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Friction Enhancing Properties of ZDDP Antiwear Additive: Part

IIInfluence of ZDDP Reaction Films on EHD Lubrication

Lisa J. Taylora; Hugh A. SpikesaaDepartment of Mechanical Engineering, Imperial College, London, United Kingdom

First published on: 01 July 2003

To cite this ArticleTaylor, Lisa J. and Spikes, Hugh A.(2003) 'Friction-Enhancing Properties of ZDDP Antiwear Additive:Part IIInfluence of ZDDP Reaction Films on EHD Lubrication', Tribology Transactions, 46: 3, 310 314, First publishedon: 01 July 2003 (iFirst)

To link to this Article: DOI: 10.1080/10402000308982631

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Friction-Enhancing Properties of ZDDP Antiwear

Additive: Part II-Influence of ZDDP Reaction

Film s on EHD ~ u b r i c a t i o n ~

LISA

J

TAYLOR AND HUGH A. SPIKES (M ember, STLE)

Imperial College

Department of Mechanical Engineering

London, SW7

2BX,

United Kingdom

KEY WORDS

Antiwcor Additives; ZDDP; Friction; Surface Roughness;

I3oundary Lubrication; Elastohydrodynamic Lubrication

INTRODUCTION

Port of this paper described a study of the friction and anti-

wear film-form ing propertics of a series of zinc dialkyldithiophos-

photc (ZDDP)-containing mineral oil solutions

8) .

n this study, it

was found that ZDDP produced a large increase in friction coeffi-

cient in mixccl lubricntion conditions. This effect is quite well

known

and

has generally bcen ascribed to the formation by ZDDP

ol n

rclativcly rough, solid-like reaction filni on the rubbing sur-

faces, which rcsults in an increase in the proportion of solid-solid

cont;lct in mixed lubrication at a given speed, with a consequent

incrcnsc in friction 7),

9).

However in Ref. 8 )

t

was also found

that thc addition of some other engine oil additives to ZDDP,

although not mitigating thc increase in friction due to the ZDDP,

rcsultctl i n thc form;ition of very smooth reaction film. This

implics that surfacc roughening is not the sole or principle mech-

anism by which ZDDP additives increase friction.

Final m anusc ript approved March

10,2003

Review led

by

Joseph Perez

The current paper describes an investigation into alternative

mechanisms by which the antiwear additive ZDDP might increase

friction in the mixed lubrication regime.

SIGNIFICANCE OF ELASTOHYDRODYNAMIC FILM

FORMATION

Figure I, taken from Part

81,

summ arises the characteristic

friction behavior of a simple ZDDP solution in base oil. Details of

the ZDDP and base oil (and also the GMO friction modifier used

in this paper) are given in Ref.

8).

This figure illustrates how fric-

tion coefficient varies with increasing mean rolling or entrainment

speed in a rolling/sliding ball-on-disc contact. Since elastohydro-

dynamic (EH D) film thickness increases with entrainment speed,

the plots shown are Stribeck-type curves, showing how friction

varies from thin film, boundary lubrication conditions at slow

speed on the left, to thick film, full EHD lubrication conditions at

high speed on the right of the figure. The initial plot in Fig. is

the measured friction response at 100C immed iately after the

start of a test. Subsequent plots show friction after a progressive-

ly longer time of rubbing of the ball on the disc in very thin film

sliding/rolling conditions. During such rubbing, in-situ interfer-

ometry shows that a solid-like antiwear film about 100 nm thick

builds up on the surfaces and this film is believed to be responsi-

ble for the change in friction behavior observed.

It can be seen that the effect of the ZDDP reaction film is not

to increase the slow speed, boundary friction coefficient but rather

to extend the boundary friction regime to higher and higher

entrainment speed. There are two m ain ways that this might occur.

One is by increasing the roughness of on e or both of the solid sur-

faces.

If

this were to occur, a thicker elastohydrodyn amic (EHD )

film (and thus a higher entrainment speed) would be needed to

begin to separate the surfaces so that the contact can enter the

mixed lubrication regime. This mechanism has, however, been

excluded by the observation that even very smooth reaction films

can increase friction. The alternative is that the presence of the

ZDD P film somehow partially prevents the entrainment of lubri-

cant to form an EHD film, so that the speed needed to entrain suf-

ficient lubricant to separate even quite smooth surfaces is

increased. To test this possibility it is necessary to measure EHD

3

10


3/6


4/6

250

I 200

3 50

o o

G

50

0

O ~ A olid reaction film

............................................................................................

,

, ,

,

; , ,

A test

3

A base oil overnight

theoretical

Entrainment speed rnls

Fig. 3-EHD fllm formatlon of a ZDDP reaction film at

100C.

reaction

film was formed on a steel ball by rubbing it for three hours

against a steel dlsc at

50

SRR

0.1

mls entrainment speed at

100C

n

1

wt. ZDDP solutlon.The steel disc was then replaced

by a coated glass one without cooling or removing the lubricant

and fllm thickness measured as a function of entrainment speed

In pure rolling at

100C.

/

A O O A

owson.

I .

t

d.

ds., Buttcrwonhs.

6) Spikcs. H. A. (1990). 'l'hcrmodynamic Approach to Viscosity. Trih. Trotu..33,

pp 140- 148.

(7)'li1ylor.

L..

Dlntv:~.A. :lnd Spikes.

H.

A. (2000). Fricti on and Wear Behaviour

of Zinc

I i:~lkylditliiopliospIi:~te

Additive.

Trih. 7 rctns..

43, pp 469-479.

8)

:~ylor.

L. J

:~ndSpikes. H.A. (2003). Friction-Enhancing Propenies of ZDDP

Anliwe:~r Atltlit ivc: P:lrt I-Friction and Morphology o f ZDD P Reaction

Films. Submitted to

P ib Trcrt~s.

( 9 ) Tripaldi, G., Veltor,

A.

and Spikes, H. (1996). Friction Behaviour of ZD DP

Films in the Mixed. Bo unda ryEH D Regime, SAE Tech. Paper

962036.

(10) Warren,

0

L., Graham. J. F.. N onan. P R.. H ouston, J. E. and Michalske, T. A.

(1998), Nanomechanical Propenies o f Films Derived from Zinc

Dialkyldithiophosphate, Trih. Lett..

4, pp 189-198.

(11) Zhu. Y. and Granick. S. (2002). Apparent Sli p of Newto nian Fluid s Past

Adsorbed Polymer Layers,

Mocrontolcc~tlcs.

5, pp 4658-4663.

(121 Yin, Z., Kas rai,

M.,

Fuller,

M.,

Bmcroft , G.

M.,

Fyfe. K., Yamaguchi, E.,

Ryason, P. R.. W illenn et. P. A. and Tan, K.

H.

(1997), Application o f Soft

X-

ray Absorption Spectroscopy in Chem ical Characterization of Antiwear Films

Generated by ZDDP: P an I-The Effect of Physical Paraneters,

Wear.

202.

pp.172-191.

friction-enhancing properties of zddp antiwear additive part

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