The thickness, friction and wear of lubricant films

(1)

The thickness, friction and The thickness, friction and

wear of lubricant films wear of lubricant films

SAE Powertrain & Fluid Systems Conference

& Exhibition, San Antonio, TX October 25^th 2005

Hugh Spikes

(2)

Why care about the thickness of Why care about the thickness of

lubricant films ? lubricant films ?

• Historically we simply measure the friction and wear properties of lubricants by rubbing bits of metal

together

• But what underlies and controls friction and wear is the protective film formed by the lubricant

• We can understand a lot more about friction and wear by studying this protective film

• And a good way to start is to measure its thickness

(3)

Layout of talk Layout of talk

• • The The Stribeck Stribeck curve curve

• • Measuring film thickness & friction Measuring film thickness & friction

• • Boundary films Boundary films - - base oil blends base oil blends

• • Antiwear films Antiwear films – – ZDDP ZDDP

• • New way to measure wear New way to measure wear

(4)

Layout of talk Layout of talk

• • The The Stribeck Stribeck curve curve

• • Measuring film thickness & friction Measuring film thickness & friction

• • Boundary films Boundary films - - base oil blends base oil blends

• • Antiwear films Antiwear films – – ZDDP ZDDP

• • New way to measure wear New way to measure wear

(5)

Stribeck

Stribeck (or (or Hersey Hersey or G or G ü ü mbel mbel ) ) curve

curve

Uη/W

Friction coefficient

Journal Bearings Thurston – found that

friction versus speed has a minimum (1879)

Stribeck – measured variation of friction coefficient with speed (1902)

Gümbel & Hersey – plotted variation of friction

coefficient with (uη/W) (both in 1914)

(6)

Origin of

Origin of Stribeck Stribeck curve curve

log(UηW)

Friction coefficient BL mixed hydrodynamic

log(film thickness)

h α (Uη/W)ⁿ

μ α (Uη/W)^1-n

n s

W k U

u U

A W

k U W

u

−

⎟ ⎠

⎜ ⎞

⎝ ⎛′

=

∝

⎟ ⎠

⎜ ⎞

⎝

⎛

⎟ ⎠

⎜ ⎞

⎝

⎛

=

η

1

μ

η η μ

W A h u

W A W

F A F

η s

μ μ τ

τ

=

(7)

Non Non - - conforming contacts conforming contacts

Film thickness and friction are decoupled.

Friction determined by highly

viscous, non-Newtonian oil in the high pressure contact

μ

_EHL

= 0.02 to 0.12

Film thickness determined by oil viscosity in the inlet

h = k ( μ U)

^0.7

α

^0.5

W

^-0.1

(8)

Stribeck

Stribeck curve in non curve in non - - conforming conforming contact

contact

(e.g. gears, cams)

log(film thickness)Friction coefficient

log(Uη) h α (Uη)^0.7

BL mixed Elastohydrodynamic (EHD)

EHD friction/traction

(9)

How How might might lubricants influence lubricants influence

friction in concentrated contacts?

• By controlling the thickness of EHL film (via viscosity and α-value)

log(Uη) BL EHL

• By controlling the EHL friction of the base fluid

(10)

How How might might lubricants influence lubricants influence

friction in concentrated contacts?

BL EHL

• By forming a thin, solid-like film with low (or high) shear strength

• By forming a viscous film on

surfaces _BL _EHL

• By changing the roughness of the surfaces

BL EHL

(11)

Layout of talk Layout of talk

• • The The Stribeck Stribeck curve curve

• • Measuring film thickness & friction Measuring film thickness & friction

• • Boundary films Boundary films - - base oil blends base oil blends

• • Antiwear films Antiwear films – – ZDDP ZDDP

• • New way to measure wear New way to measure wear

(12)

Measuring film thickness, 1885 Measuring film thickness, 1885

Goodman, Proc Inst. Civ. Eng., lxxxix, 1885-6, part III, pp 1-19

h_max = 73.5 μm ± 2.5 μm

(13)

Measuring film thickness

Measuring film thickness – – ultrathin ultrathin film interferometry

film interferometry

Video camera

Steel ball

Spectrometer

Coated glass disc

Microscope

( ^N ⁻ φ ) λ ⁼ ² ⁿ

_οι_l

^h

_oil

⁺ ² ⁿ

_spacer

^h

_spacer

(14)

1 10 100 1000

0.01 0.1 1 10

Entrainment Speed, U (m/s)

Film Thickness (nm)

40°C 80°C 120°C

Ultrathin film interferometry Ultrathin film interferometry

SN150 Group I mineral oil h = k(Uη)⁰^.⁷α⁰^.⁵

(15)

Ultrathin film interferometry Ultrathin film interferometry

Boundary films shown by enhanced film thickness at low speeds 1

10 100

0.001 0.01 0.1 1 10

Entrainment speed (m/s)

Film thickness (nm)

0.1% stearic acid, wet hexadecane

(16)

Measuring friction, 1884 Measuring friction, 1884

Tower B, Proc. I. Mech. E., 1884, pp. 29-35

(17)

Measuring friction

Measuring friction - - MTM MTM

Mixed rolling-sliding contact

Steel ball Steel disc

Test lubricant

(18)

MTM friction curve MTM friction curve

0 0.02 0.04 0.06 0.08 0.1 0.12

0.001 0.01 0.1 1 10

Entrainment speed, U (m/s)

40°C 80°C 120°C

SN150 Group I mineral oil

(19)

MTM MTM Stribeck Stribeck curves curves

0 0.04 0.08 0.12

0.1 1 10 100 1000

Film thickness (nm)

40°C 80°C 120°C

SN150 Group I mineral oil

boundary friction

EHL friction

(20)

Layout of talk Layout of talk

• • The The Stribeck Stribeck curve curve

• • Measuring film thickness & friction Measuring film thickness & friction

• • Boundary films Boundary films - - base oil blends base oil blends

• • Antiwear films Antiwear films – – ZDDP ZDDP

• • New way to measure wear New way to measure wear

(21)

Layout of talk Layout of talk

• • The The Stribeck Stribeck curve curve

• • Measuring film thickness & friction Measuring film thickness & friction

• • Boundary films Boundary films - - base oil blends base oil blends

• • Antiwear films Antiwear films – – ZDDP ZDDP

• • Measuring wear Measuring wear

(22)

Viscous boundary films

• Modern, low polarity hydrocarbon base fluids generally require addition of a significant

proportion of higher polarity base fluid (e.g. ester) to help additive solubility and deposit formation control.

• The polar base fluid molecules should be

preferentially attracted to polar, metal surfaces

• Can we make use of this ?

(23)

Surface fractionation Surface fractionation

Guangteng et al., Wear (1996)

1 10 100

0.0001 0.001 0.01 0.1 1

Film thickness (nm)

10% Ester blend PAO (low η 13 cP) Ester (high η, 72 cP )

(24)

Why ? Why ?

The more polar

molecules concentrate close to the solid

surfaces because of solid/liquid attractive forces

If the more polar fluid is more viscous, a viscous surface layer is formed

(25)

Surface fractionation Surface fractionation

1 10 100

0.0001 0.001 0.01 0.1 1

Film thickness (nm)

10% Ester blend PAO (high η 314 cP) Ester (low η, 9 cP )

(26)

Effect of surface fractionation Effect of surface fractionation

0 0.04 0.08 0.12 0.16

0.001 0.01 0.1 1 10

EHL friction coefficient

10% Ester blend PAO (low vis) Ester (high vis)

Guangteng et al., Trib. Trans. (1997)

(27)

Effect of surface fractionation Effect of surface fractionation

0 0.04 0.08 0.12 0.16

0.001 0.01 0.1 1 10

SN100

ESTH+SN100 ESTH

100 °C

(28)

Layout of talk Layout of talk

• • The The Stribeck Stribeck curve curve

• • Measuring film thickness & friction Measuring film thickness & friction

• • Boundary films Boundary films - - base oil blends base oil blends

• • Antiwear films Antiwear films – – ZDDP ZDDP

• • New way to measure wear New way to measure wear

(29)

ZDDP antiwear films ZDDP antiwear films

15mins 30mins

45mins 60mins

90mins 120mins

0 20 40 60 80 100 120

0 1 2 3 4

Rubbing time (hrs)

Film thickness (nm)

0.08% wt. P, 80°C

ZDDPs form quite thick (100 nm) solid-like films on rubbing surfaces

Fujita et al., Trib. Trans. (2005…)

(30)

0 0.04 0.08 0.12 0.16

0.001 0.01 0.1 1 10

start of test 10 minutes 30 minutes 3 hours

100°C

Increased friction due to ZDDP Increased friction due to ZDDP film film

These films result in increased friction.

Not in the boundary but in the mixed lubrication regime

(31)

How ? How ?

• Forms a high friction surface film ? or

• Inhibits formation of an EHD film ?

(32)

Does the ZDDP film inhibit Does the ZDDP film inhibit

formation of an EHL film ? formation of an EHL film ?

(a) roll/slide steel disc on steel ball to form ZDDP reaction film (50% SRR, 0.1 m/s)

(b) replace steel disc by spacer-layer one and measure film thickness /speed behaviour in pure rolling using optical interferometry

Taylor et al., Trib. Trans. (2003)

(33)

Does the ZDDP film inhibit Does the ZDDP film inhibit

formation of an EHL film ? formation of an EHL film ?

0 50 100 150 200 250 300

0 1 2 3 4

Entrainment speed m/s

Film thickness (nm) Solid ZDDP film

EHD theory

(34)

Why ? Why ?

Possibilities …

• Formation of low viscosity liquid layer next to ZDDP film

• Formation of low α-value layer next to ZDDP film

• Starvation due to loose ZDDP film material blocking the inlet

• Slip of lubricant at the ZDDP film surface

0 120 μm 225

• Increase in surface roughness due 0

to the ZDDP film

(35)

Conclusions so far Conclusions so far

We can gain insight into lubrication mechanisms by coupling measurement of both film thickness and friction taken over a range of entrainment speeds

¾ Enables us to identify the operating regime of lubrication

¾ Enables us to explore impact of lubricant rheology on film formation

¾ Enables us to see how additives influence friction

Next stage is to combine this with wear measurements

(36)

Layout of talk Layout of talk

• • The The Stribeck Stribeck curve curve

• • Measuring film thickness & friction Measuring film thickness & friction

• • Boundary films Boundary films - - base oil blends base oil blends

• • Antiwear films Antiwear films – – ZDDP ZDDP

• • New way to measure wear New way to measure wear

(37)

Measuring wear 1803 Measuring wear 1803

Hatchett, C. Phil. Trans. R. Soc. Lond. Part 1, 42-194, (1803)

(38)

Limitations of existing wear tests Limitations of existing wear tests

• most current mild wear tests are based on pure sliding, non-

conforming contact with high sliding speeds

• Wear is measured from scar dimensions at end of test

• Because of high sliding speed, test often operates in mixed-lubrication.

Thus sensitive to viscosity

• Contact area and pressure change markedly during test as wear occurs on the stationary surface

0 400 800

0 20 40 60

test time (mins)

wsd(μm)

0 2 4

pressure (GPa)

(39)

Mixed sliding

Mixed sliding - - rolling rolling

• This distributes wear around contact so there is much less change in local geometry and thus contact pressure

• One obvious solution is to rotate both surfaces, to give mixed

sliding-rolling

• But it requires a different method of measuring wear

(no large, clearly defined, localised wear scar)

(40)

Sliding speed

Sliding speed vs vs entrainment entrainment speed

speed

• However mixed sliding-rolling also provides a very important opportunity

• Wear depends on sliding distance, so to obtain lots of wear in a reasonable time you need a high sliding speed, u

_s

2 1 u u

u _s = −

Wear rate ∝ u_s Sliding speed

• But a high sliding speed normally means a high

entrainment speed and thus a high EHL film thickness, and so less wear

⎟ ⎠

⎜ ⎞

⎝

= ⎛ + 2

2 1

u U u

EHD film thickness ∝ U^0.7 Entrainment speed

(41)

1 2

1

u u

u

_s

= − =

• In “pure sliding”, where u

₂

= 0

2 1

1 0.5

2u u

U u ⎟ =

⎠

⎜ ⎞

⎝

= ⎛ +

• But is mixed sliding-rolling we can have any u

_s

and U combination we want

Contra

Contra - - rotation rotation

• If we move the surfaces in

opposite direction (contra-rotation)

we can have high u

_s

with low U

(42)

For example For example - -

u_s = 0.5 m/s U = 0.25 m/s

• speed 1 = 0.5 m/s,

• speed 2 = 0 m/s

Pure sliding

• speed 1 = 0.35 m/s,

• speed 2 = -0.15 m/s

u_s = 0.5 m/s U = 0.05 m/s

Counter-rotation

(43)

• ICP-AES (

Atomic emission spectroscopy with inductively coupled plasma)

To measure wear To measure wear

¾ Monitors wear (remove samples of oil periodically)

¾ ppb resolution

¾ Must be no metal from other wear positions (or package)

¾ Needs ICP equipment

¾ Not on line

(44)

Test protocol Test protocol

• Set up MTM with test fluid and specimens

• Reach test temperature without load applied

• Withdraw two 0.5 ml samples of test oil for ICP

• Rub surfaces in contra-rotating sliding-rolling with entrainment speed = 0.05 m/s (boundary lubrication) and sliding speed = 0.25 m/s. Load = 30 N. Temperature normally 80ºC.

• Periodically halt rubbing, bubble in N₂ gas and withdraw two samples of oil for ICP (and take Stribeck curve and interference image if needed)

• End test after 2-4 hours and take two more oil samples

• Analyse oil samples and carry out profilometry and imaging on wear tracks

(45)

Wear results

Wear results – – base oil base oil

0 10 20 30 40 50

0 0.5 1 1.5 2 2.5

Time (hrs.)

Fe conc(ppm)

Base oil, 60°C Base oil, 80°C

Base oil, 80°C (repeat) Base oil, 100°C

λ ≈ 0.4

λ ≈ 1

• Wear rate increases with temperature (lower viscosity, thinner film)

• Good repeatability

(46)

Wear results

Wear results – – ZDDP solutions ZDDP solutions

• Very low ZDDP concentrations give higher wear than base oil alone

• For high ZDDP concs, almost no wear (unless other additives present)

0 20 40 60 80 100

0 1 2 3 4 5

Time (hrs.)

Fe conc(ppm)

base oil 0.005% P 0.01% P 0.02% P 0.03% P 0.05% P

(47)

Wear results

Wear results – – ZDDP solutions ZDDP solutions

0 1 2 3 4 5 6

0 0.02 0.04 0.06

[P] wt%

k 1(m3 /J)*10-15

0.01 0.1

1 10

0 0.02 0.04 0.06

[P] wt.%

k 1(m3 /J)*10-15

Q = k

₁

u

_s

Wt

(48)

Conclusions Conclusions

• • We now have experimental techniques to We now have experimental techniques to measure the film thickness and friction

measure the film thickness and friction properties of lubricants over the range of properties of lubricants over the range of

entrainment speed on a routine basis.

• • These enables us to relate EHL and boundary These enables us to relate EHL and boundary lubricating properties to lubricant composition lubricating properties to lubricant composition and optimise these properties