Proceedings of WTC2005
World Tribology Congress III
September 12-16, 2005,
Washington,
D.C., USA
Proceedings
of WTC2005
World Tribology Congress III
September 12-16, 2005, Washington, D.C., USA
WTC2005-63246
WTC2005-63246: Dra0ft
All AFM topography images of tribofilms were collected
using a Nanoscope IIIa equipped with a Multimode head
(Digital instruments, Santa Barabara, CA). The images of the
antiwear films were recorded in contact force mode with Vshaped silicon nitride cantilevers possessing a nominal spring
constant of 0.12 N/m.
Oxygen (K-edge), phosphorus (K- and L-edge), sulfur (Kand L-edge) and Fe L-edge (XANES) spectra were collected on
the three Canadian Synchrotron Radiation Facility (CSRF)
beamlines situated on the 1 GeV Aladdin storage ring,
University of Wisconsin, Madison. Details are given elsewhere
(Yamaguchi et al.,2003)..
TRIBOFILMS GENERATED FROM ZDDP
AND DDP ON STEEL SURFACES: PART 1,
GROWTH, WEAR AND MORPHOLOGY
Z. Zhanga, E. S. Yamaguchib, M. Kasraia, and G. M. Bancrofta
a
Department of Chemistry, University of Western Ontario,
London, Ontario, Canada N6A 5B7
b
Chevron Oronite Company LLC, Richmond, California USA
94802,
ABSTRACT
The growth and morphology of tribofilms, generated
from zinc dialkyldithiophosphate (ZDDP) and an ashless
dialkyldithiophosphate (DDP) over a wide range of rubbing
times (10 seconds to 10 hours) and concentrations (0.1 to 5 wt
% ZDDP), have been examined using atomic force microscopy
(AFM), X-ray photoelectron spectroscopy (XPS), and X-ray
absorption near edge structure (XANES) spectroscopy at the O,
P and S K-edges, and the P, S, and Fe L-edges. The physical
aspects of the growth and morphology of the tribofilms will be
presented here (Part I) and the chemistry of the films will be
discussed in (Part II)
RESULTS AND DISCUUSSION
It has been shown in the past that the intensity of the P
K-edge XANES spectra can be used to measure the average
thickness of the major phosphate component of ZDDP-derived
antiwear films and thermal films (Suominen Fuller et al., 2000).
As an example, Figure 1 shows a representative of P K-edge
spectra from ZDDP antiwear films from a 1 wt % ZDDP
solution after very short rubbing times. These spectra arise
from the phosphate in the film, and not the original ZDDP,
which gives a lower energy peak at <2150 eV. Qualitatively, it
is rather remarkable that, even after 10 seconds of rubbing, a
rather intense phosphate P K-edge spectrum arises. These
intensities have been used to determine the tribofilm thickness.
INTRODUCTION
A number of surface analytical techniques have been
used to determine thickness and the morphology of tribofilms.
Techniques such as XPS, AES, and SIMS in conjunction with
ion etching, have been used to determine the physical film
thickness (Minfray et al., 2004). Unfortunately, ion etching
destroys the sample and thus the same sample cannot be used
for other characterizations. On the other hand, the XANES
technique is the only non-destructive method for thickness
measurement (Suominen Fuller et al., 2000).
ZDDP tribofilms have also been investigated using
imaging techniques such as atomic force microscopy (AFM)
(Graham et al., 1999) and imaging nanoindenters (Bec et al.
1999; Nicholls et al 2003) These techniques illustrate the
morphology of the film and are all in agreement that the ZDDP
antiwear film is laterally and vertically heterogeneous, being
composed of ridge and valley regions. The ridge regions are
composed of raised patches of film that have been termed
antiwear pads. It has been suggested that these pads are
responsible for bearing the load between the two rubbing
surfaces and limiting the contact between the asperities, thereby
reducing wear (Graham et al. 1999)
60
ZDDP tribofilm generated from different rubbing time
50
Intensity
40
30
20
10
1 min
30 sec
10 sec
0
2120
2140
2160
2180
2200
2220
Photon energy (eV)
Figure 1: P K-edge XANES spectra for ZDDP
tribofilms from short rubbing times
At very short rubbing times, the ZDDP films are much
thicker than the comparable DDP films: for example after 10
seconds rubbing, the ZDDP film is over 120 Å thick whereas
the DDP film is barely detectable. After one minute of rubbing,
the ZDDP film is well over 400 Å thick, whereas the
comparable DDP film is about 200Å thick.
The antiwear performance of ZDDP and DDP (1%) at
different rubbing times was also investigated. The WSW was
measured from the pin using a calibrated microscope. The wear
EXPERIMENTAL
Zinc
dialkyldithiophosphate
(ZDDP),
ashless
dialkyldithiophosphate (DDP) and the base oil used in this
paper were commercial products. The tribological films
(referred to as tribofilms) were generated in a Plint tribometer
on 52100 steel coupons. Experimental details for film
generation have been described before (Nicholls et al.,2003).
The testing conditions were: temperature 100 oC; Speed 25 Hz;
Load 225 N; Stroke 7 mm; rubbing time 10 seconds to 10 hrs.
1
1
Copyright
by
Copyright
© 2005©
by####
ASME
ASME
for the ZDDP additive for very short rubbing times (10-60 sec)
is relatively small (~ 75 µm) and gradually increases to 130 µm
after 1 hr rubbing. However, the WSW for DDP is always
higher than those of ZDDP’s values. It starts at ~ 100 µm and
reaches 180 µm after 1 hr rubbing. The antiwear performance
of mixed ZDDP/DDP was also investigated. It was found that a
very small concentration of ZDDP (0.1%) is sufficient to
produce a thick film and give a good wear performance.
ZDDP:100%
DDP:100%
ZDDP 10% +
DDP 90%
(A)
(B)
(C)
CONCLUSIONS
Using multiple analytical tools (XANES, XPS, and AFM), the
average thickness and the morphology of tribofilms formed
from ZDDP and DDP were investigated. For ZDDP, a
phosphate film (about 100 Å thick) forms after only 10
seconds, while the rate of DDP film formation is much slower.
The presence of a small amount of ZDDP in combination with
DDP is sufficient to provide very good wear protection, and
under all conditions used, antiwear performance of ZDDP was
superior to DDP. The morphology of the tribofilms was
measured using AFM. It was shown that at short rubbing times
(10 sec), the ZDDP films are less uniform than the DDP films.
However after 30 seconds rubbing, antiwear pads have formed
for ZDDP and not for the DDP films. Despite the difference in
chemistry and morphology of the films, the average thickness
of both films is very similar after many minutes of rubbing.
ACKNOWLEDGMENTS
This study supported by Chevron Oronite Company LLC,
(NRC), and (NSERC) of Canada. We are grateful to the
National Science Foundation (NSF) for supporting the SRC
under Award No. DMR-0084402.
REFERENCES
Bec S., Tonck A., Georges, J.M., Coy R.C., Bell J.C., Roper G.W.,
1999, “Relationship between mechanical properties and
structures of zinc dithiophosphate anti-wear films”.,
Proceedings of the Royal Society of London A; Vol., 455,
4181-4203.
Graham, J. F., McCague, C. and Norton, P.R, 1999, Topography and
nanomechanical properties of tribochemical films derived
from zinc dialkyl and diaryl dithiophosphates “Trib. Lett.
Vol. 6, pp 149-157
Minfray, C Martin, J. M. Esnouf, C. Le Mogne, T. Kersting R and.
Hagenhoff, B, 2004, “A multi-technique approach of
tribofilm characterization” Thin Solid Films Vol. 447-448,
pp. 272-277.
Najman, M.N., Kasrai, M., Bancroft G. M, Frazer B. H., and De
Stasio, G., 2004 “The correlation of microchemical
properties to antiwear (aw) performance in ashless
thiophosphate oil additives”, Trib. Lett. Vol. 17 pp. 811-22
Nicholls, M.A., Do, T., Norton, P. R., Bancroft, G. M., Kasrai, M.
Capehart, T. W,. Cheng, Y.-T and Perry, T. 2003, Chemical
and mechanical properties of zddp antiwear films on steel
and thermal spray coatings studied by XANES and
nanoindentation techniques” Trib. Lett. Vol.15, pp. 241-248.
Nicholls, M.A., Bancroft, G. M.. Norton, P. R. Kasrai, M,. De Stasio,
G. Frazer, B. H and. Wiese, L. M, 2004, “Chemomechanical
properties of antiwear films using X-ray absorption
microscopy and nanoindentation techniques” Trib. Lett.,
Vol.17 pp. 245-259
Suominen Fuller, M. L,. Fernandez, L. R., Massoumi, G. R., Lennard,
W. N., Kasrai, M. and Bancroft, G.M., 2000, “The use of Xray absorption spectroscopy for monitoring the thickness of
antiwear films from ZDDP”, Trib. Lett. Vol. 8, pp.187-192.
Yamaguchi, E.S., Zhang, Z, Kasrai, M. and. Bancroft, G. M, 2003,,
“Study of the interaction of ZDDP and dispersants using xray absorption near edge structure spectroscopy Part 2
Tribochemical reactions”, Trib. Lett. Vol. 15, pp. 385-394.
Figure 2: AFM images of tribofilm generated from: (a) ZDDP
alone, (b) DDP alone, (C) 10% ZDDP and 90% DDP
Figure 2 presents the AFM images for the ZDDP,
DDP and mixed ZDDP/DDP films. The area of high
topography (light brown in the colour image) is where the
tribofilm (polyphosphate) has formed, whereas the low area
(dark brown) represents the thin covered area or bare steel
(Nicholls et al., 2004: Najman et al., 2004). ZDDP forms large
pads elongated along the rubbing direction (Fig. 2A) whereas a
film generated from pure DDP does not show the large pad
formation and one can observe the scratch marks (Fig. 2B).
Once a small amount of ZDDP is added (10%), large pads are
formed, and further addition had no qualitative apparent effects
on the pad size and the morphology of the AFM image. The
lack of good tribofilm (pad) formation in the DDP film in the
early stage of film formation is obviously related to the
chemistry of the films which will be discussed in Part 2. Using
multiple analytical tools (XANES, XPS, and AFM), the
average thickness and the morphology of tribofilms formed
from ZDDP and DDP were investigated. For ZDDP, a
phosphate film (about 100 Å thick) forms after only 10
seconds, while the rate of DDP film formation is much slower.
The presence of small amount of ZDDP in combination with
DDP is sufficient to provide very good wear protection, and
under all conditions used, antiwear performance of ZDDP was
superior to DDP. The morphology of the tribofilms was
measured using AFM. It was shown that at short rubbing times
(10 sec), the ZDDP films are less uniform than the DDP films.
However after 30 seconds rubbing, antiwear pads have formed
for ZDDP and not for the DDP films. Despite the difference in
chemistry and morphology of the films, the average thickness
of both films is very similar after many minutes of rubbing. The
full manuscript has been submitted to Trib. Lett. (Nov. 2004)
2
2
Copyright
by
Copyright
© 2005©
by####
ASME
ASME