Introduction to Polymer Tribologydesign/SoftMatter2018_10.pdf · tribological performances. 03/07/2018 2 MAIN CLASSES OF POLYMERS Thermoplastics Thermosets Elastomers ... coated on

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Introduction to Polymer Tribology

Polymers are long chain molecules in which one unit called “mer” repeat itself by covalent bonds. The backbone of these long molecules (macromolecules) are primarily made of C or Si.

There are synthetic polymers such as polyethylene and polypropylene, and naturally occurring polymers such as starch, glucose and proteins.

poly(ethylene)

Phospholipid

Sujeet K Sinha, Department of Mechanical Engineering, Indian Institute of Technology Delhi, India

Hydrophilic head

Hydrophobic tailProtein molecule

Elemental constituent, molecular structure, shape, chain length and cross-link density determine much of the mechanical and physical properties of a polymer. It is also true for their tribological properties.

Elements such as sulphur, nitrogen, fluorine may have beneficial effects on friction.

Linear chain structure give lower COF in comparison to branched or aromatic chains

PTFE molecule

Cross-link density has complex effect and an optimum cross–link density may give the best tribological performances.

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MAIN CLASSES OF POLYMERS

Thermoplastics Thermosets Elastomers

(e.g. PE, PTFE, Nylon, UHMWPE, HDPE, thermoplastic PI)

(e.g. epoxy, phenolic, thermoset PI)

(e.g. natural rubber, styrene-butadiene rubber (SBR), Nitrile rubber, butyle rubber, silicone rubber or polysiloxanes)

Friction is defined as the force of resistance when one body slides or rolls over the other

What is Friction?

Sliding vs Rolling

Friction is the work done or energy dissipated at the interface (contact area is important)

The main works at the interface involves working against the intermolecular adhesive forces, elastic deformation of the asperities, plastic deformation and fracture of the asperities, shearing of interfacial layer which is different from the two bodies.

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Normal LoadVelocity

Interfacial zone

Rigid asperity

Polymer

Cohesive zone

Hard surface

The Two-term Model of Friction

Ftotal = Fadhesive + Fploughing

1. Interfacial 2. Cohesive or ploughing

Wear debris

Scratch left behind

Two smooth surfaces with an interfacial

FP Bowden and D Tabor, Friction and Lubrication of solids, 1950

3. Friction at the Transfer Film

00.10.20.30.40.5

0 1 2 3 4Coef

ficie

nt o

f fric

tion

Distance, mm

Silicon nitride ball on Si/DLC/UHMWPE Coating

50µm 50µm

Silicon nitride ball surfaceWear track

A silicon nitride ball (4 mm diameter) slid against PEEK (Ra=2.21 μm) with Multiply-Alkylated Cyclopentane (MAC) oil (0.4 wt%) in Hexadecane (Loy and Sinha Wear 296 (2012) 681–692 )

PEEK (Ra=2.21 μm) lubricated with Multiply-Alkylated Cyclopentane (MAC) oil (0.4 wt%) in Hexadecane

PEEK surface PEEK surface Si3N4 ball

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Coefficient of friction vs Number of cycle for 0.4wt% MAC coated on PEEK of various surface roughness (Loy and Sinha Wear 296 (2012) 681–692 ).

Bearing Steel ball of 12 mm diameter sliding against an epoxy composite

Transfer film is formed during the initial sliding

The interfacial term of friction is influenced by the interfacial shear strength at the contact,

τ = τo + αp

μadhesive = τo/p + α

The interfacial shear strength is a strong function of interfacial interactions and what goes on at the interface

Interfacial or adhesive friction is a strong function of the contact area

e.g. surface modification by physicalor chemical means can help changethe coefficient of friction or stiction

Si surface

Self-assembled monolayer (SAM)

τ = shear stressτo = shear yield strength of the interfaceα = a constantp = contact pressureμ = coefficient of friction

At high contact pressure (or load), μ = α

Hydrophilic head

Hydrophobic tailMain body of the molecule

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Tabor’s calculation of the pull-off force (Journal of Colloid and Interface Science, Vol. 58, No. I, January 1977)

The attractive force between a sphere and a flat in the non-retarded region is give as,

AH1,2 is the appropriate Hamaker constantZ is the smallest distance of approach

Contact between an elastic sphere and a hard flat surface as a function of the load between them (schematic). Because of surfaceforces the circle of contact is larger than that given by the classical equations of Hertz which assume zero adhesion between the surfaces. According to Derjaguin et al. (10), the pull-off force Fo = 4πRγ.

γπ RF o 4=

Contact Area and Adhesion

No adhesion

JKR model for contact areaJohnson, Kendall and Roberts published their model in 1971 which considered the adhesive force only within the contact and is applicable to the case when the elastic deformation by adhesive force is large.

The pull-off force at the point of separation between the two solids for the JKR model is calculated for a = 0 as,

where, ΔGad. ≈ 2γ

Ref: Johnson, Kendall and Roberts, Proc. R. Soc. Lond. A324, 301 (1971)

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Interfacial friction depends upon the surface adhesion between the two contacting bodies

McFarlane and Tabor, Relation between friction and adhesion, Proc. Roy. Soc. Lond. A, 1950, 202, 244-253.

3.03.0 22 −= ψμ

μ = Coefficient of frictionψ = Coefficient of adhesion (the ratio of adhesion force between two metals to the original normal load applied

Friction of Graphite Surface against tungsten sharp pinSkinner, J, Gane N and Tabor D., Micro-friction of graphite, Nature Physical Science, Vol. 232, August 30, 1971, pp. 195-196.

They found that at low loads (<40 mg), the coefficient of friction on basal plane was very low (μ=0.002 to 0.05). The COF was 0.3 on the edge planes.

At low loads, as there was no appreciable damage to the surface, this difference in the COF was attributed to the differences in the adhesion property between the tungsten tip and the graphite planes.

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To provide an analytical relationship between friction and surface energy for UHMWPE film and compare with the current experimental data

SiUHMWPE filmSilicon nitride ball

Relation between Adhesion and Friction for polymers

Surface Energy Modifications

The surface energy of the Si3N4 ball

- air-plasma treatment with 1 and 10 minutes exposure timeand

- 3-4 nm thick PFPE film was overcoated onto it

The surface energy of UHMWPE film

- air-plasma treatment with different exposure times (30 seconds, 5 minutes and 10 minutes)

Minn, M. and Sinha, S. K. (2010) The frictional behavior of UHMWPE films with different surface energies at low normal loads, WEAR, Vol.268 (2010) 1030-1036.

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Results and discussion

Sample Roughness Treatment Water contact angle (deg)

Surface Energy

(mJ/m2)

Si3N4 ball 5 nm

PFPE coated 95 17.9No treatment 83 21Air Plasma (10 mins) 38 29.7

UHMWPE film 0.6 μm

PFPE coated 95 23.7No treatment 93 26.9Air Plasma (30s) 43 44.92Air Plasma (5 mins) 39 45.93Air Plasma (10 mins) 36 46.52

Bare Si 0.41 nm PFPE coated 66 24.13


Surface Energy and Attractive Force between Surfaces

It is known that when two surfaces (e. g. ball and a flat surface) come into contact, there is a finite force acting between them called attractive or pull-off force, Fo

*

* R. S. Bradley, Philos. Mag. 13, 583 (1932). D. Tabor, J. Colloid Interface Sci. 58, 2 (1977).

where and are surface energies of the two surfaces and 1γ 2γ

( )2

12 1 2γ γ γ= −


Fo =2πR (γ1 + γ2 - γ12)


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Si3N4 ball UHMWPE film

Fo (mJ/m2)Treatment

Surface Energy

(mJ/m2)Treatment

Surface Energy

(mJ/m2)

PFPE coated 17.9 No treatment 26.9 0.55

No treatment 21 Air Plasma (10 mins) 46.52 0.79

Air Plasma (10 mins) 29.7 No treatment 26.9 0.71

PFPE coated 17.9 Air Plasma (10 mins) 46.52 0.72

No treatment 21 No treatment 26.9 0.6Air Plasma (10 mins) 29.7 Air Plasma (10

mins) 46.52 0.93

Attractive force, Fo



The relationship between the shear stress, τ and the contact pressure, P is obtained as,

0

0.1

0.2

0.3

0.4

50 70 90 110 130Contact Pressure, P (MPa)

Shea

r Stre

ss, τ

(M

Pa)

0.55 mN 0.71 mN 0.6 mN0.72 mN 0.79 mN 0.93 mN

Attractive force, Fo

1. All Fo obey τ = τo + αP.

2. α, the pressure coefficient is 0.0013, same for all Fo.

3. Initial shear stress, τoincreases as Fo

Observations:


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The relationship between the initial shear stress, τo and the attractive force, Fo is

y = 9E-05e8.3991x

R2 = 0.9836

0

0.05

0.1

0.15

0.2

0.25

0.3

0.5 0.6 0.7 0.8 0.9 1Attractive Force, F o (mN)

Initi

al S

hear

Stre

ss, τo

(MPa

)

τo = c1 exp (n Fo)


The relationship between initial coefficient of friction and surface energy

Since τ = τo + αP and the initial friction force, Fi = τA, we can write as Fi = τoA+ αPA

By dividing the above equation with applied load, L, we obtain the initial coefficient of friction, μi as

oi A

Lτμ α = × +

where P = L/A.


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Finally, we can correlate μi as a function of attractive or pull-off force, Fo using τo = c1 exp (n Fo) as

1 oi

exp( )c nF AL

μ α = × + For the visco-elastic materials such as UHMWPE film, Bowden and Tabor* suggested that the contact area, A is nearly proportional to L0.75

1 2 oi 0.25

exp( )c c nFL

μ α = + * F. P. Bowden and D. Tabor, The Friction and Lubrication of Solids (Oxford University Press, Oxford, 1958).


Fo =2πR (γ1 + γ2 - γ12)


0

0.05

0.1

0.15

0.2

0.25


Initi

al C

oeffi

cien

t of F

rictio

n, μ

i

15 mN 25 mN

0

0.05

0.1

0.15

0.2

0.25


Initi

al C

oeffi

cien

t of F

rictio

n, μ

i

50 mN 75 mN

Verification of the relationship between initial coefficient of friction and surface energy

1 2 oi 0.25

exp( )c c nFL

μ α = +

c1 = 9 x 10-5 Mpac2 = 0.74 ± 0.18 m2 N−0.75

n = 8.4 (mN)-1


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Data on the Relation between Adhesion and FrictionSample Water

contact angle (°)

Initial Coefficient of

FrictionSi 6 0.6

Si/Ph2CO/fPE/PFPE 71 0.066

Si/Ph2CO/fPE 80 0.11

Si/Ph2CO/PFPE 82 0.0865

Si/Ph2CO 86 0.179

M. Minn, YSG Soetanto and SK Sinha, Tribological properties of ultra-thin functionalized polyethylene film chemisorbed on Si with an intermediate benzophenone layer, Tribology Letters 41 (2011) 217-226

SAM Film thickness, Angstrom

Surface energy, mN/m (Water contact angle,degree)

Coefficient of friction

OTS 25 20.2 (111) 0.07±0.01

UTS 15 21.5 (108) 0.09±0.01

FTS 10 8.1 (105) 0.16±0.02

Organo-trichlorosilanes

1. n-octadecyltrichlorosilane (n-C18H37SiCl3 )(OTS),2. n-undecyltrichlorosilane (n-C11H23SiCl3, UTS)3. (tridecafluoro-1,1,2,2-tetrahydrooct-1-yl)trichlorosilane ( n-C6F13CH2CH2SiCl3) (FTS)

DePalma and Tillman, Langmuir, 5 (1989) 868-872

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Surfaces Water contact angle, degree


Roughness, nm

Bare Si 12 0.6 (high) 0.31

Si/OTS 108 0.19 (low) 1.957

Si/APTMS 50 0.5 (high) 1.7

Si/epoxy SAM 52 0.6 (high) 0.74

Satyanarayana and Sinha, J. Phys. D: Appl. Phys., 38 (2005) 3512–3522Satyanarayana, Gosvami, Sinha and Srinivasan, Philosophical Magazine, Vol. 87, No. 22, 1 August 2007, 3209–3227

Octadecyltrichlorosilane (OTS, CH3-(CH2)17-SiCl3)

3-aminopropyltrimethoxysilane (APTMS)

Non-polar methyl terminal group

Polar amine terminal group

Hydrophobic and hydrophilic SAMs

The substrate silicon is hydroxylated by treating the surface with a piranha solution (a mixture of 7:3 (v/v) 98% H2SO4 and 30% H2O2) at 60-70 oC for 50-60 minutes.

Epoxy SAM or GPTMS

3-glycidoxypropyltrimethoxy silane

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The relationship between the adhesive forces and work of the adhesion.

Coefficient of friction for Si(111), Au(111), and SAMs measured using the force calibration method. For each sample, the adhesive forces were measured at least six times at different locations on the surface.

Ref. Fig 5 and 7, Bhushan and Liu, Physical Review B, 63 (2001) 245412

Ref. Fig 9, Bhushan and Liu, Physical Review B, 63 (2001) 245412

HDT (on Au) shows the lowest friction and lowest wear

DHBp (on Si) has the highest wear resistance due to rigid spacer chain, stronger interfacial bond and rigid substrate.

Relative humidity (RH) does not have very strong influence on friction. RH may increase or decrease friction due to capillary force or lubrication effects of water.

Critical load for accelerated wear

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Sample Water contact

angle, degrees

Surface

roughness#, nm

Thickness,

nm

Coefficient of

friction

Wear life, number

of cycles

Bare Si 12 0.16 - 0.4 100

Si/APTMS 52 0.37 4.1 0.83 100

Si/APTMS/PE 97 1.9 15.0 0.08 4400*

Si/APTMS/PS 80 1.6 12.1 0.4 100

Water contact angle, coefficient of friction and wear life data for bare Si, Si/APTMS and polymers films.

*the lowest and highest wear life data among three tests for Si/APTMS/PE are 3000 and 7100 cycles, respectively.#RMS roughness measured using AFM over a scan area of 1 µm x 1 µm.

N. Satyanarayana, S. K. Sinha and L. Shen, Effect of molecular structure on friction and wear of polymer thin films deposited on Si surface”, Tribology Letters (in press)

Nanolubrication by composite layers: SAM +PFPEN. Satyanarayana’s PhD thesis, NUS, 2007

Sliding speed = 0.021-0.042 ms-1

Material Water contact angle, degrees


Bare Si 12 0.6Si/PFPE-as lubricated 66 0.17Si/PFPE-thermal treatment 112 0.13Si/OTS 108 0.19Si/OTS/PFPE-as lubricated 114 0.15Si/OTS/PFPE-thermal treatment 110 0.13Si/APTMS 50 0.5Si/APTMS/PFPE-as lubricated 114 0.2Si/APTMS/PFPE-thermal treatment 112 0.13Si/epoxy SAM 52 0.6Si/epoxy SAM/PFPE-as lubricated 81 0.1Si/epoxy SAM/PFPE-thermal treatment

107 0.17

Hydrophilic SAM helps bind with PFPE molecules

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 20 40 60 80 100 120

Coef

ficie

nt o

f Fric

tion

Water Contact Angle (sessile)

Si/SAMs/PFPE Lubed/Thermally-treated

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 20 40 60 80 100 120

Coef

ficie

nt o

f Fric

tion

Water Contact Angle (sessile)

Si/SAMs/Thermally-treated

Plotting the data from the previous slide

0

0.5

1

1.5

2

2.5

0 1 2 3 4 5 6 7Coef

ficie

nt o

f fric

tion

Distance, mm

Silicon nitride ball on Si

50µm 50µm

Track Ball

00.10.20.30.40.5

0 1 2 3 4Coef

ficie

nt o

f fric

tion

Distance, mm

Silicon nitride ball on Si/DLC/UHMWPE film

50µm 50µm

0

0.51

1.52

2.5

0 1 2 3 4Coef

ficie

nt o

f fric

tion

Distance, mm

Silicon nitride ball on PMMA

50μm50μm

4 mm diameter ball, 4 g normal load at 1mm/s sliding speed

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PEEK

0

2

4

6

8

10

12

14

16

18

0 1 2 3 4 5 6 7 8Scratch Distance (mm)

Scra

tch

Forc

e (N

)

0.51.02.04.06.08.0

10.0

Scratch force as a function of scratching time polymers tested at a scratching speed of 0.2 mm s-1.30o included angle conical tip of carbon tool steel; tip radius = 6 μm

PEEK

PMMA

0

2

4

6

8

10

12

14

16

18

0 1 2 3 4 5 6 7 8Scratch Distance (mm)

Scra

tch

Forc

e (N

)

PMMA

5 N 10 NNormal loads

Cohesive Friction - Scratching by a conical tip

Ref. S. K. Sinha and Desmon Lim, “Effects of normal load on single-pass scratching of polymers” Wear, 260 (2006) 751-765.

Sinha and Lim, Wear 260 (2006) 751–765

PVC

02468

101214161820

0 10 20 30 40Time(s)

Scra

tch

Forc

e(N

)

0.51.02.04.06.08.010.0

0.5 N 6 N 10 N

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ThermoplasticsLinear or branched molecules held together by weaker intermolecular forces; No cross-linking, though, some may have few % of cross-linking for mechanical property enhancement.

In comparison to thermosets, thermoplastics are weaker mechanically with lower hardness

MaterialPercent

Elongation %Modulus of Elasticity,

E (GPa)Ultimate Tensile Strength (MPa)

Indentation hardness

(MPa)

PP: poly(propylene) 150 – 300 0.9 –1.5 25 – 40PVC: poly(VinylChloride) 60 2.5 – 4.0 25 –70PC: polycarbonate 60 2.3 – 2.4 55 – 75 155 PEEK: poly(ether ether Ketone) 100 –150 3.7 – 4.0 70 –100

PMMA: poly(methylmethacrylate) 2.5 – 4 2.4 – 3.3 80 173

POM: poly(oxymethylene) 15 – 40 2.3 – 2.8 60 –70PET: poly(ethyleneterephthalate) 50 –300 2.0 – 4.0 80 147

UHMWPE:Ultra-high molecular weight poly(ethylene)

500 0.2 - 1.2 20 - 40 40

PTFE: Poly(tetrafluoroethylene) 400 0.3-0.8 10-40 27-32

HDPE:High Density poly(ethylene)

600 0.5 – 1.2 15 - 40

LDPE:Low Density poly(ethylene)

600 0.1 – 0.3 2 - 25

Thromoplastics are unique as many provide very low coefficient of friction and some with low wear

PolymerCoefficient of

frictionSpecific wear rate (x10-6 mm3/Nm)

PV value (Pa.ms-2)

PMMA 0.48 1315.9 145560PEEK 0.32 31.7 149690UHMWPE 0.19 15.5 187138POM 0.32 168.2 149690Epoxy 0.45 3506.6 153997PTFE < 0.1-0.2

What makes some thermoplastics low friction materials?

1. Specific wear rate of a pin = Wear volume in mm3 /(Load in N x Distance travelled in m)2. PV means Pressure (Pa) x Velocity (ms-1)

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Teflon (PTFE)

PTFE shows directionality in friction property after it has been rubbed slightly

(c) Specimen rotated by 90o after the first sliding and then re-slid on a different spot on the glass plate.

(a) First sliding; PTFE pin on glass plate

(b) Specimen re-slid after the first sliding but in the same direction on a different spot on the glass plate

Crystalline structure of PTFE

Electron micrograph of “granular” PTFE slowly cooled from 380 oC. Image taken of a N2 cooled broken specimen at magnification x6000

Ref. Bunn, Cobbold and Palmer, J. Polymer. Sci., 28 (1958) 365-376

Electron micrograph of “granular” PTFE cooled slowly from 500 oC. Image taken of a N2 cooled broken specimen at magnification x6000

Crystallized block of “dispersion” PTFE cooled slowly from 380 oC

Molecules in sheet form pressed together. The plane of such sheets are normal to the plane of the image. Striations are parallel to the length of the bands.

AFM image of teflon. ( taken VEECO.com website)

Molecules, with their ends level, aligned along the width of the bands

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Spherulitic structure of poly(ethylene)

Structure of crystalline polypropylene. Ref. Fig. 4-19, Page 124, Budinski & Budinski book

AFM image of a PE crystalline region

0.1

0.3

0.2

0.4

2 4 6 8 10

Coef

ficie

nt o

f fric

tion

Sliding distance (104 cm)

0.5

1.0

Wear depth (10

-1cm)

Tribology of PTFE

Wear depth and coefficient of friction as a function of sliding distance (Load 1.5 kg, speed 30 cm/s, 3mm dia ptfe pin rubbed against glass plate in vacuum)

Wea

r rat

e (1

0-7cm

/cm

)Co

effic

ient

of f

rictio

n

Band width (μm)0.1 0.2 0.3 0.4

5

10

15

0.1

0.3

0.2

All figures redrawn from Ref. Tanaka, Uchiyama and Toyooka, Wear 23 (1973) 153-172.

Higher band width gives higher wear while friction is largely unaffected

Early research [Gorokhovaskii and Agulov, Mekhanik Polimerov, 2(1) (1966) 87-92] mentioned that wear rate of PTFE was influenced by the crystallinity of PTFE (and also the size of the crystal had some effect), however, Tanaka and co-workers (1973) mention that the band size in PTFE is the main contributor to wear.

Friction coefficient

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Proposed mechanism of PTFE film formation during sliding (after Tanaka, Uchiyama and Toyooka, 1973)

Mechanism of Friction and Wear for PTFE

Easy slippage between bands

Friction is low because of the slippage between the bands.

These bands can be easily removed which can be transferred on to the counterface as a film; Thicker band will lead to high wear rate

High temperature produces lumpy transfer material with high wear rate. This is because of the reduction in the bulk strength in comparison to that of the interface.

Interfacial shear strength > Bulk shear strength (at high temperature)

The coefficient of friction depends upon a balance between the increase in the contact area due to softening and the decrease in the bulk shear strength

Sliding direction

Crystalline slice

Tribology of poly(ethylene) (PE)Low Density poly(ethylene) (LDPE) High Density poly(ethylene) (HDPE)Ultra-High Molecular Weight poly(ethylene) (UHMWPE)

μ

Sliding distance, mm

0.1

0.3

0.2

0.4

LDPE

10 20

μ

Sliding distance, mm

0.1

0.3

0.2

0.4

HDPE

10 20

Ref. Images taken from Pooley and Tabor, Proc. R. S. London A. 329 (1972) 251-274

Based on figures in Ref. Briscoe and Tabor, Friction and Wear of Polymers, Chapter 1, Polymer Surfaces, D.T. Clark and J. Feast, Ed., JohnWiley & Sons, 1978

PE slid over glass surface at 20 oC, Load = 1kg, sliding speed = 1 mms-1

Lumpy transfer film

Thin and adhering transfer film

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Isothermal transfer layer types

No transfer layer

e.g. PP

Smooth molecular profile or ‘special’ polymers

e.g. PTFE, UHMWPE

Lumpy or un-ordered transfer layer

e.g Low Density PE

Counterface

Transfer fi l m

Counterface

Transfer fi lm

Prevailing temperature can change the way transfer films are formed with consequential changes in the coefficient of friction and wear rate

Ultra-high Molecular Weight Poly(ethylene)

Yield strength: 21-28 MPa

Tensile strength: 38-48 MPa

% Elongation: 350-525

Wear rate: 15 x 10-6 mm3/N-m at PV = 0.18 MPa. ms-1

Less than half compared to other engineering polymers such as PEEK and PMMA

More than double for PEEK and more by order(s) of magnitude for other polymers

UHMWPE is a poly(ethylene) with molecular weight number in the range of 3.1-5.67 millions; less packing of the molecules in the crystal structure (semi-crystalline with 70-80% crystallinity) with density of 0.935-0.930 g/cc which is slightly lower thanthat of HDPE (0.941 g/cc).

Note: Much lower (in the range of 10-14 to 10-16

mm3N-1m-1) wear rates are also reported (see Jones and Hardy, WEAR, 70 (1981) 77-82 and references therein). This differences could arise due to many operational factors that tribological properties of materials depend upon. Therefore, for any comparison, it is always recommended that all the tests are conducted using the same machine and under the same experimental conditions. Both friction and wear properties are system dependent.

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A success story for UHMWPE

Specific Wear Rates for Polymers (Data taken from different sources)

(See next slide for experimental conditions)

Ref. S. K. Sinha, “Wear Failure of plastics” 2002, chapter in “Failure Analysis & Prevention”, ASM International Handbook, Volume 11 (Volume Editors: William T. Becker and Roch J. Shipley), Ohio, USA, pp. 1019-1027.

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Specific wear rate for a number of polymers in adhesive wear modes [101]. Specimen legends and test conditions are given below (with publisher’s permission):

Specimen Material Counterface Sliding speed 1/Se Normal pressure Temp. Ref.No. roughness Ra,

μm (v), m/s (p) MPa oC________________________________________________________________________________________________________________1. PMMA 1.2 --- 0.09 --- --- [176]2. PBI (Polybenzimidazoles) --- 1 --- 1 20 [120]3. Nylon 6 --- 5x10-3 --- 20 --- [177]4. Nylon 11 0.11 1 --- 0.65 --- [124]5. Nylon 1.2 --- 0.1 --- --- [176]6. PEEK --- 1 --- 1 20 [120]7. PEEK 0.05 0.5 --- 5 --- [166]8. Polystyrene (PS) 1.2 --- 5 --- --- [176]9. Acetal 1.2 --- 0.5 --- --- [176]10. Polypropelene (PP) 1.2 --- 0.1 --- --- [176]11. PTFE --- 0.2 --- 0.05 --- [107]12. PTFE --- 0.1 --- 5.66 29 [68]13. PTFE 1.2 --- 0.2 --- --- [176]14. UHMWPE 0.05 0.5 --- 5 --- [166]15. HDPE 0.9 0.03 --- 2.8 --- [178]16. Polyethylene (PE) 1.2 --- 0.09 --- --- [176]17.Phenolic resin 0.05 5.6 --- 0.84 --- [75]

Ref. S. K. Sinha, “Wear Failure of plastics” 2002, chapter in “Failure Analysis & Prevention”, ASM International Handbook, Volume 11 (Volume Editors: William T. Becker and Roch J. Shipley), Ohio, USA, pp. 1019-1027. (All references mentioned in the last column of the table are given in this chapter)

Continue from the last slide…

0

0.05

0.1

0.15

0.2

0.25

PS

PPS+

PTFE

ABS

POM

+PTF

E

PEEK PI

HD

PE

PTFE

PMM

A

UH

MW

PE

Spec

ific

Wea

r Rat

e (m

m^3

/N-m

)

(Abrasive wear results)

Abrasive wear results for a number of polymers. The tests were in a dry sand rubber wheel tester that uses a 228 mm diameter chlorobutyl rubber wheel (60Shore A) as an abrader. Width of the wheel = 12.7 mm, rotational speed = 20.9 rad/s, Loading force = 140 N, Abrasive: 215 to 300 μm silica sand.

B. J. Briscoe and S. K. Sinha, “Tribology of polymeric solids and their composites” 2005, in Wear – Materials, Mechanism and Practice, (ed. G. Stachowiak), pp. 223-267, John Wiley & Sons, Chichester, England.

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Ball slider on bulk UHMWPE sheet

0

0.1

0.2

0.3

0.4

0.5

0.6

0 20 40 60 80 100

Coef

ficie

nt o

f Fric

tion

No. of cycles of disk rotation

4mm dia.Si3N4 ball on bulk UHMWPE(Normal load 5g)

50 rpm

500 rpm

50μm

50μm

After 2000 cycles @50 rpm

Ball

Disk

After 2000 cycles @500 rpm

50μm 50μm

Ball Disk

A continuous or discontinuous film is formed on the ball surface (counterface) in both cases.

Friction and Wear Mechanism for UHMWPESimilar to other polymers, the role of transfer film is observed

50µm

4mm diameter Silicon nitride ball slid against a 3.4 μm thick UHMWPE film on Si/DLC substrate for 100,000 cycles at 4g normal load

50µm Same as above for 28 μm thick UHMWPE film

50µm

The film surface

Contact point in the film at very high magnification (FESEM image)

Fair strength and high toughness allows for interfacial plastic deformation and energy dissipation.

Extremely large molecular length and linearity helps in the localized deformation and relaxation of the molecules without fracture at molecular or micro scale. Large amount of local strain can be accommodated. UHMWPE has slightly lower density than HDPE which could help this material in unfolding of the long chain of molecules

Linear thermoplastic have “smooth molecular profile”, meaning they have no cross-linking and no bulky side group.

15.0kV ×500 100μm

Thick transfer layer or large plastic deformation in the bulk of the polymer leads to high friction and increased wear. (when temperature is high or after long period of sliding)

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“Smooth Molecular Profile” for polymers

(a) (b)

Chemical structure of PE and PS molecules functionalized with reactive maleic anhydride chemical groups. (a) Polyethylene-graft-maleic anhydride (PE) and (b) Poly (styrene-co-maleic anhydride) (PS).

Si wafer

3-aminopropyltrimethoxysilane (APTMS SAM)

Functionalized polymer film

Sample Water contact

angle, degrees

Surface

roughness#,

nm

Thickness,

nm

Coefficient

of friction

Wear life,

number of

cycles

Bare Si 12 0.16 - 0.4 100

Si/APTMS 52 0.37 4.1 0.83 100

Si/APTMS/PE 97 1.9 15.0 0.08 4400*

Si/APTMS/PS 80 1.6 12.1 0.4 100

Water contact angle, coefficient of friction and wear life data for bare Si, Si/APTMS and polymers films.

*the lowest and highest wear life data among three tests for Si/APTMS/PE are 3000 and 7100 cycles, respectively.#RMS roughness measured using AFM over a scan area of 1 µm x 1 µm.


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010203040506070

0 50 100 150

Normal load, g

Fric

tiona

l for

ce, g Bare Si

Si/APTMS

Si/APTMS/PE

Si/APTMS/PS

Effects of load and velocity on the performance of PE and PS films

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.5 1 1.5 2 2.5

Sliding velocity, mm/sec

Coe

ffici

ent o

f fric

tion

Bare SiSi/APTMSSi/APTMS/PESi/APTMS/PS


Effects of load and velocity on the performance of PE and PS films

The dynamic coefficients of friction with respect to the number of sliding cycles for bare Si, Si/OTS, Si/APTMS, Si/APTMS/PE and Si/APTMS/PS samples, obtained in ball-on-disk tests against 4 mm Si3N4 ball at a normal load of 5 g and sliding velocity of 0.021 ms-1.

SEM image of the wear track after tribological test on Si/APTMS/PS


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Transfer film Tribo-chemistry“Chemical reaction is essentially present in almost all tribological interactions”

Force

Rotation

Main reasons for tribo-chmeical reactions in polymer/metal interactions:

Elevated interfacial temperature

Catalytic actions of the exposed clean metal surface

Fillers either catalysing the reaction or take part in making reaction products

Mechanical straining of the materials resulting in chain scission of the organic molecules

The formation of the transfer film by thermoplastics is largely a chemical interaction

PTFE is extremely adherent to metals surfaces (freshly atomically cleaned, with oxide film and non-reactive metal surfaces) and forms few atomic layer transfer film depending upon the contact load and the duration of contact. This is true for both static and dynamic contacts.Evidences suggest that unsaturated C-C bonds in PTFE may react with metals or there may be van der Waals / electrostatic interactions between the polymer molecules and the metal surfaces. The adhesion is strong enough to pull-out metal debris from the metal counterface.

Ref. Brainhard and Buckley, WEAR 26 (1973) 75-93.

Addition of some compounds in a polymer may enhance the bonding of the transfer film

1,000

2,000

2 4 6

Wea

r, μg

Load, kg

Wear of 6 mm diameter HDPE pin against hardened steel (mean asperity height = 0.05 μm) – Effect of CuO and Pb3O4 fillers

No filler

With filler

Addition of a combination of CuO (5% by wt.) and Pb3O4(30% by wt.; both 0.1-5 μm particle)in HDPE has shown great improvement in the wear resistance of HDPE.

This improvement in wear resistance is attributed mainly to the improvement in the adhesion of the transfer film on steel surface. A combination of CuO and Pb3O4 is believed to enhance some aspect of chemical bonding between the steel and the HDPE transfer film.

Such improvement in the wear resistance did not happen for LDPE or for HDPE with glass substrate.

The presence of the fillers also reduced the sensitivity of the counterface temperature. Better wear resistance at slightly higher temperature (~50 oC) in comparison to that for HDPE without these fillers.

The coefficient of friction for HDPE was largely unaffected at lower loads (1-3 kg) but was lower for filled samples at high loads (3-5.5kg)

Ref: Briscoe, Pogosian and Tabor, WEAR 27(1974) 19-34.

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Effect of Polymer Molecular WeightZP Lu and K Friedrich, Friction and wear of PEEK and its composites, Wear 181-183 (1995) 624-631

Fixed crystallinity ~ 30%

Molecular weight varied from 14000 to 56000

Friction of Elastomers (rubbers)Rubber was the earliest polymeric material which was tested for tribological performances. In particular 1920s and 1930s saw many scientific studies on rubber tribology.

FL Roth, RL Driscoll and WL Holt, Friction properties of rubber, Journal of Research of the National Bureau of Standards, Research Paper-RP1463, Vol. 28, April 1942, 441-462

Standard abrasion and pre-gum, pressed vulcanized for 60 and 30 minutes at 143 oC

COF for standard abrasion rubber at different speeds and surfaces/roughnesses

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Friction Mechanism D. Moore, Friction and wear in rubbers and tyres, Wear, 61 (1980) 273-282.

Coefficient of friction as function of the sliding velocity at various temperatures of the acrylonitrile-butadiene rubber on wavy glass. Curves are shown in two groups for clarity.

Symbols

-15 oC

-12.5 oC

-10 oC

-5 oC

0 oC

5 oC

10 oC

20 oC 30 oC

40 oC

55 oC

70 oC

85 oC

K A Grosch, The relation between the friction and visco-elastic properties of rubber, Proc. Toya. Soc. London, Series A, Vol. 274, No. 1356 (1963), 21-39.

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Master curve of the coefficient of friction of the acrylonitrile-butadiene rubber compound on wavy glass. Reference temperature To = 20 oC

K A Grosch, The relation between the friction and visco-elastic properties of rubber, Proc. Toya. Soc. London, Series A, Vol. 274, No. 1356 (1963), 21-39.

Elastomers show large elastic deformation before tearing; Waves of detachment which is a characteristics of stick-slip can be seen in the contact area under shear

Annu. Rev. Fluid Mech. 2001. 33:265–87

Stick-slip in hot extrusion of polymers

Excellent wear resistance in the elastic range with high friction

The musical instrument violin is also based on the stick-slip motion between two strings which produces excellent sound due to the acoustics

A. Schallamach, How does rubber slide?, Wear, 17 (1971) 301-312.

Fig. 7, in Sinha and Briscoe, Encyclopedia of Polymer Science and Technology Copyright c 2006 John Wiley & Sons, Inc.

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Results on some industrial rubbersM Mofidi, Tribology of elastomers, PhD thesis 2007, Lulea University of Technology, Sweden

Counterface: steel

M Mofidi, Tribology of elastomers, PhD thesis 2007, Lulea University of Technology, Sweden

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Wear of RubberM Mofidi, Tribology of elastomers, PhD thesis 2007, Lulea University of Technology, Sweden


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Comparison of Dry and Lubricated sliding in abrasionM Mofidi, Tribology of elastomers, PhD thesis 2007, Lulea University of Technology, Sweden


Friction Wear

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Effects of filler on the tribology of rubbers

Yuqi Li, Qihua WangTingmei Wang and Guangqin Pan, Preparation and tribological properties of grapheme oxide/nitrile rubber nanocomposites, J Mat. Sci. (2012) 47, 730-738.


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Hydrogenated Acrylonitrile butadiene rubber with nanoclay

KG Gatos, K Kameo and J Karger-Kocsis, On the friction and sliding wear of rubber/layered silicate nanocomposites, eXPRESS Polymer Letters Vol. 1, No. 1 (2007) 27-31.


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Summary Few thermoplastics are extremely low friction and they can be used as bulk or as a solid lubricants

Excellent tribological properties of polymers depend upon several factors such as molecular profile, crystallinity, surface energy, ductility etc.

Suitable polymer composites can be designed if we understand the nature of the bulk polymer and then try to achieve low friction and low wear by solid lubrication or by strengthening

Rubbers tend to show very high friction and high wear beyond certain elastic limit. It is important to strengthen rubber by using strong fibres or particles for low wear while keeping high friction property

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