friction-enhancing properties of zddp antiwear additive part
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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
URL: http://dx.doi.org/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
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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.