated 20 um coating 120 um coating
Wetting properties of the
coated/uncoated aluminiums
0
5
10
15
20
25
Aluminium LM6 Aluminium LM 13 Aluminium TEKSID
Mean
Co
nta
ct
An
gle
(º)
Uncoated 20 um coating 120 um coating
At t =0 Al-7%Si better wetting
On coated samples the lubricant wets better
42
Are versatile, rapid application of high performance materials in
thicknesses from a few mils to more than 25 mm on parts of a
variety of sizes and geometries.
… requires minimal base-metal preparation
Typical applications: piston rings, journals, conveyors, shifter
forks, extrusion dies, transformer cases, ship hulls, ship tanker
compartments, …
Properties:
·Oxidation, Corrosion and Wear resistance
· Restoration of dimension
· Abradable clearance control
· Thermal barriers
· Electrical conductivity or resistivity
Heating, propulsion and
impact of particles
Thermal Spray Coatings
43
Developments of the NANO HVOF Process
Equipment NANO HVOF Materials
Application
Properties
•Less energy needed reducing heat in the work piece
•Makes Inside Diameter HVOF possible
•Higher particle velocity leads to higher compressive internal stress
•Improved wear properties and improved corrosion barrier
MBN
44
WC-CoCr 86 10 4 coating thickness 80µm
0
100
200
300
400
500
600
700
800
900
1000
0 5 10 15 20 25 30 35 40
d50 of powder particle size in µm
Saltspray
Testresult h
-10 +1µm
-25 +5µm
-30 +10µm
-45 +22µm
-53 +15µm
NANO-HVOF HVOF
-15 +5µm
single particle splat thickness < 1 µm
single particle splat thickness approx. 7 µm
45
46
Friction properties
Wear of cast iron vs HVOF
coated surface
Roughness properties
47
Coating selection, using Microtribological tests
SAMPLE WEAR SCAR MATERIALS
COFmean(*)
( )
dCOFmean
( )
Ra
(um) Length
(um)
Width
(um)
BALL WEAR SCAR
Al-7%Si 0.08 0.003 0.04 no wear track Tiny adhesión of aluminium
Al-7%Si + PEO (10 um) 0.10 0.003 0.47 no wear track No wear scar
Al-7%Si + PEO (40 um) 0.10 0.003 0.23 no wear track No wear scar
Al-7%Si + HVOF 0.09 0.003 0.05 no wear track No wear scar
Al-12%Si 0.08 0.004 0.05 no wear track Tiny adhesión of aluminium
Al-12%Si + PEO (120 um) 0.10 0.004 0.13 no wear track No wear scar
(*)Mean value of the interval 400-500 cycles
Both surface treatments
improve the wear properties
decreasing the adhesion
mechanism
Bibliography
• A. Igartua, J. Barriga y A. Aranzabe, “Biodegradable Lubricants”, The Virtual Tribology Institute, ISBN 83-7204-449-X, Poland, 2005.
• K. Holmberg, S. Hogmark, A. Igartua, B. Podgotnik, “Triboscience and tribotechnology superior friction and wear control in engines and transmissions, European Science Foundation, COST, QS-NA-23308-EN-C, Belgium, 2007
• A. Igartua, E. Fuentes, R. Bayon, V. Sáenz de Viteri, “Materials for improved wear resistance of total artificial joints, COST Tribology, ISBN 978-84-932064-4-4, Eibar, 2007.
• E. Rabinowicz, Friction and Wear of Materials, Wiley Interscience, 1995
• A. Igartua, J. Barriga, A. Aranzabe, IV Congreso Ibérico de Tribología, “Resumen de las Comunicaciones, Ibertrib, Bilbao 2007.
• A. Igartua, A. Alberdi, “3rd COST 516 Tribology Symposium, ISBN 84-699-2557-1, TEKNIKER, Eibar,2000.
• K. Homberg, S. Hsu, La. A. Ferreira, J. Seabra, “Superior Friction and wear control in engines and transmissions, COST532 Conference, NIST Conference, ISBN 972-8953-01-1, Porto, 2005.
• J. Meneve, K. Verkammen, 2nd COST 516 Tribology Symposium, ISBN 90-5857-001-0, Antwerpen, May 1999
• Curso Tribología, AIN.
• TEKNIKER EU Projects participation: KRISTAL, CLEANENGINE, NANO-HVOF, EREBIO,
• J. Vizintin, M. Kalin, “Tribology of Mechanical Systems”, ASME Press, ISBN 0-7918-0209-4, 2004.
48
